# Formation of the simplest amide in molecular clouds: formamide (NH2CHO) and
its derivatives in H2O-rich and CO-rich interstellar ice analogs upon VUV
irradiation

K.-J. Chuang Current address: Laboratory for Astrophysics, Leiden Observatory,
Leiden University, P.O. Box 9513, NL-2300 RA Leiden, the Netherlands.
Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the
Friedrich Schiller University Jena, Institute of Solid State Physics,
Helmholtzweg 3, D-07743 Jena, Germany<EMAIL_ADDRESS>C. Jäger
Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the
Friedrich Schiller University Jena, Institute of Solid State Physics,
Helmholtzweg 3, D-07743 Jena, Germany  S.A. Krasnokutski Laboratory
Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich
Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3,
D-07743 Jena, Germany  D. Fulvio Current address: Istituto Nazionale di
Astrofisica, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16,
I-80131 Naples, Italy.  Max Planck Institute for Astronomy, Königstuhl 17,
D-69117 Heidelberg, Germany  Th. Henning Max Planck Institute for Astronomy,
Königstuhl 17, D-69117 Heidelberg, Germany

###### Abstract

The astronomical detection of formamide (NH2CHO) toward various star-forming
regions and in cometary material implies that the simplest amide might have an
early origin in dark molecular clouds at low temperatures. Laboratory studies
have proven the efficient NH2CHO formation in interstellar CO:NH3 ice analogs
upon energetic processing. However, it is still under debate, whether the
proposed radical–radical recombination reactions forming complex organic
molecules remain valid in an abundant H2O environment. The aim of this work
was to investigate the formation of NH2CHO in H2O- and CO-rich ices under
conditions prevailing in molecular clouds. Therefore, different ice mixtures
composed of H2O:CO:NH3 (10:5:1), CO:NH3 (4:1), and CO:NH3 (0.6:1) were exposed
to vacuum ultraviolet photons in an ultra-high vacuum chamber at 10 K.
Fourier-transform infrared spectroscopy was utilized to monitor in situ the
initial and newly formed species as a function of photon fluence. The infrared
spectral identifications are complementarily secured by a temperature-
programmed desorption experiment combined with a quadrupole mass spectrometer.
The energetic processing of CO:NH3 ice mixtures mainly leads to the NH2CHO
formation, along with its chemical derivatives such as isocyanic acid (HNCO)
and cyanate ion (OCN-). The formation kinetics of NH2CHO shows an explicit
dependency on ice ratios and compositions; the highest yield is found in
H2O-rich ice. The astronomical relevance of the resulting reaction network is
discussed.

astrochemistry – methods: laboratory: solid state – infrared: ISM – ISM: atoms
– ISM: molecules – UV-photon Processes

##  1 Introduction

Complex organic molecules (COMs) have been widely observed in massive
molecular clouds, high- and low-mass protostars, and cometary material (see
reviews by Herbst & van Dishoeck 2009; Herbst 2017; Altwegg et al. 2019).
Among the successfully identified O-, N-, and S-bearing COMs, some of them
attract exceptional attention in the field of astrochemistry due to their
potential role in the extraterrestrial origin of building blocks of life
(Muñoz Caro et al., 2002; Meierhenrich, 2005; Nuevo et al., 2008; Fulvio et
al., 2021; Ioppolo et al., 2021). For example, formamide (NH2CHO) has been
suggested to be an important precursor for the formation of nucleobases (e.g.,
adenine, uracil, thymine, cytosine, and guanine), which are found in RNA
and/or DNA structures (Saladino et al., 2006; Barks et al., 2010; Rotelli et
al., 2016).

In astronomical observations, NH2CHO was first detected by Rubin et al. (1971)
toward Sgr B2. Later, it has been observed by follow-up searches in various
high-mass protostellar objects (Bisschop et al., 2007; Halfen et al., 2011;
Isokoski et al., 2013; Jones et al., 2013; Neill et al., 2014; Suzuki et al.,
2018; Colzi et al., 2021). NH2CHO has also been identified toward several
solar-mass stars and shock regions (Takahiro et al., 2012; Kahane et al.,
2013; Mendoza et al., 2014; López-Sepulcre et al., 2015; Taquet et al., 2015;
Coutens et al., 2016; Imai et al., 2016; Lee et al., 2017; Oya et al., 2017;
Marcelino et al., 2018). A detailed list of astronomical observations can be
found in the review by López-Sepulcre et al. (2019). Although the detection of
NH2CHO in prestellar cores is still lacking, the common detection of the
simplest amide in cometary comae, which have been considered to represent the
pristine interstellar ice composition, suggests that it could have an early
and cold origin from dense molecular clouds (Bockelee-Morvan et al., 1997;
Biver et al., 2014; Goesmann et al., 2015; Altwegg et al., 2017; Drozdovskaya
et al., 2019).

Both gas-phase and solid-state formation routes of NH2CHO have been discussed
to explain its widespread and abundant detection in the aforementioned star-
forming regions. The gas-phase reaction NH2 \+ H2CO $\rightarrow$ NH2CHO + H
has been theoretically suggested to be efficient under interstellar cloud
conditions (Barone et al., 2015; Skouteris et al., 2017). However, its
reaction rate constant is still under debate (Song & Kästner, 2016). In
addition, the gas-phase reaction CO + NH2 $\rightarrow$ NH2CO and the
following, so-called “radical disproportionation” (i.e., 2NH2CO $\rightarrow$
NH2CHO + HNCO) have been reported in early high-temperature studies (150-400
℃) simulating the primordial Earth’s conditions (Boden & Back, 1970; Yokota &
Back, 1973; Hubbard et al., 1975).

Table 1: Overview of the Experiments Performed Exp | T |  N(H2O) |  N(CO) |  N(NH3) | Ratio | VUV Fluence
---|---|---|---|---|---|---
| (K) | (molecules cm-2) | (molecules cm-2) | (molecules cm-2) | H2O:CO:NH3 | (photons cm-2)
CO:NH3+VUV | 10 | - | 2.3$\times$1016 | 3.6$\times$1016 | 0 : 0.6: 1 | 8.2$\times$1017
CO:NH3+VUV | 10 | - | 1.4$\times$1017 | 3.6$\times$1016 | 0 : 4 : 1 | 8.2$\times$1017
H2O:CO:NH3+VUV | 10 | 2.6$\times$1017 | 1.5$\times$1017 | 2.9$\times$1016 | 10 : 5 : 1 | 8.2$\times$1017
H2O:CO+VUV | 10 | 2.0$\times$1017 | 1.5$\times$1017 | - | 1 : 1 : 0 | 8.2$\times$1017
H2O:NH3+VUV | 10 | 3.6$\times$1017 | - | 3.3$\times$1016 | 10 : 0 : 1 | 8.2$\times$1017

