11institutetext: 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
22institutetext: Laboratory for Astrophysics, Leiden Observatory, Leiden
University, P.O. Box 9513, NL-2300 RA Leiden, the Netherlands. 22email:
<EMAIL_ADDRESS>33institutetext: Max Planck Institute for Astronomy,
Königstuhl 17, 69117 Heidelberg, Germany

# Formation of N–bearing complex organic molecules in molecular clouds:
Ketenimine, acetonitrile, acetaldimine, and vinylamine via the UV photolysis
of C2H2 ice

K.-J. Chuang 1122 C. Jäger 11 J. C. Santos 22 Th. Henning 33

###### Abstract

Context. The solid-state C2H2 chemistry in interstellar H2O-rich ice has been
proposed to explain astronomically observed complex organic molecules (COMs),
including ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH),
toward early star-forming regions. This formation mechanism is supported by
recent laboratory studies and theoretical calculations for the reactions of
C2H2+OH/H. However, the analog reaction of C2H2+NH2 forming N-bearing species
has been suggested to have a relatively low rate constant that is orders of
magnitude lower than the value of C2H2+OH.

Aims. This work extends our previous laboratory studies on O-bearing COM
formation to investigate the interactions between C2H2 and NH3 ice triggered
by cosmic ray-induced secondary UV photons under molecular cloud conditions.

Methods. Experiments were performed in an ultra-high vacuum chamber to
investigate the UV photolysis of the C2H2:NH3 ice mixture at 10 K. The ongoing
chemistry was monitored in situ by Fourier-transform infrared spectroscopy as
a function of photon fluence. The IR spectral identification of the newly
formed N-bearing products was further secured by a quadrupole mass
spectrometer during the temperature-programmed desorption experiment.

Results. The studied ice chemistry of C2H2 with NH2 radicals and H atoms
resulting from the UV photodissociation of NH3 leads to the formation of
several N-bearing COMs, including vinylamine (CH2CHNH2), acetaldimine
(CH3CHNH), acetonitrile (CH3CN), ketenimine (CH2CNH), and tentatively
ethylamine (CH3CH2NH2). The experimental results show an immediate and
abundant CH2CHNH2 yield as the first-generation product, which is further
converted into other chemical derivatives. The effective destruction and
formation cross-section values of parent species and COMs were derived, and we
discuss the chemical links among these molecules and their astronomical
relevance.

##  1 Introduction

Astronomical observations have revealed a rich molecular inventory in star-
forming regions, including simple volatiles (such as H2O, CO, CH4, NH3) and
complex organics consisting of molecules with six or more atoms (Herbst & van
Dishoeck, 2009; Boogert et al., 2015; McGuire, 2018). These interstellar
molecules have been proposed to be further incorporated into part of
planetesimals or cometesimals, ultimately enriching the molecular complexity
in planetary systems. The (partial) linear correlation between molecules
detected in the protostar IRAS 16923-2422 and the cometary material discovered
by the Rosetta Mission toward the comet 67P/C-G supports the concept of the
inherited molecular evolution (Drozdovskaya et al., 2019). For example,
oxygen–bearing complex organic molecules (COMs) have been unambiguously found
toward high- and low-mass protostars, and even in several prestellar stages,
such as B1-b, L1689B, and B5 (Bisschop et al., 2008; Öberg et al., 2010;
Bacmann et al., 2012; Cernicharo et al., 2012; Taquet et al., 2017). Among
them, the simultaneous detection of O–bearing COMs characterized by a degree
of hydrogenation, such as C2HnO2 (e.g., glycolaldehyde and ethylene glycol)
and C2HnO (e.g., ketene, acetaldehyde, and ethanol) hint at their solid-state
chemical history through hydrogen addition reactions on grain surfaces
(Jørgensen et al., 2012, 2016; van Gelder et al., 2020). Besides O–bearing
COMs, nitrogen-containing organics (i.e., Hn-NC-R) are also considered
essential ingredients in the formation of prebiotic compounds such as amino
acids and nucleobases. Several N–bearing COMs expressed by the formula C2HnN
(n=3 and 5), including vinylamine (CH2CHNH2), acetaldimine (CH3CHNH),
ketenimine (CH2CNH), and acetonitrile (CH3CN) have been identified in giant
molecular clouds toward Sgr B2 (Lovas et al., 2006; Loomis et al., 2013; Thiel
et al., 2017; Zeng et al., 2021). However, the fully saturated species
ethylamine (CH3CH2NH2) remains a tentative detection (Zeng et al., 2021).

Table 1: Summary of band strength values used in this work. Species | Chemical formula | IR peak position | Band strength value | References | Note
---|---|---|---|---|---
|  | (cm-1) | (cm molecule-1) |  |
Acetylene | C2H2 | 810 | 2.40$\times$10-17 | Hudson et al. (2014) | exp.
Ammonia | NH3 | 1067 | 1.95$\times$10-17 | Hudson et al. (2022) | exp.
Hydrogen cyanide | HCN | 2107 | 1.03$\times$10-17 | Gerakines et al. (2022) | exp.
Cyanid anion | CN- | 2081 | 3.70$\times$10-17 | Georgieva & Velcheva (2006) | cal.
Acetonitrile | CH3CN | 2253 | 2.20$\times$10-18 | Hudson & Moore (2004) | exp.
Ketenimine | CH2CNH | 2054 | 7.20$\times$10-17 | Hudson & Moore (2004) | est.
Vinylamine | CH2CHNH2 | 1302 | 1.20$\times$10-18 | Lammertsma & Prasad (1994) | cal.
Acetaldeimine | CH3CHNH | 1403 | 2.80$\times$10-18 | Lammertsma & Prasad (1994) | cal.

