11institutetext: Laboratory for Astrophysics, Leiden Observatory, Leiden
University, PO Box 9513, 2300 RA Leiden, The Netherlands
11email<EMAIL_ADDRESS>22institutetext: FELIX Laboratory, Radboud
University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 33institutetext:
School of Electronic Engineering and Computer Science, Queen Mary University
of London, London E1 4NS, UK 44institutetext: Institute for Molecules and
Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The
Netherlands 55institutetext: Center for Interstellar Catalysis, Department of
Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C 8000,
Denmark

# Resonant infrared irradiation of CO and CH3OH interstellar ices

J. C. Santos 11 K.-J. Chuang 11 J. G. M. Schrauwen 22 A. Traspas Muiña 33 J.
Zhang 33 H. M. Cuppen 44 B. Redlich 22 H. Linnartz 11 S. Ioppolo 55 3 3

###### Abstract

Context. Solid-phase photo-processes involving icy dust grains greatly affect
the chemical evolution of the interstellar medium by leading to the formation
of complex organic molecules and by inducing photodesorption. So far, the
focus of laboratory studies has been mainly on the impact of energetic
ultraviolet (UV) photons on ices, but direct vibrational excitation by
infrared (IR) photons is expected to influence the morphology and content of
interstellar ices as well. However, little is still known about the mechanisms
through which this excess vibrational energy is dissipated, and its
implications on the structure and ice photochemistry.

Aims. In this work, we present a systematic investigation of the behavior of
interstellar relevant CO and CH3OH ice analogues upon resonant excitation of
vibrational modes using tunable infrared radiation, leading to both the
quantification of infrared-induced photodesorption and insights in the impact
of vibrational energy dissipation on ice morphology.

Methods. We utilize an ultrahigh vacuum setup at cryogenic temperatures to
grow pure CO and CH3OH ices, as well as mixtures of the two. We expose the
ices to intense, near-monochromatic mid-infrared free-electron-laser radiation
using the LISA end-station at the FELIX free electron laser facility to
selectively excite the species. Changes to the ice are monitored by means of
reflection-absorption infrared spectroscopy combined with quadrupole mass-
spectrometry. The methods also allow to characterize the photodesorption
efficiency.

Results. The dissipation of vibrational energy is observed to be highly
dependent on the excited mode and the chemical environment of the ice. All
amorphous ices undergo some degree of restructuring towards a more organized
configuration upon on-resonance irradiation. Moreover, IR-induced
photodesorption is observed to occur for both pure CO and CH3OH ices, with
interstellar photodesorption efficiencies of the order of 10 molecules cm-2
s-1—i.e., comparable to or higher than UV-induced counterparts. Indirect
photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is
also observed to occur, particularly in environments rich in methanol. The
astrochemical implications of these IR-induced phenomena are discussed.

###### Key Words.:

astrochemistry – methods: laboratory: solid state – infrared: ISM – ISM:
molecules – molecular processes

##  1 Introduction

Interstellar icy dust grains are continuously exposed to both energetic and
non-energetic triggers that initiate solid-state processes, such as chemical
reactions. The consequent association of induced fragments—e.g., in exothermic
reactions—has been reported to result in larger molecules containing excess
energy. The ejection of solid-state chemical products to the gas phase through
so-called ”reactive desorption” and ”photo-induced chemical desorption” is a
direct consequence of it (Minissale & Dulieu, 2014; Minissale et al., 2016;
Muñoz Caro & Martín Doménech, 2018; Chuang et al., 2018; Oba et al., 2018).
This extra energy, which can be in vibrational form, must in turn be
dissipated within the ice itself. Understanding the mechanisms through which
this energy is dissipated, and its consequences to the content and morphology
of the ice, is of special relevance to construct accurate astrochemical
models. In the last decades the focus has been on ultraviolet photons, as
these are able to break chemical bonds. However, also impacting infrared
photons are expected to have substantial effects, specifically in dense
molecular clouds. The infrared interstellar radiation field (ISRF) is most
intense in the inner galaxy, and is generated both by stellar emission (mainly
for $\lambda\lesssim 8$ $\mu$m) and by re-emission from dust grains (dominant
for $\lambda\gtrsim 8$ $\mu$m). The former is accounted for by a combination
of blackbody radiation fields, while the latter requires a model of the
interstellar dust such as described by Draine & Li (2001) and Li & Draine
(2001). Within clouds, the integrated flux of secondary UV photons resulting
from the excitation of H2 upon interaction with cosmic rays is expected to be
of the order of $\sim 10^{4}$ photons cm-2 s-1 (Cecchi-Pestellini & Aiello,
1992), but that of IR photons with wavelengths in the range of $1-10$ $\mu$m
is currently estimated to be substantially higher and of the order of $\gtrsim
10^{9}$ photons cm-2 s-1 (e.g., Mathis et al. 1983; Porter & Strong 2005;
Roueff et al. 2014). However, the response of ices upon direct vibrational
excitation has been scarcely investigated in the laboratory and is typically
not included in models (e.g., Cuppen et al. 2017). For instance, vacuum
ultraviolet radiation is known for resulting in a rich photochemistry (see,
e.g., the review by Öberg 2016 and references therein), transforming the ice
structure (Kouchi & Kuroda, 1990), as well as leading to photodesorption
induced by electronic transitions (Fayolle et al., 2011, 2013; van Hemert et
al., 2015), whereas laboratory investigations of potential photodesorption and
structural changes upon direct vibrational excitation are still limited.
Structural changes are typically reflected in the spectral appearance of such
ices (e.g., Cuppen et al. 2011; Cooke et al. 2016; Müller et al. 2021), and
therefore changes in ice morphology and content due to energy-dissipation
processes will play a part in interpreting observational data from infrared
facilities—such as the James Webb Space Telescope (McClure et al., 2023).

Silicates and carbonaceous materials are the main components of (sub)micron-
sized dust grains present in the interstellar medium (ISM) (Millar & Williams,
1993). During the earlier stages of star formation, H and O atoms adsorbed
onto these grains react to form H2O, resulting in a water-rich “polar-ice”
layer (Ioppolo et al., 2010; Cuppen et al., 2010; Linnartz et al., 2015).
Besides water, carbon dioxide (CO2) is a very abundant solid-phase ISM
species, and is mainly present in the aforementioned interstellar polar-ice
layer mixed with H2O (Goumans et al., 2008; Oba et al., 2010; Ioppolo et al.,
2011; Noble et al., 2011). As the density of the collapsing cloud increases,
carbon monoxide (CO) molecules present in the gas phase (catastrophically)
freeze-out on top of the polar ice, forming a second coating known as the
“apolar-ice” layer (Tielens et al., 1991; Boogert et al., 2002; Pontoppidan et
al., 2003). Through hydrogenation reactions of solid-state CO, methanol
(CH3OH) is efficiently formed (Tielens & Hagen, 1982; Charnley et al., 1992;
Hiraoka et al., 1994; Watanabe & Kouchi, 2002; Fuchs et al., 2009; Santos et
al., 2022). This alcohol is abundantly present in interstellar ices mixed with
CO, as shown by various laboratory and observational works (e.g., Bottinelli
et al. 2010; Cuppen et al. 2011; Penteado et al. 2015). Follow-up studies have
shown that along the same hydrogenation scheme also larger complex organic
molecules (COMs), like glycerol, can be formed involving radical reactions
(Chuang et al., 2016; Fedoseev et al., 2017). Alternatively, CH3OH can also be
formed less efficiently during stages prior to CO-freeze out through reactions
involving CH4 and OH (Qasim et al., 2018).

