11institutetext:  Max Planck Institute for Astronomy, Königstuhl 17, D-69117
Heidelberg, Germany 22institutetext: Physics Department, Syracuse University,
Syracuse, NY 13244, USA 33institutetext: Institute for Astronomy, University
of Hawai’i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822–1839, USA
44institutetext: 44email<EMAIL_ADDRESS>
# Ammonia, carbon dioxide and the non-detection of the 2152 cm-1 CO band

Jiao He 1144 Giulia Perotti 11 Shahnewaz M. Emtiaz 22 Francis E. Toriello 22
Adwin Boogert 33 Thomas Henning 11 Gianfranco Vidali 22

###### Abstract

Context. CO is one of the most abundant ice components on interstellar dust
grains. When it is mixed with amorphous solid water (ASW) or located on its
surface, an absorption band of CO at 2152 cm-1 is always present in laboratory
measurements. This spectral feature is attributed to the interaction of CO
with dangling-OH bonds (dOH) in ASW. However, this band is absent in
observational spectra of interstellar ices. This raises the question whether
CO forms a relatively pure layer on top of ASW or is in close contact with
ASW, but not via dangling bonds.

Aims. We aim to determine whether the incorporation of NH3 or CO2 into ASW
blocks the dOH and therefore reduces the 2152 cm-1 band.

Methods. We performed laboratory experiments to simulate the layered structure
of the ice mantle, that is, we grew CO ice on top of 1) pure ASW, 2)
NH3:H2O=10:100 mixed ice, and 3) CO2:H2O=20:100 mixed ice. Infrared spectra
were measured to quantify the strength of the 2152 cm-1 band. In addition, a
second set of experiments were performed to determine how the incorporation of
NH3 into ASW affects the dOH band.

Results. We found that annealing the ice reduces the 2152 cm-1 band and that
NH3 blocks the dOH on ASW surface and therefore reduces the 2152 cm-1 band
more effectively than CO2. We suggest that this difference between NH3 and CO2
can be ascribed to the polarity of the guest molecule (NH3 is a polar species,
whereas CO2 is apolar). The polarity implies that the formation of an H-bond
between the N atom of ammonia and the dOH is a barrier-less reaction. We also
determined the pore surface area of the ice mixtures as a function of the
annealing temperature, and found that the nondetection of 2152 cm-1 band does
not necessarily exclude the possibility of a porous ice mantle.

###### Key Words.:

astrochemistry – Methods: laboratory: solid state – ISM: molecules – Solid
state: volatile

##  1 Introduction

The nondetection of the 2152 cm-1 (4.647 $\mu$m) band in astronomical
observations of interstellar ices is one of the long-standing puzzles of
astrochemistry. The 2152 cm-1 band is the result of the interaction between
absorbed CO and dangling-OH bonds (dOH) on amorphous water-ice surfaces
(Sandford et al., 1988; Buch & Devlin, 1991; Devlin, 1992; Palumbo &
Strazzulla, 1992; Palumbo, 1997; Collings et al., 2003a; Al-Halabi et al.,
2004; Palumbo et al., 2010; He et al., 2019). This feature has been identified
in laboratory CO:H2O ice mixtures, whereas it appears to be absent in
observational spectra of interstellar ices. The question then is why CO
molecules apparently do not bind to the dangling-OH site in pre- and
protostellar environments, resulting in spectra lacking the 2152 cm-1 band.

At first, the nondetection was attributed to the low signal-to-noise ratio of
observations in the 4 $\mu$m region, and thus, to the difficulties in
resolving this band when the abundance of the responsible interstellar
molecules is too low (e.g., Sandford et al. 1988; Ehrenfreund et al. 1997). A
second explanation ascribed the nondetection to the low spectral resolution of
the observations because of the overlap of several bands in the 4 $\mu$m
region, such as gas-phase CO rovibrational transitions, the H2 Pfund-$\beta$
emission line at $\sim$2149 cm-1 (Pontoppidan et al., 2002), and the C$-$N
stretching mode at 2165 cm-1 (Schmitt et al., 1989; Schutte & Greenberg, 1997;
Pendleton et al., 1999; Novozamsky et al., 2001).

With the advent of higher-resolution ground-based infrared instrumentation, it
has been possible to disentangle the spectral features in the 4 $\mu$m region.
In particular, the CO ice band was successfully resolved toward a sample of 39
low- to high-mass young stellar objects (YSOs) using the Very Large Telescope
Infrared Spectrometer And Array Camera (VLT-ISAAC; Pontoppidan et al. 2003).
In the VLT-ISAAC $3-5\leavevmode\nobreak\ \mu$m survey the 2152 cm-1 feature
was not detected toward any source, and only stringent upper limits were
provided (Pontoppidan et al., 2003). The large sample size (i.e., 39 objects),
high signal-to-noise ratio, and spectral resolution of this set of
observations suggested that the nondetection of the 2152 cm-1 band is not
linked to the capabilities of existing infrared facilities, but might be
linked to the structure and composition of interstellar ices. It is well
established that water-based ices in the interstellar medium (ISM) are
amorphous, and a good analog of them is amorphous solid water (ASW). ASW grown
by vapor deposition at low temperatures ($\sim$10$-$20 K) in the laboratory is
highly porous with a very large specific surface area (Stevenson et al., 1999;
Kimmel et al., 2001; Ayotte et al., 2001; He et al., 2019). As the temperature
becomes higher, the porosity decreases, and the crystallinity increases. The
blue wing of the broad OH-stretching absorption peak in the infrared contains
two weak dOH absorption features at 3696 and 3720 cm-1 (Rowland et al., 1991;
Buch & Devlin, 1991; Palumbo, 2005; Raut et al., 2007a; Palumbo et al., 2010;
Dartois et al., 2013; Bu et al., 2015; He et al., 2019), which are attributed
to 3- and 2-coordinated water sites on the pore surface, respectively. It has
been demonstrated that the 3720 cm-1 (2.688 $\mu$m) feature is unstable and
diminishes when the ASW is slightly annealed or exposed to irradiation (e.g.,
Palumbo, 2005; Raut et al., 2007b; Bu et al., 2015; He et al., 2019), while
the 3696 cm-1 (2.705 $\mu$m) feature persists even when the ASW is annealed to
as high as 130 K. The more stable 3696 cm-1 feature was searched for in the
ISM, but was not found (Keane et al., 2001). It is known that the 2152 cm-1
feature is linked to the interaction of CO with dOH (Devlin, 1992), and that
there is an anticorrelation between the 2152 cm-1 and 3696 cm-1 bands (He et
al., 2019). Therefore, a theory that explains the nondetection of 2152 cm-1
feature when CO is present in the ice should also explain the nondetection of
3696 cm-1 when CO is absent.