Several gas-phase and solid-phase formation pathways have been proposed to
explain the above observations, particularly for O–bearing COMs, either
through “bottom-up” (starting with CO or C atoms) or “top-down” (starting with
fragments of carbonaceous species) routes (Tielens, 2013). In the solid-state
scenario, atomic species or simple molecules accrete onto cold dust grains in
molecular clouds, facilitating the surface association to form larger
molecules. For example, CO hydrogenation not only results in H2CO and CH3OH
but also offers key radicals, namely HCO, CH2OH, and CH3O for O–bearing COM
formation, such as CH2OHCHO and CH2OHCH2OH, through the radical-radical
reaction mechanism (Butscher et al., 2015; Fedoseev et al., 2017; Chuang et
al., 2016, 2017; Simons et al., 2020; He et al., 2022; Tsuge & Watanabe,
2023). The intermediate species also react with other radicals or participant
gas-phase reactions through non-thermal desorption, leading to heterogeneous
COMs (Vazart et al., 2020). On the other hand, highly unsaturated hydrocarbon
alkynes, such as C2H2, which could originate from PAH photodissociation, gas-
phase production, or even sublimation of carbonaceous materials, can also
associate with OH/H radicals to form O–bearing COMs. The solid-state reactions
based on the –C$\equiv$C– molecular backbone have been investigated in
theoretical and experimental works (Tielens, 1992; Hudson & Moore, 1997;
Hiraoka et al., 2000; Wu et al., 2002; Kobayashi et al., 2017). Recently,
Molpeceres & Rivilla (2022) systematically studied the interactions of C2Hn
(n=2, 4, and 6) with several radicals, including H, OH, NH2, and CH3, and
reported the prevalence of OH–bearing products (e.g., C2H2OH and C2H4OH) on
grain surfaces. In addition, the association of C2H with H2O has been proposed
to form C2H2OH through a barrierless reaction (Perrero et al., 2022).

Our previous laboratory studies reported on the solid-state COM formation
based on C2H2 ice under molecular cloud conditions through non-energetic
(i.e., OH/H addition reactions) and energetic processing (i.e., proton
radiolysis of H2O) (Chuang et al., 2020, 2021). The direct attachment of OH
radicals to C2H2 has been suggested to efficiently form the isomers vinyl
alcohol (CH2CHOH) and acetaldehyde (CH3CHO). These newly formed O–bearing COMs
could further react with H atoms on dust grains, leading to ketene (H2CCO) and
ethanol (CH3CH2OH) through hydrogen abstraction and addition reactions,
respectively. The experimental findings align with the theoretical work of
Molpeceres and Rivilla (2022), which shows a relatively low activation barrier
of 2.39 kcal mol-1 for the reaction C2H2+OH$\rightarrow$C2H2OH. Nevertheless,
this theoretical work reports that the contribution of the analog reactions of
C2H2 with NH2 and CH3 radicals are minor (see Figure 1 in Molpeceres & Rivilla
2022). The calculated activation barrier of C2H2+NH2$\rightarrow$C2H2NH2 is
only somewhat high (i.e., 6.93 kcal mol-1), but the reaction rate constant
(i.e., $\sim$10-6 s-1) in the solid state is several orders of magnitude lower
than those for the reactions C2H2+OH$\rightarrow$C2H2OH and
C2H2+H$\rightarrow$C2H3. It is even less possible for the reaction of
C2H2+CH3$\rightarrow$C2H2CH3 (i.e., $\sim$10-13 s-1). This leads toward the
question of whether the so-called top-down mechanism based on C2H2 ice can
lead to N–bearing COMs on dust grains like its O–bearing counterpart. In
laboratory studies, a 5-keV electron beam has been used to simulate the cosmic
ray-induced ice chemistry of C2H2 and NH3 ice, showing the formation of C2H3N
isomers, such as ethynamine (HCCNH2) and 2H-azirine(c-H2CCHN), and CH2CHNH2 at
$\sim$5 and 10 K, respectively (Turner et al., 2021; Zhang et al., 2023).
Volosatova et al. (2022) reported the high-resolution IR spectroscopy of
C2H2:NH3 in noble gas (Ar, Kr, or Xe) mixtures at 5 K and identified the
CN–containing species (e.g., CH2CNH, CH3NC, and CH2NCH) upon X-ray irradiation
of the C2H2$\cdots$NH3 complex. Moreover, Canta et al. (2023) explored the
vacuum ultraviolet (VUV) irradiation of C2H2 ices mixed with NH3 and found
that CH3CHNH accounts for most of the depleted NH3 as well as a small amount
of CH3CN.

In this work, we provide an experimental follow-up study on the interactions
of C2H2 with NH2 radicals and H atoms, which are formed upon VUV
photodissociation of NH3 ice, investigated at 10 K. In the interstellar ice,
NH3 is suggested to form through successive H–atom addition reactions to N
atoms (i.e., N$\rightarrow$NH$\rightarrow$NH2$\rightarrow$NH3) along with H2O
in the translucent clouds (Fedoseev et al., 2015). Therefore, the studied
reactions can be triggered during the accretion of the atomic species N and H
onto grain surfaces or by VUV photons penetrating the interstellar H2O-rich
ice containing NH3 in molecular clouds. This work explores the relevant solid-
state chemistry for the formation of N–bearing COMs on dust grains and reports
on the possible chemical pathway. The experimental details are described in
the next section. The results and discussion are presented in Sections 3 and
4, respectively. Finally, Section 5 focuses on the astronomical relevance of
the experimental data and provides our conclusions.

##  2 Experiment

All experiments were performed using the ultra-high vacuum (UHV) apparatus at
the Laboratory Astrophysics and Cluster Physics in Jena. The details of the
experimental setups and the latest characteristics of the applied UV-photon
source (D2 lamp) have been described in previous studies (Potapov et al.,
2019; Chuang et al., 2022). Here, only the relevant information of this
laboratory work is briefly presented. A KBr substrate is mounted on a copper
sample holder connected to a closed-cycle helium cryostat through indium foil
and located at the center of the UHV chamber. The base pressure is
¡1$\times$10-9 mbar at room temperature. The substrate temperature was read
out by silicon diodes with ¡0.5 K accuracy and regulated between 10 and 330 K
through a Lakeshore temperature controller equipped with a resistive heater. A
commercial D2 lamp (Hamamatsu: D2H2–L11798), a MgF2-sealed closed cycle light
source, was used to generate VUV photons, which were further guided to the ice
sample through another MgF2 UHV-viewport installed on the chamber. The
spectrum of the VUV photons is mainly characterized by broad emission centered
at $\sim$160 nm, with negligible Ly-$\alpha$ at 121.6 nm due to the strong
MgF2 absorption at a lower wavelength. The UV flux of $\sim$7.6$\times$1013
photon cm-2s-1 was estimated in Chuang et al. (2022) and agreed with values in
the literature using a similar experimental setting (Martín-Doménech et al.,
2020).