In the solid phase, several low-temperature formation mechanisms of NH2CHO
have been investigated, including atomic addition, radical association with
interstellar molecules (e.g., CO, NH3, and H2O), and radical-radical
recombination reactions. It has been concluded that the successive
hydrogenation of isocyanic acid (HNCO) is unlikely to form NH2CHO on dust
grains under interstellar conditions, because of a relatively low rate
constant (Song & Kästner, 2016). Furthermore, the dual-cyclic mechanism of
H-atom abstraction and addition reactions connecting HNCO and NH2CHO favors
the hydrogen-poor species (Haupa et al., 2019). Following the gas-phase work
reported in Hubbard et al. (1975), the radical-molecule association of NH2 \+
CO resulting in NH2CO radicals has been adapted to explain the NH2CHO
formation in the energetic processing of CO:NH3 ice mixture (Hudson & Moore,
2000; Bredehöft et al., 2017). However, this pathway is questioned because of
the absence of NH2CO (Jones et al., 2011). Another radical-molecule
association route comprising H2CO and NH2O, which is formed in situ through
H-atoms addition/abstraction of NO/NH2OH, has been proposed in Dulieu et al.
(2019), but further validation of its efficiency is still desired. As an
alternative, barrierless radical-radical recombination of NH2 and HCO with
proper geometric orientation has been suggested to form NH2CHO in the solid
phase (Hudson & Moore, 2000). This formation mechanism has also been
investigated under different energetic sources, such as fast ions, electrons,
and UV as well as X-ray photons (Jones et al., 2011; Abplanalp et al., 2016;
Fedoseev et al., 2016; Ciaravella et al., 2019; Martín-Doménech et al., 2020).

The solid-state chemistry of the CO:NH3 ice mixture triggered by energetic
processing has been extensively investigated in the past decades (Milligan &
Jacox, 1965; Ferris et al., 1974; Hagen et al., 1979; Grim et al., 1989; Demyk
et al., 1998; Hudson & Moore, 2000). Although NH2CHO is a prevalent product
detected upon UV photon or ion impacts, the main focii of these studies were
more on the spectroscopy of [NH${}_{4}]^{+}$[OCN]- and on the identification
of refractory material of biological interests after a relatively long
irradiation time. In the last ten years, quantitative studies of the product
evolution as a function of energy have become available. For example, Jones et
al. (2011) studied the energetic electron bombardment of the CO:NH3 ice
mixture at $\sim$12 K and reported the temporal profiles of HCO and NH2CHO to
support the suggested reaction pathway, i.e., HCO + NH2 $\rightarrow$ NH2CHO.
More recently, a systematic study showed the dependence of the NH2CHO
formation on the CO:NH3 ratios and temperatures (10-24 K) upon vacuum UV (VUV)
irradiation (Martín-Doménech et al., 2020). The derived kinetics of the NH2CHO
formation was further compared to the formation of the analog product CH3CHO
following similar recombination reactions (CH3 \+ CHO) in the CH4:NH3 ice
mixture and to the data obtained by Jones et al. (2011). It has been concluded
that both VUV photons and energetic electrons have very similar effects on the
formation cross-section of formamide. In spite of these numerous studies, the
validity of the proposed solid-state chemistry leading to complex species in
an astronomically realistic icy environment comprising abundant H2O ice is
still under debate. More experimental studies are needed in order to
understand the role of water in the COM formation, as mentioned in López-
Sepulcre et al. (2019). The concern is linked to a higher diffusion barrier of
radicals and the limited orientations at the surface and in the bulk ice of
H2O (Enrique-Romero et al., 2019; López-Sepulcre et al., 2019). Agarwal et al.
(1985) studied the UV photolysis of interstellar ice containing H2O, CO, and
NH3 using a relatively high photon fluence. However, they found a list of
large prebiotic compounds, with the notable exception of formamide. More
recently, Ciaravella et al. (2019) reported the NH2CHO formation upon X-ray
irradiation of an ice mixture with a similar quantity of H2O, CO, and NH3
(1:0.9:0.7) at 13 K. Besides this, the potential role of abundant H2O on the
formation kinetics of formamide still remains unclear.

The previous studies mentioned above have devoted great effort into exploring
each specific topic, such as OCN- formation/identification, NH2CHO formation
mechanisms, and kinetics of the newly formed N-bearing products. These topics
are important to comprehensively understand the interstellar ice evolution in
molecular clouds. However, they have not been experimentally investigated in a
single chemical system considering the possible influence of the most abundant
H2O ice. The current laboratory study, for the first time, aims at verifying
the formation of NH2CHO and its chemical derivatives (e.g., HNCO and OCN-) in
CO:NH3 ice mixtures with or without H2O triggered by the cosmic ray induced
secondary VUV photons (mainly H2 emissions; Gredel et al. 1989; Cruz-Diaz et
al. 2014). The goal of this study is to reveal a potential chemical network
involving the three chemically important species H2O, CO, and NH3 in
interstellar ice and remove the doubts concerning the COM formation in
H2O-rich ice mantles. The present work is motivated by the importance of
NH2CHO in astrochemistry and highly requested quantitative data regarding the
NH2CHO formation in relevant ice compositions. In this study, the kinetic
evolution of all newly formed products, including both N- and O-bearing
species, is monitored in situ as a function of photon fluence. The
experimental details are described in the next section. The results are
presented in section 3 and discussed in section 4. The astronomical relevance
of the experimental findings and conclusions are summarized in section 5.

##  2 Experimental

Table 2: Summary of IR peak Positions and Band Strength Values Used in This Work. Species | Chemical Formula | IR Peak Position | Band Strength | References
---|---|---|---|---
|  | (cm-1) | (cm molecule-1) |
Carbon monoxide | CO | 2137 | 1.1$\times$10-17 | Jiang et al. (1975); Bouilloud et al. (2015)
Ammonia | NH3 | 1040 | 2.0$\times$10-17 | Hudson et al. (2022)
Formamide | NH2CHO | 1698 | 6.5$\times$10-17 | Brucato et al. (2006)
Isocyanic acid | HNCO | 2261 | 7.2$\times$10-17 | van Broekhuizen et al. (2004)
Cyanate | OCN- | 2150 | 1.3$\times$10-16 | van Broekhuizen et al. (2004)
Formaldehyde | H2CO | 1724 | 9.6$\times$10-18 | Schutte et al. (1993)
Formyl radical | HCO | 1855 | 1.8$\times$10-17 | Ryazantsev et al. (2017)
Carbon dioxide | CO2 | 2343 | 1.2$\times$10-16 | Gerakines & Hudson (2015)