Several laboratory studies have been devoted to studying the structure of
water ice (porosity, pore surface area, etc.) and the dOH bonds, particularly
the impact of energetic particles. Raut et al. (2007b) used 100 keV Ar ions to
bombard ASW and found compaction of the ice and a decrease in the dOH peak
area. They proposed that impacts from cosmic rays can explain the absence of
dOH features in molecular clouds. Palumbo et al. (2010) used keV ions as well
as Lyman-alpha photons (UV) to irradiate water-containing ices, and found that
both UV and ions reduce the dOH band area and thus help to explain the
nondetection of dOH feature. Dartois et al. (2013) used ions of even higher
energy (MeV to GeV) to better simulate the effect of cosmic-ray bombardment
and confirmed the compaction of ASW and the elimination of dOH bonds by ions.
Behr et al. (2020) varied the temperature of ASW between 10 and 90 K and found
that the decrease in dOH band area occurs in the whole temperature range they
explored. These experimental works mostly focused on the $\sim$3700 cm-1 dOH
feature, but not directly on the 2152 cm-1 feature. All of these experimental
studies indicate that energetic particles can make porous ASW compact and
reduce the dOH feature. However, the compaction of pure ASW by either thermal
processing or bombardment of energetic particles is unlikely to fully
eliminate the dOH feature because the dOH feature in pure water ice is
directly linked to its (pore) surface area. It has been found that regardless
of whether the water ice is amorphous or crystalline, dOH is always present
(Zhang & Buch, 1990b, a; Rowland et al., 1991; Devlin, 1992). The ice mantle
might possess a certain level of porosity while at the same time the dOH
feature is negligible. The dOH bonds might also be blocked by other molecules.
We try to address these issues in the current study.

The answer to the nondetection of the 2152 cm-1 band was also directly
investigated with laboratory experiments. Palumbo (1997) measured the infrared
spectra of H2O:CO ice mixtures of different mixing ratios and compared the
area of the two peaks at 2139 and 2152 cm-1. She found that as the
concentration of CO decreases, the 2152 cm-1 is more pronounced, suggesting
that CO favors the dOH binding sites over other sites. Fraser et al. (2004)
carried out experiments of layered CO:H2O ices with different CO coverage. The
authors inferred that layered CO:H2O ices, with significant CO, are good
interstellar-ice analogs over mixed ices because the 2152 cm-1 feature is not
evident in their spectra. This is explained by the fact that CO can only react
with a small surface area, resulting in a 2152 cm-1 band under the detection
limit. In this case, the ASW on dust grains would have a relatively compact
structure, and CO would easily form a pure layer on top of the water-dominated
layer. However, it is unclear whether the water-dominated layer is fully
compact. The absence of 2152 cm-1 peak is unlikely to be explained by a
compact ASW structure alone. For example, He et al. (2019) showed (see Fig. 9
in that paper) that CO on an ASW that was annealed to 140 K and cooled down to
20 K still has the 2152 cm-1 peak. For CO on pure ASW, the 2152 cm-1 peak is
proportional to the surface area of the ASW (He et al., 2019). Even when we
assume that all the interstellar water ice is fully compact, the surface area
could still be significant, considering that dust grains could be fluffy. As
an alternative explanation, Fraser et al. (2004) further speculated that some
species other than CO, in particular, CO2, NH3, CH4, and CH3OH, could adsorb
at dangling-OH binding sites, thus removing the 2152 cm-1 feature in
observational spectra. In this scenario, the ASW is still allowed to have a
large surface area. However, Fraser et al. (2004) did not provide experimental
confirmation of which interstellar ice components can act as effective dOH
blockers.

Cuppen et al. (2011) tested this hypothesis by performing a spectroscopic
study of CO:H2O:CO2 and CH3OH:CO mixtures. They found that a CH3OH:CO 1:1
mixture is sufficient to reproduce the observed width and peak position of the
red component of the CO band, as well as the nondetection of the 2152 cm-1
band. In this regard, CH3OH:CO mixtures provided a better fit of one low-mass
and one high-mass protostars over a H2O:CO mixture (Penteado et al., 2015).
This finding suggested that CO might reside in H2O-poor/CH3OH-rich
environments in interstellar ices; this concept is supported by laboratory
experiments and model simulations of CH3OH formation from CO ice (Watanabe &
Kouchi, 2002; Fuchs et al., 2009; Cuppen et al., 2009).