In this paragraph, we briefly describe the experimental procedure. Gaseous
C2H2 (Air Liquide; 99.6%) and NH3 (Air Liquide, 99.999%) were simultaneously
introduced into the main chamber through two separate all-metal leak valves
and deposited directly onto the pre-cooled substrate at 10 K. The deposition
and the photolysis of ice samples were monitored in situ by Fourier-transform
infrared spectroscopy (FTIR) in transmission mode in a range of 400-7500 cm-1
with 1 cm-1 resolution. The abundances of parent molecules and products in
column density (N, in molecule centimeter-2) were obtained using the modified
Beer-Lambert law (Chuang, 2018). The absorption peak area was corrected by
straight baseline subtraction and derived through Gaussian curve fitting,
accompanied with one standard deviation as the error bars and/or direct
integration of the IR feature. This estimation does not take into account the
uncertainties due to the baseline subtraction and IR absorption band strength
(A’) of pure ices acquired from the literature. Theoretical band strength
values were used for molecules such as vinylamine, acetaldeimine, ketenimine,
and cyanide anion, which have no experimental value available. The applied
band strength for the selected species is summarized in Table 1. The ratios of
the studied ice mixture are C2H2:NH3=1:2.7, 1:1.4, 1:0.5, and 1:0.2, and the
absolute abundance of the C2H2 ice is $\sim$(6-10)$\times$1016 molecule cm-2.
These reported column densities and ratios can be recalibrated when more
precise values become available.

After UV-photon irradiation of the studied ice sample, a temperature-
programmed desorption (TPD) experiment with a ramping rate of 5 K min-1 was
performed. The quadrupole mass spectrometer recorded the sublimated species’
mass signals as a function of temperature. The data of desorption temperature
and the corresponding fragmentation pattern induced by the electron impact
ionization at 70 eV were utilized to identify the newly formed products in
addition to IR spectroscopy.

##  3 Results

Figure 1: Infrared spectra of the studied interstellar ice analogs. Upper: IR
spectra obtained after UV irradiation of the C2H2:NH3 (1:1.4) ice mixture at
10 K for a fluence of 5.7$\times$1017 photon cm-2. The column densities of
C2H2 and NH3 in the ice mixture are 8.1$\times$1016 and 1.1$\times$10-17
molecule cm-2, respectively. Bottom: Infrared spectra obtained after UV
irradiation of pure ice NH3 and C2H2 at 10 K for a fluence of 5.7$\times$1017
photon cm-2 and offset for clarity. The column densities of pure C2H2 and NH3
ice are 7.3$\times$10-17 and 1.1$\times$10-17 molecule cm-2, respectively. The
shaded area is present for the IR spectra before UV irradiation. The IR peaks
of the newly formed products are labeled with an arrow.

Figure 1 presents the IR absorption spectra obtained after the UV irradiation
of the C2H2:NH3 (1:1.4) ice mixture (upper panel) and pure NH3 as well as the
C2H2 ice (bottom panel) at 10 K for a photon fluence of 5.7$\times$1017 photon
cm-2. The shaded IR spectra obtained before the UV irradiation are presented
for comparison; the newly formed IR features are shown in an unshaded area.
The absorption peaks of the parent C2H2 are observed at $\sim$810 cm-1
($\nu$5; CH bending), $\sim$1390 cm-1 ($\nu$4+$\nu$5; combination), and 1950
cm-1 (C2H2-relative band), and the peaks of NH3 are found at $\sim$1067 cm-1
($\nu$2; NH sym. deform) and 1625 ($\nu$4; NH deg. deform) cm-1. The tiny peak
of acetone at 1710 cm-1, a common contamination source in an acetylene gas
bottle, is negligible; the ratio of N(CH3COCH3) to N(C2H2) is ¡0.1%. It is
noted that C2H2 IR features are different in pure and mixed ices with NH3,
indicating that the C2H2 spectral profiles (e.g., full width half maximum and
peak position) are sensitive to the surrounding environment. The C2H2:NH3
absorption spectra with different ratios are reported in the Appendix (Figure
8) for future comparisons with experimental and observational data.

Figure 2: Difference IR spectra obtained after UV irradiation of C2H2:NH3 ice
mixtures with ratios of (a) 1:0, (b) 1:0.2, (c) 1:0.5, (d) 1:1.4, and (e)
1:2.7 at 10 K for a fluence of 5.7$\times$1017 photon cm-2. Infrared spectra
are offset for clarity. The shaded area marks the absorption features of the
parent species (i.e., blue is for C2H2 and red is for NH3) now visible as
negative bands.

