# Adsorbate Dissociation Due to Heteromolecular Electronic Energy Transfer
from Fluorobenzene Thin Films

E.T. Jensen  0000-0001-8030-4204<EMAIL_ADDRESS>Department of Physics
University of Northern British Columbia, 3333 University Way, Prince George
B.C. Canada V2N 4Z9

(February 13, 2024)

###### Abstract

Study of the near-UV photodissociation dynamics for monolayer (ML) quantities
of CH3I on thin films of a series of fluorobenzenes and benzene (1–25ML) grown
on a Cu(100) substrate finds that in addition to gas-phase-like neutral
photodissociation, CH3I dissociation can be enhanced via photoabsorption in
several of the thin films studied. Distinct CH3 photofragment kinetic energy
distributions are found for CH3I photodissociation on C6H5F, 1,4-C6H4F2 and
C6H6 thin films, and distinguished from neutral photodissociation pathways
using polarized incident light. The effective photodissociation cross section
for CH3I on these thin films is increased as compared to that for the higher
F-count fluorobenzene thin films due to the additional photodissociation
pathway available. Quenching by the metal substrate of the photoexcitation via
this new pathway suggests a significantly longer timescale for excitation than
that of neutral CH3I photodissociation. The observations support a mechanism
in which neutral photoexcitation in the thin film (i.e. an exciton) is
transported to the interface with CH3I, and transferring the electronic
excitation to the CH3I which then dissociates. The unimodal CH3 photofragment
distribution and observed kinetic energies on the fluorobenzene thin films
suggest that the dissociation occurs via the ${}^{3}Q_{1}$ excited state of
CH3I.

##  I Introduction

Electronic energy transfer (EET) in molecular systems is a phenomenon found in
a variety of contexts. For example, gas-phase photosensitization resulting in
enhanced UV photodissociation of CH3I in mixtures with benzene vapour was
noted over 70 years ago[1]. There are many heteromolecular systems where the
role of EET has been studied in detail via fluorescence (e.g.
p-difluorobenzene:p-xylene[2] and benzene:biacetyl[3] dimers) in molecular
beam experiments. In condensed-phase systems of aromatic molecules the
language of solid-state physics is used to describe the transport of localized
electron-hole excitations in terms of Frenkel excitons (electron-hole on the
same molecular site) or charge-transfer (CT) excitons when the electron-hole
pair are separated on nearest-neighbour sites[4]. In the astrophysical
context, non-thermal photodesorption of H2O and benzene molecules from mixed
benzene:H2O ices due to near-UV absorption by the benzene has been
demonstrated[5, 6]. The present study expands upon an earlier work[7] in which
an EET process was identified by the dynamics seen with time-of-flight
spectroscopy in which contribution to the photodissociation of CH3I adsorbed
on benzene thin films was observed, but absent for CH3I on hexafluorobenzene
thin films. Here we study CH3 photofragment kinetic energies from the UV
photodissociation of CH3I adsorbed on series of different fluorobenzene thin
films and report systematic changes in the observed photodissociation
dynamics.

###  I.1 Near-UV Photoabsorption by Benzene and Fluorobenzenes

The near-UV photoabsorption of condensed C6H6 films at 90K has been studied in
detail[8, 9]. In the region of interest for this work, the vibronic states
within the $\pi$–$\pi$* transition to the ${}^{1}\\!B_{2u}$ ($S_{1}$)
electronic state are redshifted relative to the gas-phase, with the $S_{1}$
band origin at 4.69eV (264nm) and $6^{1}_{0}1^{2}_{0}$ absorption observed at
248.85nm, close to the excimer laser wavelength used in this work. Broad
spectral absorption to the triplet ${}^{3}\\!E_{1u}$ ($T_{2}$,
$\sim$225–269nm) state was also observed in addition to the ${}^{3}\\!B_{1u}$
($T_{1}$, $\sim$263–310nm) state. The vibronic structure of the triplet state
excitations has also been detailed for condensed-phase C6H6 in thin films
using inelastic electron scattering[10].

Near-UV photoabsorption for gas-phase fluoro-substituted benzenes are similar
to that of C6H6, with the $S_{1}$ absorptions generally red-shifted relative
to benzene and with larger cross sections[11, 12]. There are few spectroscopic
studies reported for condensed-phase fluorobenzenes– a study of crystalline
monofluorobenzene found the $S_{1}$ band origin at 4.691eV (264.3nm) with
excitonic absorption features and the vibronic series extending to higher
energies including the 248nm wavelength used in the present work[13].
Inelastic electron scattering has been used in studying the vibronic structure
of the $T_{1}$ states for monofluorobenzene (mFBz) and p-difluorobenzene
(dFBz) in the energy region around 3.6eV (340nm)[14], which lie below the
energy region of interest in this work. It would be expected that the excited
states of the condensed fluoro-substituted benzenes will broadly follow that
of their gas-phase counterparts and have perturbation similar to that observed
for benzene. We expect that the dipole-allowed absorption to the $S_{1}$
vibronic states will also have an overlapping $T_{2}$ state in the region
surrounding the 248nm wavelength used in the present work[12].

###  I.2 Near-UV Photodissociation of Methyl Iodide

Photodissociation of gas-phase CH3I in the near-UV region occurs via the
‘A-band’, a set of $n\rightarrow\sigma^{*}$ transitions observed as three
overlapping states between $\sim$4.2eV and 5.6eV (${}^{3}Q_{1}$, ${}^{3}Q_{0}$
and ${}^{1}Q_{1}$ in order of increasing energy) above the ground state in the
Franck-Condon region[15]. At the 248nm wavelength ($h\nu$=4.99eV) used in the
present work, the $X\rightarrow{{}^{3}Q_{0}}$ excitation dominates gas-phase
photoabsorption and the $X\rightarrow{{}^{1}Q_{1}}$ is a minor channel. The
subsequent dissociation can proceed via two principal pathways:

$\begin{split}CH_{3}I+h\nu&\rightarrow CH_{3}+I({{}^{2}\\!P_{3/2}})\textrm{
\\{ground state I\\}}\\\ &\rightarrow CH_{3}+I^{*}(^{2}\\!P_{1/2})\textrm{
\\{excited I\\}}\end{split}$ (1)

The energy difference between ground state $I$ and excited $I^{*}$ is 0.943eV,
leading to significant differences in the translational energies imparted to
the fragments and which can be resolved in our time-of-flight measurements.
There are also vibrational and rotational energy partitioning differences for
the CH3 photofragments along the two pathways. Another significant factor for
this system is that the $X\\!-\\!{{}^{3}Q_{0}}$ excitation is a parallel
transition (requiring a component of the incident $\vec{E}$-field along the
C–I bond axis), while the ${{}^{1}Q_{1}}$ and ${}^{3}Q_{1}$ excitations are
perpendicular. This polarization dependence for optical absorption allows
utilizing polarization and molecular orientation to aid in understanding the
photodissociation dynamics at 248nm[16]. The ${}^{3}Q_{0}$ state correlates to
the I∗ outcome in Equ. 1, but a curve-crossing with the ${}^{1}Q_{1}$ state
(which correlates to the I pathway) during dissociation enables non-adiabatic
transitions that result in both pathways being observed subsequent to an
initial ${}^{3}Q_{0}$ excitation[15].

###  I.3 Energetics of Stimulated Dissociation

The dissociation of a CH3I molecule in free space requires momentum and energy
conservation, which determines how the excess kinetic energy is partitioned
between the CH3 fragment and the halogen atom. For neutral photodissociation,
a starting point for rationalizing the CH3 photofragment kinetic energy is:

$\displaystyle T_{CH_{3}}$ $\displaystyle=\frac{m(I)}{m(CH_{3}I)}\\{$
$\displaystyle h\nu-D_{0}(C-I)-E_{int}(I)$ (2) $\displaystyle-
E_{int}(CH_{3})\\}$

where $m()$ is the mass of the particular species, $h\nu$ is the photon
energy, $D_{0}$ is the bond energy, and $E_{int}(I)$ allows for the possible
electronic excitation of the departing halogen atom and $E_{int}(CH_{3})$ is
the internal energy (vibration and rotation) of the departing methyl fragment.

For surface adsorbate systems the parent molecule is not in free space, but
embedded at or near the vacuum interface of the system being studied. The
departing CH3 fragments must also escape the surface attractive forces, so the
kinetic energies will be reduced by an amount on the order of 0.1eV[17].
Further, it is known from prior work in gas-phase cluster and surface
photochemistry that the observed fragment kinetic energy distributions can be
altered by chemical or post-dissociation interactions, however Equ. 2 provide
a basis to begin rationalization of the observed kinetic energy distributions.

###  I.4 Adsorption and Valence Band Structure of Fluorobenzenes on the
Cu(100) Surface

The adsorption behaviour of monolayer and thin films of C6H6 and C6F6 on
coinage metal surfaces has been studied and described in detail in the
literature. Studies of C6H6 on copper surfaces has revealed differences in
structure for the first layer that depend on the particular surface (material
and crystal face), most commonly with molecules lying flat for the first layer
and tilting upright in the second layer[18, 19], behaviour ascribed to the
quadrupole moment for the molecule and a propensity for a T-motif in the
solid[19]. For thicker benzene films a herringbone structure is believed to
occur[20], similar to the crystal structure of the bulk solid. By contrast,
C6F6 is believed to adsorb in flat-lying layers due to the F-atoms forming
halogen bonds with C–C bonds of neighbouring molecules[21, 22]. In contrast to
the case for C6H6 and C6F6, there are relatively few surface science studies
in the literature that have characterized the adsorption of other
fluorobenzenes on coinage metal surfaces. The various factors that affect the
thin film structure, such as the surface corrugation and lattice spacing as
well as the intermolecular interactions such as electrostatic multipole
moments, hydrogen bonding or halogen bonding will be significant for the
fluorobenzenes. For example, infrared spectroscopy of C6H5F on Cu(111) finds
that the molecules lie flat until the first layer is 2/3 complete, and then
the molecules distort and tilt with the C–F bond coming out-of-plane[23]. A
scanning probe microscopy study of the adsorption of 1,3,5-C6H3F3 on the more
corrugated Ag(110) surface observed a rectangular unit cell with alternate
molecules rotated to facilitate halogen bonding with its neighbours[24].