Although convincing, a mixing ratio of CH3OH:CO 1:1 does not reflect the
average composition of interstellar ices (Boogert et al., 2015). The CH3OH:CO
ratio in high-mass YSOs is in fact subjected to large variations (0.2$-$3;
Dartois et al. 1999; Boogert et al. 2002, 2008; Pontoppidan et al. 2003,
2008). In addition, for fewer than a handful of low-mass YSOs, the CH3OH:CO
ratio is $\geq 0.7$ (Pontoppidan et al., 2004; Perotti et al., 2020, 2021),
and it is only 0.2$-$0.3 for starless and prestellar cores (Boogert et al.,
2011; Chu et al., 2020; Goto et al., 2021).

Additionally, although it is plausible that a fraction of CO settles in CH3OH-
rich ices, a significant amount of it inevitably migrates and interacts with
the H2O matrix (Collings et al., 2003b; He et al., 2018b; Zamirri et al.,
2018), contributing to the 2152 cm-1 band. This consideration is not taken
into account in Cuppen et al. (2011) due to the selection of a CH3OH:CO
mixture that does not contain H2O. In summary, although large quantities of
CH3OH can act as a dangling-OH bond blocker, other more abundant species
present in the ices, such as NH3 and CO2, might also play a role in the
suppression of the 2152 cm-1 band.

According to mid-infrared observations, the abundance of NH3 ice in low- and
high-mass star-forming regions is $\sim 5-7\%$ relative to H2O (Dartois et
al., 2002; Bottinelli et al., 2010; Öberg et al., 2011). However, laboratory
constraints on optical constants and band strengths have shown that its
abundance has been underestimated by up to 30% (Kerkhof et al., 1999; Zanchet
et al., 2013). NH3 is thought to form on the ice surface by the H-addition to
N atoms (Hiraoka et al., 1995) while competing with the H-addition to O atoms.
This suggests that NH3 is formed simultaneously with water and is intimately
mixed with water. This might also result in newly formed NH3 molecules
reacting with dOH. Similarly, the abundance of CO2 ice in the ISM has been
estimated to between $10-34\%$ compared to H2O (D’Hendecourt & Jourdain de
Muizon, 1989; de Graauw et al., 1996; Gibb et al., 2004; Pontoppidan et al.,
2008; Noble et al., 2013). The most efficient formation pathways of CO2 are
proposed to be the reaction between CO and OH radicals (Noble et al., 2011;
Oba et al., 2011; Ioppolo et al., 2011) or between CO and oxygen atoms (Roser
et al., 2001). Because water is formed from O and OH as well, it follows that
CO2 should also be intimately mixed with water. It is possible that CO is
binding to the dOH sites before forming CO2, and therefore blocking the dOH
sites (Fraser et al., 2004).

A related question that must be addressed is the quantification of the surface
area of ices in the ISM. Ices play an important role in chemical reactions in
the ISM because they provide a surface to adsorb and retain small reactive
species, and because they stabilize reaction products by taking away the
energy released in chemical reactions. The reaction rate depends on whether
reactions occur in bulk ice or on the ice surface. The diffusion rate, which
largely determines the rate of chemical reactions, is usually much faster on
the surface than in bulk. When the chemistry in the ISM is modeled, it is
necessary to distinguish surface reactions from reactions in bulk ice. For
this reason, laboratory measurement of the surface area of astrophysically
relevant ices provides key information for an accurate modeling of grain-
surface reactions. As demonstrated by He et al. (2019), the pore surface area
of ASW decreases with increasing annealing temperature, and the surface area
is proportional to the band area of the 3696 cm-1 dOH feature. It is therefore
important to examine the pore surface area together with the dOH feature at
3696 cm-1 or with the 2152 cm-1 feature.

In the present paper, we test whether the absence of the 2152 cm-1 feature is
due to the interaction of CO with water ice and whether molecules such as NH3
and CO2 could block the dOH signatures of ASW. We performed a systematic study
of different H2O:NH3 and H2O:CO2 ice mixtures, using porous ASW as the main
ice analog component. CO was deposited on top of both mixtures. NH3 and CO2
were selected because they are ubiquitous species in astrophysical
environments. In particular, they have been detected in the gas reservoirs of
inner and outer regions of protoplanetary disks and in protostellar cores
(Gürtler et al., 2002; Gibb et al., 2004; Öberg et al., 2010; Mandell et al.,
2012; Salinas et al., 2016; Bosman et al., 2017; Pontoppidan et al., 2019;
Najita et al., 2021) and on comets and planets (Mumma & Charnley, 2011;
Altwegg et al., 2020; Swain et al., 2008; Millan et al., 2021). This work
focuses on the 2152 cm-1 feature; we do not attempt to tackle the red
component at $\sim$2136 cm-1. We do not compare the measured spectra with
observations either because our measured spectra are in reflection mode, which
should not be compared directly with observations.

This paper is structured as follows. We begin in Section 2 by describing the
experimental setup used to record the laboratory spectra. In Section 3 we test
the effectiveness of CO2 and NH3 as dOH bond blockers and present the key
experimental results. Section 4 discusses the results and explains them by
comparing the chemical behaviors of CO2 and NH3 embedded in H2O. Additionally,
it supplies a list of additional dOH-blocker candidates. We summarize our
conclusions in Section 5.

