[orcid=0000-0002-2271-1781] [1] [cor1]Corresponding author

[orcid=0000-0003-4985-8254] [orcid=0000-0002-4304-2628]
[orcid=0000-0003-4397-0739] [orcid=0000-0002-0275-7272]
[orcid=0000-0002-7770-9531]

# Infrared free-electron laser irradiation of carbon dioxide ice

Sergio Ioppolo<EMAIL_ADDRESS>Jennifer A. Noble  Alejandra Traspas Muiña
Herma M. Cuppen  Stéphane Coussan  Britta Redlich  School of Electronic
Engineering and Computer Science, Queen Mary University of London, London E1
4NS, UK  CNRS, Aix-Marseille Univ, PIIM, Marseille 13397, France  School of
Physical Sciences, University of Kent, Canterbury CT2 7NH, UK  Radboud
University, Institute for Molecules and Materials, Nijmegen 6525 AJ, The
Netherlands  FELIX Laboratory, Radboud University, Nijmegen 6525 ED, The
Netherlands

###### Abstract

Interstellar ice grains are believed to play a key role in the formation of
many of the simple and complex organic species detected in space. However,
many fundamental questions on the physicochemical processes linked to the
formation and survival of species in ice grains remain unanswered. Field work
at large-scale facilities such as free-electron lasers (FELs) can aid the
investigation of the composition and morphology of ice grains by providing
novel tools to the laboratory astrophysics community. We combined the high
tunability, wide infrared spectral range and intensity of the FEL beam line
FELIX-2 at the HFML-FELIX Laboratory in the Netherlands with the
characteristics of the ultrahigh vacuum LISA end station to perform
wavelength-dependent mid-IR irradiation experiments of space-relevant pure
carbon dioxide (CO2) ice at 20 K. We used the intense monochromatic radiation
of FELIX to inject vibrational energy at selected frequencies into the CO2 ice
to study ice restructuring effects _in situ_ by Fourier Transform Reflection-
Absorption Infrared (FT-RAIR) spectroscopy. This work improves our
understanding of how vibrational energy introduced by external triggers such
as photons, electrons, cosmic rays, and thermal heating coming from a nascent
protostar or field stars is dissipated in an interstellar icy dust grain in
space. Moreover, it adds to the current literature debate concerning the
amorphous and polycrystalline structure of CO2 ice observed upon deposition at
low temperatures, showing that, under our experimental conditions, CO2 ice
presents amorphous characteristics when deposited at 20 K and is unambiguously
crystalline if deposited at 75 K.

###### keywords:

Astrochemistry Methods: laboratory Techniques: free-electron laser Techniques:
spectroscopic Infrared: ISM ISM: molecules

##  1 Introduction

To date, more than 240 different species have been identified in space in the
gas phase [1]. Since gas-phase reaction rates are often found to be too low to
account for the observed abundances of species in space, the icy mantles of
interstellar dust grains are believed to give rise to a rich prebiotic
chemistry, which feeds complex organic molecules (COMs) to star-forming
regions and ultimately to planetary systems [2, 3, 4]. Current infrared (IR)
observations show that laboratory spectral data on peak position and feature
width can be used to extract information on layering and mixing of
interstellar ices [5, 6]. The NASA James Webb Space Telescope (JWST) mission
will enable the observation of interstellar ices over a wide wavelength range
(0.6-28.3 $\mu$m) toward a large variety of sources. Its high spectral
resolution will allow the observation of spectral characteristics depending on
different ice morphologies, thermal histories, and mixing environments in
star-forming regions, beyond simply measuring feature width and peak position.
However, the large amount of new observational data that will be provided by
JWST can only be correctly interpreted if a parallel effort is made in
laboratories to develop new experimental methodologies, for instance, at free-
electron laser (FEL) large-scale facilities, to provide unprecedented
spectroscopic details on morphologies and dynamics within interstellar ice
analogs.

Believed to be largely amorphous in nature, the structure of interstellar ices
is still under investigation. In space, dust grains covered in icy material
are continuously exposed to ‘energetic’ particles such as cosmic rays,
electrons, and UV photons that release excess energy into the ice target
causing physicochemical changes within the grain mantle. Exothermic surface
chemical reactions induced by gas-phase species landing on the grain surface
can also affect the structure and dynamics within interstellar ices.
Understanding how exothermic chemical reactions deposit large amounts of
energy into icy mantles covering dust grains containing some of the molecular
building blocks for life, could help explain the formation of prebiotic
compounds in space. Particularly, surface chemical reactions can be broken
down into various elementary processes, namely, adsorption, diffusion,
dissociation, product formation, energy dissipation, and desorption. The
mechanisms underlying energy dissipation at the atomic scale thus decide the
fate of possible subsequent processes.

To date, little is known about how the energy released into an icy grain after
a surface reaction or cosmic ray interaction dissipates within the grain [7].
Direct experimental quantification of energy dissipation by time-resolved
techniques with concurrent atomic resolution is still an ongoing challenge [8,
9]. Moreover, the effect of low energy photon irradiation, _i.e._ , in the
infrared regime, on – for instance – the hydrogen bonding network of water ice
has been scarcely addressed. Only a couple of IR irradiation studies have been
performed on crystalline water ice at elevated temperatures, the main purpose
of which was to investigate desorption efficiencies and products [10, 11]. In
terms of porous amorphous solid water (pASW), some of us have previously
studied the pASW surface via the ‘dangling’ OH stretching modes [12, 13, 14],
revealing an irreversible restructuring of these surface modes upon selective
IR irradiation by means of a table-top pulsed optical parametric oscillator
(OPO) laser. Our recent combined laboratory and theoretical work on selective
FEL irradiation of pASW showed that ices can absorb vibrational energy in the
broad IR (4000-400 cm-1, 2.5-25 $\mu$m) and terahertz (THz, 0.1-10 THz,
30-3000 $\mu$m) spectral ranges, causing local heating within the ice
structure and at the surface that leads to a reorganization of the ice toward
a more ordered material [15].

Selective IR/THz experiments at FEL facilities are needed to better understand
the nature of IR/THz modes in solids as well as to time-resolve dynamics and
energy relaxation processes within condensed matter. Particularly important to
the fields of astrochemistry, astrobiology, surface science, and catalysis are
physicochemical studies of diffusion, segregation, reaction, and ultimately
desorption of molecules at the surface and in the bulk of space-relevant ice
layers. Here, we present the first laboratory selective FEL irradiation study
of pure solid amorphous and crystalline carbon dioxide (CO2) ices at 20 K.
Based on our previous work on ASW, cubic ice (Ic), and hexagonal ice (Ih), we
selected CO2 as our target for the following reasons. ASW is the most abundant
form of ice in the Universe and provides the surface upon, and matrix within
which, volatile species react to form prebiotic molecules, as well as
absorbing some of the excess reaction energy [16, 13]. After H2O, CO2 is one
of the most abundant solid-phase species in the interstellar medium (ISM),
with abundances of between 20 and 30% that of H2O ice, depending on the object
observed [6]. In dense molecular clouds, CO2 is formed in the solid phase
primarily through the CO+OH surface reaction [17, 18, 19, 20]. Therefore, CO2
tends to reside mostly in a H2O-rich layer, where OH radicals are more
abundant and available for reaction with CO molecules. Additionally, CO2 and
more complex species form in mixed ices that undergo ‘energetic’ processing
such as thermal heating, photon, electron, and ion irradiation that
dramatically affect the bulk and surface structure of the ice [21, 22, 23, 24,
25, 26, 27, 28, 29]. Therefore, understanding changes induced upon chemical or
physical perturbation in CO2 ices is pivotal to unraveling molecular evolution
in space.