Although we report some data for fluorobenzene thin films of 1ML coverage, the
focus of the present work is on multilayer thin films, with the majority of
the data presented using 10ML thin films. As such we are interested in the
bulk-like crystal structures that are likely to be formed under our
experimental conditions. In the present work a monolayer of CH3I is adsorbed
on top of the fluorobenzene thin film, so the structure of the CH3I layer will
be influenced by the underlying thin film structure, primarily through the
electrostatic multipole moments of the particular species on the condensed
solid. Given the large number of possible fluorobenzene molecules, we
restricted the present study to those with dipole moments of zero: C6F6
(hFBz), 1,2,4,5-C6H2F4 (tetraFBz), 1,3,5-C6H3F3 (triFBz), 1,4-C6H4F2 (dFBz)
and C6H6 (benzene); plus C6HF5 (pFBz) and C6H5F (mFBz) which possess non-zero
dipole moments. The systematics of bulk crystal structures for the
fluorobenzenes are described in detail in Ref. 25.

As for the studies of physical structure, previous studies of the electronic
structure of these molecules in the adsorbed state has been extensive for
benzene as well as for hFBz, these have been studied using one-photon and two-
photon photoemission[26, 27, 28], inverse photoemission[29] and the subject of
some theoretical electronic bandstructure calculations[22, 24]. There is
relatively smaller publication list on the electronic structure for the
intermediate fluorobenzenes[30], with work published using inelastic electron
scattering for studies of excitations in dFBz and mFBz thin films[14].

##  II Experimental Details

The experiments were performed in an ultra-high vacuum (UHV) system that has
been described previously[16]. The Cu(100) single crystal sample is 12mm in
diameter and was cooled by liquid nitrogen (base temperature $\sim$90K) and
heated by radiative heating from a filament up to 300K and electron
bombardment heating to 920K for cleaning. Sample temperatures were monitored
by a type K thermocouple spot-welded to the tungsten sample mounting wire.
Sample cleanliness and order were monitored by Auger electron spectroscopy
(AES) and low energy electron diffraction (LEED) measurements respectively.
The crystal was prepared in UHV by cycles of Ar+ ion bombardment and electron
bombardment heating and annealing until the sample AES spectra indicated a
clean copper substrate with the LEED patterns of a (1$\times$1) surface.

Deposition of molecules on the sample was done using a micro-capillary array
directed doser[31], with the sample held normal to the doser, 25mm away. This
arrangement was found to enhance the deposition by a factor of 10 compared to
background dosing. The pressure in the UHV chamber was measured using
uncorrected ionization gauge readings. The dosing (in Langmuirs, L) was
calibrated in terms of equivalent monolayers for the different species used by
Temperature Programmed Desorption (TPD) measurements as discussed in Section
III.1 below. The liquids used in this work were degassed by multiple freeze-
pump-thaw cycles. Vapour from CH3I (Sigma-Aldrich, 99.5%), benzene (Sigma-
Aldrich, 99.8%), mFBz (Sigma-Aldrich, 99%), dFBz (Sigma-Aldrich, >99%), triFBz
(TCI, >98%), tetraFBz (Sigma-Aldrich, >99%), pFBz (TCI, >98%) and hFBz (Sigma-
Aldrich, >99.5%) was obtained from the liquid in a pyrex vial a few cm from
the precision leak valve used to admit the room-temperature vapour to the
directed doser. The deposition of the molecules was done using substrate
temperatures below 100K.

Temperature programmed desorption (TPD) measurements were made by positioning
the sample to face a quadrupole mass spectrometer (QMS). In early experiments,
a UTI 100C QMS was used, and for later experiments it was replaced by a
Stanford Research Systems RGA 200 QMS. The QMS ionizer was located $\sim$80mm
away from the sample and behind an aperture that limits the ionizer line-of-
sight to the central region of the sample. The sample was heated using a
filament at ground potential, located a few mm behind the sample mount.

The time-of-flight (TOF) photodissociation measurements were performed using a
second QMS (Extrel). Neutral products from the Cu(100) surface travel 185mm to
pass through a 4mm diameter aperture to a differentially pumped region with an
axial electron bombardment ionizer. The sample to ionizer distance is 203mm.
Ions created in the ionizer travel through the quadrupole region and are mass
selected, in the present experiments using m/q=15amu. Ion arrivals are
recorded using a multichannel scaler that begins counting 50$\mu s$ prior to
the initiating laser pulse, and the counts recorded from multiple laser pulses
are summed. Unless otherwise indicated, the spectra shown in the present work
are the result of summing data from 1000 laser pulses into 1000 1$\mu s$ time
bins. In order for the ion arrival times to reflect the neutral fragment time-
of-flight, they are corrected for the ion flight time (for CH${}_{3}^{+}$,
17$\mu s$ at the 50eV ion energy used in the QMS). This is the leading
systematic uncertainty in the recorded flight times ($\pm 1.5\mu s$) which
does not affect comparisons between different TOF spectra but does lead to
fixed nonlinear systematic uncertainty in the reported fragment kinetic
energies $(KE\propto 1/(TOF)^{2})$, which is most problematic at short flight
times. The TOF spectra $N(t)$ were converted to probability distributions
$P(E)$ versus CH3 kinetic energy using the Jacobian transformation with an
added factor $1/t$ to account for the higher ionization probability for slower
neutral CH3 fragments[32].

The laser pulses ($\sim$5ns duration) are produced by a small excimer laser
(MPB PSX-100) operating at 20Hz. In this work KrF ($\lambda$=248nm,
$h\nu$=4.99eV) laser light was used, with laser fluences on the sample of
$\sim$0.8mJ/cm2. The intrinsic bandwidth of the laser emission for excimer
lasers is rather broad– for a free-running KrF excimer laser the center
wavelength is approximately 248.4nm (4.992eV) and has a fwhm bandwidth of
$\sim$0.40nm (0.008eV).

Linearly polarized laser light has been used exclusively in this work for the
reasons described in Section I.2. To create the polarized light, the beam is
incident upon a birefringent MgF2 crystal prism at the Brewster angle to
separate the p- and s-polarized components. The p-polarized beam was aligned
on the sample. The s-polarized light was derived from the p-polarized beam by
inserting an antireflection coated zero-order half-waveplate into the beam.
The laser light was collimated using a 6mm diameter aperture and was unfocused
on the sample. The laser light is incident upon the sample at a fixed angle of
45∘ from the TOF mass spectrometer axis– for example, when the Cu crystal
sample is oriented to collect desorption fragments along the surface normal
direction, the light is incident at 45∘.

Cross sections for the photodissociation of molecular thin films examined in
this work were determined by measuring the depletion rate of the CH3
photofragment yields (summed CH3 photofragment counts). These “depletion cross
sections” are obtained by recording CH3 photofragment yields from
photodissociation for a sequence of TOF spectra. Time-of-flight spectra are
obtained using 200–400 laser pulses per scan, then repeated for 10 or more
successive scans. In the systems studied here, the photofragment yields are
observed to monotonically diminish as the net laser photon flux was increased,
and the resulting yield vs. photon flux curves were then fit using a single
exponential decay model. Reasonable fits to the data were obtained, at least
in the low photon flux region. This procedure allows the possibility that
other photochemical processes involving CH3I removal but not seen in the TOF
data might be occurring in the heterogeneous thin films. In the present work,
the dissociated species was always from a dose equivalent to 1ML CH3I film,
while the underlying fluorobenzene thin film was varied. We used a 1ML CH3I on
a 10ML C6F6 thin film as a reference system for comparison when studying the
cross sections of the other fluorobenzene thin films. The absolute cross
sections measured this way have fairly large errors (we estimate $\pm 50\%$),
but the errors in comparative relative cross sections (i.e. vs CH3I on C6F6)
between the different systems is much lower, on the order of $\sim$10% based
on repeatability of measurements.

##  III Results and Observations

###  III.1 Temperature Programmed Desorption of Fluorobenzenes on Cu(100)

In order to be able to adsorb known quantities of the various molecules used
in the present work, TPD has been used to characterize the relationship
between the amounts dosed (in Langmuirs, L) and the formation of a complete
monolayer (1ML). After establishing the dose required to form 1ML for a
particular molecule, we have assumed that for the temperature used in these
experiments (T<100K) the sticking coefficient for these molecules is close to
unity for multilayer films, so dose amounts in multiples of the monolayer dose
to produce the desired multilayer thin film. We have characterized TPD for
CH3I, C6H6 and C6F6 in our apparatus previously[7] so have performed TPD
experiments for the other fluorobenzenes used in the present work.

The TPD spectra observed for the sequence from mFBz through pFBz were similar
in terms of general features to those observed for benzene[7, 18, 19] and
hFBz[7, 26]. Submonolayer quantities of the fluorobenzenes were observed to
desorb from the Cu(100) substrate with a peak near 210K (ranging from 200K to
220K). As the dose was increased, the monolayer desorption peak broadened
toward lower temperatures, characteristic of repulsive intermolecular
interactions. At the completion of the first monolayer, the monolayer
desorption peaks extended from $\sim$160K to just above 220K. A peak
corresponding to desorption from the second layer emerged at 150-165K
depending the particular fluorobenzene, and this peak shifted to slightly
higher temperature as the second layer coverage was increased, with a peak in
the desorption temperature 160K-170K. For mFBz, a peak near 158K emerged for
doses that were compatible with growth of the third layer (where the second
layer peak was at 164K), and as this peak grew it merged with the bilayer peak
at higher doses. For the other fluorobenzenes in this work, we did not observe
differences in desorption peaks between the second and third or subsequent
layers, aside from growing peak height with dose. For the fluorobenzenes in
the current study, doses corresponding to the completion of 1ML were found to
be in the range 0.38L to 0.43L though with small uncertainties (5-10%). This
was used as the basis for creating the multilayer films– doses are quoted in
nominal monolayer (ML) amounts and as will be discussed in Section III.3,
there are reasons to believe that some of the multilayer fluorobenzene films
exhibit Stranski-Krastanov growth.