In the upper panel of Fig. 1, several IR peaks are exclusively detected after
UV photolysis of the C2H2:NH3 (1:1.4) ice mixture in addition to the
absorption features of hydrocarbons (e.g., C2H4, C4H2, and C4H4) and
NH2/NH${}_{4}^{+}$, which are also observed after UV irradiation of individual
C2H2 and NH3 ice, respectively (see the bottom panel). Therefore, these IR
peaks must represent the newly formed products containing CN bonds. In the
2300-2000 cm-1 region, where CN stretching modes are present, the absorption
peak at 2253 cm-1 has been assigned to acetonitrile CH3CN ($\nu$2). This is
the strongest absorption feature, and its band strength is only
2.2$\times$10-18 cm molecule-1, which is an order of magnitude less than that
of common interstellar molecules (Hatch et al., 1992; Hudson & Moore, 2004;
Hattori et al., 2005; Abdulgalil et al., 2012). The rest of the CH3CN IR
features are most likely blended with other species due to their weak
absorption coefficient. This detection is in agreement with Canta et al.
(2023), who have reported the formation of CH3CN upon the VUV irradiation of
C2H2:NH3 = 1:1 ice mixtures. Furthermore, multiple overlapping peaks at 2107
cm-1, 2081 cm-1, and 2054 cm-1 have been successfully deconvoluted using
Gaussian curve fitting and assigned to hydrogen cyanide (HCN, $\nu$3; CN
stretching mode), cyanide anion (CN-, $\nu$3; CN stretching mode), and
ketenimine (CH2CNH, $\nu$3; C=C=N stretching mode), respectively (Jacox, 1979;
Mungan et al., 1991; Ito & Nakanaga, 2010; Gerakines et al., 2022). The
detailed IR identification of HCN and CN- refers to the early work by
Gerakines et al. (2004). Besides these (highly) unsaturated CN–bearing
products (e.g., containing the moieties C-C$\equiv$N/C=C=N), the IR features
of C2H5N isomers, for example, vinylamine (CH2CHNH2) and acetaldimine
(CH3CHNH), are found at 1302, 1283, 996 cm-1 in Fig. 1, while other
characteristic peaks are blended with the parent NH3 and C2H2 features.
Acetaldimine has also been detected in previous VUV-irradiation works on
C2H2:NH3 ice mixtures (Canta et al., 2023). It is important to note that
CH2CHNH2 and CH3CHNH are tautomers (interconverted chemical structure; amine
$\leftrightarrow$ imine). Aziridine (c-C2H4NH), one of the C2H5N isomers with
the highest chemical potential energy (i.e., $\sim$49 and $\sim$43 kcal mol-1
higher than CH3CHNH and CH2CHNH2, respectively), is absent (Stolkin et al.,
1977).

Figure 3: Infrared spectrum obtained at the interval of 130-160 K during the
TPD experiment of the UV-irradiated ice mixture C2H2:NH3 (1:1.4) at 10 K for a
fluence of 5.7$\times$1017 photon cm-2.

The newly formed molecules are expected to result from the association of
several intermediate species upon UV photolysis of the C2H2:NH3 ice mixture.
Therefore, the intensity of these designated IR peaks should be correlated
with the concentration of C2H2 and/or NH3. Figure 2 shows the IR difference
spectra obtained after UV irradiation of C2H2:NH3 ice mixtures with different
ratios, including (a)1:0, (b) 1:0.2, (c) 1:0.5, (d) 1:1.4, and (e) 1:2.7 at 10
K for a fluence of 5.7$\times$1017 photon cm-2. The parent and product
features appear as negative and positive bands in the difference spectra,
respectively. For example, the negative peaks of C2H2 are found at $\sim$1950,
$\sim$1390, and $\sim$810 cm-1, and NH3 peaks are shown at $\sim$1626 and
$\sim$1067 cm-1. In spectrum (a) of Fig 2, the detection of C4H4 ($\sim$1940,
$\sim$977, and $\sim$935 cm-1) and C4H2 ($\sim$2133, $\sim$2138, and $\sim$857
cm-1) is in line with previous studies on photolysis (and radiolysis) of pure
C2H2 (Compagnini et al., 2009; Abplanalp & Kaiser, 2020; Lo et al., 2020;
Pereira et al., 2020). A tiny C2H4 peak can be seen at 961 cm-1 and is due to
a limited amount of H atoms for the successive H-addition reactions
C2H2$\rightarrow$C2H3$\rightarrow$C2H4. However, in the spectra (b)-(e) of
Fig. 2, C2H4 IR features at 961 and 1438 cm-1 become more dominant with the
increasing concentration of NH3 in C2H2:NH3 ice mixtures. This is because the
photodissociation of NH3 mainly results in NH2 ($\tilde{A}$ or $\tilde{X}$)
radicals and H atoms (Evans et al., 2012). The newly generated H atoms enhance
the C2H2 hydrogenation forming C2H4, while C2H6 remains absent. In contrast,
C4H4 and C4H2 features become less dominant than other products. Besides the
hydrocarbon formation, the photofragment NH2 could react with C2H2 or its
hydrogenated species, C2Hn, forming N–bearing COMs. The IR spectral
assignments of products containing CCN bonds are supported by observing their
characteristic peaks, which increase with increasing NH3 concentration. For
example, in the spectrum (e) with the highest NH3 concentration
(C2H2:NH3=1:2.7), the IR features of CH3CN (2253 cm-1), HCN (2107 cm-1), CN-
(2081 cm-1), and CH2CNH (2054 cm-1) are the most abundant. The IR features of
CH2CHNH2 at 1302, 1283, and 996 cm-1 and of CH3CHNH at 1367 cm-1 are also
enhanced, but their strongest peaks at $\sim$1660 cm-1 overlap strongly with
the negative feature of NH3 at 1626 cm-1.