All experiments have been performed in the Laboratory Astrophysics and Cluster
Physics Group in Jena using the ultra-high vacuum (UHV) apparatus. The
experimental details have been described in previous work (Potapov et al.,
2019), and here only relevant information as well as further modifications are
mentioned. The base pressure in the UHV chamber is $\sim 5\times 10^{-10}$
mbar at room temperature. A copper sample holder is mounted on the tip of a
closed-cycle helium cryostat (ColdEdge CS-204) and positioned at the center of
the UHV chamber. The temperature at the sample holder monitored by a silicon
diode with $\leq$0.5 K accuracy can be varied between 10 and 330 K, and is
controlled by a resistive heater manipulated by a Lakeshore temperature
controller. The thermal contact between the cryostat and the substrate is
improved using an indium foil to minutesimize the temperature difference. In
this work, a KBr window, which has a high rate of transmission in the mid-IR
range, has been used as a bare substrate that allows the intended ice samples
to condense and the bulk ice chemistry triggered by VUV photons at 10 K to be
studied.

The VUV photons have been generated by a commercial deuterium lamp (Hamamatsu:
H2D2-L11798), a closed-cycle (MgF2-sealed) light source, and further guided to
the ice sample through a MgF2 UHV viewport installed on the main chamber. The
transmission of a MgF2 window decreases from more than $\sim$88% at 160.0 nm
to $\sim$66% at 121.6 nm (for 2 mm thickness). After calibrating the default
emission spectrum provided by the Hamamatsu with the standard transmission of
the MgF2 window, the contribution of Ly$\alpha$ emission in the range of
115-128 nm drops significantly, making it negligible compared to the D2
emissions at 128-170 nm. The spectral energy distribution with a stronger
emission band at $\sim$160 nm represents the H2 Lyman band system induced by
cosmic rays interacting with hydrogen molecules in dense molecular clouds
(Prasad & Tarafdar, 1983; France et al., 2005). The UV photon chemistry
induced by the H2 Werner band system remains for future studies. Although the
in situ measurement of UV flux at the sample position is still lacking, a
lower photon flux is expected. The estimated UV flux ($\phi$) of
$\sim$7.6$\times$1013 photons cm-2 s-1 has been determined by comparing the
formation rate constant ($k^{\prime}$) of formamide in this work to the
averaged formation cross-section ($\sigma$) value reported in Martín-Doménech
et al. (2020), who used the same D2 lamp under very similar experimental
settings: $\phi$=$k^{\prime}$/$\sigma$.

Pure CO (Westfalen, 99.97%) or CO mixed with H2O vapor (Sigma-Aldrich, UHPLC
grade) and NH3 (Air Liquide, 99.999%) were simultaneously introduced into the
main chamber through two separate all-metal leak valves and deposited onto the
precooled KBr substrate, which is positioned at an angle of 30∘ to the
deposition tubes and 90∘ to the IR beam. In order to investigate CO-poor, CO-
rich, and H2O-rich interstellar ices, three different ice compositions were
intentionally selected: (1) CO:NH3=0.6:1, (2) CO:NH3=4:1, and (3)
H2O:CO:NH3=10:5:1. The deposited ice mixtures were then irradiated with UV
photons at 10 K. The applied ice compositions and column densities are
summarized in Table 1.

The growth of the deposited ices and the chemistry induced by the photolysis
were monitored in situ using Fourier transform infrared spectroscopy (FTIR) in
transmission mode in a range of 7500-400 cm-1 with 1 cm-1 resolution. The
quantitative analysis of the IR spectra followed the commonly used method
described in previous studies (see Jones et al. 2011 and Martín-Doménech et
al. 2020) on the formamide formation, and is only briefly explained below. The
selected IR absorption peaks were corrected by straight baseline subtraction,
and the overlapping peaks were deconvoluted using Gaussian functions, which
are both available in the OriginPro software. Their corresponding absorption
areas were obtained directly by Gaussian curve fitting. The absolute
abundances of parent molecules and products were derived by converting IR
absorption peak areas to column densities utilizing the modified Beer-Lambert
Law (Bennett et al., 2004; Terwischa van Scheltinga et al., 2018),

$\textit{N}=\frac{\ln 10\cdot\int\textit{Abs}(\nu)d(\nu)}{\textit{A'}}$ (1)

where N is the column density in molecules cm-2, Abs($\nu$) is the IR
absorbance, and A’ is the corresponding IR band strength in cm molecule-1. The
error bars were defined as the standard deviation of the Gaussian fit. This
estimation does not account for uncertainties originating from the baseline
subtraction procedure and IR absorption band strengths, which have been
reported in the literature. The used IR band strengths are summarized in Table
2. The derived column densities of these species can be further refined as
soon as more precise values become available.

After VUV irradiation of predeposited (H2O:)CO:NH3 ice mixtures for 180
minutes, the photon-processed ice samples were sublimated by a temperature-
programmed desorption (TPD) experiment with a ramping rate of 5 K minute-1.
The desorbed species were analyzed with a quadrupole mass spectrometer (QMS;
XT300M, Extorr Inc.). Characteristic desorption temperatures and fragmentation
patterns induced by the ionization with 70 eV electrons were recorded in order
to identify the newly formed products by comparing them with standard samples
in the NIST database and literature. Such a QMS-TPD diagnostic technique has
demonstrated a much higher sensitivity with respect to FTIR.

##  3 Results

###  3.1 NH2CHO formation in CO:NH3 ice mixtures

Figure 1: Top panel: IR spectra obtained after deposition of ice mixtures (a)
CO:NH3 (0.6:1) and (b) CO:NH3 (0.6:1) after VUV irradiation for a fluence of
8.2$\times$1017 photons cm-2 (180 minutes) at 10 K. Bottom panel: IR spectra
obtained after deposition of (c) CO:NH3 (4:1) and VUV irradiation of (d)
CO:NH3 (4:1) ice mixture for a fluence of 8.2$\times$1017 photons cm-2 (180
minutes). IR spectra are offset for clarity. IR spectrum (e) is shown for pure
NH2CHO ice at 10 K. The shadowed areas are representative for the
contamination with Ni(CO)4 from the gas regulator and the FTIR defect features
at 2046 and $\sim$1200 cm-1, respectively (Meunier, 2019).