###  III.2 Photodissociation of CH3I on Fluorobenzene Thin Films

The photodissociation experiments were performed on thin films (1–10ML) of the
selected fluorobenzene with 1ML of CH3I adsorbed on top. Unless otherwise
noted, the samples were oriented so that the surface normal was in the
direction of the QMS so that the laser light was incident at 45∘ from the
surface normal. Figure 1 shows TOF spectra obtained using p- and s-polarized
light from 1ML CH3I adsorbed on 3ML pFBz. The TOF spectrum obtained using
p-polarized light (red trace) displays two peaks, labelled I (at 43$\mu$s) and
I* (at 53$\mu$s). These peaks are qualitatively similar to those observed in
gas-phase photodissociation of CH3I at this wavelength and their origin can be
understood from the description for neutral photodissociation outlined in
Section I.2. The inset plot shows the same data plotted in terms of the CH3
photofragment kinetic energy– the ‘I’ peak pathway centered at 1.75eV and the
‘I*’ pathway peak centered at 1.15eV. These peaks are prominent for
p-polarized light when collecting CH3 photofragments in the surface normal
direction when adsorbed CH3I molecules have their C–I bond axis oriented in
the surface normal direction, so that the parallel transition
$X\rightarrow{{}^{3}Q_{0}}$ is accessible from a component of the p-polarized
light. When s-polarized light is used (blue trace) the parallel transition is
forbidden in this detection geometry and a much smaller CH3 photofragment
yield is observed. The contributions to the observed s-polarized TOF spectrum
likely come from several sources. The smaller ‘I’ pathway peak and absence of
a distinct I* peak appears to be a consequence of the
$X\rightarrow{{}^{1}Q_{1}}$ (perpendicular) transition that has a smaller
oscillator strength than the ${}^{3}Q_{0}$ counterpart, and leads exclusively
to the I pathway in the dissociation limit. It is also observed that there is
a continuum of CH3 photofragment energies (i.e. a broad background), in the
ToF spectra which we interpret as primarily due to orientation effects for the
adsorbed CH3I, which would include antiferroelectric ordering or disorder.
Such order effects can lead to CH3 photofragments undergoing collisions as
they depart the surface region, so that their kinetic energies are reduced and
CH3 photofragments that were initially aimed in other directions are detected
in the surface normal direction. The initial photodissociation event for these
inelastically scattered CH3 might also have a contribution from the stronger
${}^{3}Q_{0}$ transition from molecules not oriented in the surface normal
direction.

Figure 1: Time-of-flight spectra for CH3 photofragments from the
photodissociation of 1ML CH3I adsorbed on 3ML C6HF5 (pFBz) on Cu(100). The red
and blue traces are obtained using p- and s-polarized 248nm light
respectively. The CH3 photofragments are detected in the surface normal
direction. The vertical dashed lines indicate the nominal flight times for the
CH3 photofragment peaks from the I and I* A-band photodissociation pathways as
described in the text. The inset plots show the data plotted as a function of
the CH3 photofragment translational energy.

Time-of-flight spectra for CH3I adsorbed on 10ML thin films for the series of
fluorobenzenes and benzene are shown in Fig. 2. There are systematic changes
in the TOF spectra for the sequence of fluorobenzene thin films, most clearly
observed in the s-polarization spectra that show the emergence of additional
TOF spectrum features for dFBz, mFBz and benzene, and also in the
corresponding p-polarization spectra where the I and I* peaks become less well
resolved due to this same new feature overlapping in these spectra.

Figure 2: Time-of-flight spectra for CH3 photofragments due to the
photodissociation of 1ML CH3I on 10ML thin films of (a) C6F6 (hFBz); (b) C6HF5
(pFBz); (c) 1,2,4,5-C6H2F4 (tetraFBz); (d) 1,3,5-C6H3F3 (triFBz); (e)
1,4-C6H4F2 (dFBz); (f) C6H5F (mFBz) and (g) C6H6 (benzene), obtained using
248nm light. The red traces are obtained using p-polarized light, while the
blue traces are obtained using s-polarized light. The CH3 photofragments are
detected in the surface normal direction. The vertical dashed lines indicate
the nominal flight times for the CH3 photofragment peaks from the I and I*
A-band photodissociation pathways as described in the text.

For CH3I adsorbed on 10ML hFBz and pFBz (Fig. 2(a) and (b)) the TOF features
are broadly similar to that described above for Fig. 1 with the p-polarized
spectra (red traces) showing well-resolved peaks for the I and I* dissociation
pathways and a small I-pathway peak in the s-polarization data (blue traces).
The broad CH3 distribution at longer flight times is somewhat larger for the
10ML C6F6 film, which we believe is due to increased orientational disorder
for the CH3I as compared to the spectra from the thinner pFBz films (e.g. Fig.
1) and also for thinner C6F6 films which we described previously[7]. Based on
this and other recent work studying the photodissociation of CH3Br and CH3Cl
on pFBz at 193nm[33], we ascribe the somewhat lower background to better
orientational ordering for the dipolar CH3I molecules on the 10ML pFBz films
as compared to hFBz.

The TOF spectra for CH3I adsorbed on 10ML tetraFBz (Fig. 2(c)) and triFBz
(Fig. 2(d)) are quite similar– the I and I* photodissociation peaks are
clearly visible in the TOF spectra from p-polarized light (red traces) but
less prominent than for the cases discussed above. There is a relatively
higher intensity low-energy tail of CH3 photofragment counts ($\tau$>60$\mu$s)
as compared to the other spectra in Fig. 2. Similarly the TOF spectra for
s-polarized light (blue traces) have higher relative CH3 counts but in a broad
TOF distribution without distinctive peaks. For the CH3I on tetraFBz and
triFBz thin films we conclude that the dissociation is dominated by neutral
CH3I photodissociation via the A-band mechanism described in Section I.2, with
orientational disorder for the CH3I likely accounting for the higher CH3
counts at long flight times.

The TOF spectra for CH3I adsorbed on dFBz, mFBz and benzene in Figs. 2(e)-(g)
differ substantially from the other spectra in the series. Using p-polarized
light (red traces) the I and I* channel peaks are not well resolved as
separate peaks, while the s-polarized light spectra (blue traces) show
distinct features in the TOF spectra that are not observed for the other thin
films. For dFBz (Fig. 2(e)) the new feature appears to peak at roughly
51$\mu$s flight time, which is intermediate to that of the I and I* flight
time peaks. For mFBz (Fig. 2(f)) the new peak is at 49$\mu$s flight time. For
benzene (Fig. 2(g)), new peaks appear at 47$\mu$s and 60$\mu$s (peaks ”A” and
”B”) with the first between the I and I* peaks and the second appearing at a
later time[7]. In contrast to the I and I* peaks, which are most prominent
when p-polarized light is used, these additional peaks are of similar
intensity for both p- and s-polarized incident light. The TOF spectra obtained
using p-polarized light are composed of both the neutral CH3I
photodissociation channel (I and I*) features, plus the new dissociation
channel that has an intensity and peak location that substantially overlaps
with the I and I* features.

More detailed comparison of TOF spectra from CH3I photodissociation on 10ML
films of pFBz and mFBz are shown in Fig. 3. Using p-polarized light (red
traces) the I and I* features are well separated for CH3I on pFBz, while that
for CH3I on mFBz has strongly overlapped peaks. In comparing the p- and
s-polarized TOF spectra, one can see that the I and I* pathways are present in
the p-pol spectrum for CH3I on mFBz with the additional intermediate TOF
feature that overlaps the I and I* peaks. The inset figures show the same TOF
data in terms of the CH3 photofragment kinetic energies– the I and I* peaks
appear at 1.75eV and 1.15eV respectively, with the intermediate peak in
centered near 1.40eV. In the TOF spectra we also observe that the total yield
(sum of CH3 counts above background) is substantially larger when using the
mFBz thin film as compared to the pFBz thin film. A robust measure of this are
the depletion cross sections obtained by the method described in Section II.
In comparing the depletion cross sections for the various thin films, we used
CH3I on the 10ML hFBz thin film as a reference so that the relative cross
sections were compared to this system. To convert the cross sections to
absolute values, the measured reference cross section for CH3I on hFBz is
$\sim\\!8.0\times 10^{-19}$cm2, similar to what was reported using 3ML hFBz
films[7]. The relative depletion cross sections for CH3I on the various 10ML
thin films are shown in Table 1. In comparison to the reference hFBz thin
film, that from pFBz is very similar. In contrast, the relative depletion rate
for CH3I on the 10ML mFBz thin film is about 3 times larger. The larger
depletion rate observed for CH3I on the 10ML mFBz thin film and the features
observed in TOF spectra for p- and s-polarized incident light support the
proposal that a new dissociation pathway is present for the CH3I adsorbed on
top of the mFBz thin film.

hFBz | pFBz | tetraFBz | triFBz | dFBz | mFBz | C6H6
---|---|---|---|---|---|---
1 | 0.9 | 1.0 | 1.1 | 1.8 | 3.0 | 2.0
Table 1: Relative depletion cross sections for 1ML CH3I adsorbed on 10ML thin
films of the selected fluorobenzenes and benzene using 248nm light. The data
were normalized to the cross section observed from the system with hFBz
(C6F6).  Figure 3: Time of flight spectra for CH3 photofragments due to the
photodissociation of 1ML CH3I on 10ML (a) C6HF5 (pFBz) and (b) C6H5F (mFBz)
thin films, obtained using 248nm light. The red traces are obtained using
p-polarized light and the blue traces are obtained using s-polarized light.
The vertical dashed lines indicate the nominal flight times for CH3
photofragment peaks from the I and I* A-band photodissociation pathways. The
inset plots show the corresponding CH3 kinetic energy distributions. The CH3
photofragments are detected in the surface normal direction.