Figure 1: Cartoon of the procedure of the first set of experiments. Step 1:
Water-dominated ice was deposited at 10 K, forming a highly porous amorphous
ice. Three different ice compositions were chosen: (1) 200 ML H2O, (2) mixture
of 200 ML H2O and 20 ML NH3, and (3) mixture of 200 ML H2O and 40 ML CO2. Step
2: Ice was warmed up to a higher temperature to anneal for 30 minutes. The
following temperatures were chosen: 20, 40, 60, 80, 100, or 120 K. Step 3: Ice
was cooled down to 20 K and exposed to CO deposition. RAIRS spectra were
recorded during the CO deposition, which is shown in Fig. 2.

##  2 Experimental setup

Experiments were performed using an ultra-high vacuum (UHV) chamber that has
been described previously (He et al., 2018a, b, 2019). The main UHV chamber
was pumped to a base pressure of $4\times 10^{-10}$ Torr when the cryostat was
off. A gold-coated copper disk was used as the substrate onto which ice
samples were grown. The temperature of the substrate can be controlled between
5 and 400 K by using a closed-cycle helium cryostat and a cartridge heater
installed right underneath the substrate. The temperature was measured using a
calibrated silicon diode sensor to an accuracy of 0.05 K. Gas or vapor was
deposited through one of the two UHV variable leak valves, which were
controlled by stepper motors interfaced to a computer. A LabVIEW program
automates the gas and vapor deposition and calculates the thickness in real-
time. A more detailed description of the gas deposition is given in the
Appendix of He et al. (2018a). Although the UHV system is equipped with two
highly collimated molecular beam lines for gas deposition, we only used the
leak valves because the background deposition from leak valves is a better
simulation of the omnidirectional deposition of gases in interstellar space.
Water vapor was evaporated from distilled water, which underwent at least
three freeze-pump-thaw cycles to remove dissolved air. Ice samples were
analyzed by a Nicolet 6700 Fourier Transform InfraRed (FTIR) Spectrometer in
the reflection absorption infrared spectroscopy (RAIRS) configuration.
Infrared spectra were obtained by averaging eight scans every 10 seconds in
the range 650–4000 cm-1. The infrared beam is at a 78 °incidence angle
relative to the surface normal. In this reflection mode, the infrared band
profile may differ from the transmission mode, such as that in Cuppen et al.
(2011). Particularly, the longitudinal optical (LO) mode of CO at 2143 cm-1,
which appears when the coverage of CO on the surface is relatively high, is
only present in reflection mode. In this study, we mostly focus on the
relatively weak dOH features at low coverage of CO. Here the difference
between reflection and transmission geometry is not important.

##  3 Experimental results and analysis

###  3.1 CO deposition on water-dominated ice mixtures

Two sets of experiments were performed in the current study; they are
described in this subsection and Sect. 3.2. The first set is an extension to
the experiments reported in He et al. (2019). He et al. (2019) grew 200
monolayers (ML; defined as $10^{15}$ molecule$\cdot$cm-2) of water by vapor
condensation on the gold surface with the surface at 10 K. At this
temperature, the water ice is amorphous and highly porous. Afterward, the
water ice was warmed up to an annealing temperature $T_{\rm ann}$ and remained
at this temperature for 30 minutes to stabilize the ice structure. The
following annealing temperatures were chosen: $T_{\rm ann}$ = 20, 40, 60, 80,
100, and 120 K. After annealing, the ice was cooled down to 20 K before
depositing CO on top of the water ice, and RAIRS spectra were collected during
the CO deposition. We chose to make a layered ice rather than a uniformly
mixed one to better represent the layered ice mantle on dust grains
(Pontoppidan et al., 2008; Boogert et al., 2015). This is different from other
studies, in which gases were premixed and then deposited (e.g., Ehrenfreund et
al., 1997; Cuppen et al., 2011). In addition to the experiments in He et al.
(2019), in the present study, we replaced pure water with (1) mixed ice of 200
ML H2O and 20 ML NH3 or (2) mixed ice of 200 ML H2O and 40 ML CO2. The ratios
CO2:H2O and NH3:H2O are representative of that in the ice mantle on dust
grains (Pontoppidan et al., 2008; Öberg et al., 2011; Zanchet et al., 2013;
Boogert et al., 2015). Other than the difference in ice composition, the
experiments follow the same procedure as in He et al. (2019). The deposition
of H2O:CO2 and H2O:NH3 ice mixtures was accomplished by codeposition of the
two gas and vapor components within 20 minutes and 25 minutes, respectively.
Fig. 1 demonstrates the procedure of the experiments. RAIRS spectra were
measured during CO deposition on ices, a selection of which is shown in Fig.
2.

Figure 2: Selected RAIRS spectra of CO deposited on water-dominated ice after
the ice was annealed to temperature $T_{\mathrm{ann}}$ and then cooled to 20
K. $T_{\mathrm{ann}}$ is 20, 40, 60, 80, 100, and 120 K from top to bottom.
The three columns are for (1) 200 ML of pure H2O ices, (2) mixed ices of 200
ML H2O and 20 ML NH3, and (3) mixed ices of 200 ML H2O and 40 ML CO2. The
insets of the figure refer to the amount of deposited CO.