The top panel of Fig. 1 presents the IR absorption spectra of a CO:NH3 (0.6:1)
ice mixture obtained before (a) and after (b) VUV irradiation for 180 minutes
at 10 K, as well as that of (e) pure NH2CHO as a reference. Besides the
absorption features corresponding to the parent molecules, such as CO at 2143
cm-1 and NH3 at 1030 cm-1 as well as 1626/1649 cm-1 (monomer/aggregate),
several IR peaks were only observed upon photolysis of the deposited ice
mixture in the spectrum (b), such as the absorption peaks at 1698, 1390, and
1324 cm-1 corresponding to C=O stretching ($\nu_{4}$), C-H bending
($\nu_{6}$), and C-N stretching ($\nu_{7}$) modes of NH2CHO, respectively
(King, 1971; Torrie & Brown, 1994; Brucato et al., 2006; Sivaraman et al.,
2013). Other less intense features of NH2CHO are overlapped with IR peaks of
NH3. The absorption feature at 2155 cm-1 is assigned to OCN- (Demyk et al.,
1998; Bernstein et al., 2000; Hudson & Moore, 2000; Raunier et al., 2003; van
Broekhuizen et al., 2004; Gerakines et al., 2004). Historically, it was
labeled as ”XCN” in astronomical observations and then successfully identified
as the ionic species OCN- by comparing with laboratory (isotope-labeled)
studies (Grim & Greenberg, 1987; Schutte & Greenberg, 1997). The counterpart
cation NH${}_{4}^{+}$ is shown at $\sim$1495 cm-1.

In the bottom panel of Fig. 1, the IR absorption spectra of the CO:NH3 (4:1)
ice mixture obtained before (c) and after (d) VUV irradiation for 180 minutes
at 10 K as well as of (e) pure NH2CHO are shown. In spectrum (c), the full
width at half maximum (FWHM) of the NH3 symmetric deformation ($\nu_{2}$) mode
becomes more narrow (10$\%$) compared to that in the CO-poor case (spectrum
(a)), due to a cage effect of overabundant CO. This is also supported by the
observed splitting of the antisymmetric and symmetric deformation modes
($\nu_{4}$) of the NH3 monomer. All spectral characteristics clearly suggest
that NH3 resides in the CO-rich ice.

Besides the previously labeled products, several new IR peaks are additionally
present in spectrum (d). The IR feature at 2260 cm-1 is assigned to HNCO
($\nu_{2}$), a precursor of OCN- (Milligan & Jacox, 1965; Teles et al., 1989;
Lowenthal et al., 2002; van Broekhuizen et al., 2004). The possible isomer
cyanate acid (HOCN) is absent, which is consistent with the previous studies
suggesting a less efficient isomerization from HNCO to HOCN (Theule et al.,
2011; Jiménez-Escobar et al., 2014). The vibrational peaks at 1855 and 1090
cm-1 originate from the CO stretching ($\nu_{3}$) and CH bending ($\nu_{2}$)
modes of HCO, which is the first intermediate product of the CO hydrogenation
scheme (Ewing et al., 1960; Milligan & Jacox, 1964). The detection of HCO is
consistent with the well-known H-atom addition reactions to CO ice, i.e.,
CO$\rightarrow$H2CO$\rightarrow$CH3OH (Watanabe & Kouchi, 2002; Fuchs et al.,
2009). The strongest C=O ($\nu_{2}$) vibrational band of H2CO is clearly
observable at 1722 cm-1 along with other relatively weak absorptions at 1500
($\nu_{3}$) and $\sim$1245 ($\nu_{5}$) cm-1 (Khoshkhoo & Nixon, 1973;
Nelander, 1980). The most intense absorption peak of the saturated product
CH3OH (i.e., C-O stretching $\nu_{8}$) cannot be deconvoluted easily from the
broad feature of parent NH3 around 1030 cm-1. In addition to reactions with
free H atoms originating from the NH3 photolysis, CO is expected to be
photoexcited (i.e., CO$\rightarrow$CO*) and followed by a reaction with
another CO resulting in the formation of CO2, which is observed at 2343 cm-1.
However, this mechanism is considered inefficient compared to the reaction of
the hydrocarboxyl radical with atomic hydrogen HOCO + H $\rightarrow$ CO2 \+
2H (Watanabe & Kouchi, 2002; Ioppolo et al., 2011; Chen et al., 2014). Here,
we cannot exclude the contribution of this reaction to the CO2 formation,
given the tiny peak of HOCO observed at 1840 cm-1.

Figure 2: Left: Ion signals obtained during TPD experiment after VUV
irradiation of CO+NH3 (4:1) for a fluence of 8.2$\times$1017 photons cm-2 (180
minutes) at 10 K. Only relevant m/z channels are shown, and they have been
shifted for clarity. The dashed lines indicate the peak positions of the
corresponding molecules. Right: Comparison of the electron-ionization
fragmentation patterns for the QMS-TPD peaks at $\sim$100, $\sim$200, and
$\sim$205 K (black) with standard data of H2CO, NH2CHO, and NH2CH2OH.

The left panel of Fig. 2 presents the ion signals, which are obtained during
the TPD experiment after VUV irradiation of CO:NH3(4:1) ice at 10 K. Only ion
signals at relevant masses are selected to complementarily secure the IR
identification. For example, H2CO is observed through the mass desorption
peaks 29 and 30 m/z at $\sim$100 K, corresponding to the most intense fragment
and the molecular mass, respectively. The CH3OH desorption is demonstrated by
the most intense mass fragment of 31 m/z at $\sim$156 K. The other two
desorption peaks at higher temperatures are due to N-bearing species. The peak
at $\sim$200 K (starting from $\sim$170 K; i.e., 43, 44, and 45 m/z) and the
peak at $\sim$205 K (starting from $\sim$180 K; i.e., 30, 31, and 46 m/z) are
consistent with the reported desorption temperature of NH2CHO and NH2CH2OH,
respectively (Bossa et al., 2009; Ligterink et al., 2018; Martín-Doménech et
al., 2020). A somehow different desorption profile has been observed for the
mass signal of 43 m/z, which starts increasing at a slightly lower temperature
(i.e., $\sim$165 K) than those observed for the mass signals of 44 and 45 m/z.
This points to an additional contribution of 43 m/z from HNCO, which is
expected to desorb at $\sim$185 K (Fischer et al., 2002; Fedoseev et al.,
2015). Furthermore, the identifications are confirmed by comparing their
(selected) mass fragmentation patterns induced by electron impact in the right
panel of Fig 2. The relative intensities between different mass signals are in
good agreement with standard electron (70 eV) ionization patterns as available
from the NIST database for H2CO and NH2CHO and from the study by Duvernay et
al. (2014) for NH2CH2OH, except for the 43 m/z, which has contributions from
both HNCO and NH2CHO. The experimental results from the ion signals during the
TPD experiment support the IR-spectral assignments of H2CO and NH2CHO and
further point out the possible formation of their saturated derivatives such
as CH3OH and NH2CH2OH, whose IR vibrational features might largely overlap
with their precursors and whose intensities are below the FTIR detection
limits.