Finally, there is an ongoing debate in the literature on the structure of
deposited CO2 ice under laboratory conditions and its corresponding
spectroscopic fingerprint assignments that can help interpreting present and
future observations of CO2 ice in space [30, 31, 32, 33, 34, 35, 36]. This
work adds to the scientific discussion on the topic by providing new
independent evidence of the morphology of CO2 ice deposited at different
temperatures with implications for the ISM and the Solar System.

##  2 Experimental Methods

Experiments are carried out using the laboratory ice surface astrophysics
(LISA) ultrahigh vacuum (UHV) end station at the HFML-FELIX Laboratory,
Radboud University, The Netherlands. Details of the experimental setup are
given in the Supporting Material. Here we discuss the experimental methodology
used during a standard experiment. Briefly, pure CO2 ices are grown on the
gold substrate of the main chamber at temperatures of 20 and 75 K. Carbon
dioxide ($99.995\%$ CO2, CANgas) is used as received from Sigma-Aldrich.
Firstly, a few mbar of CO2 gas are introduced in the dosing line, while
monitoring the pressure mass-independently. Then, gases are admitted into the
main chamber by a manual all-metal needle valve. During deposition, the
pressure in the main chamber is measured by means of the cold cathode gauge
and the QMS and it is kept constant at 1$\times$10-6 mbar by adjusting the
aperture of the all-metal inlet valve. QMS calibration experiments are
performed at room temperature to find a correlation between residual mass
signals detected with the cryostat on and off.

At the beginning of a standard experiment, the cryostat is turned on, the MCT
detector is filled with liquid nitrogen, and the nitrogen purge is activated.
Once the base temperature of the substrate is reached in a matter of a few
hours, the gold substrate is flash heated to 200 K to further clean its
surface prior to gas deposition. Then, upon reaching the selected substrate
temperature (20 or 75 K), a background FTIR spectrum comprising 512 co-added
scans with a resolution of 0.5 cm-1 is acquired in the 4000-600 cm-1 range to
remove any signal along the IR beam path that does not arise from the ice
sample. During deposition, FTIR spectra with 8 co-added scans are acquired
every 20 seconds to monitor the ice growth, while residual gas is measured by
the QMS. At the end of all depositions, an FTIR spectrum with 256 co-added
scans is acquired. If the deposition temperature is not 20 K, the ice is then
cooled and kept at this temperature and another FTIR spectrum with 256 co-
added scans is acquired prior to any exposure to FEL radiation. All spectra
are acquired at a resolution of 0.5 cm-1. Figure 1 shows the FT-RAIR spectra
of pure CO2 deposited at 20 K and at 75 K and then cooled to 20 K. The figure
highlights the overtone ($\nu_{1}+\nu_{3}$ at 3708 cm-1 and $2\nu_{2}+\nu_{3}$
at 3599 cm-1), stretching ($\nu_{3}$ at 2376 cm-1), and bending ($\nu_{2}$ at
675 cm-1) modes of pure 12CO2 as well as the natural presence of 13CO2
($\nu_{3}$ at 2283 cm-1) in the deposited sample. Differences in spectral
profiles and absorption peak positions compared to some literature values are
due to the fact that here spectra are acquired by means of RAIR spectroscopy
as opposed to transmission spectroscopy [32, 34].

Figure 1: FT-RAIR spectra of pure CO2 deposited at 20 K (black solid line) and at 75 K and then cooled to 20 K (red solid line) marked with absorption modes in the MIR. Solid 13CO2 is also visible.  Table 1: Experimental details of CO2 ice exposed to FEL radiation. Molecule | Deposition T | FEL irradiation | TPD
---|---|---|---
| K |  $\mu$m | K min-1
CO2 | 20 | 3.2, 4.19, 4.21, | 2.5
|  | 4.22, 4.28, 14.76, |
|  | 15.18 |
CO2 | 20 | — | 2.5
CO2 | 75 | 3.2, 4.2, 4.22, | —
|  | 4.25, 14.8 |

The molecular column density $C$ (molecules cm-2) of the deposited CO2 ices at
both 20 and 75 K is calculated by integrating a modified Beer-Lambert Equation
(Eq. 1) to account for the RAIR mode configuration over the $\nu_{3}$
absorbance band of CO2:

$C=\ln(10)\frac{\sin(\theta)}{2A^{\prime}(\nu)}\int{\tau(\nu)d\nu},$ (1)

where $A^{\prime}(\nu)$ is the integrated band strength, $\tau(\nu)$ is the
optical depth (cm-1), and $\theta$ is the FTIR incident angle of 13∘. Based on
the work of Gerakines and Hudson [32], we chose
$A^{\prime}(\nu_{3})~{}=~{}1.18\times 10^{-16}$ cm molecule-1 and
$A^{\prime}(\nu_{3})~{}=~{}7.6\times 10^{-17}$ cm molecule-1 as the integrated
band strengths for pure CO2 deposited at 20 K and at 75 K and then cooled to
20 K, respectively [32, 37, 38]. In our experiments, the column densities of
CO2 deposited at 20 and at 75 K are 11$\times 10^{15}$ and 17$\times 10^{15}$
molecule cm-2, respectively. Apart from the deposition temperature, all other
parameters are kept the same during deposition, _e.g._ , deposition time
($\sim$190 s) and residual pressure in the main chamber ($1\times 10^{-6}$
mbar). Once calculated, the column densities are used to determine the ice
thicknesses in nm as per Eq. 2:

$d=\frac{CZ}{\rho N_{A}}\times 10^{7},$ (2)

where $Z$ is the molecular mass of the CO2 molecule (g mol-1), $\rho$ is the
density of the ice, which we have taken to be 1.2 g cm-3 and 1.67 g cm-3 for
CO2 deposited at 20 K and at 75 K, respectively [39], and $N_{A}$ is the
Avogadro constant ($6.02\times 10^{23}$ molecule mol-1). Hence, we estimate
pure CO2 deposited at 20 and 75 K to be both $\sim$7 nm thick with a
deposition rate of $\sim$0.037 nm s-1.

Figure 2: FEL irradiation of pure amorphous CO2 ice at 20 K. Left panels show
an FT-RAIR spectrum of pure CO2 ice after deposition in the $\nu_{3}$ and
$\nu_{2}$ spectral ranges (upper and lower panels, respectively). Colored
arrows and strips indicate the frequencies and FWHM of FELIX-2 used during
irradiation, respectively. Right panels show FT-RAIR difference spectra before
and after FEL exposure. Spectra and FEL irradiation frequencies are color-
coded. Difference spectra are offset for clarity.