Given that the desorption temperatures of the newly formed N–bearing COMs are
(much) higher than those of the parent C2H2 ($\sim$80 K) and NH3 ($\sim$100
K), the corresponding IR features of CH2CHNH2 and CH3CHNH can be revealed
after the sublimation of the parent species. Figure 3 shows the IR difference
spectrum obtained between 130 and 160 K, where CH2CHNH2 and CH3CHNH are
expected to desorb under the current experimental conditions. As pure ice
spectra of these species are still lacking, the following assignment of
vibrational modes mainly relies on theoretical studies and matrix isolation
spectroscopy. In Fig. 3, the strongest absorption feature is at $\sim$1660
cm-1, which is due to vibrational transitions of CH2CHNH2 ($\nu$6; C=C
stretching mode) and CH3CHNH ($\nu$5; C=N stretching mode) (Stolkin et al.,
1977; Hamada et al., 1984; Hashiguchi et al., 1984; Lammertsma & Prasad,
1994). A broad feature at $\sim$1070 cm-1 also has dual contributions from the
NH2 rocking mode ($\nu$11) of CH2CHNH2 and the CH3 wagging mode ($\nu$10) of
CH3CHNH2. Several isolated peaks of CH2CHNH2 were clearly detected at
$\sim$1600 ($\nu$7; NH2 deformation mode), 1302 ($\nu$9; CH bending mode),
1280 ($\nu$10; CN stretching mode), 986 ($\nu$12; CH waging mode), and
$\sim$810 ($\nu$14; NH2 wagging) cm-1 (Hamada et al., 1984; Lammertsma &
Prasad, 1994; McNaughton & Evans, 1999). The unblended IR features of CH3CHNH
are observed at 1435/1410 ($\nu$14; CH3 deformation modes), 1367/1231 ($\nu$8;
trans-/cis-mixed modes), and 1112 ($\nu$9; CH deformation mode) cm-1 (Stolkin
et al., 1977; Hashiguchi et al., 1984). The presented IR spectrum at the
interval of 130-160 K complementarily supports the previous N–bearing COM
identification at 10 K, as shown in Fig. 1. Moreover, it provides a
temperature domain to ensure the spectral identification.

Figure 4: Derivatives of the species’ absorbance area during the TPD
experiment are presented as a function of temperature.

Similarly, during the TPD experiment, the absorption peak intensities (i.e.,
absorbance area) of the assigned species, including C2H4 (960 cm-1), HCN (2107
cm-1), CH3CN (2252 cm-1), CH2CNH (2053 cm-1), CH3CHNH (1367 cm-1), and
CH2CHNH2 ($\sim$810 cm-1), were monitored as a function of the substrate
temperature. Each obtained data point spans over $\sim$5 K due to the FTIR
measurement time (i.e., 60 seconds). An overview of the IR absorbance area of
the selected species during the TPD experiment is reported in the Appendix
(see Figure 9). Moreover, their derivatives of absorbance area as a function
of temperature are reported in Fig. 4, pointing to the desorption efficiency
of the selected species. For example, in Fig. 4, the C2H3N isomers, namely
CH2CNH and CH3CN, desorb at quite different temperatures (i.e., $\sim$85 and
$\sim$135 K, respectively; left panel), and HCN desorbs mainly at $\sim$120 K
(middle panel). The desorption peaks of the C2H5N isomers, namely CH3CHNH and
CH2CHNH2, are at $\sim$130 K and $\sim$135 K, respectively (right panel). Due
to the low resolution of the temperature readout for IR data, the desorption
peaks of CH3CHNH and CH2CHNH2 are challenging to pinpoint.

Figure 5: Species identification in QMS-TPD data. Left: Desorption signals of
interested masses obtained during the TPD experiment after UV irradiation of
C2H2:NH3 (1:1.4) for a fluence of 5.7$\times$1017 photon cm-2 at 10 K. The ion
signals have been shifted for clarity. The dashed lines indicate the
desorption peak of the assigned molecules. Right: Comparison of the obtained
mass fragmentation pattern at 136.4, 137.3/137.7, and 155.2 K with standard
data for CH3CN (NIST), CH2CHNH2 (SpectraBase), and CH3CH2NH2 (NIST).

The left panel of Fig. 5 shows the desorption mass signals obtained by QMS
during the TPD experiment after the UV irradiation of the ice mixture C2H2:NH3
(1:1.4) for a fluence of 5.7$\times$1017 photon cm-2 at 10 K. Since QMS has a
fast-scanning cycle (approximately every five seconds), each data point only
spans over $\sim$0.4 K, giving a better temperature resolution. The prominent
desorption peaks of CH3CN, for example, 40 (C2H2N+) and 41 (C2H3N+) m/z, are
clearly shown at $\sim$136.4 K, which is in line with the IR data. The
desorption peaks of 15 (CH3+/NH+) and 28 (CH2N+) m/z centered at $\sim$137.3 K
as well as 42 (C2H4N+) and 43 (C2H5N+) m/z centered at $\sim$137.7 K are most
likely from two C2H5N isomers. Although the desorption temperature of the
above mass signals is very close to CH3CN, the selected mass signals are
exclusively from C2H5N isomers due to the higher molecular masses. At higher
temperatures, we found two main desorption mass signals of 44 (C2H6N+) and 45
(C2H7N+) m/z at 155.2 K, which can be attributed to CH3CH2NH2. In the right
panel of Fig. 5, the identification of QMS-TPD data is carefully confirmed by
comparing these molecular-relevant mass signals with the species’ standard
fragmentation pattern induced by electron impact. The data of CH3CN and
CH3CH2NH2 are from the NIST database,111https://doi.org/10.18434/T4D303 and
CH2CHNH2 is from the SpectralBase.222https://spectrabase.com/ It is important
to note that the reported CH2CHNH2 fragment spectrum has no visible
contribution from 28 and 15 m/z mass signals. However, Vinogradoff et al.
(2012) showed that the primary fragment mass of CH3CHNH is 28 m/z (CH2N+) due
to the loss of the CH3 fragment (15 m/z), while mass signals of 42 and 43 m/z
only account for ¡10%. Therefore, the 28 and 15 m/z mass signals detected at
$\sim$137.3 K are expected to originate primarily from the CH3CHNH. The
desorption temperatures obtained from IR-TPD and QMS-TPD data, including
parent and product species, are summarized in Fig. 6. A linear fit to the data
set shows an excellent agreement (i.e., the fit slope of $\sim$0.99$\pm$0.01)
between two independent detection methods, complementarily identifying these
N–bearing products.

Figure 6: Desorption temperatures of selected species obtained from IR and QMS
methods during the TPD experiment. The dashed line represents a one-to-one
correlation, and the solid line (red) shows the linear fitting result. The
vertical bar indicates the desorption range in the IR-TPD data.