Figure 3: Evolution of (a) parent molecules and (b) newly formed products
after VUV irradiation of CO:NH3 (4:1) ice mixture over a fluence of
8.2$\times$1017 photons cm-2 (180 minutes). The dashed lines present the
fitting results, and the solid lines connecting data are only for clarity.

###  3.2 Kinetic Evolution of CO:NH3 Ice Mixture under VUV Irradiation

In addition to the qualitative identifications of the photolysis products, the
absolute column density (N) of the parent molecules and newly formed species
were monitored in situ as a function of photon fluence. In Fig. 3, the
evolutions of parent molecules (e.g., CO and NH3 in Fig. 3(a)) and major
products (e.g., NH2CHO, HCO, OCN-, H2CO, HNCO, and CO2 in Fig. 3(b)) after VUV
irradiation of the CO:NH3 (4:1) ice (fluence of 8.2$\times$1017 photons cm-2)
are shown. The consumption of CO and NH3 is fit by a single exponential
equation:

$\Delta\textit{N}\text{(molecules)}=\alpha(1-\text{exp}(\sigma\cdot\phi\cdot
t))\\\ ,$ (2)

where $\alpha$ is the saturation value (i.e., the maximum change when reaching
the equilibrium state) in molecules cm-2, $\sigma$ is the photolysis cross-
section in cm2 photon-1, $\phi$ is the UV flux in photons cm-2 s-1, and t is
the irradiation time in seconds. The derived cross-section values are
(1.0±0.2)$\times$10-18 and (2.5±0.2)$\times$10-18 cm2 photon-1 for CO and NH3,
respectively. The value of NH3 is larger than that of CO, due to the direct
photodissociation of NH3 forming NH2 and H.

In Fig. 3(b), NH2CHO and HCO are the two products that form most efficiently
at the beginning of VUV irradiation. The abundances of NH2CHO and HCO increase
rapidly up to a fluence of 2.7$\times$1017 photons cm-2. The formation cross-
section values of NH2CHO and HCO are (2.9±0.2)$\times$10-18 and
(6.0±0.7)$\times$10-18 cm2 photon-1, respectively. Once the HCO production
rate slows down, a similar saturation behavior is also observed in NH2CHO. The
obtained ratio of NH2CHO/HCO shows a relatively constant value of 2.7±0.2
after reaching a fluence of 2.7$\times$1017 photons cm-2 implying a chemical
correlation between these two. The formation of H2CO and HNCO is also observed
upon VUV processing, but with much lower production yields compared to NH2CHO.
Unlike the aforementioned species, the production of OCN- and CO2 is
considered as a high-order generation of photolysis products, given the
different kinetic curves and the delay of formation. The abundances were only
detectable after reaching a fluence of 2.7$\times$1017 photons cm-2. However,
at the end of the photolysis experiment, the absolute column density of OCN-
was higher than that of HNCO. Similar product evolution as a function of
photon fluence was also found in the CO:NH3 (0.6:1) ice mixture for NH2CHO,
H2CO, and OCN-, but their intermediate products HCO and HNCO were absent.

Figure 4: IR difference spectra obtained before and after UV irradiation of
(a) H2O:NH3 (10:1), (b) H2O:CO (1:1), and (c) H2O:CO:NH3 (10:5:1) ice mixtures
for a fluence of 8.2$\times$1017 photons cm-2 at 10 K. IR spectra are offset
for clarity.

###  3.3 NH2CHO Formation in H2O-rich Ice Mixture

Figure 4 shows the difference IR absorption spectra before and after 180-min
VUV irradiation of (a) H2O:NH3 (10:1), (b) H2O:CO (1:1), and (c) H2O:CO:NH3
(10:5:1) ice mixtures at 10 K. The newly formed products are present in
positive peaks and the parent molecules are present in negative features,
which are intentionally omitted in Fig. 4, except for H2O at $\sim$1633 cm-1.
The control experiments (a) H2O:NH3 (10:1) and (b) H2O:CO (1:1) provide
spectral evidence to identify possible carriers of the newly formed peaks,
which are exclusively detected in the H2O:CO:NH3 ice mixture (spectrum (c) in
Fig. 4).

As shown in spectrum (a), VUV photolysis of H2O:NH3 ice mixture only results
in a broad N-H bending feature around 1497 cm-1, which has been assigned to
NH2 and/or NH${}_{4}^{+}$ in [NH${}_{4}]^{+}$[OH]- salt (Schutte et al., 1999;
Zheng et al., 2008). The peaks of the possible product (NH2OH) originating
from OH + NH2 are absent. This is probably due to a low formation yield or
high destruction cross-section of NH2OH upon VUV-photon impact (Fedoseev et
al., 2015). In spectrum (b), the new IR features are assigned to C-bearing
species appearing upon VUV irradiation of H2O:CO. Besides CO2, H2CO is another
C-bearing product, presenting absorption bands at 1719, 1498, and 1244 cm-1.
The IR features of the intermediate radicals HCO and HOCO (i.e., the
precursors of H2CO and CO2 , respectively) are largely overlapped, as shown at
$\sim$1853/1846 cm-1 (Milligan & Jacox, 1971; Bennett et al., 2010). The
spectral deconvolution of these two peaks was realized by Gaussian curve
fitting and further confirmed during the TPD experiment because these two
species have different desorption temperatures (IR absorptions at 1853 and
1846 cm-1 disappeared at 15-30 and 30-45 K, respectively). The IR features of
the formate anion (HCOO-) are visible at 1587 and 1355 cm-1 (Milligan & Jacox,
1971; Bennett et al., 2010). This suggests a possible acid-base reaction
between HCOOH and H2O (Schutte et al., 1999; Hudson & Moore, 2000). All
aforementioned features shown in spectra (a) and (b) are also unambiguously
present in spectrum (c) of the VUV irradiated H2O:CO:NH3 (10:5:1) ice mixture.
In addition, the newly appeared IR features present in spectrum (c) must be
attributed to photolysis products containing CN bonds; the IR peaks at 1693,
1392, and 1333 cm-1 have been assigned to NH2CHO. However, OCN- or HNCO, the
chemical derivatives of NH2CHO, are absent.