Ices are irradiated at 20 K using the FELIX-2 infrared FEL source with
macropulses of 5 Hz repetition rate and at frequencies in the MIR range
(3.2-15.2 $\mu$m). All irradiations of pure CO2 ice deposited at 20 K and at
75 K are carried out for five minutes to ensure the complete saturation of any
structural changes in the ice. The spectral FWHM of the FELIX beam is on the
order of 0.8 % $\delta\lambda/\lambda$ for all wavelengths. Unless otherwise
specified, the laser macropulse energy at longer wavelengths ($\sim$130 mJ in
the 15 $\mu$m wavelength region) is attenuated to $\sim$20 mJ to match that in
the 3-4 $\mu$m region. Thus, at all wavelengths investigated, the average
power is $\sim$0.03 W, with a laser fluence at the sample of approximately
$\sim$0.2 J/cm2. Table 1 shows the main experiments and their FEL irradiation
details.

FTIR spectra are acquired before and after irradiation, from which difference
spectra are obtained to highlight changes in the IR band profiles. All data
analysis and spectral manipulation is performed using _in house_ python
scripts. Unless specified, FEL irradiations are performed on unirradiated ice.
During FEL irradiation, the QMS is used to monitor any possible desorption of
species from the ice. Upon completion of FEL irradiations, a TPD experiment is
usually performed by means of the QMS as well as the FTIR with spectra of 128
co-added scans collected every 120 seconds. Since a heating ramp of 2.5 K
min-1 is used, each FTIR spectrum is acquired every 5 K from 20 to 200 K. A
TPD control experiment of a CO2 ice deposited at 20 K and not exposed to FEL
irradiation is performed for comparison and interpretation of the FEL exposure
experiments.

##  3 Results and discussion

###  3.1 FEL irradiation of CO2 ice deposited at 20 K

The left panels of Figure 2 show an FT-RAIR spectrum of pure CO2 ice deposited
at 20 K in the $\nu_{3}$ and $\nu_{2}$ spectral ranges (upper and lower
panels, respectively). Both $\nu_{3}$ and $\nu_{2}$ modes exhibit tranverse
optical (TO) and longitudinal optical (LO) phonons in which the normal
vibrations propagate through the ice lattice perpendicular and parallel to the
direction of the IR field, respectively. In our FT-RAIR spectra, the LO
phonons at 4.2 and 14.8 $\mu$m are stronger than the TO phonons at 4.27 and
15.2 $\mu$m for the $\nu_{3}$ and $\nu_{2}$ modes, respectively [30, 40, 41].
Colored arrows indicate the frequencies at which the ice has been exposed to
FELIX-2 radiation for five minutes of continuous exposure at a time and always
on fresh, unirradiated ice spots. Here and throughout the rest of the paper,
we will use the same color scheme. Vertical color-coded strips indicate the
spectral FWHM of the FELIX beam at each selected wavelength. Around the peak
of the $\nu_{3}$ mode, the spectral FWHM of the FELIX beam is such that at all
three selected FEL irradiation frequencies the three irradiations overlap with
one other. This can explain the similar effects seen in the ice after FEL
exposure for the three irradiations at 4.19, 4.21, and 4.22 $\mu$m by
comparing FT-RAIR spectra before and after FEL exposure (upper-right panel of
Fig. 2). Such difference spectra reveal that the ice has undergone a degree of
restructuring as well as perhaps some desorption. The latter is highlighted by
the fact that, upon exposure to FEL radiation, both $\nu_{3}$ and $\nu_{2}$
modes decreased in intensity, as shown by the negative peaks in the upper
three difference spectra of the upper-right panel of Fig. 2. Moreover, the
$\nu_{3}$ and $\nu_{2}$ band shapes present modified profiles suggesting
restructuring of the ice, as shown by the positive peaks in the three
difference spectra. More pronounced negative peaks are observed for
irradiations around the LO phonon mode peak of the $\nu_{3}$ band. The fourth
FEL irradiation at 4.28 $\mu$m on the red wing (_i.e._ , TO phonon) of the CO2
stretching mode presents only weaker negative peaks. FEL irradiation can
depend on the absorption cross section of the $\nu_{3}$ band, showing weaker
effects at 4.28 $\mu$m. Moreover, a larger restructuring effect seen after FEL
irradiations of the LO versus the TO phonon modes of the $\nu_{3}$ band can be
explained by the fact that the FELIX-2 optical configuration used for the
experiments presented here is such that the FEL radiation is p-polarized,
_i.e._ with the electric vector parallel to the incidence plane and in
resonance with LO phonon modes.

FEL irradiations at the CO2 bending mode (14.76 and 15.18 $\mu$m, lower-left
panel of Fig. 2) provoke spectral changes that are comparable to those seen
upon irradiating $\nu_{3}$ at its red wing, namely, negative peaks.
Differences in spectral changes upon FEL radiation of the $\nu_{2}$ and
$\nu_{3}$ bands suggest that on-resonance injection of vibrational energy into
the CO2 ice is a frequency dependent process. As in the case of the $\nu_{3}$
band, the effect is relatively stronger for FEL irradiation at the LO phonon
mode peak of the $\nu_{2}$ mode and weaker at its red wing (_i.e._ , TO phonon
mode). Much like in the case of pASW, FEL irradiations performed at
frequencies where the ice molecules do not present any vibrational absorption
do not cause any change in the ice structure nor its thickness [15]. Hence,
the difference spectra acquired before and after FEL exposure at non-resonant
frequencies appear flat (spectra not shown in Fig. 2). This is a further
confirmation that it is the ice that absorbs vibrational energy and not the
gold substrate underneath.

Figure 3: Spectral fits (red solid lines) of two distinct FEL irradiations at
the stretching (left panel) and bending (right panel) modes of amorphous CO2
ice at 20 K. FT-RAIR difference spectra acquired before and after FEL exposure
are presented as solid black lines. Vertical blue and orange dotted lines
indicate the selected FEL radiation frequencies. The dashed red line is a FT-
RAIR spectrum acquired during the TPD of an unirradiated ice at 80 K, whereas
the dashed blue line is a FT-RAIR spectrum acquired at 20 K. Both spectra are
used as fit components.