It has been shown that CO molecules can diffuse and penetrate porous ASW
already at below 16-20 K, reaching all the surface area pores (Mispelaer et
al., 2013; Karssemeijer & Cuppen, 2014; Lauck et al., 2015; He et al., 2018b,
2019). The same is true in the H2O:CO2 and H2O:NH3 mixtures. The second and
third columns of Fig. 2 show that when the CO coverage is below saturation of
the surface, the peak profile is similar to the profile that is usually seen
in CO and H2O mixed ice. At the beginning of the CO deposition, the coverage
of CO on the pore surface is small, and all the CO molecules interact with the
water surface, which is shown by the two peaks in the infrared centered at
$\sim$2139 and $\sim$2152 cm-1. As CO gas deposition proceeds, the pore
surface area is gradually covered by CO, and eventually, CO builds up as pure
CO ice; at this stage, CO primarily interacts with other CO molecules rather
than with the water surface. This is indicated by the emergence of a 2143 cm-1
absorption peak due to the LO mode of CO. Similar to He et al. (2019), we took
the CO deposition amount at which the 2143 cm-1 peak emerges as the pore
surface area of the ASW. Fig. 3 shows the pore surface area, in units of ML,
of the three ices at different annealing temperatures. The error bar accounts
for the uncertainty in determining the emergence of the LO peak. The
introduction of 20% CO2 into water ice lowers the pore surface area by almost
half, while 10% of NH3 has only a small impact on the surface area. One
possible reason why CO2 more effectively blocks the pore surface area is that
CO2 forms clusters on the surface of ASW (He et al., 2017), which block some
micropores.

Figure 3: Pore surface area of water-dominated ice (mixture) that is annealed
to different temperatures, as determined from Fig. 2. The surface area is the
CO deposition dose at which the 2143 cm-1 peak emerges.  Figure 4: Zoom of the
fifth row (100 K) of Fig. 2.

The band area of the 2152 cm-1 peak in Fig. 2 can also be analyzed. By visual
inspection, we can see that the peak area decreases at higher annealing
temperatures for all three ices. When annealed at 120 K, the 2152 cm-1 peak
becomes insignificant. In H2O:NH3 mixtures, the decrease in 2152 cm-1 peak
area is more pronounced. Even if the annealing temperature is only 60 K, the
2152 cm-1 peak appears to be the tail of the 2143 cm-1 peak rather than a
separate peak. When the H2O:NH3 mixture is annealed to 100 K, the 2152 cm-1
peak disappears almost completely, while in the H2O:CO2 mixture, the 2152 cm-1
peak is still significant. Fig. 4 shows a zoom-in of the fifth row of Fig. 2,
corresponding to the ices annealed to 100 K. Because the 2152 cm-1 peak is
directly correlated to CO interacting with the dOH bonds (He et al., 2019), it
is evident that 10% of NH3 blocks the dOH bonds more effectively than 20% of
CO2, provided that the ice is annealed to higher than 80 K. As discussed
above, 20% of CO2 reduces the surface area of ASW by half, which should also
reduce the dOH sites proportionally. In the CO2:H2O mixture we note that the
surface area decreases to half throughout the experiment (Fig. 3), but in
contrast to expectations, the 2152 cm-1 peak is not equally reduced to half.
This observation strengthens the argument that CO2 is a less efficient dOH
blocker.

To quantify it, we calculated the 2152 cm-1 peak area by using the sum of two
Gaussian functions to fit the spectra in Fig. 2. Because the 2143 cm-1 peak
only emerges after the surface is saturated by CO, that is, the 2152 cm-1 peak
is saturated before the emergence of the 2143 cm-1 peak, we only focused on
the spectra before the emergence of the 2143 cm-1 peak. The advantage is that
we only need two components for the fitting. Fig. 5 shows examples of the
fitting. With a small amount of CO on the surface, the fitting by the sum of
two Gaussian functions is excellent. However, after CO fully covers the
surface, the fitting becomes worse, as illustrated by the top trace in the
left panel. This is also demonstrated in the right panel. As the surface is
fully covered by CO, the error bar of the 2152 cm-1 band area becomes very
large. From the right panel, we can determine the saturation 2152 cm-1 peak
areas. This analysis procedure is repeated for the ices with different
annealing temperatures, and the results are shown in Fig. 6. Here we can see
that 10% of NH3 effectively blocks the dangling OH bonds as long as the ice
mixture is annealed to 100 K. We note that the fitting only calculates the
area of the 2152 cm-1 peak, but does not consider its shape. For H2O:NH3
mixtures annealed to 60 K and above, the 2152 cm-1 component looks as if it is
the tail of the 2140/2143 cm-1 peak rather than a separate peak, making it
more difficult to discern than for H2O:CO2 mixtures in astronomical
observations.

Figure 5: Illustration of the method for determination of 2152 cm-1 band area.
Left panel: Examples of the fitting of the CO absorption profile using the sum
of two Gaussian functions. The red lines are the measured spectra, the dashed
green lines are the two Gaussian components, and the orange lines are the
total fitting. Spectra are offset for clarity. Right panel: Band area of the
2152 cm-1 component during CO deposition; the error bar of the fitting is also
shown. The large error bar at larger thickness is due to the emergence of the
LO peak at 2143 cm-1.  Figure 6: Saturation band area for the 2152 cm-1 band,
as shown in Fig. 2. The band area is calculated by applying the fitting
protocol illustrated in Fig. 5. Figure 7: Saturation band area for the 2152
cm-1 band as shown in Fig. 6, divided by the pore surface area, as shown in
Fig. 3.

Introducing CO2/NH3 into ASW has an impact on both the pore surface area and
the fraction of dOH sites on the pore surface. It is necessary to determine
whether the decrease of dOH is due to the blocking by CO2/NH3 or due to the
decrease of the pore surface area. For this reason, the saturation band area
for the 2152 cm-1 band in Fig. 6 was divided by the pore surface area in Fig.
3 to obtain the normalized dOH blocking efficiency. The result is shown in
Fig. 7. NH3 lowers the 2152 cm-1 band area per pore surface area compared with
pure ASW. In contrast, CO2 even increases the 2152 band area per pore surface
area because it reduces the pore surface area more effectively. This confirms
that NH3 is an effective dOH blocker on ASW surface.