The infrared irradiation of crystalline water ice has been previously studied
with a focus on its resonant desorption at elevated temperatures ($\gtrsim$100
K) (Krasnopoler & George, 1998; Focsa et al., 2003). These first studies
suggested that on-resonance irradiation of crystalline ices leads to
selectivity effects, with a strong wavelength-dependent H2O desorption as a
result of IR-photon absorption. Infrared-induced matrix assisted laser
desorption and ionization (MALDI) experiments ensued, evincing the
photodesorption of polycyclic aromatic hydrocarbons (PAHs)—as well as their
hydrated clusters—embedded in water ice matrices and subjected to resonant
vibrational excitation (Focsa et al., 2006; Mihesan et al., 2006; Henderson &
Gudipati, 2014). More recently, porous amorphous solid water (pASW) has been
studied in works combining IR-free electron laser (FEL) irradiation
experiments and molecular dynamics simulations, which revealed a
reorganization of the ice structure towards a more ordered configuration upon
the absorption of photons in the broad IR regime (Noble et al., 2020; Coussan
et al., 2022; Cuppen et al., 2022). Amorphous CO2 ices have shown a similar
behavior as a result of IR-FEL irradiation, whereas this effect was barely
observed in the crystalline counterparts (Ioppolo et al., 2022). Moreover,
both Noble et al. (2020) and Ioppolo et al. (2022) hint at possible
photodesorption upon resonant irradiation of the pure ices.

Vibrational energy pooling (VEP) is also an interesting phenomenon that takes
place as a result of vibrational excitation. In VEP, vibrationally excited CO
molecules pool their energy through a base-camp mechanism, resulting in highly
excited CO species that constitute a small portion of the group (DeLeon &
Rich, 1986; Corcelli & Tully, 2002; Chen et al., 2017, 2019; DeVine et al.,
2022). DeVine et al. (2022) have explored the effects of VEP of crystalline CO
under interstellar conditions in a combined laboratory and theoretical work,
and reported the formation of CO2 and C2O3 species as a result of spin-
forbidden reactions involving highly vibrationally excited CO molecules. The
reaction pathways they explored also led to CO desorption due to the energy
release from the dissociation of an intermediate CO dimer species only
accessible at extremely high vibrational excitation levels.

Infrared spectroscopic properties such as band width and peak position can
provide powerful information on the mixing and layering conditions of ices
(Boogert et al., 2015). With the James Webb Space Telescope (JWST), infrared
observations of interstellar ices are available at a wide spectral range
($0.6-28.3$ $\mu$m) and with unprecedented spatial resolution, and
sensitivity, enabling to focus on specific areas in a proto-planetary disk or
to investigate dense interstellar clouds for extinction values as high as 90
(McClure et al., 2023). The interpretation of these observations requires
highly detailed laboratory data on the spectroscopic properties, morphology,
and dynamics of ices. Moreover, understanding the effects of vibrational
energy on the structure and dynamics of the CO-rich ice layer plays a key role
in developing thorough models of the chemical evolution of the interstellar
medium.

In the present work, we explore the effects of IR-photon irradiation and the
consequences of vibrational-energy release in interstellar apolar ice
analogues. We provide the first selective IR-FEL irradiation study on pure
amorphous and crystalline CO ices (henceforth aCO and cCO), as well as pure
amorphous CH3OH ices and mixtures of the two species. In section 2, the
experimental setup and procedures employed in this work are detailed. The main
results are described and discussed in section 3. In section 4 we explore the
astrophysical implications, and our main findings are summarized in section 5.

##  2 Experimental Methods

Table 1: Overview of the experiments performed in this work. Experiment | T${}_{\text{deposition}}$ a | T${}_{\text{irradiation}}$ a | N(CO) b | N(CH3OH) b | FEL irradiation
---|---|---|---|---|---
| (K) | (K) | (molecules cm-2) | (molecules cm-2) | ($\mu$m)
CH3OH | 20 | 20 |  | $6\times 10^{17}$ | 2.96, 3.07, 3.14, 4.67, 9.62
aCO | 20 | 20 | $1\times 10^{17}$ |  | 4.67, 5.20
cCO | 29 | 20 | $1\times 10^{17}$ |  | 4.67, 5.20
CO:CH3OH = 1:0.3 | 20 | 20 | $2\times 10^{17}$ | $6\times 10^{16}$ | 3.07, 4.67, 9.62
CO:CH3OH = 1:3.0 | 20 | 20 | $2\times 10^{17}$ | $5\times 10^{17}$ | 3.07, 4.67, 9.62
a Measured temperature values varied by $\pm$0.8 K.
b Upper limits derived using transmission band strengths.

The experiments are performed at the HFML-FELIX laboratory at Radboud
University, the Netherlands, directing tunable free electron laser radiation
towards the laboratory ice surface astrophysics (LISA) ultrahigh vacuum (UHV)
end station. At HFML-FELIX, LISA is connected to FELIX-2 and -1 to perform
selective irradiation experiments in the broad infrared ($3700-220$ cm-1,
$2.7-45$ $\mu$m) and terahertz (THz, $10-2$ THz, $30-150$ $\mu$m) spectral
ranges respectively, shedding light on the relaxation mechanisms that follow
the excitation of particular vibrational modes within solid matter. LISA has
been described in detail elsewhere (Noble et al., 2020; Ioppolo et al., 2022),
and here we provide a brief explanation of the experimental procedure. The
experiments described here have been obtained during four 7.5-hours long beam
shifts, focusing on the basic concept of (off/on) resonant IR photodesorption
of interstellar ices. Extended measurements are scheduled for later this year
in another run of 4 beam shifts.

A gold-plated optically-flat copper substrate in the center of the UHV chamber
is in thermal contact with the head of a closed-cycle helium cryostat, which
allows to control the substrate temperature in the range of $20-300$ K through
resistive heating. The temperature is monitored by a silicon diode thermal
sensor fixed at the bottom of the substrate. At room temperature, the base
pressure in the chamber is $\sim 2\times 10^{-9}$ mbar. Gaseous CO
(Hoekloos/Praxair, purity 99.9%) and CH3OH (Sigma-Aldrich, purity 99.9%) vapor
are introduced in the dosing line, either separately—in the case of pure
ices—or simultaneously for a binary ice. Beforehand, the methanol sample is
purified through multiple freeze-pump-thaw cycles. The ices are then grown on
the substrate through background deposition by admitting the pure species or
mixtures into the main chamber via an all-metal leak valve that faces the
walls of the chamber. This deposition method leads to more uniform ices and
better simulates the deposition conditions on interstellar dust grains.
Amorphous CO ice is deposited at the lowest substrate temperature of 20 K,
while crystalline CO is grown at 29 K (see, e.g., Kouchi 1990). The different
solid-state configurations are confirmed by the clearly sharper CO-stretching
feature of the cCO IR spectra. After deposition, all ices are maintained at 20
K during the irradiations.

The ice growth and the effects of the IR irradiation are monitored in situ
through reflection-absorption infrared spectroscopy (RAIRS) using a Fourier
Transform IR spectrometer. The FTIR spectra cover a range of $5000-600$ cm-1
with a resolution of $0.5$ cm-1. To calculate the column density $(N_{X})$ of
the species in the ice, a modified Beer-Lambert law is applied to convert the
IR integrated absorbance ($\int Abs(\nu)d\nu$) to the absolute abundance:

$N_{X}=\ln 10\frac{\int Abs(\nu)d\nu}{A^{\prime}_{X}}$ (1)

where $A^{\prime}_{X}$ is the absorption band strength of a given species.
Since the band strengths of CO and CH3OH have not been estimated for the LISA
setup in reflection mode, we use the transmission values from the literature
instead. Namely, $A(\ce{CO})_{\nu_{1}=2142}=1.1\times 10^{-17}$ cm molecule-1
(Jiang et al., 1975) and $A(\ce{CH3OH})_{\nu_{8}=1032}=1.8\times 10^{-17}$ cm
molecule-1 (Hudgins et al., 1993). Since the RAIRS technique is more sensitive
than transmission infrared spectroscopy, the column densities derived here
should be regarded as upper limits. The details of each experiment performed
in this work are listed in Table 1.