To understand whether FEL radiation effects cause a global heating of the ice
or not, we have compared FEL irradiations at 4.19 and 14.76 $\mu$m (left and
right panels of Figure 3, respectively) and fitted their profiles with
selected FT-RAIR spectra of pure CO2 ice deposited at 20 K and then heated
during a TPD with a ramp of 2.5 K min-1 (see Table 1). Each panel shows an
independent fit with two spectral components, namely, a cold negative
contribution at 20 K and a warm positive component at 80 K, which provided the
best qualitative fits. The spectral component at 80 K shows that the ice in
the control experiment has undergone restructuring during TPD toward a more
crystalline-like form. The fact that a more crystalline ice (warm component)
gives a positive contribution, while the amorphous ice (cold component) is a
negative contribution to the fit suggests that there is a loss in amorphous
ice because the ice structure reorganizes toward a more ordered configuration
upon FEL exposure. Particularly, the fit at the $\nu_{3}$ mode of the ice
irradiated at 4.19 $\mu$m shows a 2% overall loss of column density of
amorphous ice and a comparable increase in the column density of reorganized,
crystalline-like, ice. When these numbers are scaled to the FEL exposed ice
area on the substrate, morphological changes occur in $\sim$50% of the
irradiated molecules. Moreover, although the fit of the FEL irradiation at the
$\nu_{3}$ mode (4.19 $\mu$m) seems to qualitatively well reproduce the
stretching mode region, changes at the $\nu_{2}$ mode are not satisfactorily
reproduced by the same fit. Furthermore, the FEL irradiation at the $\nu_{2}$
mode (14.76 $\mu$m) cannot be reproduced by any combination of all available
FT-RAIR spectra acquired during the TPD control experiment as listed in Table
1. This is because losses at the $\nu_{2}$ mode due to irradiation are always
larger relative to the $\nu_{3}$ mode than in the case of global heating.
Hence, changes in the ice induced by the FEL radiation are frequency dependent
and cannot be simply explained by global thermal heating of the ice or global
desorption.

Based on our previous results in similar studies, the most likely scenario is
a selective vibrational excitation, that is a local increase in vibrational
energy equivalent to local heating, and possibly minor desorption of specific
molecules excited by the FEL radiation at the surface and/or within the bulk
of the ice. FEL irradiation at the $\nu_{3}$ mode seems to induce mainly
restructuring effects as shown by a combination of new positive and negative
peaks in the difference spectra before and after FEL exposure, whereas
irradiations at the $\nu_{2}$ mode seems to lead to an additional process,
wherein a vibrational selectivity gives rise to the observation of only
negative peaks [42]. FEL-induced photodesorption could contribute to the
observed reduction in the $\nu_{2}$-mode intensity upon irradiation. However,
we should not exclude other possibilities such as hole-burning effects,
particularly at the irradiated frequencies. Here, QMS data is key to directly
detect FEL-induced photodesorption of CO2. Unfortunately, although QMS data
were successfully recorded during ice deposition and TPD experiments, during
FEL irradiation, the acquisition speed of the instrument was not optimized for
the detection of gas desorption plumes ejected from the ice in short bursts
due to the pulsed nature of the FEL. Hence, during FEL irradiation, QMS
results were inconclusive. The issue is beyond the scope of this paper and it
will be addressed in future work.

Figure 4: Power dependence of FEL radiation and spectral changes of amorphous
CO2 ice. Left panel: in solid black, blue, and red lines, FT-RAIR difference
spectra acquired before and after FEL exposure of fresh spots of CO2 ice with
0, 3, and 8 dB attenuation, respectively. Right panel: linear fit (solid black
line) of the integrated areas (black full squares) of the three FT-RAIR
spectra as a function of the FEL beam power. Both axes of the right panel are
on a natural logarithmic scale.

As for the case of pASW [15], all morphological changes in the CO2 ice are
localized within the area exposed to the laser beam and do not cause a global
effect throughout the ice. This is verified by acquiring FT-RAIR spectra of
the ice in regions in close proximity to irradiated areas and observing no FEL
radiation effects. Moreover, to test whether in this case morphological
changes in the ice are due to a single-photon process, we performed a FEL
power dependence study on the irradiation of the CO2 bending mode and measured
changes at both the $\nu_{3}$ and $\nu_{2}$ modes. The left panel of Figure 4
shows FT-RAIR difference spectra acquired before and after FEL exposure of
three fresh ice spots at three different FEL beam power settings controlled by
attenuating the beam power with 0, 3, and 8 dB, corresponding to powers of
130, 65, and 20 mJ as measured by a powermeter at the FELIX control station.
The right panel of Figure 4 shows that spectral changes measured by
integrating the region around the $\nu_{3}$ mode are nearly linearly
proportional to the increase in FEL power, in agreement with the hypothesis
that changes in the ice are caused by a single-photon process. However, since
local temperature at the irradiated ice spot cannot be measured, we cannot
completely rule-out a contribution to the changes in ice morphology being
caused by laser-induced global thermal heating. However, our FT-RAIR data show
that full ice desorption, hole burning and ice melting effects do not occur at
the FEL exposed spot of the deposited CO2 layer as the spectral changes
observed in our experiments reveal an IR photon-induced process that is
selective and frequency dependent. This is also confirmed in preliminary
experiments with more volatile species, such as CO and N2, that are more
sensitive to thermal changes in the ice than CO2. Hence, our data indicate
that thermal heating is a local process and vibrational energy dissipation
likely affects only a few species in the ice per impinging IR photon. Future
theoretical work on molecular dynamics simulations of our experiments will
provide further insights on the IR photon-induced processing of space relevant
ices.

###  3.2 FEL irradiation of CO2 ice deposited at 75 K

Figure 5: FEL irradiation of pure crystalline CO2 ice deposited at 75 K and
cooled to 20 K. Left panel shows a FT-RAIR spectrum of pure CO2 ice after
deposition in the $\nu_{3}$ and $\nu_{2}$ spectral ranges. Colored arrows and
strips indicate the frequencies and FWHM of FELIX-2 used during irradiation,
respectively. Right panel shows FT-RAIR difference spectra before and after
FEL exposure. Spectra and FEL irradiation frequencies are color-coded.
Difference spectra are offset for clarity.

The left panel of Figure 5 shows a FT-RAIR spectrum of pure CO2 ice deposited
at 75 K and then cooled to 20 K in both the $\nu_{3}$ and $\nu_{2}$ spectral
ranges. Colored arrows indicate the frequencies at which the ice has been
exposed to FELIX-2 radiation for five minutes of continuous exposure at a time
and always on fresh ice spots. Vertical color-coded strips indicate the
spectral FWHM of the FELIX beam at each selected wavelength. As in Figure 2,
the color of the arrows in the left panel of Figure 5 corresponds to the color
of the FT-RAIR difference spectra acquired before and after FEL exposure as
shown in the right panel of the same figure. Spectral changes highlighted in
the difference spectra of the right panel of Figure 5 are all qualitatively
comparable regardless of whether the ice is irradiated at the $\nu_{3}$ or
$\nu_{2}$ mode. This suggests that reorganization of the ice is unlikely to
occur, which was expected since the ice is already crystalline due to its
deposition temperature of 75 K.