##  4 Discussion

Figure 7: Kinetic evolution of (a) parent and (b) products derived from UV
photolysis of the C2H2:NH3 (1:1.4) ice mixture at 10 K over a fluence of
1.1$\times$1018 photon cm-2. The dashed lines present the fitting results, and
the solid lines connecting data are only for clarity.

Figure 7 shows the temporal evolution of the parent and newly formed products
obtained in the UV photolysis of a C2H2:NH3 (1:1.4) ice mixture at 10 K over a
fluence of 1.1$\times$1018 photon cm-2. The molecular abundances are derived
from their corresponding IR absorption features. The obtained products’
kinetic evolution in the UV-photolyzed C2H2:NH3 ice mixture shows possible
formation routes.

In the left panel of Fig. 7, the depletion of C2H2 and NH3 are reported and
fit with a single exponential equation:

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

where $\alpha$ is the saturation value (i.e., the maximum abundance when
reaching the equilibrium state) in molecule centimeter-2, $\sigma$ is the
effective cross-section in centimeter2, $\phi$ is the UV flux in photon
centimeter-2 s-1, and t is the irradiation time in seconds. The derived
photolysis cross-section values are (3.7$\pm$0.4)$\times$10-19 and
(7.1$\pm$1.2)$\times$10-19 cm2 for C2H2 and NH3, respectively. Given that the
applied UV photons dominate at 130-160 nm (i.e., photon energy of 7.7-9.5 eV)
with a negligible amount of Ly-$\alpha$ photons, the direct photoionization of
C2H2 and NH3 is excluded under the current experimental conditions due to
their relatively high threshold energies (i.e., $\sim$11.4 and $\sim$10.2 eV
for C2H2 and NH3, respectively; (Collin & Delwiche, 1967; Xia et al., 1991;
Locht et al., 1991)). Moreover, the C2H2 photodissociation primarily results
in the formation of an ethynyl radical (CCH) instead of forming two
methylidyne radicals (CH) because of a strong C$\equiv$C bond requiring a
threshold energy of 9.9 eV (photon wavelength $\sim$125.3 nm; Okabe 1975) for
the dissociation. This explains the main product formation with an even number
of carbon atoms, such as C4H2, through recombining two C2H radicals and the
absence of CH4 in the pure C2H2 ice experiment. The depletion mechanism of NH3
is most likely due to photodissociation resulting in NH2 and H (Okabe & Lenzi,
1967);

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

As mentioned previously, the detected products result from interactions among
photofragments of the C2H2:NH3 ice mixture. For example, H reacts with C2H2,
leading to the formation of C2H3 through H–atom addition reactions. The C2H2
hydrogenation on dust grains at 10-20 K has been investigated in theoretical
and laboratory studies, and their activation energies are overcome by quantum
tunneling at low temperatures (Hiraoka et al., 2000; Kobayashi et al., 2017);

$\text{C${}_{2}$H${}_{2}$+H}\rightarrow\text{C${}_{2}$H${}_{3}$}.$ (3)

The estimated reaction barrier of reaction 3 (4.30-4.78 kcal mol-1) is very
similar to the value of the reaction CO+H $\rightarrow$ HCO (3.25 kcal mol-1),
which is one of the critical surface reactions on interstellar dust grains
(Miller & Klippenstein, 2004; Kobayashi et al., 2017; Álvarez-Barcia et al.,
2018). The efficient conversion from triple-bond (C$\equiv$C) into double-bond
(C=C) species is expected upon H–atom addition under molecular cloud
conditions.

The radical-molecule association between NH2 and C2H2 has been experimentally
reported in the gas phase and further supported by theoretical calculation
showing a relatively high activation barrier of 6.93-8.24 kcal mol-1 (Bosco et
al., 1984; Lesclaux et al., 1985; Hennig & Wagner, 1995; Moskaleva & Lin,
1998);

$\text{C${}_{2}$H${}_{2}$+NH${}_{2}$}\rightarrow\text{CHCHNH${}_{2}$}.$ (4)

Moreover, it has been pointed out that the reaction rates of reaction 4 in the
gas phase and solid state are several orders of magnitude lower than that of
C2H2+H and C2H2+OH (Brunning & Stief, 1985; Molpeceres & Rivilla, 2022).
Therefore, reaction 4 is a critical step, and whether this route plays a
relevant role in interstellar ice chemistry probably depends on the excess
energy or excited state of NH2 after NH3 photodissociation. In addition to a
lower activation barrier for reaction 3, H atoms are more mobile than NH2 at
10 K, which gives them a higher chance to meet C2H2 in the interstellar ice.
Consequently, the competition between reactions 3 and 4 favors the C2H3
formation. Since the radical-radical associations are generally considered
barrierless reactions, the recombination between C2H3 and nearby NH2 could
lead to the CH2CHNH2 formation through the reaction

$\text{C${}_{2}$H${}_{3}$ +
NH${}_{2}$}\rightarrow\text{CH${}_{2}$CHNH${}_{2}$}.$ (5)

This reaction route has been reported in gas-phase laboratory studies and used
to explain the CN–bearing species in planetary chemical models (Ferris &
Ishikawa, 1988; Kaye & Strobel, 1983). Very recently, Zhang et al. (2023) also
reported this reaction route forming CH2CHNH2 in their laboratory work
studying the 5–keV electron bombardment of a C2H2:NH3 ice mixture. In Fig. 7
(b), CH2CHNH2 is immediately and abundantly formed upon the impact of UV
photons, suggesting that CH2CHNH2 is a first-generation product in the
C2H2:NH3 ice mixture. Furthermore, its tautomer CH3CHNH can also be observed
and has a nearly linear formation. The ratio of CH3CHNH/CH2CHNH2 increases
with UV fluence. A similar formation curve has been found for the formation of
keto-enol tautomers, namely CH2CHOH and CH3CHO, in the previous study of H+
radiolysis of a C2H2:H2O ice mixture at 17 K (see Figure 4 in Chuang et al.
2021).