For the IR irradiation of the ice, we use the IR-FEL source FELIX-2 to
generate nearly monochromatic mid-IR photons in the ranges of $2.9-4.5$ $\mu$m
(fundamental mode) and $4.5-9.7$ $\mu$m (third harmonic mode). The photons are
bunched in $5-10$ microseconds-long macropulses with a repetition rate of 5
Hz, consisting of a train of micropulses of several picoseconds spaced by 1
nanosecond. Unless otherwise specified, at longer wavelengths ($\gtrsim$4.5
$\mu$m), the energy of the macropulses is attenuated to a constant value of
$\sim$20 mJ, as measured at the diagnostic station located before the LISA
end-station area of the facility. At shorter wavelengths ($\lesssim$4.5
$\mu$m), the laser energy is kept at its highest attainable value of $\sim$5
mJ, as it is generated in the third harmonic mode. The fluence of infrared
photons with wavelength $\lambda$ in each experiment can be calculated by the
expression:

$\text{Fluence}_{\lambda}=\frac{E_{\lambda}*r*t}{S}$ (2)

where $E_{\lambda}$ is the energy and $r$ is the repetition rate of the
macropulses, $S$ is the area of the FELIX-2 beam spot on the substrate, and
$t$ is the duration of the irradiation. The FEL beam impinges the gold-plated
flat substrate at an angle of $45^{\circ}$ with respect to the surface,
resulting in an elliptical spot size of $\sim$1.0 x $\sim$1.5 mm in semi-axes.
Comparatively, the FTIR beam angle with the gold substrate is of $13^{\circ}$,
resulting in an elliptical spot size of $\sim$1.5 x $\sim$6.5 mm in semi-axes.
Consequently, the FTIR RAIRS beam fully covers the region irradiated by the
FEL beam, with a FTIR/FEL area ratio of $\gtrsim$6\. The spectral FWHM of
FELIX-2 is estimated to be $\sim$0.8% $\delta\lambda/\lambda$ for the entire
wavelength range.

A z-translator allows the movement of the substrate in height, providing
multiple ice spots for systematic IR irradiation. In this work we utilize
seven different substrate positions, each separated by 4 mm from the
neighboring spot. Each FEL irradiation is thereby performed at an unirradiated
ice spot, unless specified. The irradiations are subsequently repeated, on
average two additional times for each spot, to ensure reproducible
results—with the exception of the long aCO and cCO experiments. As stated
above, the FTIR beam is $\gtrsim$6x larger in area than the FEL beam, and thus
part of the ice probed by the FTIR is not exposed to FEL irradiation. Hence,
difference FTIR spectra acquired before and after FEL irradiation are utilized
to visualize changes in the ice. During the irradiation experiments, possible
desorption of species from the ice is monitored by a quadrupole mass
spectrometer (QMS). Control temperature-programmed desorption (TPD)
experiments with analogous deposition conditions and a ramping rate of 2.5 K
min-1 are performed to assist in interpreting the IR-FEL irradiation results.

In this paper, we discuss FEL irradiations in terms of wavelength and FTIR
spectra in wavenumbers (both in vacuum) to reflect the higher spectral
resolution of the FTIR data as opposed to the transform-limited bandwidth of
the FEL photons.

##  3 Results and Discussion

Figure 1 shows the spectra of the ices of pure CH3OH (panel a), pure CO (both
aCO and cCO, panel b), and 1:0.3 and 1:3.0 mixtures of CO:CH3OH in panels c
and d, respectively. Also indicated in this figure are the involved
vibrational modes. The wavelengths of FELIX-2 used during the irradiations are
marked by the dashed vertical lines, and their respective FWHMs are shown by
the shadowed areas. Given the broad wavelength coverage of the FTIR
measurements, the laser bandwidths are hard to see in panels a, c, and d, but
the shadowed area is substantially larger in panel b because of the smaller
spectral range shown here. For each experiment (i.e., pure CO, pure CH3OH, and
mixtures), we use the same color scheme throughout the paper. Below the
individual ices are discussed.

Figure 1: Spectra and irradiation maps of the ices explored in this work, all
taken at 20 K. Panel a) Pure amorphous CH3OH. Panel b) Pure amorphous and
crystalline CO. Panel c) Mixture of CH3OH and CO with a CO:CH3OH ratio of
1:0.3. Panel d) Same as c), but with a 1:3.0 ratio. All spectra are taken
after deposition and before irradiation. The wavelengths of FELIX-2 used later
for on and off-resonance irradiation are shown with dashed lines, and the FWHM
is visible from the shadowed areas. The fundamental mode of CO is denoted in
boldface, and the other labels correspond to the ($\nu_{\text{X}}$)
vibrational modes of CH3OH.

###  3.1 CH3OH

####  3.1.1 Morphology

In Figure 2, the difference spectra obtained before and after continuous
exposure for 5 minutes of pure CH3OH ice to the FEL light at 2.96 ($\nu_{1}$),
3.07 ($\nu_{1}$), 3.14 ($\nu_{1}$), 4.67 (off-resonance), and 9.62 ($\nu_{8}$)
$\mu$m are shown. Intermolecular interactions give rise to the OH-stretching
features of CH3OH at 3.07 and 3.14 $\mu$m (see, e.g., Jakobsen et al. 1967;
Hudgins et al. 1993), while the shoulder feature at 2.96 $\mu$m has been
suggested to originate from molecules with weakly bound O-H bonds, or
potentially dangling bonds, due to pores in the ice (Luna et al., 2018). The
macropulse energies of all irradiations are adjusted to a constant value of
$\sim$5 mJ. Modifications to the CH3OH band profiles are clearly observed for
all exposures in which the IR wavelength is on resonance with absorption bands
of methanol. Comparatively, no differences are observed after the off-
resonance irradiation (i.e., at 4.67 $\mu$m). Thus, the observed spectral
changes are a consequence of the absorption of IR radiation by the methanol
ice, and not by the golden substrate underneath—in agreement with what has
been previously observed for pASW and CO2 ices (Noble et al., 2020; Ioppolo et
al., 2022). Overall, each CH3OH absorption feature shows similar spectral
changes in the difference spectra of all on-resonance irradiations, with only
variations in their intensities. The most pronounced effects are observed for
the irradiations at the OH-stretching region. Indeed, one may expect that
irradiations at the OH-stretch—which has a higher band strength—would result
in more photon-absorption events than those on resonance with the CO-
stretching mode. Furthermore, the exposure at the CH3OH $\nu_{1}$ band at 2.96
$\mu$m, in particular, yields changes significantly stronger than the other OH
stretches. The fact that neighboring irradiations in the OH-stretching region
yield such different results highlights the high dependence of the energy-
dissipation mechanisms on the type of vibrational mode excited, even within
similar energy ranges. Irradiations at the CH-stretching modes of CH3OH are
also performed (not shown) and result in similar spectral changes to the other
on-resonance spectra, albeit with lower intensities.

Figure 2: Difference spectra obtained before and after 5-minutes IR-FEL
irradiation on methanol ice at 20 K. The difference between spectra acquired
at 80 K and 20 K (without IR irradiation) during a TPD control experiment is
also shown in cyan and included in the 2.96 um plot. To allow direct
comparison, all spectra are plotted for the same scale; the differences in the
noise level between 3 $\mu$m and 9 $\mu$m are due to the intrinsic wavelength-
dependent S/N of the FTIR. The irradiation spectra are offset for clarity.