Figure 6 shows the FT-RAIR difference spectrum of the pure CO2 ice deposited
at 75 K and then cooled to 20 K before and after being exposed to FEL
radiation at 4.2 $\mu$m. The profile of the difference spectrum is fitted with
two spectral components, namely, a cold negative contribution of the same ice
before FEL irradiation at 20 K and a warm positive contribution of the ice as
deposited at 75 K. Unlike the case of amorphous CO2 ice, which is a metastable
material, in the case of crystalline CO2 ice, changes in the spectral profile
at different temperatures are reversible and can be obtained during multiple
temperature cycles. As the cryostat cools the ice sample after deposition of a
crystalline ice, IR features generally become sharper and more intense at
lower temperatures (10-20 K) relative to the same features at higher
temperatures (_e.g._ , 75 K in the case of CO2) because cooling the sample
will reduce the thermal population of hot-band states of the vibrations. Such
reversible process is also seen in the THz spectral range [31, 43]. The
lowering of the temperature reduces the thermal agitation of the lattice and
consequently the phonon population in the THz range, which lowers the number
of nearly equivalent classes of oscillators. This allows for a better
minimization of the local potential energy.

It is worth noting that the two crystalline spectral components acquired at
different temperatures of the same deposited ice prior to FEL exposure can
partially fit the changes seen in the crystalline ice after FEL irradiation.
However, as opposed to the case of amorphous ice seen in Figure 2, spectral
changes upon FEL exposure of crystalline CO2 ice seem to be less dependent on
the selected absorption mode. A closer look at the fit of Figure 6 suggests
that some negative peaks in both the $\nu_{3}$ and $\nu_{2}$ modes are never
reproduced by the chosen fit components, indicating that some other processes
such as desorption may be occurring in the ice. Here, the loss in the
$\nu_{3}$ mode is larger than that in the $\nu_{2}$ band, as expected if the
ice desorbed. FEL photoinduced desorption was already observed in the case of
crystalline H2O ice [15, 11]. As mentioned before, QMS data are not conclusive
concerning the detection of IR photodesorption. Nevertheless, indirect
evidence is shown in the FT-RAIR difference spectra for both amorphous and
crystalline CO2 ice. Dedicated studies of infrared-induced photodesorption of
solid molecules will be the focus of future research campaigns at HFML-FELIX
Laboratory.

The difference in behavior of the pure CO2 ices – that deposited at 20 K and
that at 75 K and then cooled to 20 K – upon FEL irradiation of selected
absorption vibrational modes in the MIR is new, independent evidence that CO2
ice deposited at 20 K under our experimental conditions is an amorphous
material, because it undergoes reorganization and minor desorption, whereas
crystalline CO2 ice does not show any clear sign of restructuring but mainly
desorption. McGuire $et~{}al.$ [33] conducted a thorough investigation of the
THz features of amorphous and crystalline CO2 ice, finding unambiguous
evidence that the only two features of CO2 ice in the far-IR at 3.3 and 2.1
THz are due solely to crystalline CO2, and that amorphous CO2 is featureless.
Our work confirms that a background slow deposition of CO2 molecules under UHV
conditions leads to the formation of amorphous CO2 ice. However, we cannot
rule-out that solid CO2 samples grown under different conditions at low
temperature, for instance, through direct deposition (_i.e._ , normal to the
substrate face) and at higher deposition rates, may lead to the formation of
the polycrystalline form of ice already reported in literature [30, 32, 34,
35, 36]. A further systematic investigation of different deposition conditions
and ice thicknesses is needed to clarify this point.

Figure 6: Spectral fit (red solid line) of a FEL irradiation at the stretching
mode of crystalline CO2 ice deposited at 75 K and cooled to 20 K. A FT-RAIR
difference spectrum acquired before and after FEL exposure is reported as a
solid black line. Vertical blue dotted line indicates the selected FEL
radiation frequency. Dashed red line is a FT-RAIR spectrum acquired after
deposition of the ice at 75 K, whereas the dashed blue line is a FT-RAIR
spectrum acquired after the ice is cooled to 20 K. Both spectra are used as
fit components.

##  4 Conclusions

This study focuses on the selective MIR irradiation of pure CO2 ices deposited
at 20 K and at 75 K then cooled to 20 K by means of the intense, tunable
radiation of the FELIX-2 free-electron laser at the HFML-FELIX Laboratory.
Experiments are carried out using the LISA end station, where ice samples are
grown and monitored $in~{}situ$ by Fourier Transform Reflection-Absorption
Infrared spectroscopy in the MIR range and by quadrupole mass spectrometry. We
have shown that amorphous CO2 ice goes through a restructuring process
reorienting some of its molecules into a more ordered structure upon resonant
absorption of IR radiation. The exposure of crystalline CO2 ice to FEL
radiation induces mostly desorption, while frequency-dependent restructuring
of the ice material is not clearly observed. This work adds to the current
debate on the nature of CO2 ice in laboratory experiments and in space.

## Supplementary Material

### Details of the Experimental Setup

Experiments are carried out using the Laboratory Ice Surface Astrophysics
(LISA) ultrahigh vacuum (UHV) end station at the HFML-FELIX Laboratory,
Radboud University in the Netherlands. The experimental setup has been rebuilt
since its original description in Noble $et~{}al.$ [15]. In the present paper,
we present a detailed description of the experimental apparatus. The LISA
system has been designed and optimized to perform selective IR/THz irradiation
of solid phase molecules when coupled to the tunable, high-power, and short-
pulsed radiation from the FELs FELIX-1 ($\sim$30-150 $\mu$m) and FELIX-2
($\sim$3-45 $\mu$m). Figure 7 shows a schematic of the experimental
configuration used at the time of the present work. The UHV system
($\sim$10-10 mbar), substantially larger than in previous iterations,
comprises a $\sim$500 mm diameter cylindrical main chamber with upper and
lower DN100CF and DN200CF flanges, respectively, as well as another nine
DN40CF ports and a DN63CF connection on its sides. At its base, the chamber is
connected to a DN200CF turbo molecular pump (TURBOVAC MAG $W~{}1500$, Oerlikon
Leybold Vacuum) through a DN200CF all-metal gate valve. The turbo pump is then
connected to a foreline pump (D 40 BCS, Oerlikon Leybold Vacuum) with an
ultimate pressure of 10-4 mbar by means of a DN40KF manual ball valve and an
activated alumina oil trap.

On top of the DN100CF flange of the main chamber there is an $xyz$-translator
(LewVac) with a $z$-stroke of 50.8 mm and $xy$-fine control of $\pm 12.7$ mm
connected to a two stage differentially-pumped rotary platform DN100CF with
half nipple base flange (LewVac), and a closed-cycle helium CH-204SB cryostat
head with a HC-4E water-cooled compressor (Sumitomo). Two T-Station 85H pumps
(Edwards Vacuum) are used to differentially pump the rotary platform and to
evacuate the DN100ISO-K end connections between LISA and the FEL beam line,
which also include a DN100ISO-K/DN40CF adapter, a series of three DN40CF gate
valves, and a DN40CF/DN63CF adapter (see Fig. 7). In particular, two of the
three manual UHV DN40CF gate valves mounted in series in the FEL beam path
before the main chamber have CsI (Crystran) and TPX (Tydex) windows,
respectively transparent in the IR and THz spectral ranges, to allow for a
faster change between FELIX-1 and -2 during a single beam time shift (8 hours)
with no need to open the vacuum line. The blank DN40CF gate valve is used to
block the FEL beam when ices are not being irradiated. Moreover, the use of
these valves between the chamber and the FEL beam line allows LISA to remain a
truly UHV system at all times, with a negligible water contamination in the
main chamber, because the valves isolate it from the high vacuum pressure of
the DN100ISO-K FEL beam line ($\sim$10-6 mbar).