Figure 5: Upper panel: (a) Abundance evolution of the newly formed NH2CHO in
the VUV photolysis of three investigated ice mixtures: CO:NH3 (0.6:1), CO:NH3
(4:1), and H2O:CO:NH3 (10:5:1), over a fluence of 8.2$\times$1017 photons
cm-2. The dashed lines present the best fitting. Bottom panel: The derived
fitting values of (b) saturation, $\alpha$, and (c) formation cross-section,
$\sigma$, for NH2CHO are present for three investigated CO:NH3 ice mixtures

The kinetic analysis shows that NH2CHO is one of the first-generation products
and is observed in all studied CO:NH3 ice mixtures with or without the
presence of H2O. The evolution of N(NH2CHO) normalized by the initial parent
species (i.e., N0(NH3)) as a function of photon fluence and the fit according
to Eq. 2 are shown in Fig. 5(a). The corresponding values of the fitting, such
as the saturation $\alpha$ (unitless) and the formation cross-section $\sigma$
(cm2 photon-1) are presented in Fig. 5(b) and (c), respectively. It is
important to note that the derived fitting outcomes are only considered as
effective results that represent the combination of product’s formation and
destruction channels. The photon-induced cross-section values ($\sigma$) that
indicate the efficiency of reaching the steady state are
(3.6±0.4)$\times$10-18, (2.9±0.2)$\times$10-18, and (0.8±0.1)$\times$10-18 cm2
photon-1 for CO:NH3 (0.6:1), CO:NH3 (4:1), and H2O:CO:NH3 (10:5:1) ice
mixtures, respectively. Although a relatively low formation cross-section was
obtained for the H2O-rich experiment, the highest saturation value ($\alpha$)
of 0.10±0.01 was found in the H2O:CO:NH3 (10:5:1) ice. In the CO:NH3 (4:1) and
CO:NH3 (0.6:1) experiments, these values are considerably smaller, i.e.,
0.028±0.001 and 0.001±0.001, respectively. The experimental results suggest
that the NH2CHO formation is strongly influenced by initial ice compositions
and by the number of photons absorbed in the bulk ice. Ice mixtures, which are
H2O- or CO-rich, may play an important role in preventing the NH2CHO
destruction by absorbing incoming photons. Moreover, UV irradiation of H2O ice
can also lead to the formation of mobile (nonthermal) H atoms that can diffuse
in the bulk ice and chemically convert photofragments, such as NHCHO or NH2CO,
back to NH2CHO. The shielding from further destruction explains the absence of
HNCO and OCN- in the H2O:CO:NH3 (10:5:1) experiment and enhances the NH2CHO
abundance as shown in Fig 5(a).

Figure 6: Proposed reaction diagram linking interstellar simple molecules and
the newly formed N-bearing COMs formed at 10 K under VUV irradiation of
H2O:CO:NH3 ice mixtures. The dashed boxes represent undetected intermediate
species.

##  4 Discussion

In this work, since there are two MgF2 windows between the ice sample and the
UV-photon source, the photon energy distribution is mainly characterized by
$\sim$160 nm ($\sim$79%) with a tiny contribution from Ly$\alpha$ ($\sim$6%)
as mentioned in the Experimental section. Therefore, the ionization of NH3 by
photons having an energy above 10 eV (i.e., $<$124 nm) is minutesimal compared
to the photodissociation of NH3. We only focus on the ice chemistry
originating from reactions between neutral radicals in solid state in the
following discussion. The possible ion/electron chemistry induced by
photoionization in binary ice containing NH3 is much more complex and has been
experimentally investigated by Demyk et al. (1998) and Hudson & Moore (2000).
On the other side, the direct photodissociation of CO is forbidden due to the
higher threshold energy required (i.e., 11.09 eV). However, CO has relatively
strong absorption cross-section values (up to $\sim$1.5$\times 10^{-17}$ cm2
photon-1) in the range of 127-157 nm coinciding with the molecular (D2/H2)
emission ($\sim$130-165 nm). These photons can excite some CO molecules into
the first electronically excited state (A${}^{1}\Pi$ $\leftarrow$
X${}^{1}\Sigma^{+}$; CO*), which carries excess energy of up to 7.9 eV (Lu et
al., 2005; Mason et al., 2006; Cruz-Diaz et al., 2014). In contrast, NH3, with
a much lower threshold energy of 4.4 eV, is expected to be efficiently
dissociated upon the impact of VUV photons resulting in amino radicals (NH2)
and H atoms via the dissociation reaction (Okabe & Lenzi, 1967):

$\text{NH${}_{3}$}\xrightarrow{\text{+h}\nu}\text{NH${}_{2}$ + H}.$ (3)

Therefore, in a CO:NH3 ice mixture, the consumption of CO is through either
the so-called nonthermal desorption following the photon energy transfer from
CO* to surface CO or chemical reactions with H atoms or NH2 in the ice. For
example, the interactions between CO and H atoms has been proposed to form HCO
via H-atom addition reactions (Watanabe & Kouchi, 2002):

$\text{CO + H}\rightarrow\text{HCO}.$ (4)

The newly formed HCO can immediately recombine with surrounding NH2 radicals,
resulting in NH2CHO via the radical-radical recombination:

$\text{HCO + NH${}_{2}$}\rightarrow\text{NH${}_{2}$CHO}.$ (5)

In addition, the successive hydrogenation of HCO could also take place,
forming H2CO:

$\text{HCO + H}\rightarrow\text{H${}_{2}$CO}.$ (6)

The two HCO consumption channels (i.e., reactions (5) and (6)) compete with
each other. However, reaction (5) can occur when HCO and NH2 radicals are
nearby, and reaction (6) requires additional H atoms from another NH3
photodissociation. Therefore, the formamide route is favored under the current
experimental conditions and the derived ratio of N(NH2CHO)/N(H2CO) is $>$3 at
steady state in all CO:NH3 experiments. For each NH3 dissociation event, one
may expect that, after CO hydrogenation (reaction (4)), NH2 is the only
radical near the newly formed HCO. The prompt recombination following reaction
(5) is expected without further diffusion. The second H atom requires the
photodissociation of another NH3 and additional diffusion. It is important to
note that radical-radical recombination reactions could also lead back to NH3
and CO through the direct H transfer (Enrique-Romero et al., 2019). However,
the repetitive NH3 dissociation followed by the CO hydrogenation effectively
favors the formation of formamide.

Besides interactions between radicals and ground-state molecules, a fraction
of electronically excited carbon monoxide (CO*) can directly react with NH2
radicals followed by H-atom recombination, providing an additional formation
pathway to NH2CHO through the reactions:

$\text{CO* + NH${}_{2}$}\rightarrow\text{NH${}_{2}$CO}.$ (7)

and

$\text{NH${}_{2}$CO + H}\rightarrow\text{NH${}_{2}$CHO}.$ (8)

A higher activation barrier reported for reaction (7) with respect to that of
reaction (4) (i.e., 15 kJ mol-1 versus 8 kJ mol-1, in the gas phase) can be
easily compensated by the absorbed VUV-photon energy. However, if NH2CO is
formed as an intermediate product, it is strongly expected to be converted
backward, forming HNCO (Noble et al., 2015; Haupa et al., 2019).