The features around the OH- and CO-stretching bands of methanol in the
difference spectra show both positive and negative components that yield a
negligible net integral absorbance. Those features are thus attributed to the
restructuring of the ice upon exposure to the IR-FEL with a power of $\sim 5$
mJ, leading to an overall loss of the weakly bound O-H modes at 2.96 $\mu$m
and corresponding gain of bulk O-H modes at 3.07 and 3.14 $\mu$m. A similar
behavior was observed for O-H dangling modes versus bulk modes of IR-FEL
irradiated pASW (Coussan et al., 2022). Moreover, the CO-stretching band
profiles show an overall narrowing trend, typically of the order of $0.2\%$.
It should be noted that the significantly larger area of the FTIR beam in
comparison with the IR-FEL beam results in an appreciable ($\gtrsim$6x)
dilution of the laser effect on the recorded IR band shapes, meaning that the
aforementioned percentage is in fact a lower limit.

To investigate the character of the morphological changes induced by the IR-
FEL, we performed a control TPD experiment of unirradiated pure CH3OH ice
deposited under the same conditions as in the irradiation study. Such an
experiment allows to compare controlled thermally induced morphology changes
with those induced upon IR irradiation. The IR spectra acquired during the TPD
experiments are reported in the Appendix A. The spectra as obtained for
different temperatures were compared to the difference spectra before and
after irradiation. The best qualitative fit to the most effective irradiation
at $\nu_{1}$—i.e., the TPD result that best reproduces the irradiation—is
obtained by the difference between the spectra taken at 80 K and 20 K, as
shown in Figure 2. The profile of the IR spectrum obtained during the TPD
experiment reproduces quite well that of the irradiation spectrum for the OH-
stretching modes, whereas discrepancies arise in the intensity of the changes
for the CO-stretching counterpart. Nonetheless, the overall profiles of both
difference spectra acquired during irradiation and the TPD experiment are
remarkably similar. This is a strong indication that, for pure methanol ices,
the excess vibrational energy is efficiently dissipated throughout the bulk of
the ice in an equivalent manner to thermal heating of an extended area. This
energy dissipation leads to restructuring towards a more organized morphology,
as was concluded for pASW and CO2 ices (Noble et al., 2020; Ioppolo et al.,
2022; Coussan et al., 2022).

The polarity of the ice could play a role in the effectiveness of the
vibrational energy dissipation through restructuring: similarly to the case of
methanol ice reported here, Noble et al. (2020) have observed that the
difference spectra upon IR-FEL irradiation of pASW can be satisfactorily
reproduced by the subtraction of theoretical spectra simulating a
restructuring of the ice. However, Ioppolo et al. (2022) found that for CO2
ices local restructuring alone is not sufficient to explain the changes
induced by the IR-FEL. Alternatively, this difference could also be related to
the type of intermolecular interactions within the ice, in which H-bonding
networks could potentially facilitate the vibrational energy dissipation
through restructuring, while generally weaker van der Waals interactions could
be less efficient in doing so. Indeed, transfer of vibrational energy has been
shown to occur in H2O ices through hydrogen-bonded water molecules with
resonant O-H stretches—which leads to local heating and restructuring of the
ice (Cuppen et al., 2022).

####  3.1.2 Power-dependence analysis and photodesorption

It is possible to assess whether the changes to the CH3OH ice are due to
single- or multi-photon processes through a power-dependence analysis. We
performed IR-FEL irradiations of a pure CH3OH ice sample at 9.62 $\mu$m with
three different power settings (5.76 mJ, 57.6 mJ, and 115 mJ) for 5 minutes
and compared the resulting integrated net areas around the $\nu_{8}$
absorption band of methanol as a function of IR-FEL power. Figure 3 shows a
fairly linear (R${}^{2}=0.98$) correlation between beam power and area,
further suggesting that the effect of the IR-FEL laser on the ice morphology
is due to single-photon processes (Noble et al., 2020; Ioppolo et al., 2022;
Coussan et al., 2022). The decreasing trend in the integrated absorption
signal at the $\nu_{8}$ band with increasing beam power suggests that
photodesorption might be observed in the infrared spectra once enough
vibrational excitations take place in the ice. The extent of the desorption is
however not high enough to be detectable with the sensitivity of the current
QMS. From the integrated absorption signal of the CH3OH $\nu_{8}$ band lost
after the 115 mJ irradiation and the total beam fluence—$\sim 1.8\times
10^{23}$ photons cm-2—we derive a tentative upper limit to the photodesorption
rate of $r=(1.6\pm 0.5)\times 10^{12}$ molecules J-1, or $r=(3\pm 1)\times
10^{-8}$ molecules photon-1. We consider only the uncertainties in the band
strength to derive the errors in the photodesorption rates, although other
error sources could also have an influence. The restructuring of the ice is
expected to have a minor effect on the band strength of CH3OH, as it was shown
to only change appreciably at temperatures above the crystallization of
methanol (Luna et al., 2018).

The resulting rate is orders of magnitude lower than typical (non-
dissociative) CH3OH UV photodesorption counterparts, of $10^{-5}$ molecules
photon-1 (Bertin et al., 2016), but it is still measurable because of the much
higher IR flux of FELIX-2 compared to the UV-broadband flux of a microwave H2
discharge lamp (Ligterink et al., 2015; Cruz-Diaz et al., 2016) or
monochromatic radiation of a synchrotron VUV beam (Fayolle et al., 2011). At a
first glance, the astronomical relevance of such a low photodesorption rate
may be considered negligible, but one should realize that in dense
interstellar clouds the IR photon fluxes are $\gtrsim 10^{4}$ times higher
than that of the secondary UV, resulting in comparable efficiencies for IR-
induced photodesorption phenomena in the interstellar medium. These
efficiencies and their impacts on astrochemical models are further discussed
in section 4. Furthermore, whereas UV studies show that CH3OH largely
fragments upon electronic excitation (Bertin et al., 2016), it is likely that
upon IR excitation desorption follows a non-dissociative pathway—further
influencing the predicted abundance of CH3OH in the gas phase.

Figure 3: Power-dependence analysis of the IR-FEL effect on CH3OH ice. The
three different power settings—5.76, 57.6, and 115 mJ—are controlled by
attenuating a 115 mJ beam with 13, 3, and 0 dB, respectively. Error bars
correspond to the three sigma limit of the instrumental error as derived from
the integrated noise signal of the off-resonance difference spectrum in the
same band width.

###  3.2 CO

####  3.2.1 Morphology

Figure 4: Difference spectra obtained before and after 10-minutes IR-FEL
irradiation on aCO ice (left panel) and cCO ice (right panel), both at 20 K.
The difference between spectra acquired at 25 K and 20 K during a TPD control
experiment is shown in brown. The irradiation spectra are offset for clarity.

The difference spectra obtained before and after 10-minutes IR-FEL exposure of
amorphous and crystalline CO ice at 4.67 and 5.2 $\mu$m are shown in Figure 4.
Since in the CO-ice case there is only one vibrational absorption band
available to resonant irradiation—limiting the number of excitations to
perform and thus leaving more time to perform longer irradiations—we opted for
a higher IR-FEL fluence (i.e., 10 instead of 5 minutes irradiation) in order
to further increase the signal-to-noise ratio in the IR difference spectra.
Irradiations of CO ice at wavelengths off-resonance (i.e., at 5.20 $\mu$m)
show a feature that turns out to be due to the ongoing stabilization of the
ice—a process that extends to a time frame of several hours after deposition.
This is confirmed by recording spectra taken 10 minutes apart (without any
exposure to the IR-FEL) directly before performing the irradiations. Their
difference spectra shows similar features with identical intensities to the
one observed in the 5.20 $\mu$m case. For direct comparison, the infrared
spectra of the 4.67 $\mu$m and 5.20 $\mu$m irradiations corrected for the
artifacts due to ice stabilization are shown in Appendix B.