Figure 7: Schematic top view of the Laboratory Ice Surface Astrophysics (LISA)
end station at the HFML-FELIX Laboratory in the Netherlands.

The cryostat head is in thermal contact with a custom-made 258 mm long oxygen-
free high thermal conductivity (OFHC) copper circular rod (GoodFellow) with a
diameter of 38 mm. At one end of the rod, a single optically flat gold-coated
38$\times$19$\times$57 mm (l$\times$w$\times$h) face is used as substrate to
ensure maximal FEL beam reflection in the IR/THz spectral range at the center
of the main chamber. Connected to the first stage of the cryostat head, an
OFHC copper radiation shield surrounds the copper rod leaving only the
substrate face uncovered. With the cryostat on, the temperature of the
radiation shield is kept constant at $\sim$77 K. In this experimental
configuration, the selected substrate face exposed to the FEL beam can be
moved in height by means of the $z$-translator to allow for the irradiation of
the gold-coated surface at different spots on the substrate face. The
$z$-translation stage is also used to remove the substrate from the line of
sight of the FEL beam such that the alignment of the Helium-Neon (HeNe) laser
of FELIX-1 and -2 is possible at any time through a ZnSe viewport (Hositrad)
at the back of the chamber, opposite the entrance port of the FEL source.

The substrate temperature is controlled in the range 15-300 K by means of a
model 336 cryogenic temperature controller (LakeShore) that reads temperature
through an uncalibrated DT-670B-SD silicon diode (LakeShore) mounted with a CO
adapter on the upper backside of the gold-coated flat substrate and regulates
a $25~{}\Omega/100$ W cartridge heater (HTR-25-100) mounted inside the copper
rod, at the mid-height of the flat substrate. Although not currently in use, a
second silicon diode can be mounted on the lower backside of the substrate, in
a position symmetric to the first diode and the center of the flat substrate.
Pressure in the main chamber is monitored by means of a wide range
(5$\times$10-3 to 1$\times$10-11 mbar) cold cathode gauge DN40CF (IKR 070,
Pfeiffer Vacuum) coupled to a TPG 300 Total pressure system with a CP 300 T11
Pirani/Cold cathode measurement board (Pfeiffer Vacuum). Active Pirani vacuum
gauges (TPR 018, Pfeiffer Vacuum) are connected at various points of the
system, for instance in the main chamber, between the turbo pump and the
foreline pump, and in the dosing line.

A DN16CF tubing dosing line is used to prepare pure gases and gas mixtures
prior to their dosing inside the main chamber. The dosing line is first
roughly pumped by means of a dry scroll pump (Edwards Vacuum) and then base
pressure better than 10-4 mbar is achieved using a T-Station 85H (Edwards
Vacuum). Apart from a Pirani gauge that measures the base pressure in the
dosing line, two active capacitance transmitters (0.001-10 and 0.1-1000 mbar,
models CCR 363 and CCR 361, respectively, Pfeiffer Vacuum) coupled to a
CenterThree display and control unit (Pfeiffer Vacuum) are used to measure
gases mass-independently during gas mixture preparation, ensuring
reproducibility of the desired ice mixture ratios. Gases are introduced into
the main chamber through an all-metal leak valve (LewVac) connected to a
custom-made double-sided DN40CF flange (LewVac) with a 6 mm central tube
facing the walls of the chamber to allow for background deposition of gases
onto the substrate. In this way, molecules are deposited at random angles with
respect to the substrate surface to better simulate deposition in space on
interstellar dust grains. A background deposition also allows for a more
uniform ice deposition all over the substrate surface. Hence, once a selected
ice is deposited, FEL irradiation can be carried out at multiple unirradiated
spots of the same deposited ice, maximizing the use of a beam time shift and
allowing for more systematic and reproducible studies.

Four DN40CF flanges of the main chamber have ZnSe viewports (Hositrad) mounted
to perform infrared spectroscopy and measure FELIX-2 beam power after the
sample. When the FEL beam power is not measured, a beam damper is placed at
the exit of the ZnSe viewport to block any radiation exiting the chamber, as
shown in Figure 7. An external liquid nitrogen cooled mercury cadmium
telluride (MCT) detector is used to acquire mid-IR (MIR) data (4000-600 cm-1,
2.5-16.6 $\mu$m) by means of a Fourier transform infrared spectrometer (FTIR)
in reflection-absorption infrared mode (RAIR) and at an angle from the gold
surface to the normal of 13∘ (VERTEX 80v, Bruker). Flat and off-axis parabolic
gold mirrors are used to steer and focus, respectively, the FTIR beam onto the
substrate and the MCT (see Fig. 7). While the spectrometer is pumped with a
dry scroll pump (Edwards Vacuum), external optics and the MCT detector are in
purge boxes filled with pure nitrogen gas. Figure 7 shows a residual gas
analyzer quadrupole mass spectrometer (QMS, MKS Instruments) facing the gold
flat substrate and at a distance of $\sim$55 mm from its center to detect
desorbing species during FEL irradiation of the ice and temperature programmed
desorption (TPD) experiments. Alignment of FEL and FTIR beams is done by
manipulating the $xyz$-translator to measure changes in the FTIR signal and in
the intensity of the FEL HeNe, and by steering a series of gold mirrors in the
two lines to maximize their overlap at the substrate central spot. The FEL and
FTIR beams as well as the QMS are all aligned to the center of the chamber and
are all in the same horizontal plane. We measured the FELIX beam spot size at
the substrate surface to be approximately 2 mm in height (diameter), while the
FTIR beam height (diameter) is $\sim$3 mm, with both beams elongated in width
due to their incident angles on the substrate, 45∘ and 13∘, respectively.
Assuming ellipsoidal cross sections of the two beams at the substrate face,
the FEL beam area covers only $\sim$14% of the area of the FTIR beam. The
current FEL and FTIR beam spot sizes allow for the irradiation and
investigation of at least seven fresh, unirradiated spots per single ice
deposition. Although not used in this work, LISA also includes a 50 eV to 5
keV electron gun (EFG-7/EGPS-2017, Kimball Physics) with a FC-7 manual Faraday
cup at its end to induce chemical reactions within deposited ices.

## CRediT authorship contribution statement

S. Ioppolo initiated and managed the project (FELIX-2020-01-51) at HFML-FELIX
Laboratory. He wrote the manuscript with assistance from H.M. Cuppen, J.A.
Noble, and B. Redlich. S. Ioppolo and J.A. Noble performed all laboratory
experiments. A. Traspas Muiña assisted during assembling and testing of the
LISA end station and performed control experiments for the project. S. Coussan
performed some preliminary experiments. All authors contributed to data
interpretation and commented on the paper.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported
in this paper.