The keto–enol tautomerization is expected to proceed through intermolecular H
relocations enhanced by H2O (Chuang et al., 2020). The amine–imine
tautomerization of C2H5N isomers has been theoretically investigated and
suggests the preference of the imine (acetaldimine) over the amine
(vinylaminy) form. The potential energy of CH3CHNH is $\sim$4 kcal mol-1 lower
than that of CH2CHNH2 (Lammertsma & Prasad, 1994). However, the activation
energy via intramolecular displacement is around 66.43-84.52 kcal mol-1 in the
gas phase, which is similar to the intramolecular shift barrier from CH2CHOH
to CH3CHO (see Table 4 in Apeloig 1990; Lin et al. 1995; Andres et al. 1998).
It has been suggested that both interconversions require a catalyst (Raczyńska
et al., 2005). In this work, the simultaneous detection of CH3CHNH and
CH2CHNH2 hints at a possible tautomerization taking place in the studied
chemical system as explained for the enol-keto species;

$\text{CH${}_{2}$CHNH${}_{2}$ (amine-form)}\leftrightarrow\text{CH${}_{3}$CHNH
(imine-form)}.$ (6)

The consequent photodissociation of C2H5N isomers has been reported to form
HCN in the gas phase (Kaye & Strobel, 1983). In Fig. 7 (b), the kinetic
evolution of HCN and its more stable ionic format, CN- are correlated. That is
because that part of the HCN is expected to be converted into CN- through the
acid-base reactions with NH3 (Noble et al., 2013):

$\text{HCN+NH${}_{3}$}\leftrightarrow\text{[NH${}_{4}^{+}$][CN${}^{-}$]}.$ (7)

The yields of HCN and CN- are shown upon the formation of C2H5N isomers; the
derived product ratios of CH2CHNH2/HCN=8.35$\pm$0.36 and CN-/HCN=5.39$\pm$0.38
are relatively constant over the entire UV fluence. These results indicate the
efficient UV photodissociation of the first-generation products (H2CCHNH2 or
CH3CHNH) leading to HCN and [NH${}_{4}^{+}$][CN-].

The detection of unsaturated C2H3N molecules (e.g., CH2CNH and CH3CN) at a
later irradiation stage is probably caused by the direct H2 (or 2H)
elimination induced by UV photons:

$\text{CH${}_{2}$CHNH${}_{2}$}\xrightarrow{+h\nu}\text{CH${}_{2}$CNH +
H${}_{2}$ (or 2H)}.$ (8)

and

$\text{CH${}_{3}$CHNH}\xrightarrow{+h\nu}\text{CH${}_{2}$CNH / CH${}_{3}$CN +
H${}_{2}$ (or 2H)}.$ (9)

The obtained abundance N(CH3CN) is about four times higher than the N(CH2CNH),
which aligns with their order in potential energy (CH2CNH is $\sim$26.1 kcal
mol-1 higher than CH3CN; Dickerson et al. 2018). In addition to direct UV
dissociation, other possible mechanisms might affect the ratio of CH3CN and
CH2CNH. The isomerization from CH3CN to CH2CNH and CH3NC with high activation
barriers of $\sim$99.5 and $\sim$64.4 kcal mol-1, respectively, have been
found to be caused only by proton, X-ray, or N-atom bombardment, but
investigation of this is outside the scope of this work (Hudson & Moore, 2004;
Mencos & Krim, 2016; Kameneva et al., 2017).

Apart from UV photodissociation, the newly formed CH2CHNH2 or CH3CHNH are
expected to react with H atoms, forming CH3CH2NH2:

$\text{CH${}_{2}$CHNH${}_{2}$ (or CH${}_{3}$CHNH) + 2H
}\rightarrow\text{CH${}_{3}$CH${}_{2}$NH${}_{2}$}.$ (10)

However, this analog reaction is speculated based on the H–atom addition
reactions to vinyl alcohol (or acetaldehyde) forming ethanol. The derived QMS
data suggest the CH3CH2NH2 formation and support a general chemical conversion
in the interstellar ice from H–poor into H–rich species (Chuang et al., 2021).
It is important to note that a direct laboratory study of the H-addition and
H-abstraction reactions linking C2HnN (n=3, 5, and 7) is still desired.

The total abundance of N-bearing COMs, including CH2CHNH2, CH3CHNH, CH3CN, and
CH2CNH, is presented in the right panel of Fig. 7 and fit with a single
exponential equation (Eq. 1). The CH3CH2NH2 yield is below the current IR
detection sensitivity. Therefore, it is not counted in the total COM
abundance. The derived effective formation cross-section for the total
abundance of N-bearing COMs is (9.0$\pm$0.7)$\times$10-19 cm2. The sound
pseudo-first-order fit suggests that these N-bearing COMs are formed through
the common precursor C2H2. The conversion ratio of the total abundance of COMs
to depleted C2H2, that is, N(COMs)/$\Delta$N(C2H2), changes with UV fluence,
varying from 0.60 to 0.46. The decrease in the ratio is probably due to the
further UV photolysis of N-bearing COMs.

##  5 Astrochemical implication and conclusions

In this work, we extended our previous studies on the reactions between
acetylene and OH/H, which results in the formation of O–bearing COMs, to
reactions between C2H2 and NH2/H. This study provides experimental evidence on
the solid-state formation of N–bearing COMs described by the formula C2HnN,
including vinylamine (CH2CHNH2), acetaldimine (CH3CHNH), acetonitrile (CH3CN),
and ketenimine (CH2CNH), in the interstellar C2H2:NH3 ice analog upon UV
irradiation at 10 K. The identification of the above COMs was realized by IR
spectroscopy and mass spectrometry. The correlation of the desorption
temperatures of the species obtained from IR and QMS data during the TPD
experiment complements the assignments. The initial molecules and newly formed
products were monitored in situ as a function of the UV fluence, suggesting
that CH2CHNH2 is the first-generation product that might tautomerize into
CH3CHNH, as found for the analog O–bearing species (e.g., CH2CHOH and CH3CHO)
in the C2H2:H2O ice mixture. The chemical derivatives with different degrees
of hydrogenation, namely CH3CN/CH2CNH and CH3CH2NH2, are formed through
photolysis and hydrogenation of C2H5N isomers, respectively. The studied ice
chemistry is of astronomical relevance, as it aids in explaining the
complexity of N-bearing molecules in early star-forming stages and draws out
the chemical relation among these observed species.