Due to the intrinsic band width, the irradiation at 4.67 $\mu$m overlaps with
both the longitudinal optical (LO) and transverse optical (TO) phonons of the
vibrational mode of CO—at $\sim$2142 and $\sim$2138 cm-1, respectively. The
on-resonance FEL exposure at 4.67 $\mu$m yields more pronounced ($\sim$2x)
modifications to the ice in the amorphous case than in the crystalline
counterpart. This is in line with the hypothesis that the ice undergoes some
degree of restructuring toward a more organized configuration, in which case
cCO would reach a saturation point more quickly, as its structure is already
largely organized. Similar saturation effects have been reported in previous
studies on crystalline H2O and CO2 ices (Noble et al., 2020; Ioppolo et al.,
2022).

The beam exposure at 4.67 $\mu$m is compared to the TPD difference spectrum
between 25 K and 20 K, also shown in both panels in Fig. 4. As shown by the
difference spectrum from the TPD experiment, heating of the ice causes the CO
molecules to reorganize in a more compact configuration, resulting in a
narrower band profile. Additionally to the narrowing effect, the irradiation
spectrum also shows a slight overall blueshift of the CO band, suggesting that
repulsive interactions between the molecules might become more prominent upon
restructuring. Concomitant ice stabilization could contribute to the apparent
blueshift, albeit to a smaller extent. Ultimately, the spectral changes
induced by the IR-FEl cannot be simply reproduced by the TPD-IR results—in
contrast to the case of CH3OH shown in Figure 2. This suggests that the
dissipation of vibrational energy throughout the CO ice is not as efficient as
for CH3OH, and thus the effects of the IR-FEL exposure in the former case
cannot be described solely by an extended heating that transforms the
structure of the ice.

####  3.2.2 Photodesorption

To thoroughly investigate the temporal effects of vibrational energy injection
into pure CO ice and the possibility of photodesorption, we systematically
irradiated both the amorphous and crystalline samples at 4.67 $\mu$m for
different irradiation durations. In Figure 5, the integrated net area around
the CO stretching mode obtained from the difference spectra before and after
each irradiation are shown as a function of irradiation time (i.e., fluence).
The error bars include the fluctuations in integrated absorbance area due to
the stabilization of the ice, such as the artifacts seen after the irradiation
at 5.20 $\mu$m in Figure 4.

Figure 5: Plot of the integrated area of the stretching mode of CO in the
difference spectra obtained as a function of irradiation time. Left panel:
Irradiation of amorphous CO ice. The solid line shows the linear fit to the
points. Right panel: Irradiation of crystalline CO ice. The averaged
integrated noise is denoted by the dashed line.

The overall absorbance area of the stretching feature of aCO decreases
linearly with irradiation fluence, strongly suggesting a continuous loss of
material upon exposure to the beam due to IR-induced photodesorption. On the
contrary, there is no significant loss in the intensity of the stretching mode
of crystalline CO due to irradiation for the fluences explored in this work.
Thus, cCO does not seem to photodesorb (as effectively) as a result of IR-
photon absorption to the same extent that aCO does. This contrasts with
results on H2O ices irradiated by IR-FEL reported by Noble et al. (2020), in
which case pASW was observed to mainly restructure upon vibrational
excitation, with only a minor potential contribution from photodesorption. On
the other hand, irradiated cubic crystalline ices underwent pure
photodesorption, totaling a loss of $\sim$15% of the deposited material. The
different nature of the intermolecular interactions among CO and H2O ices
could be the reason behind this discrepancy. van Hemert et al. (2015)
predicted the UV-induced photodesorption probability of cCO to be around 5
times smaller than that of aCO, which they attributed to the stronger binding
of the molecules in the crystalline structure. This value is remarkably
consistent with experimental measurements of CO photodesorption as a function
of deposition temperature (Öberg et al., 2009). Likewise, the same effect
could also contribute to the significantly lower desorption efficiency of cCO
upon FEL irradiation. Since the observed decrease in the CO band intensity
does not happen at the precise wavelength of irradiation—which is shifted from
the CO peak—we can rule out the contribution from hole-burning effects to the
negative integrated areas.

The transfer of vibrational energy to translational energy (i.e.,
vibrationally induced photodesorption) by CO molecules has been previously
predicted to be inefficient (van Hemert et al., 2015; Fredon et al., 2021). In
this work, we observe a total absolute loss of $\sim 2.6\times 10^{15}$
molecules cm-2 of aCO as a result of 80 minutes of irradiation, which yields a
relatively small amount of desorbed species that is below the detection limit
of the QMS—rendering the mass spectrometry data inconclusive. Nonetheless, our
infrared spectroscopy data provide clear evidence of a CO photodesorption
process that has not been predicted by the aforementioned models. From the
total column density loss of aCO after 80 minutes of irradiation and the total
beam fluence integrated over the entire exposure time—$\sim 2.4\times 10^{23}$
photons cm-2—we derive a tentative upper limit to the photodesorption rate of
$r\sim(2.6\pm 0.8)\times 10^{11}$ molecules J-1, or $\sim(1.1\pm 0.3)\times
10^{-8}$ molecules photon-1. The errors were estimated from the band strength
uncertainties. Similarly to CH3OH, changes in the band strength of CO due to
restructuring are expected to be minor, and the derived rate translates to a
CO desorption efficiency induced by IR photons comparable to the UV value
under interstellar conditions (see section 4). Moreover, the linear trend
between the decrease in integrated absorption signal and photon fluence is
further evidence of a zeroth-order, single-photon interaction of the IR beam
with the ice leading to desorption—as has been observed before for the UV-
induced photodesorption of CO (Öberg et al., 2007; Muñoz Caro et al., 2010;
Fayolle et al., 2011; Chen et al., 2014; Muñoz Caro et al., 2016; Paardekooper
et al., 2016). Similar to this work, other studies of FEL exposure to pASW and
CO2 ices suggest the occurrence of IR-induced photodesorption (see, e.g, Noble
et al. 2020; Ioppolo et al. 2022; Coussan et al. 2022).

In the case where multiple CO species are vibrationally excited
simultaneously, effects such as VEP phenomena can arise. In the interstellar
medium this is rather unlikely, but for the high fluxes used in the laboratory
this should not be a priori neglected. As a consequence, some desorption
mechanisms could in principle become accessible to CO molecules that collect
energy from their neighbors and reach higher vibrational levels (DeVine et
al., 2022). Differently from DeVine et al. (2022), however, we do not detect
any photoinduced products upon irradiation of aCO nor cCO. This is likely due
to the differences between the experimental conditions of both studies—e.g.,
ice morphology, thickness, IR-photon generation method—that result in a
significant quenching of the pooling effect during our experiments in
comparison to DeVine et al. (2022). Indeed, simultaneously excited species at
any given time are expected to be very diluted within the ice in this work, as
the number of absorption events per IR-FEL pulse is estimated to be much lower
($\sim$0.1%) than the total ice column density. Excited species are therefore
expected to be sufficiently apart to prohibit effective interactions..
Moreover, it is worth emphasizing that multi-photon excitations are unlikely
under our experimental conditions, as is shown by the linear trend in Figure
3.