## Acknowledgements

The authors thank the HFML-FELIX Laboratory team for their experimental
assistance and scientific support. The LISA end station is designed,
constructed, and managed at the HFML-FELIX Laboratory by the group of S.
Ioppolo. This work was supported by 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).

## References

  * [1] B. A. McGuire, 2021 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules, ApJS259 (2) (2022) 30.  arXiv:2109.13848, doi:10.3847/1538-4365/ac2a48.
  * [2] E. Herbst, E. F. van Dishoeck, Complex Organic Interstellar Molecules, ARA&A47 (1) (2009) 427–480.  doi:10.1146/annurev-astro-082708-101654.
  * [3] K. I. Öberg, E. A. Bergin, Astrochemistry and compositions of planetary systems, Phys. Rep. 893 (2021) 1–48.  arXiv:2010.03529, doi:10.1016/j.physrep.2020.09.004.
  * [4] S. Ioppolo, G. Fedoseev, K. J. Chuang, H. M. Cuppen, A. R. Clements, M. Jin, R. T. Garrod, D. Qasim, V. Kofman, E. F. van Dishoeck, H. Linnartz, A non-energetic mechanism for glycine formation in the interstellar medium, Nature Astronomy 5 (2021) 197–205.  arXiv:2011.06145, doi:10.1038/s41550-020-01249-0.
  * [5] E. M. Penteado, A. C. A. Boogert, K. M. Pontoppidan, S. Ioppolo, G. A. Blake, H. M. Cuppen, Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects, MNRAS454 (1) (2015) 531–540.  doi:10.1093/mnras/stv1987.
  * [6] A. C. A. Boogert, P. A. Gerakines, D. C. B. Whittet, Observations of the icy universe., ARA&A53 (2015) 541–581.  arXiv:1501.05317, doi:10.1146/annurev-astro-082214-122348.
  * [7] A. V. Ivlev, T. B. Röcker, A. Vasyunin, P. Caselli, Impulsive Spot Heating and Thermal Explosion of Interstellar Grains Revisited, ApJ805 (1) (2015) 59.  arXiv:1503.05012, doi:10.1088/0004-637X/805/1/59.
  * [8] M. Dell’Angela, T. Anniyev, M. Beye, R. Coffee, A. Föhlisch, J. Gladh, T. Katayama, S. Kaya, O. Krupin, J. LaRue, A. Møgelhøj, D. Nordlund, J. K. Nørskov, H. Öberg, H. Ogasawara, H. Öström, L. G. M. Pettersson, W. F. Schlotter, J. A. Sellberg, F. Sorgenfrei, J. J. Turner, M. Wolf, W. Wurth, A. Nilsson, Real-Time Observation of Surface Bond Breaking with an X-ray Laser, Science 339 (6125) (2013) 1302–1305.  doi:10.1126/science.1231711.
  * [9] H. Öström, H. Öberg, H. Xin, J. LaRue, M. Beye, M. Dell’Angela, J. Gladh, M. L. Ng, J. A. Sellberg, S. Kaya, G. Mercurio, D. Nordlund, M. Hantschmann, F. Hieke, D. Kühn, W. F. Schlotter, G. L. Dakovski, J. J. Turner, M. P. Minitti, A. Mitra, S. P. Moeller, A. Föhlisch, M. Wolf, W. Wurth, M. Persson, J. K. Nørskov, F. Abild-Pedersen, H. Ogasawara, L. G. M. Pettersson, A. Nilsson, Probing the transition state region in catalytic CO oxidation on Ru, Science 347 (6225) (2015) 978–982.  doi:10.1126/science.1261747.
  * [10] A. Krasnopoler, S. M. George, Infrared resonant desorption of h2o from ice multilayers, J. Phys. Chem. B 102 (5) (1998) 788–794.  arXiv:https://doi.org/10.1021/jp972016q, doi:10.1021/jp972016q.
URL https://doi.org/10.1021/jp972016q

  * [11] C. Focsa, B. Chazallon, J. L. Destombes, Resonant desorption of ice with a tunable LiNbO 3 optical parametric oscillator, Surf. Sci. 528 (1) (2003) 189–195.  doi:10.1016/S0039-6028(02)02631-6.
  * [12] J. A. Noble, C. Martin, H. J. Fraser, P. Roubin, S. Coussan, Ir selective irradiations of amorphous solid water dangling modes: Irradiation vs annealing effects, J. Phys. Chem. C 118 (35) (2014) 20488–20495.  arXiv:https://doi.org/10.1021/jp506943k, doi:10.1021/jp506943k.
URL https://doi.org/10.1021/jp506943k

  * [13] J. A. Noble, C. Martin, H. J. Fraser, P. Roubin, S. Coussan, Unveiling the surface structure of amorphous solid water via selective infrared irradiation of oh stretching modes, J. Phys. Chem. Lett. 5 (5) (2014) 826–829, pMID: 26274073.  arXiv:https://doi.org/10.1021/jz5000066, doi:10.1021/jz5000066.
URL https://doi.org/10.1021/jz5000066

  * [14] S. Coussan, P. Roubin, J. A. Noble, Inhomogeneity of the amorphous solid water dangling bonds, Phys. Chem. Chem. Phys. 17 (2015) 9429–9435.  doi:10.1039/C5CP00662G.
URL http://dx.doi.org/10.1039/C5CP00662G