Figure 4 shows TOF spectra and photofragment kinetic energy distributions for
CH3I adsorbed on 10ML dFBz and mFBz. For CH3I on the dFBz film, the
s-polarization data (blue traces) show the peak to be somewhat lower intensity
and a bit slower (51$\mu$s or 1.3eV) as compared to that for the mFBz thin
film. The p-polarized TOF spectrum shows the I and I* neutral
photodissociation pathways plus the new pathway that is observed for both
polarizations. The relative CH3I depletion cross section measured (Table 1)
for the dFBz thin film was enhanced by a factor of roughly 1.8 compared to the
hFBz thin film. These observations support the notion that there is also an
enhanced dissociation mechanism operating for CH3I on 10ML dFBz thin films
that is mediated by photoabsorption in the dFBz.

Figure 4: Time-of-flight spectra for CH3 photofragments due to the
photodissociation of 1ML CH3I on 10ML (a) 1,4-C6H4F2 (dFBz) and (b) C6H5F
(mFBz) thin films, obtained using 248nm light. The red traces are obtained
using p-polarized light and the blue traces are obtained using s-polarized
light. The vertical dashed lines indicate the nominal flight times for CH3
photofragment peaks from the I and I* A-band photodissociation pathways. The
inset plots show the corresponding CH3 kinetic energy distributions. The CH3
photofragments are detected in the surface normal direction.

For CH3I adsorbed on 10ML tetraFBz and triFBz thin films, TOF spectra are
shown in Fig. 5. As noted above, the CH3 photofragments in these thin film
systems show a higher intensity broad background– in the p-polarization light
data the I and I* peaks are clearly visible and well-separated while the
s-polarization data is almost featureless aside from small peaks that are
present at the I and I* peak positions. The TOF data are not definitive in
identifying any new dissociation pathways for CH3I, such as seen for the mFBz
and dFBz thin films. Measurements of the depletion cross sections for CH3I on
the tetraFBz and triFBz thin film systems in Table 1 also do not show a
significant change in depletion rates as compared to that of the reference
hFBz thin film, although the triFBz has a slightly larger rate ($\sim$10%)
which is small enough that if there is a contribution from a new dissociation
pathway, we are unable to resolve it in the TOF data, given the broad spectra
as seen in Fig. 5.

Figure 5: Time-of-flight spectra for CH3 photofragments due to the
photodissociation of 1ML CH3I on 10ML (a) 1,2,4,5-C6H2F4 (tetraFBz) and (b)
1,3,5-C6H3F3(triFBz) thin films, obtained using 248nm light. The red traces
are obtained using p-polarized light and the blue traces are obtained using
s-polarized light. The vertical dashed lines indicate the nominal flight times
for CH3 photofragment peaks from the I and I* A-band photodissociation
pathways. The inset plots show the corresponding CH3 kinetic energy
distributions. The CH3 photofragments are detected in the surface normal
direction.

The novel photodissociation pathways observed for CH3I adsorbed on dFBz, mFBz
and benzene thin films are only observed for multilayer films. Figure 6 shows
TOF spectra obtained using p- and s-polarized light for 1ML CH3I on varying
thickness (1–10ML) films of mFBz on Cu(100). As was observed for CH3I on
benzene[7], CH3 photofragments from CH3I A-band photodissociation pathways are
observed for all the films from 1ML through to 10ML. In the p-polarization TOF
data, the I and I* pathways are observed distinctly for the thinner films
(1–3ML), and as the film thickness is further increased (5 and 10ML) the new
dissociation feature appears and grows in intensity such that the three peaks
overlap. In the s-polarization TOF spectra, the new dissociation feature is
clearly resolved and has high intensity for the 5 and 10ML thin films,
possibly visible as a shoulder for the 3ML film in Fig. 6(b). These
observations suggest that the new dissociation mechanism found for mFBz and
dFBz films require a separation of the photoabsorbing molecules from the
Cu(100) substrate in order to cause the dissociation of the coadsorbed CH3I.
In considering all of the features observed for this process, we conclude that
the photochemical process occurs by photoabsorption in the underlying
fluorobenzene thin film with subsequent electronic energy transfer to the CH3I
at the vacuum interface.

Figure 6: Time-of-flight spectra for CH3 photofragments following the
photodissociation of 1ML CH3I adsorbed on varying thickness C6H5F (mFBz) thin
films, obtained using 248nm light. The spectra in (a) are obtained using
p-polarized light and the spectra in (b) are obtained using s-polarized light.
Successive spectra are offset vertically by 150 counts for clarity. The CH3
photofragments are detected in the surface normal direction. The vertical
dashed lines indicate the nominal flight times for CH3 photofragment peaks
from the I and I* A-band photodissociation pathways.

The growth of the photodissociation yield with the film thickness suggests
that the initial photoabsorption is followed by energy transport, so leads to
the question of what range is possible for this transport process before
quenching occurs[34]. The data of Fig. 6 substantiate different timescales for
the quenching of neutral photodissociation compared to the EET mechanism. For
example, the p-polarization spectrum for CH3I on 2ML mFBz clearly shows the
I/I* pathways similar to Fig. 1 with little or no observable contribution from
the EET dissociation pathway, but as the mFBz film thickness is increased, the
contribution from the EET pathway grows and eventually dominates the p- and
s-polarized TOF spectra. At short separations, the quenching rate depends
sensitively on wavefunction overlap between the excited state and substrate
valence states. For the thinnest films (a few ML) resonant energy transfer in
which electrons in the metal tunnel to fill the adsorbate hole state and the
excited electron tunnels to unoccupied metal states[34, 35, 36] is the most
likely quenching mechanism. The timescale for the dissociation of the CH3I
moiety on purely repulsive potential energy surfaces (e.g. ${}^{3}Q_{0}$) will
be roughly the same independent of mechanism. This suggests that the
characteristic timescale for the thin film excitation is significantly longer
for the EET-driven dissociation than the neutral photodissociation, which
occurs in $\sim$100 fs[15, 37]. A recent study of luminescence from exciton
states in organic thin films grown on single crystal metal substrates found
that lifetimes (varying from picosecond to nanosecond timescales) increased by
an order of magnitude for films $\sim$10ML thick as compared to those few
monolayers thick[38]. In order to focus on the EET mechanism and minimize the
contribution from CH3I neutral photodissociation, the CH3 yield in the EET
peak (centered around 48$\mu$s flight time) from CH3I on mFBz using
s-polarized light was used (similar to the spectra in Fig. 6b). The CH3 EET
photoyield shown in Fig. 7 is found to increase roughly linearly from 1ML up
to $\sim$10ML doses, and continue to increase with reduced slope ($\sim$1/3
that at lower doses) up to the thickest films used (25ML doses). That the
observed yield curve does not saturate within the range of doses studied could
be due to further reduction of longer range dipole quenching[39], but it is
more likely due to the morphology of the thin films grown under the conditions
of our experiments.

Figure 7: Yield of CH3 photofragments in the EET peak region from CH3I on
C6H5F (mFBz) films created from varying the dose. The yields were obtained
from TOF spectra using s-polarized light.

###  III.3 Effects of Fluorobenzene Film Morphology

The increasing CH3 yield observed for CH3I on 1–10ML mFBz films is due in part
to the reduced quenching of excitations as the distance to the Cu(100) metal
surface increases for the reacting species. For thicker films, quenching is
less efficient so that the excitation can be formed and have sufficient
lifetime and mobility to diffuse and cause dissociation of CH3I by EET from
mFBz. A complication is that the effect of the thin film morphology appears to
become more significant as the mFBz dose increases. The most satisfactory
explanation for the observed behaviour at higher doses in Fig. 7 is that the
mFBz thin films have 3D island crystallites forming at higher coverages,
analogous to the behaviour found for benzene multilayer films grown under
similar conditions[20]. In the present experiments, we have grown the mFBz
(and the other fluorobenzene) thin films at substrate temperatures below 100K,
so that these films are crystalline (i.e. grown above temperatures where
amorphous films would be expected, <60K). The morphology of benzene multilayer
films grown in similar conditions has been studied in some detail[18, 19, 20,
40], but there is little in the current literature specific to fluorobenzene
film growth. It is quite likely that the first layer on Cu(100) fully covers
the metal surface, given the observations from TPD described in Section III.1.
For higher doses, the structure of the thin films is less clear, but most
likely the Stranski-Krastanov type where crystallites grow on the surface so
that regions have differing thickness, dictated in part by the mobility of the
molecules as they arrive at the surface. We have done a limited study of the
effect of annealing the mFBz thin films. Figure 8 shows the effect of
annealing on the TOF spectra from mFBz thin films. The experimental procedure
used was to dose the mFBz thin film (10ML) at T<100K, then warm and maintain
the substrate at a target temperature for a minute, and re-cool to T<100K
followed by dosing the CH3I layer. Using the TOF spectra as a probe, we found
little change until annealing temperatures of 120K or higher were used. The
highest annealing temperature used was 130K to prevent mFBz desorption, which
begins near 150K. The TOF spectra from mFBz thin films annealed to 120K<T<130K
were significantly altered from the as-deposited thin films– the large signal
from EET causing CH3I dissociation (peak at 48$\mu$s) was effectively absent
(s-polarization) as were the I/I* pathway peaks characteristic direct
photodissociation of CH3I (p-polarization). The effect of annealing the mFBz
at T>120K appears to be to allow adsorbate mobility to grow larger
crystallites of mFBz and leave large terraces of the Cu(100) substrate with
low coverage (presumably monolayer mFBz) so that much of the subsequently
dosed CH3I is close enough to the metal surface for efficient quenching of the
excitation– similar to the low coverage (1ML) traces in Fig. 6. The TOF
spectra from annealed films have CH3 translational energy distributions very
different from that seen for as-deposited 10ML mFBz films. Our findings
suggest a similar conclusion to that made for benzene thin films: “extra
thermal annealing leads to a destabilization of smaller crystalline domains in
favour of larger crystallites which then grow even more”[20]. Thus for the
thin films grown at low temperature (T<100K), we expect that these are
crystalline and for multilayer doses, have varying thickness crystalline
regions with an average thickness as indicated by the flux of molecules used.
The primary evidence for this from Fig. 7 is that the CH3 photoyield does not
saturate for some mFBz dose, which would be expected for layer-by-layer
growth[34]. To the extent we have investigated, annealing these films leads to
growth of a small number of larger crystallites that cover much less of the
surface that leaves the majority of the substrate covered by monolayer
quantities of the mFBz.