###  3.3 Mixtures

####  3.3.1 Morphology

To further explore the impact of the chemical environment on the dissipation
of vibrational energy, we performed irradiation experiments on ice mixtures of
CO and CH3OH with ratios of CO:CH3OH = 1:0.3 (CO-rich) and 1:3.0 (CH3OH-rich).
As methanol is expected to form through CO hydrogenation, such mixtures are of
direct astronomical relevance (Watanabe & Kouchi, 2002; Fuchs et al., 2009;
Cuppen et al., 2011). The difference spectra of both mixtures obtained before
and after 5 minutes of FEL exposure at 20 K are shown in Figure 6. For
comparison, we maintain the same absolute column densities of CO in both CO-
rich and methanol-rich ices (see Table 1).

Figure 6: Difference spectra obtained before and after 5-minutes IR-FEL
irradiation on ice mixtures of CO and CH3OH at 20 K. Upper panel: Mixture
ratio of CO:CH3OH = 1:0.3. Lower panel: Mixture ratio of CO:CH3OH = 1:3.0. The
control TPD difference spectra between 25 K and 20 K are also shown in cyan.
The irradiation spectra are offset for clarity.

The FEL exposures of the CO-rich ice do not result in any significant changes
around the methanol features (i.e., CO and OH stretches), namely above the
detection limit (top panel of Figure 6). This is likely because the impact of
restructuring changes of the deposited amount of methanol remains within the
spectral noise level. The CO feature, conversely, is modified by the IR-FEL
irradiation at both 4.67 and 9.62 $\mu$m, displaying the same spectral profile
in both cases. After 5 minutes of irradiation with $\sim$20 mJ, the ice
becomes saturated to the IR-FEL effects (see Figure 13 in Appendix C). This
indicates that the absorption of photons by either CO or CH3OH molecules (at
4.67 and 9.62 $\mu$m, respectively) leads to similar modifications of the ice,
which result in a slight redshift of the overall CO band profile. The IR
difference spectrum obtained during TPD between 25 K and 20 K reproduces the
CO shape reasonably well (see upper graph in the top panel), meaning that most
ice changes can be explained by an extended heating-like event upon photon
absorption that leads to a more organized configuration. However, the negative
features in the irradiation spectra are not completely reproduced by the TPD
control experiment, which suggests that additional desorption could also be
taking place. It is worth noting that both the irradiation and TPD difference
spectra of the CO-rich mixture display a different profile around the CO band
compared to the pure aCO ice. In the TPD experiments, pure amorphous CO
becomes narrower with higher temperatures as a result of an increasingly
ordered configuration, but no change in peak position is observed. In the
binary ice, conversely, the CO stretching mode becomes slightly redshifted at
higher temperatures, indicating that (more) attractive interactions dominate
the new CO sites. These observations highlight the crucial role played by the
surrounding species on solid-state IR band shapes. Moreover, the on-resonance
irradiation at the $\nu_{1}$ vibrational mode of CH3OH at 3.07 $\mu$m results
in no apparent changes around the CO feature, despite it yielding the most
pronounced effects in the case of pure methanol ice. These observations
provide further evidence of the strong dependence of the dissipation of
vibrational energy on the involved excited vibrational mode and on
interactions with the surrounding ice matrix.

In the case of the methanol-rich ice, modifications to both species’ bands
upon irradiation are observed above the detection limit of the instrument
(bottom panel of Figure 6). The higher absolute abundance of methanol in this
case compared to the CO-rich ice allows for more IR photons to be absorbed
when irradiation is on resonance with methanol absorption wavelengths. As a
consequence, the exposures at 9.62 $\mu$m and especially at 3.07 $\mu$m result
in significantly more intense changes to the CO band than in the CO-rich ice.
Whereas in the latter case the irradiation at the $\nu_{1}$ mode of CH3OH
leads to no differences in the CO band, in a more methanol-rich
environment—likewise in pure methanol ice—it yields the most pronounced
profile changes. The methanol features in this mixture, however, display a
remarkably different behavior upon irradiation at its OH stretching mode in
comparison to the pure methanol case. Clearly, the changes in the band
profiles of both CH3OH and CO species are highly dependent on the excited
vibrational mode, as found by the differences of the shapes resulting from the
three irradiation frequencies. Interestingly, when irradiated at 3.07 $\mu$m
and 4.67 $\mu$m, the profile of the stretching feature of CO in the methanol-
rich mixture is the inverse of what is observed for the CO-rich counterpart:
while the IR excitation of the former leads to a blueshift of the peak, in the
latter it results in a redshift. In fact, the blueshifted profile better
resembles the one observed in the irradiation of pure aCO, despite their very
different surrounding conditions. These profiles are repeatedly observed
during additional measurements of the CH3OH-rich ice mixture (see Figure 14 in
Appendix C). However, the mechanisms that result in such effects are still
unclear. Further systematic investigation of the dissipation of vibrational
energy by CO:CH3OH ice mixtures with various ratios are warranted to fully
understand these observations.

Both irradiations at 3.07 and 4.67 $\mu$m lead mainly to a decrease in the
area of the OH-stretching band of methanol, as well as a narrowing of its CO
stretching mode. Comparatively, the irradiation at 3.07 $\mu$m results in a
more complex difference spectrum, which shows methanol vibrational modes that
do not follow a clear trend in overall band shift nor width change.
Furthermore, despite both ices containing the same column density of CO
molecules, the higher abundance of surrounding CH3OH species in the methanol-
rich ice results in more intense changes to the CO band, even upon irradiation
at 4.67 $\mu$m. As was observed by Cuppen et al. (2022) in H2O ices, it is
likely that the higher fraction of hydrogen-bonded species in the CH3OH-rich
mixture facilitates the vibrational energy transfer in comparison to the CO-
rich counterpart, thus resulting in more intense spectral changes. In both ice
mixtures, irradiations at the CH-stretching bands of methanol (not shown)
yield similar changes to the other modes, but less intense.

Fredon et al. (2021) have explored the dissipation of different types of
energy (i.e., vibrational, rotational and translational) by admolecules on top
of an ASW surface using molecular dynamics simulations. They observed that the
distribution of vibrational energy among the species’ vibrational modes
heavily impacts the energy dissipation channels, in agreement with our
experimental results and those of previous IR-FEL exposure works (Noble et
al., 2020; Ioppolo et al., 2022; Coussan et al., 2022; Cuppen et al., 2022).
They also concluded that the dissipation of vibrational energy from the
admolecule to the surface occurs through the excitation of a surface-
admolecule bond, and therefore intermolecular interactions play a key role in
this process—which is corroborated by our experimental results. Further
theoretical works on the dissipation of vibrational energy in ice mixtures are
needed to fully understand the profile changes observed in Figure 6.

As for the CO-rich case, the irradiation spectrum of the methanol-rich ice at
9.62 $\mu$m is best reproduced by the corresponding TPD difference spectrum
between 25 and 20 K. Yet, neither the band profiles nor the relative
intensities can satisfactorily match that of the 9.62 $\mu$m irradiation
spectrum, which displays overall more negative features than the TPD control
experiment. This indirectly implies that consumption of the ice material
should take place, for instance, through photodesorption of CO, CH3OH, or
both. We do not detect any new signals in the IR spectra after irradiation,
and thus IR-induced photochemistry is unlikely to contribute to this effect.
Additionally, the irradiations at 3.07 $\mu$m and 4.67 $\mu$m yield difference
spectra that cannot be reproduced by any combination of the TPD control
spectra, and thus must be dominated by a process other than extended heating.

####  3.3.2 Photodesorpion

As discussed above, the 9.62 $\mu$m difference spectrum of the methanol-rich
mixture hints at the possibility of photodesorption taking place. We further
investigate this hypothesis by performing successive FEL 5-minutes exposures
at 9.62 $\mu$m on a same spot and varying the power settings. For the first
irradiation, the $\sim$60 mJ beam is attenuated by 5 dB to yield a total power
of $\sim$20 mJ. The second irradiation is performed at the full beam power of
$\sim$60 mJ. The resulting difference IR spectra and corresponding QMS data
are presented in Figure 7.