###  3.2 Dangling-OH bonds in H2O:NH3 ice mixtures

The results of the first set of experiments presented above suggest that
introducing NH3 into water-dominated ice can effectively decrease the 2152
cm-1 peak and that NH3 is an effective dOH bond blocker. In the second set of
experiments, we further explore how the concentration of NH3 and the
temperature of the ice affect the dOH bonds. We fixed the amount of water to
be 100 ML, and deposited NH3:H2O ice mixtures with mixing ratios ranging from
0:100 to 30:100. After the deposition of the mixture at 10 K, the ice was
warmed up at a ramp rate of 6 K/minute, and RAIRS spectra were collected.

The spectra of ice with mixing ratio of NH3:H2O=0:100, 10:100, and 20:100
during warm-up are shown in Fig. 8. In pure water ice, both the dOH bands at
3696 and 3720 cm-1 are present at the lowest temperature. The 3720 cm-1 band
disappears by about 60 K, and the 3696 cm-1 band persists until above 120 K.
For a more detailed discussion of the dOH bands of pure ASW, see He et al.
(2019). When 10% or 20% of NH3 is added to the mixture, the 3720 cm-1 peak is
indiscernible, except for the 10% mixture at the lowest temperature. The 3696
cm-1 peak is much lower than that for pure ASW, particularly when the ice
mixture is warmed up.

Following a similar method as in He et al. (2019), we calculated the area for
the 3696 cm-1 peak. A broad Gaussian function was used to fit the blue wing of
the OH-stretch peak, and two narrow Gaussian functions were used to fit the
peaks at 3696 and 3720 cm-1. The band area of the 3696 cm-1 peak was obtained
from the fitting. The results are shown in Fig. 9. With an NH3:H2O mixing
ratio between 0 and 15%, the dOH band area decreases with increasing NH3
concentration at all temperatures. At higher mixing ratios, the dOH band area
becomes less dependent on the concentration. When the mixing ratio is higher
than 10%, the dOH band is almost negligible when annealed to 100 K or above.
This agrees with the results of the 2152 cm-1 peak in Section 3.1. This is
expected because it is known that the 2152 cm-1 peak is correlated with the
dOH band at 3696 cm-1 (He et al., 2019).

Figure 8: Dangling-OH bond region of the RAIRS spectra for NH3:H2O mixtures
during warming up. The mixing ratios and temperatures are indicated in the
figure.  Figure 9: Dangling-OH bond 3696 cm-1 band area in NH3:H2O mixtures
during warming up. The mixing ratios and temperatures are indicated in the
figure.

###  3.3 H2O:CO2 ice mixtures

Similar experiments as in Section 3.2 might be performed for CO2:H2O mixtures,
and the dOH region might be monitored in the infrared. Such experiments have
already been performed and presented in Figure 3 of He et al. (2018a). The dOH
bonds of water are shadowed by the $\nu_{1}+\nu_{3}$ combination mode, and
therefore it is challenging to analyze the dOH at 3696 cm-1 similar to that in
Section 3.2. Previously, Cuppen et al. (2011) measured the infrared spectra of
H2O:CO:CO2 mixtures, and found that it takes a large amount of CO2 to suppress
the 2152 cm-1 band at low temperatures. Even with a mixture of
H2O:CO:CO2=2.75:1:10, that is, three to four times more CO2 than H2O, there is
still a non-negligible 2152 cm-1 peak. It is clear from both Cuppen et al.
(2011) and this study that CO2 is a less effective dOH blocker.

##  4 Discussion

###  4.1 The surface area and the nondetection of dOH bands

The porosity of the ice mantle covering dust grains in the ISM is still
debated. Porous water ice provides a large catalytic surface area for grain
surface reactions to take place, while reactions in compact ice are challenged
by high-diffusion energy barriers for most particles inside bulk compact water
ice. The detection of a large variety of complex organic molecules seems to
favor the argument that the water-rich ice is somewhat porous and provides a
large surface area. However, similarly to the 2152 cm-1 band, also the 3696
cm-1 dOH band, which is usually present in vapor deposited ASW, has never been
detected in interstellar ices (Keane et al., 2001). This raises the questions
why the dOH bands disappear, and whether there is a dOH blocker in the ice.
Closely related is the nondetection of the 2152 cm-1 peak for CO ice on water
surface.