Figure 8: Time-of-flight spectra for CH3 photofragments from the
photodissociation of 1ML CH3I adsorbed on 10ML C6H5F (mFBz) thin films as-
deposited (at T<100K) or that have subsequently been annealed and re-cooled to
T<100K. The vertical dashed lines indicate the nominal flight times for CH3
photofragment peaks from the I and I* A-band photodissociation pathways. The
spectra are offset vertically by 50 counts for clarity.

##  IV Additional Discussion

The observation of an EET dissociation mechanism for CH3I adsorbed on the
mFBz, dFBz and benzene thin films but absent for the higher fluorobenzenes is
at first surprising as the photoabsorption cross sections for the higher
fluorobenzenes are generally as large or larger than for benzene at the 248nm
wavelength used in the present work[8, 11]. However as the fluorine atom
number increases the ‘perfluoro effect’ lowers an unoccupied $\pi\sigma$*
level toward the LUMO $\pi\pi$*. The lowered $\pi\sigma$* state increases the
rate of internal conversion $S_{1}\rightarrow S_{0}$, reducing the excited
state lifetime and fluorescence yield significantly as compared to lower
F-count fluorobenzenes[41]. This effect seems to be responsible for the
absence of the EET pathway being observed for CH3I on the triFBz through hFBz
thin films.

For CH3I adsorbed on the pFBz and the other fluorobenzene thin films we found
no evidence for dissociative electron attachment (DEA) of the CH3I due to
photoelectrons using 248nm light, which is consistent with the findings we
previously reported for a series of halomethanes adsorbed on benzene and hFBz
thin films[7]. We have observed features consistent with the photoelectron DEA
mechanism for CH3Br and CH3Cl for very thin films of benzene and several of
the fluorobenzenes (1-2ML films, using both 248nm and 193nm light)[33], but
which diminishes as the film thicknesses are increased and was not observable
for the thicker multilayer films. Further, 248nm illumination of CH3Br on 10ML
mFBz and dFBz thin films found no CH3 photofragment signal in TOF spectra,
which we interpret as being due to the analogous ‘A-band’ dissociative states
for CH3Br being at higher energies[42] than for CH3I and not accessible via
248nm photoexcitation.

The energies observed for the CH3 photofragments from the EET process can be
used to estimate the excitation energy causing dissociation (equivalent to
$h\nu$ in Equ. 2). The excitation energies found are 4.71eV (C6H6), 4.60eV
(mFBz) and 4.49eV (dFBz) if $E_{int}(CH_{3})$ is assumed to be the same as
seen in the neutral photodissociation (e.g. Fig. 1 (top)) as well as the
I-pathway outcome, so represent a rough upper bound on the excitation energy
causing dissociation. These can be used as a basis for considering more
specific detail for the mechanism of dissociation.

The evidence for the observed EET to CH3I in the thin film points to an
initial photoabsorption by the $S_{1}$ state of the aromatic molecules. In
solid benzene at 100K the $S_{1}$ fluorescence lifetime is roughly 80ns[43],
and though no data exists for the mFBz or dFBz in the solid state, gas-phase
$S_{1}$ fluorescence lifetimes are longer (10–20ns) than for the higher
fluorobenzenes[12]. In EET systems the energy transfer between donor and
acceptor generally occurs via states of the same multiplicity. For CH3I the
only available singlet state in the A-band is the ${}^{1}Q_{1}$, which leads
to the I-pathway in Equ. 1. However in the Franck-Condon region, the
preponderance of the ${}^{1}Q_{1}$ state is located at higher energy[37, 42]
than the available excitation energy, so seems unlikely to be the acceptor
state for EET. For mFBz and dFBz, only a single TOF peak is observed, at lower
CH3 photofragment kinetic energy than the ${}^{3}Q_{0}$ neutral
photodissociation I-pathway but higher energy than the I*-pathway. This
suggests an excited state energy less than the initial $h\nu=4.99$eV photon
that leads to the I-pathway only. There are several lower energy electronic
excited states for CH3I exclusively leading to the I-pathway that are derived
from the ${}^{3}Q$ multiplet, including the ${}^{3}Q_{1}$ as well as others
that are optically dark[37]. In contrast, the CH3I on benzene system displays
bimodal EET features in the TOF spectrum (peaks “A”and “B”), as discussed
previously[7], and was ascribed to EET via the CH3I ${}^{3}Q_{0}$ excited
state. These TOF peaks would correspond to the I and I*-pathways, and the
higher initial excitation energy is consistent with the ordering of the
${}^{3}Q$ states in the Franck-Condon region. To access CH3I triplet state
excitations through EET, the heavy-atom effect[44, 45] due to the presence of
I-atoms would be required to facilitate the mixing of states of differing
multiplicity and intersystem crossing, such as that seen in the
$X\,^{1}\\!A_{1}\rightarrow{{}^{3}Q_{0}}$ photoexcitation and in solution-
phase fluorescence quenching of pyrene by CH3I[45].

For the arenes used in the present work, the lowest energy triplet states lies
well below the $S_{1}$ states and are at too low an excitation energy to cause
the observed CH3 photofragment kinetic energies, but the second triplet states
are located in the same energy region as the $S_{1}$ states. For solid benzene
the triplet $T_{1}$ origin is at 3.67eV, and the $T_{2}$ origin is at 4.59eV
($S_{1}$ at 4.69eV)[8, 10]. The corresponding states for the mFBz and dFBz
states are at similar or slightly lower energies in the gas-phase[12] and thin
films solids[14].

The suggestion that the lower energy EET processes seen for CH3I on mFBz and
dFBz films leads to a different excited state than that seen on benzene
requires some scrutiny– for optical excitation throughout the A-band region,
the ${}^{3}Q_{0}$ excitation dominates. For the listed ${}^{3}Q$ states to be
significant in the EET process, the wavefunction overlap with arene excited
states must be the critical factor, which will depend on the state energies,
adsorption site and geometry for CH3I on the different molecular thin films.

It is useful to briefly consider comparisons with some findings from the
photochemistry of bimolecular clusters, though these represent the converse
from the extended condensed-phase systems of the present work. The benzene:I2
dimer (as well as other arene:I2 dimers) has been studied extensively due to
the formation of a well-known charge-transfer (CT) complex. Photodissociation
dynamics following excitation of the CT state find that the ionic and neutral
state mixing leads to a significant dissociation pathway C6H6-I + I with
substantial kinetic energy release. Modelling of this process finds that the
pathways and outcome are sensitive to the initial geometry of the dimer[46,
47]. Another cluster system of relevance is that of the CH3I:O2 dimer, where
near-UV photodissociation led to CH3I dissociation ascribed to the
${}^{3}Q_{1}$ state, as the I-pathway outcome of Equ. 1 was the only one
observed[48], similar to the observations and argument made above for CH3I on
mFBz and dFBz thin films.

##  V Summary and Conclusions

The 248nm illumination of CH3I adsorbed on various fluorobenzene films grown
on a Cu(100) substrate displays an additional CH3I photodissociation mechanism
for 1,4-difluorobenzene, monofluorobenzene and benzene thin films but absent
for the higher F-count fluorobenzenes studied. The use of s-polarized incident
light allows this mechanism to be examined separately from gas-phase-like
neutral photodissociation of CH3I. This novel photodissociation mechanism
displays a unimodal CH3 photofragment kinetic energy distribution for the
fluorobenzenes, with peak kinetic energies that are distinct for each
molecular thin film. The substrate-induced quenching behaviour observed
suggests that the excitation has a notably longer lifetime than that for the
neutral photodissociation channel. These observations can be understood by a
model in which the photoexcitation creates excitons in the fluorobenzene thin
films that transfer this energy to the CH3I, which promptly dissociates. As
such, we anticipate that this phenomenon should be operative for a wide
variety of aromatic molecules that display similar photoexcitations in the
near-UV region.

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# Raining in MKW 3s: a Chandra-MUSE analysis of X-ray cold filaments around
3CR 318.1