Figure 7: IR and QMS data obtained for two successive 9.62 $\mu$m FEL
exposures of the methanol-rich ice performed at the same substrate position
(i.e., without moving the translation stage) and at 20 K. The first (black)
irradiation was attenuated to a power of $\sim$20 mJ, whereas the second (red)
is performed at the full attainable power of $\sim$60 mJ. Left panel:
Difference of the spectra acquired before and after the irradiations. The
spectra are offset for clarity. Right panel: Superposition of the QMS data for
the mass signal of CO (m/z = 28) measured concomitantly to the IR spectra
during the FEL exposures (green lines) and their corresponding smoothed signal
(black and red lines). The dashed line indicates the beginning of the
irradiations.

The difference spectrum of the first irradiation is consistent with the 9.62
$\mu$m irradiation spectrum shown in the lower panel of Figure 6, which was
performed at similar conditions. In the case of UV experiments, photo-induced
codesorption (i.e., the excitation of one species leading to the desorption of
another) has been observed in the past for CO:N2 and CO:CO2 ice mixtures
(Bertin et al., 2013; Fillion et al., 2014), but did not proceed upon the
excitation of CO in CO:CH3OH mixtures (Bertin et al., 2016). By contrast, both
CH3OH and CO seem to potentially desorb from the ice to some extent upon IR
excitation of methanol, given by the decrease in the intensity of their
vibrational modes after the exposure to the free-electron laser. This
desorption, however, is expected to be small, as no signals of CO nor CH3OH
were detected in the QMS (see the right panel in Figure 7). From the areas of
the negative peaks in the IR difference spectrum, we estimate an absolute loss
in column density of $\sim 1.1\times 10^{15}$ and $\sim 0.6\times 10^{15}$
molecules cm-2 for CO and CH3OH, respectively. Upon increasing the beam power
to its highest attainable value of $\sim$60 mJ, the second irradiation results
in a difference spectrum with much more pronounced features. The CO stretching
mode decreases in intensity drastically, as shown by its prominently negative
peak. Moreover, we detect an increase in the mass signal of CO clearly above
noise level upon the start of the irradiation. Comparatively, no increase in
mass signal is detected for CH3OH (not shown), and no new signals are observed
in the IR spectrum. These two techniques together confirm the IR-induced
photodesorption of CO from the methanol-rich ice. The negative peaks in the IR
difference spectrum after the 5-minutes irradiation yield respective CO and
CH3OH absolute losses of $\sim 8.7\times 10^{15}$ and $\sim 1.3\times 10^{15}$
molecules cm-2 for a fluence of $\sim 9.2\times 10^{22}$ photons
cm-2—corresponding to photodesorption rates of $\sim(9\pm 3)\times 10^{-8}$
molecules photon-1 and $\sim(1.4\pm 0.4)\times 10^{-8}$ molecules photon-1,
respectively. The former is $\sim 8$x higher than the estimated
photodesorption rate of pure CO, whereas the latter is $\sim 2$x lower than
pure CH3OH. Since the irradiation frequency (9.62 $\mu$m) is on resonance with
the $\nu_{8}$ vibrational mode of CH3OH, its energy dissipation mechanisms
must lead to the desorption of CO in a more effective way than by direct
vibrational excitation. This can happen, e.g., as a result of the
reorganization of the methanol species towards a more stable configuration,
which might spatially force CO species out of the ice. Alternatively, energy
transfer effects between interacting neighboring CO and CH3OH could also play
a part in the sublimation of CO—for instance through the higher density of
H-bond interactions among species in a CH3OH-rich ice. In contrast, the
photodesorption of methanol becomes less effective in the mixture than in the
pure ice. These hypotheses will be investigated more deeply in future IR-FEL
studies with interstellar ice analogues.

##  4 Astrophysical implications

In this work we have focused on the infrared irradiation of interstellar ice
analogues. We observe that, upon resonant vibrational irradiation, ices of
pure CO, pure CH3OH, and mixtures of the two will generally undergo
restructuring towards a more compact configuration as a result of vibrational
energy dissipation. Thus, even in regions with low typical temperatures (such
as dense molecular clouds), we would nonetheless expect the icy grains to
contain some degree of organization—either as a result of infrared photon
absorption or other phenomena leading to vibrational excitation in the ice.
This could potentially influence not only the infrared band profiles of the
species in the mantle, but also their diffusion, adsorption, desorption, and
reaction rates—which in turn can significantly impact astrochemical models and
observations. Evidently, the extent of this effect strongly depends on how
localized the changes are in the ice, and how frequently it is being
processed. The implications of it will therefore be increasingly intense in
regions with higher radiation fluxes.

We also provide compelling evidence of the desorption of carbon monoxide
induced by both vibrationally excited CO and CH3OH molecules, as well as the
IR-induced photodesorption of pure CH3OH ice. Those species are the main
constituents of the apolar ice layer that shrouds interstellar dust grains,
and therefore their photoprocesses are of great relevance to the chemistry of
the interstellar medium. Given the available specific fluxes of IR photons
with $\sim$4.7 and $\sim$9.6 $\mu$m inside interstellar clouds— $\sim 3\times
10^{9}$ and $\sim 4\times 10^{8}$ photons cm-2 s-1, respectively (Mathis et
al., 1983)—and the hereby estimated upper limits to the photodesorption rates
for pure aCO and CH3OH ices, we derive tentative IR-induced photodesorption
efficiencies of $\sim 3.3\times 10^{1}$ molecules cm-2 s-1 for carbon monoxide
and $\sim 1.2\times 10^{1}$ molecules cm-2 s-1 for methanol in molecular cloud
conditions. Comparatively, the total UV flux (i.e., integrated for all
frequencies) inside molecular clouds is estimated to be of the order of $\sim
10^{4}$ photons cm-2 s-1 (Cecchi-Pestellini & Aiello, 1992) which, multiplied
by the derived UV-induced non-dissociative photodesorption rates of CO and
CH3OH, yields respective efficiencies of $\sim(1.4-89)\times 10^{1}$ and $\sim
1.0\times 10^{-1}$ molecules cm-2 s-1. These values are summarized in Table 2.
Notably, the UV-induced photodesorption efficiencies for CO are comparable to
the IR-induced counterparts, and for CH3OH they are two orders of magnitude
lower. The former values are, on the one hand, presented as upper limits due
to the utilization of transmission instead of reflection band strengths to
derive the photodesorption rates. On the other hand, the fact that the
specific IR fluxes at the species’ peak absorption frequencies—instead of the
integrated flux over the IR absorption width—are used in the calculations
leads to an underestimation of the photodesorption efficiencies. The values
listed in Table 2 should be considered tentative, and a more thorough
quantification of such efficiencies is warranted.

A caveat to these estimations is that the density of excitations (i.e., the
number of simultaneously excited species in the ice) is considerably higher in
laboratory experiments compared to the interstellar medium, which could play a
part in the mechanisms that lead to IR-induced desorption. Still, only
$\sim$0.1% of the species in the ice are estimated to be excited per IR-FEL
pulse, and therefore they should be sufficiently diluted that their
interactions will have a minor effect. Moreover, the total photon fluence
employed here correspond to $10^{6}-10^{7}$ yrs of exposure in interstellar
conditions, thus within molecular cloud timescales. Therefore, we expect that
IR-induced photodesorption can greatly affect the abundance of CO and CH3OH in
both the solid and gas phase, and we advise to take this into consideration
when modeling the chemistry of interstellar environments. In particular, IR-
induced photodesorption can potentially lead to an enhanced abundance of
larger species, potentially even COMs, being preserved as they are ejected to
the gas-phase from the ice, since the lower-energy IR photons are less likely
to result in dissociation compared to UV rays. This could then help explain
the observed abundances of CH3OH in the gas phase. More systematic works
focused on the quantification of the IR-induced photodesorption rates of
interstellar ice analogues are evidently of high interest and more research is
needed, both to learn about the processing taking place at a molecular level
and to extrapolate these findings to interstellar environments and timescales.