The chemistry of electronically excited CO with other strongly bound
molecules, such as H2 and N2 with high activation barriers has been
experimentally investigated in the solid state by Chuang et al. (2018) and
Martín-Doménech et al. (2020). The interactions between CO* and NH3 might also
be relevant in the present study. However, the presence of available CO* in
the ice mixture strongly depends on the impacting photon fluence and the time-
scale of energy dissipation in the bulk ice. Additionally, the direct proton
transfer from NH3 to CO could also contribute to the formation of NH2CHO,
given that the reaction barrier of 314.5 kJ mol-1 calculated in current work
for the isolated CO-NH3 complex at the b3lyp/6-311G+(d,p) level can be easily
overcome by the absorption of photons. Despite the nondetection of the
vibrational feature of the CO-NH3 complex ($\sim$2145 cm-1; Lundell et al.
1998), this formation route cannot be excluded.

The continued UV irradiation leads to further chemical modification of first-
generation products. For example, the photolysis of NH2CHO results in the HNCO
formation through the H2 loss channel (Lundell et al., 1998; Duvernay et al.,
2005):

$\text{NH${}_{2}$CHO}\xrightarrow{\text{+h}\nu}\text{HNCO + H${}_{2}$}.$ (9)

A very similar photodissociation of H2CO is also expected, enriching the
content of reactive radicals like HCO, H, or even CO. Moreover, the
dehydrogenation pathway through H-atom abstraction reactions has also been
reported via (Haupa et al., 2019; Suhasaria & Mennella, 2020)

$\text{NH${}_{2}$CHO}\xrightarrow{\text{+H(-H${}_{2}$)}}\text{NH${}_{2}$CO}\xrightarrow{\text{+H(-H${}_{2}$)}}\text{HNCO}.$
(10)

The product HNCO can react with the surrounding NH3 ice and convert into OCN-,
through the acid-base reaction:

$\text{HNCO + NH${}_{3}$}\leftrightarrow\text{[NH${}_{4}]^{+}$[OCN$]^{-}$}.$
(11)

This is supported by observing a delay of the OCN- formation shown in Fig. 3.
The efficient conversion from the neutral isocyanic acid (HNCO) to the cyanate
ion (OCN-) has been experimentally confirmed at 10 K (Jiménez-Escobar et al.,
2014).

These secondary products (e.g., HNCO and OCN-) are absent in the photolysis of
the H2O-rich ice mixture (H2O:CO:NH3=10:5:1). Instead, the abundance of NH2CHO
rapidly increases with photon fluence. NH2CHO, which is formed by either
reactions (4)-(5) or (7)-(8), is largely preserved in H2O ice instead of being
photodissociated into simpler products.

In addition to the formation of dehydrogenated species, H2CO could also be
hydrogenated into saturated molecules through successive addition reactions of
H atoms:

$\text{H${}_{2}$CO}\xrightarrow{\text{+2H}}\text{CH${}_{3}$OH}.$ (12)

However, hydrogenation of NH2CHO forming NH2CH2OH has been shown to be less
likely (Noble et al., 2015). Since aminomethanol is only tentatively observed
in the QMS-TPD data, its formation could originate from the thermal chemistry
(Duvernay et al., 2014):

$\text{NH${}_{3}$}+\text{H${}_{2}$CO}\rightarrow\text{NH${}_{2}$CH${}_{2}$OH}.$
(13)

As a consequence of the H2O dissociation, some OH radical association
reactions might take place as well. The interaction between OH radicals and CO
is expected to result in the intermediate product HOCO through a barrierless
reaction:

$\text{OH + CO}\rightarrow\text{HOCO}.$ (14)

The hydrogenation of HOCO leads to the formation of CO2 and HCOOH (Ioppolo et
al., 2011; Qasim et al., 2019):

$\text{HOCO + H}\rightarrow\text{CO${}_{2}$ + H${}_{2}$}$ (15a) and
$\text{HOCO + H}\rightarrow\text{HCOOH}.$ (15b)

Alternatively, HCOOH is also formed through the recombination of HCO with OH
radicals:

$\text{HCO + OH}\rightarrow\text{HCOOH}.$ (16)

A similar acid-base reaction has also been reported for a HCOOH ice mixed with
H2O- or NH3-rich environments, leading to ionic species (Schutte et al., 1999;
Hudson & Moore, 2000; Bergner et al., 2016):

$\text{HCOOH + H${}_{2}$O (or
NH${}_{3}$)}\rightarrow\text{[HCOO$]^{-}$[H${}_{3}$O]${}^{+}$ (or
[NH${}_{4}$]${}^{+}$)}.$ (17)

The proposed reactions induced by VUV irradiation of astronomically relevant
ice mixtures containing H2O, CO, and NH3 are summarized in Fig. 6.

##  5 Astrochemical Implications and Conclusions

The present laboratory study demonstrates the solid-state chemistry of forming
NH2CHO and its derivatives, including HNCO and [OCN]-[NH${}_{4}]^{+}$, by the
VUV photolysis of CO:NH3 ice mixtures at 10 K. We investigated the proposed
formation mechanism involving HCO radicals and NH2 radicals in two different
interstellar-relevant ice compositions in molecular clouds: H2O-rich and CO-
rich ice analogs. The parent molecules and newly formed products were
monitored in situ by IR spectroscopy as a function of photon fluence and
complementarily confirmed by mass spectrometry during the TPD experiments.
Besides the identifications of the first-generation products in both ice
scenarios, the further interactions between NH2CHO and energetic H atoms
triggered by continuous VUV irradiation result in hydrogen-poor HNCO (or OCN-
when reacting with NH3). The kinetic analysis of N-bearing products offers a
hint at the solid-state chemical network from the most abundant species (H2O,
CO, CO2, and NH3) after energetic processing in molecular clouds before
evaporation.