A. Jimenez-Gallardo Dipartimento di Fisica, Università degli Studi di Torino,
via Pietro Giuria 1, I-10125 Torino, Italy  European Southern Observatory,
Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile  Istituto
Nazionale di Fisica Nucleare, Sezione di Torino, I-10125 Torino, Italy  INAF-
Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese,
Italy  F. Massaro Dipartimento di Fisica, Università degli Studi di Torino,
via Pietro Giuria 1, I-10125 Torino, Italy  Istituto Nazionale di Fisica
Nucleare, Sezione di Torino, I-10125 Torino, Italy  INAF-Osservatorio
Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
Consorzio Interuniversitario per la Fisica Spaziale, via Pietro Giuria 1,
I-10125 Torino, Italy  B. Balmaverde INAF-Osservatorio Astrofisico di Torino,
via Osservatorio 20, 10025 Pino Torinese, Italy  A. Paggi Dipartimento di
Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino,
Italy  Istituto Nazionale di Fisica Nucleare, Sezione di Torino, I-10125
Torino, Italy  INAF-Osservatorio Astrofisico di Torino, via Osservatorio 20,
10025 Pino Torinese, Italy  A. Capetti INAF-Osservatorio Astrofisico di
Torino, via Osservatorio 20, 10025 Pino Torinese, Italy  W. R. Forman Center
for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA
02138, USA  R. P. Kraft Center for Astrophysics $|$ Harvard & Smithsonian, 60
Garden Street, Cambridge, MA 02138, USA  R. D. Baldi Istituto di
Radioastronomia, INAF, via Gobetti 101, 40129, Bologna, Italy  Department of
Physics & Astronomy, University of Southampton, Hampshire SO17 1BJ,
Southampton, UK  V. H. Mahatma Thüringer Landessternwarte, Sternwarte 5, 07778
Tautenburg, Germany  C. Mazzucchelli European Southern Observatory, Alonso de
Córdova 3107, Vitacura, Región Metropolitana, Chile  V. Missaglia Dipartimento
di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125
Torino, Italy  Istituto Nazionale di Fisica Nucleare, Sezione di Torino,
I-10125 Torino, Italy  INAF-Osservatorio Astrofisico di Torino, via
Osservatorio 20, 10025 Pino Torinese, Italy  F. Ricci Dipartimento di Fisica e
Astronomia dell’Università degli Studi di Bologna, via P. Gobetti 93/2,
I-40129 Bologna, Italy  INAF- Osservatorio di Astrofisica e Scienza dello
Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy  G. Venturi
Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica
de Chile, Casilla 306, Santiago 22, Chile  INAF - Osservatorio Astrofisico di
Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy  S. A. Baum  University of
Manitoba, Dept. of Physics and Astronomy, Winnipeg, MB R3T 2N2, Canada  E.
Liuzzo Istituto di Radioastronomia, INAF, via Gobetti 101, 40129, Bologna,
Italy  C. P. O’Dea University of Manitoba, Dept. of Physics and Astronomy,
Winnipeg, MB R3T 2N2, Canada  M. A. Prieto Departamento de Astrofísica,
Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain  Instituto de
Astrofísica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain  H. J. A.
Röttgering Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden,
The Netherlands  E. Sani European Southern Observatory, Alonso de Córdova
3107, Vitacura, Región Metropolitana, Chile  W. B. Sparks SETI Institute,
Mountain View, CA 94043  Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218, USA  G. R. Tremblay Center for Astrophysics $|$
Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  R. J. van
Weeren Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The
Netherlands  B. J. Wilkes Center for Astrophysics $|$ Harvard & Smithsonian,
60 Garden Street, Cambridge, MA 02138, USA  J. J. Harwood Centre for
Astrophysics Research, School of Physics, Astronomy and Mathematics,
University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB,
UK  P. Mazzotta Dipartimento di Fisica, Università di Roma “Tor Vergata” Via
della Ricerca Scientifica 1, I-00133 Rome, Italy  J. Kuraszkiewicz Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA

###### Abstract

We present the analysis of X-ray and optical observations of gas filaments
observed in the radio source 3CR 318.1, associated with NGC 5920, the
Brightest Cluster Galaxy (BCG) of MKW 3s, a nearby cool core galaxy cluster.
This work is one of the first X-ray and optical analyses of filaments in cool
core clusters carried out using MUSE observations. We aim at identifying the
main excitation processes responsible for the emission arising from these
filaments. We complemented the optical VLT/MUSE observations, tracing the
colder gas phase, with X-ray Chandra observations of the hotter highly ionized
gas phase. Using the MUSE observations, we studied the emission line intensity
ratios along the filaments to constrain the physical processes driving the
excitation, and, using the Chandra observations, we carried out a spectral
analysis of the gas along these filaments. We found a spatial association
between the X-ray and optical morphology of these filaments, which are colder
and have lower metal abundance than the surrounding intra-cluster medium
(ICM), as already seen in other BCGs. Comparing with previous results from the
literature for other BCGs, we propose that the excitation process that is most
likely responsible for these filaments emission is a combination of star
formation and shocks, with a likely contribution from self-ionizing, cooling
ICM. Additionally, we conclude that the filaments most likely originated from
AGN-driven outflows in the direction of the radio jet.

galaxies: active — X-rays: general — radio continuum: galaxies — galaxies:
clusters: intracluster medium

##  1 Introduction

The Third Cambridge Catalog of radio sources and its revised versions (3C,
3CR, 3CRR, Edge et al. 1959; Bennett 1962; Spinrad et al. 1985; Laing et al.
1983) constitute one of the most valuable samples of radio-loud active
galactic nuclei (AGN). In 2018, Balmaverde et al. started the MUse RAdio Loud
Emission line Snapshot (MURALES) survey to observe all southern 3CR sources
with the Multi-Unit Spectroscopic Explorer (MUSE; Bacon et al. 2010) and
enrich the multifrequency coverage of the 3CR catalog.

During the MURALES survey, an optical filamentary structure was detected in
3CR 318.1 (Balmaverde et al. 2019), the brightest cluster galaxy (BCG) of the
galaxy cluster MKW 3s (Morgan et al. 1975), in agreement with observations by
Edwards et al. (2009). This structure consists of two ionized gas filaments
extending $\sim$25 kpc in the south and the south-west directions, both with
almost constant velocities along their projected lengths ($\sim$230 km s-1 and
$\sim$280 km s-1, respectively). The filament that points to the south
presents a bright knot at its southern end. In contrast with the typical
narrow-line regions seen in other radio galaxies, these filaments extend
beyond tens of kiloparsecs and present drastically different emission line
ratios (e.g., [O III]/H${}_{\beta}\sim$0.1 instead of the typical value of
$\sim$10).

Similar filamentary structures were discovered in other BCGs, as NGC 1275 in
the center of the Perseus galaxy cluster (Lynds 1970, Conselice et al. 2001
and Fabian et al. 2008), spatially associated with an X-ray excess (Fabian et
al., 2011). The same situation occurs for other BCGs, where a tight connection
between X-ray and optical filament emission was found (McDonald et al., 2010).

However, the physical processes responsible for the ionization in these
optical filaments is still debated. While Voit et al. (1994) proposed
ionization due to hot cooling ICM as the excitation mechanism behind these
filaments; McDonald et al. (2010), who analyzed a sample of 23 filaments
around BGCs, suggested that these types of filaments are due to ICM thermal
conduction. In the case of the Perseus cluster, Fabian et al. (2011) rejected
this scenario, due to the lack of a thick interface between the cold and hot
gas, proposing that the origin of cold X-ray filaments is due to penetration
of cold gas by the hot surrounding gas through reconnection diffusion.

Here we present a comparison between optical and X-ray images of 3CR 318.1 to
investigate the main ionization processes underlying the origin of these
optical filaments, thus focusing only on the first $\sim$30 kpc at the center
of MKW 3s (for works on the large scale structures see e.g., Mazzotta et al.
2002, 2004 and Bîrzan et al. 2020).

This manuscript is organized as follows. A brief description of 3CR 318.1 is
given in § 2. Optical and X-ray data analyses are reported in § 3. Results are
presented in § 4 while § 5 is devoted to our conclusions.

Unless otherwise stated, we adopted cgs units for numerical results and
assumed a flat cosmology with $H_{0}=69.6$ km s-1 Mpc-1, $\Omega_{M}=0.286$
and $\Omega_{\Lambda}=0.714$ (Bennett et al., 2014). Spectral indices,
$\alpha$, are defined by flux density, S${}_{\nu}\propto\nu^{-\alpha}$.

##  2 3CR 318.1

3CR 318.1 is the radio source associated with the nearby ($z=0.0453$, which
corresponds to 0.896 kpc/arcsec) galaxy NGC 5920, the BCG of the galaxy
cluster MKW 3s. Based on its optical line emission, it can be classified as an
extremely low-excitation galaxy (see Capetti et al. 2013). The central source
presents a steep spectrum at low radio frequencies (i.e.,
$\alpha^{150\text{MHz}}_{1.4\text{GHz}}=0.63$) with a luminosity of $\log
L_{150\text{MHz}}=23.16$ W Hz-1 (Capetti et al. 2020). The surrounding diffuse
radio emission also shows an extremely steep radio spectrum
($\alpha^{1.28\text{GHz}}_{235\text{MHz}}=2.42$), leading to a classification
of relic radio galaxy by Giacintucci et al. (2007). The host galaxy of 3CR
318.1 presents an AB UV magnitude at 2600 Å of 19.04 $\pm$ 0.04, corrected for
Galactic reddening (Cardelli et al. 1989), with $E(B-V)=0.0356\pm
0.0010$111https://irsa.ipac.caltech.edu/applications/DUST/. This magnitude
yields a UV luminosity at 2600 Å of $L_{UV}=(4.96\pm 0.17)\times 10^{42}$ erg
s-1. An overview of 3CR 318.1 and its environment is shown in Fig. 1.

Peres et al. (1998) found that MKW 3s presents a moderate “cooling flow”
($\dot{M}\sim$170 M⊙ yr-1). Using deep ($\sim$57 ks) Chandra X-ray
observations of the central 200 kpc, Mazzotta et al. (2002) identified the
presence of a cavity, located at $\sim$90 kpc south of the X-ray nucleus,
hotter than the $\sim$3 keV average cluster temperature and filled by radio
emission arising from the southern lobe (Mazzotta et al. 2004), as recently
confirmed by Bîrzan et al. (2020) using Low Frequency Array observations.

Figure 1: r-band SDSS image with SWIFT UV contours at 2600 Å overlaid in blue
(left) and 3.4 $\mu$m WISE image with 328 MHz VLA radio contours overlaid in
black (right), showing 3CR 318.1 at the center and other sources in the field.
The red squares show MUSE field of view. Radio contours were drawn at 5, 10,
20, 40, 80 times the root mean square (rms) level of the background in the
radio map (beam size of 5.4″). Sources in the field with known spectroscopic
redshifts are marked in magenta. Ambient parameters, shown in both panels,
were derived according to Massaro et al. (2019, 2020a, 2020b). $N_{cn}$ is the
number of cosmological neighbors within a radius of 500 kpc, 1 Mpc and 2 Mpc,
respectively, $\sigma_{v}$ is the velocity dispersion of the cosmological
neighbors, $M_{env}$ is the environmental mass (i.e., galaxies, IGM and dark
matter) estimated from the velocity dispersion, $d_{proj}$ is the projected
distance between the central radio galaxy and the centroid of the position of
the cosmological neighbors within 2 Mpc (marked by the black cross) and
$\Sigma_{5}$ is the cosmological parameter used to trace the dark matter halo
density (see Sabater et al. 2013 and Worpel et al. 2013). SWIFT-UVOT data were
reduced according to the standard procedure (see Massaro et al. 2008a, b, for
more details).