  * [15] J. A. Noble, H. M. Cuppen, S. Coussan, B. Redlich, S. Ioppolo, Infrared resonant vibrationally induced restructuring of amorphous solid water, J. Phys. Chem. C 124 (2020) 20864–20873.  doi:10.1021/acs.jpcc.0c04463.
  * [16] H. J. Fraser, M. P. Collings, M. R. S. McCoustra, D. A. Williams, Thermal desorption of water ice in the interstellar medium, MNRAS327 (4) (2001) 1165–1172.  arXiv:astro-ph/0107487, doi:10.1046/j.1365-8711.2001.04835.x.
  * [17] T. P. M. Goumans, M. A. Uppal, W. A. Brown, Formation of CO2 on a carbonaceous surface: a quantum chemical study, MNRAS384 (3) (2008) 1158–1164.  doi:10.1111/j.1365-2966.2007.12788.x.
  * [18] Y. Oba, N. Watanabe, A. Kouchi, T. Hama, V. Pirronello, Experimental Study of CO2 Formation by Surface Reactions of Non-energetic OH Radicals with CO Molecules, ApJ712 (2) (2010) L174–L178.  doi:10.1088/2041-8205/712/2/L174.
  * [19] S. Ioppolo, Y. van Boheemen, H. M. Cuppen, E. F. van Dishoeck, H. Linnartz, Surface formation of CO2 ice at low temperatures, MNRAS413 (3) (2011) 2281–2287.  doi:10.1111/j.1365-2966.2011.18306.x.
  * [20] J. A. Noble, F. Dulieu, E. Congiu, H. J. Fraser, CO2 Formation in Quiescent Clouds: An Experimental Study of the CO + OH Pathway, ApJ735 (2) (2011) 121.  arXiv:1104.0031, doi:10.1088/0004-637X/735/2/121.
  * [21] M. H. Moore, R. Khanna, B. Donn, Studies of proton irradiated H2O+CO2 and H2O+CO ices and analysis of synthesized molecules, J. Geophys. Res.96 (E2) (1991) 17541–17545.  doi:10.1029/91JE01682.
  * [22] V. Mennella, M. E. Palumbo, G. A. Baratta, Formation of CO and CO2 Molecules by Ion Irradiation of Water Ice-covered Hydrogenated Carbon Grains, ApJ615 (2) (2004) 1073–1080.  doi:10.1086/424685.
  * [23] V. Mennella, G. A. Baratta, M. E. Palumbo, E. A. Bergin, Synthesis of CO and CO2 Molecules by UV Irradiation of Water Ice-covered Hydrogenated Carbon Grains, ApJ643 (2) (2006) 923–931.  doi:10.1086/502965.
  * [24] M. J. Loeffler, G. A. Baratta, M. E. Palumbo, G. Strazzulla, R. A. Baragiola, CO2 synthesis in solid CO by Lyman-$\alpha$ photons and 200 keV protons, A&A435 (2) (2005) 587–594.  doi:10.1051/0004-6361:20042256.
  * [25] O. Gomis, G. Strazzulla, CO 2 production by ion irradiation of H 2O ice on top of carbonaceous materials and its relevance to the Galilean satellites, Icarus177 (2) (2005) 570–576.  doi:10.1016/j.icarus.2005.04.003.
  * [26] S. Ioppolo, M. E. Palumbo, G. A. Baratta, V. Mennella, Formation of interstellar solid CO{_2} after energetic processing of icy grain mantles, A&A493 (3) (2009) 1017–1028.  doi:10.1051/0004-6361:200809769.
  * [27] S. Ioppolo, I. Sangiorgio, G. A. Baratta, M. E. Palumbo, Solid CO2 in low-mass young stellar objects. Comparison between Spitzer and laboratory spectra, A&A554 (2013) A34.  doi:10.1051/0004-6361/201321176.
  * [28] T. Suhasaria, G. A. Baratta, S. Ioppolo, H. Zacharias, M. E. Palumbo, Solid CO2 in quiescent dense molecular clouds. Comparison between Spitzer and laboratory spectra, A&A608 (2017) A12.  doi:10.1051/0004-6361/201730504.
  * [29] C. R. Arumainayagam, R. T. Garrod, M. C. Boyer, A. K. Hay, S. T. Bao, J. S. Campbell, J. Wang, C. M. Nowak, M. R. Arumainayagam, P. J. Hodge, Extraterrestrial prebiotic molecules: photochemistry vs. radiation chemistry of interstellar ices, Chem. Soc. Rev. 48 (2019) 2293–2314.  doi:10.1039/C7CS00443E.
URL http://dx.doi.org/10.1039/C7CS00443E

  * [30] R. M. Escribano, G. M. Munoz Caro, G. A. Cruz-Diaz, Y. Rodriguez-Lazcano, B. Mate, Crystallization of CO2 ice and the absence of amorphous CO2 ice in space, Proceedings of the National Academy of Science 110 (32) (2013) 12899–12904.  doi:10.1073/pnas.1222228110.
  * [31] M. A. Allodi, S. Ioppolo, M. J. Kelley, B. A. McGuire, G. A. Blake, The structure and dynamics of carbon dioxide and water containing ices investigated via THz and mid-IR spectroscopy, Phys. Chem. Chem. Phys. 16 (8) (2014) 3442.  doi:10.1039/C3CP53767F.
  * [32] P. A. Gerakines, R. L. Hudson, First Infrared Band Strengths for Amorphous CO2, an Overlooked Component of Interstellar Ices, ApJ808 (2) (2015) L40.  doi:10.1088/2041-8205/808/2/L40.
  * [33] B. A. McGuire, S. Ioppolo, M. A. Allodi, G. A. Blake, THz time-domain spectroscopy of mixed CO2-CH3OH interstellar ice analogs, Phys. Chem. Chem. Phys. 18 (30) (2016) 20199–20207.  arXiv:1606.03058, doi:10.1039/C6CP00632A.
  * [34] G. A. Baratta, M. E. Palumbo, The profile of the bending mode band in solid CO2, A&A608 (2017) A81.  doi:10.1051/0004-6361/201730945.
  * [35] M. Tsuge, T. Nguyen, Y. Oba, T. Hama, A. Kouchi, N. Watanabe, UV-ray irradiation never causes amorphization of crystalline CO2: A transmission electron microscopy study, Chem. Phys. Lett. 760 (2020) 137999.  doi:10.1016/j.cplett.2020.137999.
  * [36] A. Kouchi, M. Tsuge, T. Hama, Y. Oba, S. Okuzumi, S.-i. Sirono, M. Momose, N. Nakatani, K. Furuya, T. Shimonishi, T. Yamazaki, H. Hidaka, Y. Kimura, K.-i. Murata, K. Fujita, S. Nakatsubo, S. Tachibana, N. Watanabe, Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains, ApJ918 (2) (2021) 45.  arXiv:2109.03404, doi:10.3847/1538-4357/ac0ae6.
  * [37] H. Yamada, W. B. Person, Absolute Infrared Intensities of the Fundamental Absorption Bands in Solid CO2 and N2O, J. Chem. Phys.41 (8) (1964) 2478–2487.  doi:10.1063/1.1726291.
  * [38] P. A. Gerakines, W. A. Schutte, J. M. Greenberg, E. F. van Dishoeck, The infrared band strengths of H2O, CO and CO2 in laboratory simulations of astrophysical ice mixtures., A&A296 (1995) 810.  arXiv:astro-ph/9409076.
  * [39] M. J. Loeffler, M. H. Moore, P. A. Gerakines, The Effects of Experimental Conditions on the Refractive Index and Density of Low-temperature Ices: Solid Carbon Dioxide, ApJ827 (2) (2016) 98.  doi:10.3847/0004-637X/827/2/98.
  * [40] G. A. Baratta, M. E. Palumbo, Infrared optical constants of CO and CO2 thin icy films, Journal of the Optical Society of America A 15 (12) (1998) 3076–3085.  doi:10.1364/JOSAA.15.003076.
  * [41] I. R. Cooke, E. C. Fayolle, K. I. Öberg, CO2 Infrared Phonon Modes in Interstellar Ice Mixtures, ApJ832 (1) (2016) 5.  arXiv:1610.00294, doi:10.3847/0004-637X/832/1/5.
  * [42] J. S. Shirk, C. L. Marquardt, Vibrational selectivity in the single-photon infrared photochemistry of matrix-isolated 2-fluoroethanol, The Journal of Chemical Physics 92 (12) (1990) 7234–7240.  arXiv:https://doi.org/10.1063/1.458209, doi:10.1063/1.458209.
URL https://doi.org/10.1063/1.458209

  * [43] S. Ioppolo, B. A. McGuire, M. A. Allodi, G. A. Blake, THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups, Faraday Discussions 168 (2014) 461.  doi:10.1039/C3FD00154G.