In the early stage of molecular clouds (1$\leq A_{V}\leq$5), the gas density
increases to 104 cm-3 leading to a decrease of the cloud temperature down to
$\sim$20 K. Atomic reactions are considered to dominate the surface chemistry
on submicron-sized interstellar dust grains. For example, in translucent
clouds (A${}_{V}\geq$3), H atoms and O atoms diffuse on the surfaces of dust
grains, forming H2O ice via the reactions
O$\xrightarrow{\text{+H}}$OH$\xrightarrow{\text{+H}}$H2O. This occurs along
with other H-atom addition reactions to N and C atoms, leading to NH3 and CH4,
respectively (Hidaka et al., 2011; Fedoseev et al., 2015; Qasim et al., 2020).
In the meantime, a fraction of gaseous CO is also expected to participate in
the first H2O-rich ice chemistry, leading to CO2 formation (Ioppolo et al.,
2011). As the gas density continuously increases in dense clouds, the
temperature of dust grains drops further(i.e., there is a negative correlation
between AV and temperature; Hocuk et al. 2017), allowing more CO to condense
onto the previous H2O-rich ice layer. This is the so-called “CO
catastrophically freeze-out stage,” forming a second CO-rich ice layer
(Boogert et al., 2015). With the significantly increased lifetime of H atoms
on surfaces, the H-atom addition reactions play an essential role in
converting inorganic CO into (complex) organic species, such as aldehydes and
alcohols (Watanabe & Kouchi, 2002; Fuchs et al., 2009; Chuang et al., 2016,
2017; Fedoseev et al., 2017; Simons et al., 2020). The accumulation of
interstellar ice is expected to take place within 105 yr, based on theoretical
estimations (i.e., 109/n${}_{\text{(H)}}$, where n${}_{\text{(H)}}$=104 cm-3;
see Willacy & Millar 1998). Besides the surface chemistry dominated by
atom(radical)-molecule reactions, the cosmic rays (CR) and the secondary UV
photons induced by CR interacting with H2 have been proven to act as efficient
chemical triggers to facilitate the molecular complexity in the ice mantles in
the remaining 105-107 yr, as the typical lifetime of molecular cloud has been
estimated up to $\sim$107 yr (Chevance et al., 2020). Considering higher
penetration depths of UV or CRs than the thickness of ice mantles, in contrast
to atoms, these energetic sources can efficiently trigger the bulk ice
chemistry involving the preserved molecules in the deep ice mantles, both in
H2O-rich and CO-rich ice layers (Cruz-Diaz et al., 2014; Cruz Diaz et al.,
2014).

The investigated photochemistry of ices leading to N-bearing organic species
is expected to take place in different astronomically relevant ice layers. The
applied UV fluence is comparable to the accumulated UV photons in molecular
clouds, i.e., 3$\times$1017-18 photons cm-2 assuming a UV flux of 103-4
photons cm-2s-1 and a cloud lifetime of 107 yr (Prasad & Tarafdar, 1983;
Cecchi-Pestellini & Aiello, 1992; Shen et al., 2004). In the H2O-rich ice
mixture, the interactions between CO and H2O photofragments (e.g., OH radicals
and H atoms) mainly contributed to the formation of C-bearing species,
providing additional chemical routes to enrich the abundance of CO2, H2CO,
HCOOH/HCOO-, HCO, and HOCO in H2O ices besides (non-energetic) atom addition
reactions. The only N-bearing species formed in the ice was NH2CHO. HNCO (or
OCN-) was not observed, suggesting that the newly formed NH2CHO was most
likely protected in the H2O-rich environment. The experimental findings
demonstrate that the NH2CHO formation pathway proposed for the photolysis of
CO:NH3 ice mixtures is still valid in interstellar H2O-rich ice layers.
Apparently, the binding/diffusing energy of radicals in H2O-rich ice can be
overcome in the UV-photon induced ice chemistry. Moreover, this reaction
channel takes place simultaneously with other reactions, such as CO+2H
$\rightarrow$ H2CO and CO+OH $\rightarrow$ HOCO induced by VUV-photons. In the
interstellar CO-rich ice layer, the UV-photon induced production of C-bearing
species is limited due to the scarcity of H2O photofragments. CO2 and H2CO are
minor products. Their precursor species, such as HCO and HOCO, are trapped in
CO ice. The N-bearing products include NH2CHO, HNCO, and OCN-.

In astronomical observations, the vibrational feature of OCN- (4.64 $\mu$m)
was first detected in the solid phase by Lacy et al. (1984) and later
identified by Demyk et al. (1998). The correlation between HNCO and OCN- has
been established (Hudson & Moore, 2000; Jiménez-Escobar et al., 2014). The
present laboratory study also supports the chemical link between these two
species at low temperatures without thermal processing. The absence of HNCO in
the interstellar H2O-rich ice probably implies fast acid-base reactions
involving a proton transfer, which may not be limited to NH3. An attempt to
search for the solid-state IR absorption features of NH2CHO has been reported
by Schutte et al. (1999) and Raunier et al. (2004), but only resulted in a
tentative identification. However, NH2CHO and its chemical derivative HNCO
have been detected simultaneously in cometary material, which is believed to
inherit the most pristine ice species in molecular clouds (Biver et al., 2014;
Altwegg et al., 2017). Future observation of N-bearing organic molecules,
including NH2CHO and its (de-)hydrogenated species (e.g., HNCO and NH2CH2OH)
in interstellar ice mantles using the JWST is desired in order to reveal the
full chemical fingerprints of N-bearing species in star-forming regions.
Astronomical observations show a strong abundance correlation between HNCO and
NH2CHO in star-forming regions (see Figure 1 in López-Sepulcre et al. 2019).
The chemical transformation from NH2CHO to HNCO has been found in this work.
However, the derived ratio of NH2CHO/HNCO $>$ 1, which is in contradiction to
observations, suggests that there are additional destruction routes, such as
H-atom abstraction, high-energetic photons, and cosmic ray dissociation,
influencing products’ degree of hydrogenation. This work acts as a case study
on the UV photolysis of interstellar ice analogs containing CO:NH3 in an
H2O-rich environment. A systematic comparison of the conversion rate between
NH2CHO and HNCO induced by different (non-)energetic sources is highly desired
to better understand the full interstellar ice evolution.

In conclusion, based on the experimental results on the photolysis chemistry
of CO:NH3 ice mixtures in H2O-rich and CO-rich ice analogs, the main findings
of this work can be summarized as below:

  1. 1.

VUV irradiation of CO:NH3 ice mixtures in H2O-rich and CO-rich environments at
10 K shows an efficient formation pathway leading to N-bearing molecules
(NH2CHO, HNCO, and OCN-) under molecular cloud conditions.

  2. 2.

The formation kinetics of NH2CHO in H2O-rich and CO-rich ice are derived (Fig.
5). It shows a strong dependency on the initial ice compositions. The highest
production yield of NH2CHO is observed in the H2O-rich ice mixture.

  3. 3.

NH2CHO is reported as a first-generation product of CO:NH3 ice mixtures upon
VUV irradiation. The continuous photon impact or the photon-induced formation
of free (energetic) H atoms from H2O and NH3 can further influence the final
product composition including the formation of HNCO and OCN-.

Th.H. acknowledges support from the European Research Council under the
Horizon 2020 Framework Program via the ERC Advanced Grant Origins 83 24 28.
Authors thank the Leiden Ice Database for providing the solid-state IR
spectrum of formamide.

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