##  3 Data reduction and analysis

X-ray data reduction was performed following standard procedures of the
$Chandra$ Interactive Analysis of Observations v4.10 (CIAO; Fruscione et al.
2006) threads222http://cxc.harvard.edu/ciao/threads/, adopting the Chandra
Calibration Database v4.8.4.1, and using the $\sim$57 ks Chandra observation
(ObsID 900). The MUSE dataset was obtained from the MURALES survey (ID
099.B-0137(A); two exposures of 10 min each), with a mean seeing in the V band
at zenith of 1.38″(see Balmaverde et al. 2019 for details on the data
reduction and analysis). Here we focus on the analysis of the [N II]$\lambda
6584$ emission since it is more luminous and extended than the H$\alpha$
emission. The [N II]/H$\alpha$ ratio is constant along filaments with [N
II]/H$\alpha\sim$1.9.

Astrometric registration to align radio, optical and X-ray images was
performed by measuring the radio centroid at 1.4 GHz (beam size of 4.5″), and
the X-ray centroid in the 0.5–7 keV energy range and then aligning them
following the procedure from Massaro et al. (2011). We shifted the X-ray image
0.568″ to the north-east, then, we aligned the position of the optical host
galaxy in the MUSE continuum image with the radio centroid shifting it by
1.62″. These shifts are consistent with those previously reported for the 3CR
Chandra Snapshot Survey (Massaro et al. 2011, 2013, 2015, 2018; Jimenez-
Gallardo et al. 2020). Registration was then verified by checking the
alignment of other point-like field sources in the Sloan Digital Sky Survey
(SDSS) and in the MUSE continuum images. The comparison between registered
X-ray and [NII]$\lambda 6584$ optical images is shown in Fig. 2.

In the same figure, we show the regions selected to perform X-ray spectral
analysis, based on the morphology of the [N II]$\lambda 6584$ emission. These,
labeled according to the nomenclature of Massaro et al. (2011), are:

  1. 1.

two circles of 2″ radius, centered on (i) the radio core and (ii) the [N
II]$\lambda 6584$ emission line knot at the end of the south filament (core
and s14, respectively);

  2. 2.

two polygons along the south-west filament, namely: sw4 (inner 4 kpc) and sw14
(remaining 13 kpc).

We chose, as background for the spectral analysis of the filament regions, a
larger circle located $\sim$45″ to the north-east where no point-like and/or
extended X-ray sources are detected, to obtain the spectrum of the excess
emission only instead of the line-of-sight average. We also performed the
spectral analysis of the galaxy cluster emission, considering two polygonal
regions, one at each side of the filaments and a background region at
$\sim$100″ to compare the inner and outer galaxy cluster emission.

Background subtracted X-ray spectra in the 0.5–4 keV energy range were
extracted using the CIAO routine specextract and the background region
containing neighboring ICM emission to the filament and analyzed using Sherpa
v4.12.1 (Freeman et al. 2001). To ensure the validity of Gaussian statistics,
spectral data were binned to at least 25 photons per bin.

We tried fitting the final spectra using an absorbed bremsstrahlung model,
however, it failed to reproduce the soft X-ray excess below 2 keV. Thus, we
adopted a collisional ionisation gas model
(xsapec333https://cxc.cfa.harvard.edu/sherpa/ahelp/xsapec.html), absorbed by
Galactic hydrogen column density
(xswabs444https://cxc.cfa.harvard.edu/sherpa/ahelp/xswabs.html;
$N_{H,Gal}=3.8\times 10^{20}$ cm-2; Kalberla et al. 2005). The xsapec model
had three free parameters: normalization, plasma temperature $kT$, and metal
abundance $Z$.

We confirmed the presence of a soft X-ray (i.e., 0.5 - 3 keV) excess spatially
coincident with the [N II]$\lambda 6584$ filament (regions sw4 and sw14) at a
level of confidence $>5\sigma$ with respect to the surrounding ICM emission.
Detection significance, reported in Gaussian equivalent, was computed assuming
a Poisson distribution of photons in the inner background region.

Assuming the presence of emission lines below 2 keV (see § 4), we created
narrow band X-ray images, shown in Fig. 3 (see e.g., Massaro et al. 2013,
2015, 2018). We chose three energy ranges mainly attributable to (i) Fe
(0.9–1.2 keV), (ii) Mg (1.2–1.5 keV) and (iii) S+Si (1.6–2.1 keV) ionized
elements. X-ray flux maps were created by using monochromatic exposure maps
set to the nominal energies of 1, 1.3 and 1.8 keV for the Fe, Mg, S+Si,
respectively, to take into account the detector effective area at different
energies.

Figure 2: 0.5–3 keV Chandra image with [N II]$\lambda 6584$ emission line
contours overlaid in cyan. The Chandra image was smoothed with a 2″Gaussian
kernel. [N II]$\lambda 6584$ contours were drawn at 1, 2, 4, 8, 16 and 32
$\times 10^{-20}$ erg s-1 cm-2 (starting at 3 times the rms). Regions chosen
for the X-ray spectral analysis (described in § 3) and the results for each
region are shown in black. The fit for each region yielded the following
reduced $\chi_{\nu}^{2}=1.304$ ($\nu=8$), with 237 net photons, for the core,
$\chi_{\nu}^{2}=0.900$ ($\nu=5$), with 167 net photons, for region s14,
$\chi_{\nu}^{2}=0.996$ ($\nu=10$), with 268 net photons, for region sw4,
$\chi_{\nu}^{2}=0.588$ ($\nu=36$), with 845 net photons, for region sw14, and
$\chi_{\nu}^{2}=0.906$ ($\nu=207$) for the cluster region.

Figure 3: Narrow-band Chandra flux images in the 0.9–1.2 keV (left), 1.2–1.5
keV (center) and 1.6–2.1 keV (right) energy bands to show Fe, Mg and S+Si
X-ray emissions with [N II]$\lambda 6584$ contours overlaid. Chandra images
were smoothed with a Gaussian kernel of 2″. [N II]$\lambda 6584$ emission line
contours are the same as in Fig. 2. X-ray emission due to Mg seems to trace
the [N II]$\lambda 6584$ morphology the best. X-ray fluxes along the filament
(regions sw4 and sw14) are $F_{\text{Fe}}=2.9^{+0.2}_{-0.3}\times 10^{-14}$
erg s-1 cm-2 in the Fe band, $F_{\text{Mg}}=1.9^{+0.1}_{-0.1}\times 10^{-14}$
erg s-1 cm-2 in the Mg band, and $F_{\text{S+Si}}=2.3^{+0.1}_{-0.2}\times
10^{-14}$ erg s-1 cm-2 in the S+Si band.

From the optical perspective, we modeled the main emission lines present in
the MUSE spectra, i.e., $H\beta$, $H\alpha$, [O III]$\lambda$5007, [O
I]$\lambda$6300, [N II]$\lambda 6584$ and the [SII] doublet at
$\lambda\lambda$6716, 6731 using Gaussian functions at different locations
across the filament and the knot. Top panel of Fig. 4 shows the MUSE spectrum
of the optical knot sw14, as an example. The $H\beta$ line was modelled using
an additional broad component only present in the core and in region s4.

##  4 Results

###  4.1 Optical

We computed the emission line intensity ratios for all regions (see Fig. 2)
and plotted them in the diagnostic diagrams defined by the intensity ratios
shown in Fig. 4, used to distinguish between different ionization mechanisms
occurring in H II regions, AGN, or LINER/shocks (Baldwin et al. 1981 and
Dopita & Sutherland 1995). Measurements obtained for all four regions in 3CR
318.1 are closely clustered and located where no emission line galaxies are
found (see e.g., Kewley et al. 2006 and Capetti & Baldi 2011).

Similarly to what was found for filaments in different BCGs by McDonald et al.
(2012), line ratios provide a contradictory classification: at the boundary
between LINER and H II from the [O III]/H$\beta$ vs [N II]/H$\alpha$ and [O
I]/H$\alpha$ diagrams and in the region of ionization due to star-formation
from the [S II]/H$\alpha$ ratio.

We also estimated the gas density along the filaments, using the ratio of the
[S II]$\lambda$6716 and the [S II]$\lambda$6731 lines (see Osterbrock 1989).
Assuming a typical temperature of $10^{4}$ K, we obtained $n_{e}\sim 1380$
cm-3 in the core and $n_{e}<100$ cm-3 in s14 and along the filament (sw4 and
sw14). We estimated the total mass of the ionized gas as $M=7.5\times
10^{-3}\left(\frac{10^{4}}{n_{e}}\frac{L_{\text{H}_{\beta}}}{L_{\odot}}\right)$
M⊙, where $L_{\text{H}_{\beta}}$ is the H$\beta$ luminosity (see Osterbrock
1989). Thus, we estimated a total ionized gas mass of $M\sim 2\times 10^{5}$
M⊙ in the core, $M>4\times 10^{5}$ M⊙ in s14, and $M>10^{6}$ M⊙ along the
filament.

Figure 4:  Top: MUSE spectrum (black histograms) of region s14, see Fig. 2, in
rest frame, shown as an example. We show in red the Gaussian fit of the
emission lines (top panels) and the residuals (bottom panels). Bottom:
Location of 3CR 318.1 regions “core”, s14, sw4 and sw14 (red filled circle,
green circle, blue triangle and magenta square, respectively) in the BPT
diagnostic diagrams. The red solid curves represent the Kewley et al. (2001)
theoretical upper bound for pure star formation, the red straight line shows
the Kewley et al. (2006) separation between AGN and LINERs, while the dashed
red curve in the [N II]$\lambda 6584$ BPT is the Kauffmann et al. (2003)
empirical classification separating star-forming galaxies and AGN. Contours
represent the iso-densities of all SDSS/DR7 emission line galaxies (Capetti &
Baldi 2011). The three black dots mark the location of the filaments in 3CR
317, the central galaxy of A2052, also observed as part of the MURALES survey
(see Balmaverde et al. 2018). Purple dashed regions mark the location of
filaments in other cool core galaxy clusters as measured by McDonald et al.
(2012).