Previous studies have found that energetic particles bombardment as well as
thermal processing reduce the dOH bonds in ASW and make the ice more compact
(e.g., Raut et al., 2007b; Palumbo, 2006; Dartois et al., 2013; He et al.,
2019, and references therein). There is also evidence that the ability of CO
to diffuse into the ASW is affected by ion processing, which then reduces the
2152 cm-1 peak (Palumbo, 1997). However, these previous studies did not
explore the possibility of having a somewhat porous ice structure without the
simultaneous presence of the 2152 cm-1 peak (or the 3696 cm-1 peak). Our study
fills the gap and determines whether dOH might be blocked by other molecules
in the ice. We determine whether the ice mantle covering dust grains can have
an insignificant 2152 cm-1 peak (or equivalently, insignificant 3696 cm-1 dOH
band), but still have a significant surface area. Figs. 3, 6, and 9 show that
this is possible if 10% of NH3 is mixed with water and the ice is annealed to
100 K (this is on the laboratory timescale, it will be lower on the timescale
of a molecular cloud). In the ISM, ices are not condensed from gas and
annealed like in the experiments, but are formed by chemical reactions on the
surface. The energy released from chemical reactions can be absorbed by the
ice and can create some localized “annealing”, which makes the ice less porous
(Oba et al., 2009; Accolla et al., 2011). The bombardment of cosmic rays can
also induce some local heating of ice and therefore change the ice structure
(Palumbo, 2006; Raut et al., 2007b; Dartois et al., 2013; Mejía et al., 2015;
Behr et al., 2020). Eventually, the state of the ice might be similar to the
ice that is condensed from gas and then annealed to a high temperature, such
as 100 K. In this scenario, the 3696 cm-1 dOH peak or the 2152 cm-1 peak
becomes negligible when ammonia is present. We speculate that this may explain
the nondetection of these two features.

Figure 10: Schematic illustration of the interactions between A) dOH and
ammonia (NH3) and B) dOH and carbon dioxide (CO2). NH3 is a polar molecule, it
tightly binds to the dOH via H-bonding between the N atom and the dOH. CO2 is
an apolar species, with binding energy three times lower than that of NH3.
This implies that NH3 is a more effective dangling-OH band blocker over CO2.
The reported binding energies are from Ferrero et al. (2020). The simplified
geometries are an adaptation of real geometries obtained with quantum chemical
calculations in Ferrero et al. (2020).

###  4.2 Effect of polarity on the nondetection of the dOH bands

The important question that arises after analyzing the different responses of
H2O:NH3 and H2O:CO2 ice mixtures is what makes NH3 a more effective dOH
blocker than CO2. The answer to this question can be found by comparing the
hydrophilicity and the chemical characteristics and behaviors of NH3 and CO2.

The blocking of dOH bonds on deuterated ASW (d-ASW) has been studied by Devlin
(1992). In this work, d-ASW was precovered with CF4 or ethylene oxide, and
then the infrared spectra of the dOH band at $\sim$2730 cm-1 and the band at
2152 cm-1 were monitored to investigate the interaction of CO with dOH. Devlin
(1992) found that precovering with CF4 enhances the 2152 cm-1 band, whereas
precovering with ethylene oxide reduces it. This behavior was attributed to
their hydrophilicity. CF4 is a hydrophobic molecule and therefore avoids dOH
bonds and associates strongly with surface oxygen. In contrast, ethylene
oxide, as a hydrophilic molecule, primarily binds to surface hydrogen atoms
that belong to OH groups, effectively decreasing the number of available dOH
groups.

The current study investigates whether NH3 and CO2 block the dOH. NH3 is a
hydrophilic molecule such as ethylene oxide, which wets the whole ASW surface
before building up as pure layers, as indicated from temperature programmed
desorption (TPD) of NH3 on water ice (He et al., 2016). In comparison, CO2 is
hydrophobic, and it forms clusters on ASW surface before the whole surface is
covered. The TPDs of CO2 also show that the interaction force between CO2 and
CO2 is stronger than that between CO2 and H2O (He et al., 2018a). This
explains why NH3 blocks dOH while CO2 does not.

We can also view it from a different aspect by examining their chemical
characteristics. NH3 and H2O are structural analogs and isoelectronic species,
as they possess the same number of electrons. Both species are nucleophiles:
the O atom in H2O and the N atom in NH3 have two unpaired electrons, which can
be donated to form hydrogen bonds. Most importantly, because they are
characterized by an unequal sharing of valence electrons (i.e., a difference
in electronegativity among the bonded atoms), NH3 and H2O are defined as
polar. They have permanent dipole moments. In contrast, CO2 is a linear
molecule with an equal sharing of valence electrons, hence it is apolar. The
fact that NH3 is polar and CO2 is apolar implies that NH3 can bind more
tightly to H2O molecules compared to CO2, as confirmed by laboratory
measurements using TPD (He et al., 2016, 2018a), which found that a low
coverage of NH3 desorbs from water ice at $\sim$140 K, much higher than CO2 ,
which desorbs at $\sim$80 K. The binding energy on water ice was also studied
using quantum chemistry calculations. Ferrero et al. (2020) modeled an
interstellar ice surface by simulating both crystalline and amorphous water
ices with DFT (B3LYP-D3 and M06-2X). The authors of this work determined
binding energies for 21 astrochemically relevant species. On ASW ices, they
found that NH3 has binding energy ranges from 4314 to 7549 K, which is several
times higher than CO (1109–1869 K) and CO2 (1489–2948 K), corroborating the
strong binding between NH3 and water found in experiments. This strong binding
makes NH3 a more effective dOH band blocker over CO2 (Fig. 10).

Henkelman & Feibelman (2016) performed density functional theory (DFT)
calculations of the interaction of NH3 with the dOH of water ice and showed
that the bond between ammonia and the dOH is promoted by the attraction of the
lone pair of the N atom to the dOH of H2O. This is a zero-energy barrier
reaction that results in the formation of an OH· · ·NH3 hydrogen bond. This
theoretical study strongly supports laboratory results that proved that NH3
flags the dOH binding sites (Lechner et al., 2015). Both studies revealed that
the sole presence of an NH3 molecule in the vicinity of H2O molecules causes
the latter to reorient with zero-energy barrier to expose a dOH bond toward
the N atom of NH3.