Table 2: Comparison of the estimated fluxes, desorption rates, and desorption efficiencies of CH3OH and CO species induced by IR and UV photons. |  | Interstellar flux | Rate | Estimated efficiency
---|---|---|---|---
|  | (photons cm-2 s-1) | (molecules photon-1) | (molecules cm-2 s-1)
CO | IR |  $>3\times 10^{9}$ a |  $\lesssim(1.1\pm 0.3)\times 10^{-8}$ c | $\sim 3.3\times 10^{1}$
UV |  $\sim 1\times 10^{4}$ b |  $\sim(0.14-8.9)\times 10^{-2}$ d | $\sim(1.4-89)\times 10^{1}$
CH3OH | IR |  $>4\times 10^{8}$ a |  $\lesssim(3\pm 1)\times 10^{-8}$ c | $\sim 1.2\times 10^{1}$
UV |  $\sim 1\times 10^{4}$ b |  $\sim 1\times 10^{-5}$ e | $\sim 1.0\times 10^{-1}$
a Mathis et al. (1983).
b Cecchi-Pestellini & Aiello (1992).
c This work.
d Öberg et al. (2007); Muñoz Caro et al. (2010); Fayolle et al. (2011); Chen
et al. (2014); Paardekooper et al. (2016).
e Bertin et al. (2016); Cruz-Diaz et al. (2016).

##  5 Conclusions

In the present work, we use FELIX-2 with the LISA end station at the HFML-
FELIX Laboratory to perform selective mid-infrared irradiations of CO and
CH3OH ices, as well as mixtures of the two, under interstellar conditions. The
results are monitored using both reflection-absorption infrared spectroscopy
and mass spectrometry techniques. Additionally, control temperature-programmed
desorption experiments on identical ices are performed to assist in the data
interpretation. This study with both resonant and off-resonance infrared
irradiations offers tools to elucidate the physico-chemical processes that
take place in the ice as a result of vibrational energy dissipation. Our main
findings are summarized below:

  * •

The vibrational excitation of the species in the ice and the subsequent
dissipation of this excess energy affects the ice morphology.

  * •

The ices explored here are restructured to a more organized configuration upon
irradiation.

  * •

In the case of pure CH3OH ice, most of the changes in the irradiation spectra
can be attributed to restructuring. However, for the pure CO ices and mixtures
with methanol, additional phenomena such as photodesorption are needed to
fully explain the difference spectra.

  * •

The changes in band shape upon irradiation are highly dependent on the FEL
wavelength and the composition of the ice. Thus, the excited vibrational mode
and the surrounding species must strongly affect the mechanism through which
the excess vibrational energy is dissipated.

  * •

We find compelling evidence of IR-induced photodesorption of pure CO ice with
a tentative estimated rate of $\sim(1.1\pm 0.3)\times 10^{-8}$ molecules
photon-1 upon excitation of its stretching mode. The photodesorption of CH3OH
is also suggested upon excitation of it $\nu_{8}$ mode, with a tentative
estimated rate of $\sim(3\pm 1)\times 10^{-8}$ molecules photon-1. Both (low)
rates yield desorption efficiencies up to two orders of magnitude higher than
UV-induced counterparts inside molecular clouds, because of the much higher IR
fluxes in such environments. So even though the absolute rate is lower in the
IR, the overall effect is expected to be around the same order of magnitude,
or larger.

  * •

Furthermore, the indirect photodesorption of CO upon IR irradiation on
resonance with CH3OH is strongly suggested.

Given the influence of the ice morphology and composition in the rates of the
processes that take place on icy grains, chemical models that involve solid-
phase reactions would especially benefit from the inclusion of vibrational-
energy dissipation mechanisms into the network. The tentative rates presented
here will be further investigated in a follow-up study in which time is made
available to derive accurate absorption cross sections for the applied
reflection mode settings.

###### Acknowledgements.

The authors thank the HFML-FELIX Laboratory team for their experimental
assistance and scientific support, Prof. Liv Hornekær’s group for sharing data
for comparison, and Dr. Thanja Lamberts for the insightful discussion on the
interpretation of the data presented here. The LISA end station is designed,
constructed, and managed at the HFML-FELIX Laboratory by the group of S.
Ioppolo and the group of B. Redlich. This work was supported by the Danish
National Research Foundation through the Center of Excellence “InterCat”
(Grant agreement no.: DNRF150); the Netherlands Research School for Astronomy
(NOVA); the Dutch Astrochemistry Network II (DANII); the Royal Society
University Research Fellowships Renewals 2019 (URF/R/191018); the Royal
Society University Research Fellowship (UF130409); the Royal Society Research
Fellow Enhancement Award (RGF/EA/180306); and the Royal Society Research Grant
(RSG/R1/180418). Travel support was granted by the UK Engineering and Physical
Sciences Research Council (UK EPSRC Grant EP/R007926/1 - FLUENCE: Felix Light
for the UK: Exploiting Novel Characteristics and Expertise). K.-J.C. is
grateful for support from NWO via a VENI fellowship (VI.Veni.212.296).

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##  Appendix A Control IR spectra

The difference IR spectra of the CO, CH3OH, CO:CH3OH = 1:0.3, and CO:CH3OH =
1:3 ices acquired during the control TPD experiments are shown in Figures 8,
9, 10, and 11, respectively. Differences are taken between a temperature T (as
labeled in the legend) and the deposition temperature of 20 K.

Figure 8: Control IR difference spectra of pure CO ice acquired during TPD.
Figure 9: Control IR difference spectra of pure CH3OH ice acquired during TPD.
Figure 10: Control IR difference spectra of the CO:CH3OH = 1:0.3 ice mixture
acquired during TPD. Figure 11: Control IR difference spectra of the CO:CH3OH
= 1:3 ice mixture acquired during TPD.

##  Appendix B aCO ice stabilization

The difference IR spectra before and after irradiation of aCO ice corrected
for stabilization are shown in Figure 12. The correction is made by
subtracting the signal due to ice stabilization—as obtained from two spectra
taken 10 minutes apart and without IR-FEL exposure—from the irradiation
difference spectra.

Figure 12: Difference spectra obtained before and after 10-minutes IR-FEL
irradiation on aCO ice at 20 K, corrected for ice stabilization.

##  Appendix C Repeated irradiations on ice mixtures

The irradiations of the ice mixtures were performed a total of three times in
a same substrate position each, i.e., the translator stage was only used when
the irradiation wavelength was changed. The difference spectra of the CO-rich
and CH3OH-rich ice mixtures are shown in Figures 13 and 14, respectively. In
the CO-rich case, the ice becomes saturated after the first irradiation at
each frequency, and no significant effect—besides ice stabilization—can be
seen in the repeated measurements. The irradiation at 9.62 $\mu$m was
performed significantly later in the experiment shift, in which case the ice
had had appreciably more time to stabilize—hence the smaller signal in its
second and third measurements. In the CH3OH-rich ice case, saturation takes
higher fluences to occur, and the effects of the IR-FEL are still observable
in the repeated irradiations.

Figure 13: Difference irradiation spectra of repeated measurements on the CO-
rich ice mixture. Irradiations at the same frequency were performed at the
same substrate position, and the ice is already saturated on each second and
third measurement. Figure 14: Difference irradiation spectra of repeated
measurements on the CH3OH-rich ice mixture. Irradiations at the same frequency
were performed at the same substrate position.