Simulations carried out by Qiu et al. (2019, 2020) show that cold filaments in
cool core clusters can originate from warm AGN-driven outflows with shorter
cooling than rising times. These simulations are consistent with the filaments
in 3CR 318.1 extending in the direction of the southern radio lobe.

###  4.2 X-rays

We performed the X-ray spectral analysis in all regions described in § 3,
along optical filaments and in the galaxy cluster (excluding filaments, core
and knots). Best fit results, obtained with the absorbed xsapec model, are
reported in Fig. 2. We also fitted regions sw4 and sw14 together and obtained
a gas temperature of $2.4^{+0.3}_{-0.3}$ keV (with $Z=0.5^{+0.3}_{-0.2}$
$Z_{\odot}$ and reduced $\chi_{\nu}^{2}=0.857$ with $\nu=59$ degrees of
freedom) for the south-western filament. This is colder than the surrounding
ICM, which has $kT=3.2^{+0.1}_{-0.1}$ keV (reduced $\chi_{\nu}^{2}=0.906$;
$\nu=207$). The ICM temperature is consistent with the average temperature
found by Mazzotta et al. (2002) for the inner regions of MKW 3s.

Metal abundance of the optical filament, $Z=0.3^{+0.2}_{-0.2}$ for sw14 (with
reduced $\chi_{\nu}^{2}=0.588$; $\nu=36$), appears to be marginally lower than
that of the surrounding ICM ($Z=0.72^{+0.08}_{-0.07}$). This is in agreement
with literature results on other filaments (see e.g., Fabian et al. 2011 and
McDonald et al. 2010).

Assuming that narrow band images trace the X-ray emission lines, the Mg
emission has a better spatial association with the optical filaments, while Fe
and S+Si show X-ray emission more extended than the filaments, as well as
strong emission at the core and the s14 (knot) regions (see Fig. 3). We
computed the X-ray flux along the filament (regions sw4 and sw14) in the Fe,
Mg and S+Si band and compared them with optical and X-ray fluxes. The X-ray
fluxes along the filament (regions sw4 and sw14) are
$F_{\text{Fe}}=2.9^{+0.2}_{-0.3}\times 10^{-14}$ erg s-1 cm-2 in the Fe band,
$F_{\text{Mg}}=1.9^{+0.1}_{-0.1}\times 10^{-14}$ erg s-1 cm-2 in the Mg band,
$F_{\text{S+Si}}=2.3^{+0.1}_{-0.2}\times 10^{-14}$ erg s-1 cm-2 in the S+Si
band, and $F_{\text{0.5-3keV}}=1.32^{+0.05}_{-0.08}\times 10^{-13}$ erg s-1
cm-2 in the soft band (0.5 - 3 keV). Detection significance of the excess
X-ray emission along optical filaments is above $5\sigma$ confidence level for
both Mg and Fe, while it is not significant for the S+Si narrow band image,
thus the S+Si emission along the filament is consistent with the ICM emission.

We also obtained $F_{\text{H}\alpha}/F_{\text{Fe}}$ =
2.9${}^{+0.4}_{-0.4}\times 10^{-2}$ and
$F_{\text{H}\alpha}/F_{\text{0.5-3keV}}$ = 6.5${}^{+0.7}_{-0.8}\times
10^{-3}$. In contrast with the filaments of 3CR 318.1, Sanders & Fabian (2007)
and Fabian et al. (2011) found that those in Perseus have soft X-ray emission
of the same order as the H$\alpha$ emission, while the H$\alpha$ emission is
an order of magnitude above the Fe emission, which highlights the much lower
ionization state here. Additionally, we obtained
$F_{\text{[OIII]}}/F_{\text{0.5-3keV}}$ = 1.15${}^{+0.12}_{-0.14}\times
10^{-3}$, which is three orders of magnitude below the value obtained by
Balmaverde et al. (2012) for the emission line regions in nine 3CR radio
galaxies.

Lastly, we derived the 0.5–3 keV, exposure corrected, X-ray surface brightness
profile with azimuthal bins of 10° centered on 3CR 318.1 core, excluding the
inner 2″ and extending up to $\sim 30$ kpc (see Jimenez-Gallardo et al. 2021
for additional details). We chose to make the bin containing the [N
II]$\lambda 6584$ filament correspond to the 180° bin. The resulting X-ray
surface brightness profile is shown in Fig. 5. The background level is two
orders of magnitude below the filament emission. The smooth increase of the
surface brightness towards the [N II]$\lambda 6584$ filament supports the
hypothesis that the filament could have originated due to outflows in the
direction of the radio jets.

Figure 5: 0.5 - 3 keV, exposure corrected X-ray surface brightness profile
with azimuthal bins of 10° centered on the core of 3CR 318.1. The bins were
chosen so that the bin at 180° is the one containing the [N II]$\lambda 6584$
filament. Background level is two orders of magnitude below the filament
emission. The increase of surface brightness corresponds to the [N II]$\lambda
6584$.

##  5 Discussion and conclusions

The MURALES survey revealed the presence of optical filaments in 3CR 318.1,
the BCG of MKW 3s. Here we compared optical VLT/MUSE and X-ray Chandra
observations to shed light on their physical origin.

From an optical perspective, intensity ratios of various rest-frame emission
lines were used to distinguish between different ionization mechanisms.
Possible explanations of filamentary emission include: ionization due to hot
cooling ICM (Voit et al. 1994), ICM thermal conduction (McDonald et al. 2010)
and reconnection diffusion (Fabian et al. 2011). However, individually, none
of the ionization mechanisms listed can account for all line ratios
simultaneously. As shown in Fig. 4, line ratios measured in 3CR 318.1 show
extremely low values of [O III]/H$\beta$ in the diagnostic diagrams,
similarly, McDonald et al. (2012) found systematically lower [O III]/H$\beta$
values in filaments of cool core galaxy clusters and higher values of [N
II]/H$\alpha$, [O I]/H$\alpha$ and [S II]/H$\alpha$ than those found in
galaxies in the SDSS. Although the diagnostic diagrams are used to
discriminate between star-forming and AGN-dominated galaxies, they tend to
fail when the ionization is due to more complex situations and/or different
ionization mechanisms (see e.g., Stasińska et al. 2008, Capetti & Baldi 2011
and Balmaverde et al. 2018) and therefore they may have a limited validity in
assessing the ionisation conditions of BCGs. Thus, the line ratios in 3CR
318.1 imply that the emission from the filaments is due to a combination of
ionization mechanisms. McDonald et al. (2012) suggested that line ratios are
due to a combination of star formation and ionization from slow shocks
($\sim$100 - 400 km s-1; see also Allen et al. 2008). Since our results are
similar to those of McDonald et al. (2012), we argue that their conclusion
applies also to the case of 3CR 318.1. An additional contribution from self-
ionizing cooling ICM is suggested by the low velocity dispersion ($\sim 60$ km
s-1) of the optical filaments in 3CR 318.1, which implies a small contribution
of shocks to the total ionization. Therefore, we conclude that the underlying
ionization mechanisms include a combination of photoionization due to star
formation, self-ionizing cooling ICM, and a small contribution of ionization
due to slow shocks. Additionally, although McDonald et al. (2012) already
found a decrease of the emission line width with radius, thanks to the MUSE
data, it was discovered that this decrease occurs sharply in the case of 3CR
318.1 (from $\sim$200 km s-1 in the core to $\sim$60 km s-1 along the
filaments; see Balmaverde et al. 2019).

We detected an excess of X-ray emission above the ICM along the [N II]$\lambda
6584$ filaments. This X-ray filament is colder and despite the large
uncertainties, appears to have a lower metal abundance than the surrounding
ICM, in agreement with literature results (McDonald et al. 2010). The
association between the X-ray and optical filamentary morphologies, together
with the radio structure, suggests that the filaments could have originated
from AGN-driven outflows in the direction of the radio jet. This scenario is
in agreement with the smooth 0.5 - 3 keV X-ray surface brightness profile at
the location of the [N II]$\lambda 6584$ filament and with simulations carried
out by Qiu et al. (2019, 2020), in which cold filaments originate from warm
outflows (104 \- 107 K). Additionally, works such as Gaspari et al. (2018) and
Voit (2021) predict that optical emission line nebulae would present velocity
dispersions of $\sim$100 - 200 km s-1 in the cases where the nebulae
originated from compression and catastrophic cooling (see also Gaspari et al.
2012, 2013, 2015, 2017 and Voit et al. 2017 for previous works on multiphase
condensation). However, the [N II]$\lambda 6584$ filaments in 3CR 318.1
present very low velocity dispersions ($\sim$60 km s-1; see Fig. 8 in
Balmaverde et al. 2019), so the AGN uplift scenario could be favoured.

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We thank the anonymous referee for their useful comments that led to the
improvement of the paper. This work is supported by the “Departments of
Excellence 2018 - 2022” Grant awarded by the Italian Ministry of Education,
University and Research (MIUR) (L. 232/2016). This research has made use of
resources provided by the Ministry of Education, Universities and Research for
the grant MASF_FFABR_17_01. This investigation is supported by the National
Aeronautics and Space Administration (NASA) grants GO9-20083X and GO0-21110X.
F. M. is in debt with S. Bianchi for his valuable input on X-ray
photoionizaton scenarios. A. J. acknowledges the financial support
(MASF_CONTR_FIN_18_01) from the Italian National Institute of Astrophysics
under the agreement with the Instituto de Astrofisica de Canarias for the
“Becas Internacionales para Licenciados y/o Graduados Convocatoria de 2017”.
A.P. acknowledges financial support from the Consorzio Interuniversitario per
la fisica Spaziale (CIFS) under the agreement related to the grant
MASF_CONTR_FIN_18_02. W.F. and R.K. acknowledge support from the Smithsonian
Institution and the Chandra High Resolution Camera Project through NASA
contract NAS8-03060. G.V. acknowledges support from ANID programs FONDECYT
Postdoctorado 3200802 and Basal-CATA AFB-170002