This is not the case for apolar molecules such as CO or N2, which are not
capable of binding as strongly as NH3 to the dOH bonds. Consequently, becausee
their attraction to dOH is weaker, their H-bonding with H2O proceeds with
nonzero energy barriers. The highest barrier and thus weakest binding to water
is found for N2 , followed by CO (Henkelman & Feibelman, 2016). This implies
lower probabilities for the adsorption of N2 and CO at the dOH sites over NH3,
and therefore less effective dOH blocking capabilities at the low temperatures
of the interstellar medium (ISM). This result agrees with the inefficient dOH
blocking behaviors of CO (Fraser et al., 2004) and CO2 (this work and Cuppen
et al. 2011).

Previous laboratory studies of the nondetection of the 2152 cm-1 band have
proposed that good dOH blockers are species that form from CO molecules at the
dOH sites (e.g., CO2 and CH3OH; Fraser et al. 2004; Cuppen et al. 2011; see
Section 4.3). By working with NH3 and CO2, we have discovered that an
important requirement for a good dOH-bond blocker is the polarity of a
molecule, its ability to form H-bonds and to reside tightly at the dOH site,
therefore impeding CO molecules from reacting with the dOH. As a general rule,
we can state that polar molecules, especially NH3 and CH3OH, are efficient dOH
blockers, whereas apolar species (e.g., CO2) are not good dOH blockers, unless
significantly more abundant than polar species, in agreement with the findings
of Cuppen et al. (2011). Based on the evidence that ammonia is one of the most
abundant polar species of interstellar ices after H2O (its abundance is
approximately a factor of 3 higher than that of CH3OH; Zanchet et al. 2013;
Boogert et al. 2015), we propose that in addition to the impact of thermal
annealing and energetic particles, ammonia is one of the main contributors if
probably not the only one in the suppression of the 2152 cm-1 band, and more
generally of all the dOH bands.

###  4.3 Other dOH-blocker candidates

The subsequent hydrogenation of N atoms leading to NH3 at the onset of
interstellar ice surfaces is not the only reaction competing with the
hydrogenation of O atoms, producing dangling-OH binding sites (Hiraoka et al.,
1995; Herbst & van Dishoeck, 2009). For instance, the C+H reactions forming
CH4 occur concurrently with the O+H reactions (Qasim et al., 2020). This
implies that CH4 could be a good candidate for the removal or blockage of the
2152 cm-1 band binding sites. However, compared to NH3 and CO2, CH4 is more
volatile, with similar desorption characteristics as CO (Collings et al.,
2004). Laboratory studies have shown that CH4 molecules can easily diffuse on
the pore surface and do not bind to the ASW surface and hence to the dangling-
OH sites tighter than CO (He et al., 2018b). We further strengthen this
argument based on the apolarity of CH4, which in turn does not make it an
efficient dOH blocker. Other species might act as suppressors of the 2152 cm-1
band. Potential candidates are molecules that form onto the ices before the CO
condensation stage, which steadily bind to the dOH sites because of their
polarity. Examples of this class are OCN and OCS. Finally, one last category
of species that could block the dOH sites are polar molecules that are
produced by the interaction of CO at the dOH sites with other species present
on the ice surfaces. This includes species such as H2CO, HCOOH, and CH3OH or
even more complex organic species.

At the current stage, it is unclear whether one species contributes to the
suppression of the 2152 cm-1 band observed in astronomical spectra more than
others, or if this is the result of multiple contributions. Further systematic
laboratory experiments of ternary and quaternary mixtures are needed to
constrain the composition and structure of ice mantles. In addition, quantum
chemical modeling of properly mixed ices, representatives of real astronomical
ices (i.e., which include at least CO, CO2, NH3, CH4, and CH3OH), may also
help to elucidate the structure and spectroscopic features of the CO band,
accounting for the interaction of multiple species in different H2O–ice
structural environments. Finally, high-sensitivity near- and mid-infrared
observations of the dOH bands with the James Webb Space Telescope will provide
spectra with a higher signal-to-noise ratio to compare with model predictions
and experimental data.

##  5 Conclusions

We investigated one of the conundrums of astrochemistry: the absence of the
2152 cm-1 band in observational spectra of pre- and protostellar environments.
This band is due to the interaction between CO and dangling OH bonds on the
surface of water ice. By making composite ices in the laboratory that resemble
the icy mantles on dust grains, we examined the impact of NH3 and CO2 on the
2152 cm-1 band and the pore surface area of the mixed ice. We found that
introducing 10% of NH3 in ASW effectively blocks the dOH bonds as long as the
ice is annealed to 100 K. At the same time, introducing NH3 exerts only a
small impact on the pore surface area of ASW. In comparison, introducing 20%
of CO2 in ASW reduces the pore surface area by half, but does block all the
dOH bonds regardless of the annealing temperature. We discussed from a
chemistry point of view why polar molecules such as NH3 are likely more
efficient dOH blockers than apolar molecules. We propose that NH3 is one of
the most important dOH blockers, which helps to explain the nondetection of
the 2152 cm-1 band, and the water-dominated layer of the ice mantle can be
slightly porous, while the 2152 cm-1 peak is insignificant.

##  6 Acknowledgements

The authors wish to thank Prof. Piero Ugliengo for the fruitful discussion on
the binding energies of ammonia and carbon dioxide in interstellar ices. J.H.,
G.P. and T.H. acknowledge support from the European Research Council under the
Horizon 2020 Framework Program via the ERC Advanced Grant Origins 83 24 28.
G.V., F.E.T. and S.M.E acknowledge support from NSF Astronomy and Astrophysics
Research Grant No. 1615897.

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