Gaseous C2H2 is commonly detected in the circumstellar shell of carbon-rich
AGB stars such as IRC+10216 and several star-forming regions. Its abundance is
about a few percent of H2O: 1-5% in massive young stellar objects, $\sim$1% in
protoplanetary disks, and 0.2-0.9% in cometary comae (Lacy et al., 1989;
Brooke et al., 1996; Cernicharo et al., 1999; Lahuis & van Dishoeck, 2000;
Mumma et al., 2003; Carr & Najita, 2008). Photofragments of ionized polycyclic
aromatic hydrocarbons (PAHs) forming C2H2 and C2H by hard UV photons have been
theoretically investigated and confirmed in the laboratory (Jochims et al.,
1994; Allain et al., 1996; Kress et al., 2010; Zhen et al., 2014; West et al.,
2018). Very recently, the JWST-MINDS team reported the abundant detection of
C2H2 in a very low-mass star, 2MASS-J16053215-1933159, along with other
complex hydrocarbons, C4H2 and C6H6 (Tabone et al., 2023; Kamp et al., 2023).
The high abundance has been explained by the efficient (thermal) destruction
of carbonaceous materials leading to the formation of C2H2 (Anderson et al.
2017; van Dishoeck et al. 2023 and reference therein). In addition, C2H2 can
be formed in the gas phase through dissociative electron recombination or
proton transfer of C2H${}_{3}^{+}$. So far, however, the detection of C2H2 ice
is limited to one observational study toward Serpens BG1 (see details in Knez
et al. 2008), suggesting an abundance of $\sim$3% with respect to H2O ice. The
lack of (more) solid detections of C2H2 ice has been explained by a difficult
spectral identification. For example, the surrounding environment can
significantly affect its absorption profile, and this is in addition to a
severe blending with strong silicates and H2O features at $\sim$13.5 $\mu$m
(Boudin et al., 1998; Knez et al., 2012).

In translucent clouds, atomic species accrete and associate on grain surfaces,
resulting in an H2O-rich ice layer containing CO2, NH3, and CH4. For example,
successive H–atom addition reactions to oxygen, nitrogen, and carbon
consequently lead to H2O, NH3, and CH4 ice formation, respectively (see review
paper Hama & Watanabe 2013; Linnartz et al. 2015). The solid-state reactions
involving the simplest alkyne C2H2 with atomic species (H and O atoms) have
been proposed in theory to form various O-bearing COMs through the
intermediate product C2H3 (Tielens 1992, see also Figure 2 in Charnley 2004).
Furthermore, Charnley et al. (2001) has suggested that C2H3 could also react
with N and H atoms to form several N-bearing cyclic species, including
1H-azirine, 2H-azirine, and aziridine, in addition to the reported molecules
in this work. Recently, the interactions of C2H2 with simple radicals (e.g.,
OH, NH2, and CH3) have been theoretically studied to report their reaction
rate constants in the solid state (Molpeceres & Rivilla, 2022). In the
laboratory, the hydrogenation of C2H2 ice has been investigated, showing the
formation of (semi-)saturated hydrocarbons, such as C2H4 and C2H6 (Hiraoka et
al., 2000; Kobayashi et al., 2017). Also, C2H2 ice can react with OH radicals,
an important intermediate species in forming H2O or a common fragment upon
energetic processing of H2O. In our previous studies, several O–bearing COMs,
including vinyl alcohol, acetaldehyde, and ethanol, were detected in reactions
between C2H2 and H/OH, which were formed through atomic association or proton
radiolysis of H2O ice (Chuang et al., 2020, 2021). These results also provide
one of the explanations for the observational discrepancy of COM compositions
between early and late star-forming stages. The present experimental work
further investigated possible reactions of C2H2 with NH2 radicals and H atoms
produced in the interstellar bulk ice upon impact of UV photons on a C2H2:NH3
ice mixture. Given the higher activation barrier (i.e., $\sim$7.6 kcal mol-1)
required for the reaction C2H2+NH2$\rightarrow$C2H2NH2 reported by Molpeceres
& Rivilla (2022), the formation of N–bearing COMs in the solid state most
likely takes place through the recombination between NH2 and C2H3 radicals.
The latter are efficiently formed through the reaction C2H2+H$\rightarrow$C2H3
with a rate constant of $\sim$104 s-1 (Kobayashi et al., 2017; Molpeceres &
Rivilla, 2022).

Although the external interstellar radiation field is shielded by dust grains
in clouds, a non-negligible UV flux can be induced by direct cosmic ray
impacts on H2 molecules through electronic excitations. The subsequent
electron relaxation from the excited states results in the emissions of Lyman
and Werner band photons (Prasad & Tarafdar, 1983). The calculated cosmic ray-
induced secondary UV flux varies between $\sim$103 and $\sim$3$\times$104
photon cm-2s-1, depending on the CR flux and Rv (i.e., dust extinction curve
in the optical region)(Prasad & Tarafdar, 1983; Cecchi-Pestellini & Aiello,
1992; Shen et al., 2004). A recent paper by Padovani et al. (2024)
recalculated the secondary UV emissions using various CR ionization rates and
Rv values and reported the photon flux in molecular clouds as a function of H2
column density (see Figure 11 in the paper). Given the typical molecular cloud
lifetime of $\sim$107 yr before ice sublimation due to the central star
(Chevance et al., 2020), the total UV photons accumulated in clouds are
expected to be in the range of 3$\times$1017-1$\times$1019 photon cm-2.
Therefore, the studied photochemistry over UV fluence of $\sim$1.1$\times$1018
photon cm-2 in this work is likely to take place within the lifetime of
molecular clouds. This work reports on the possible interstellar (bulk) ice
chemistry triggered by secondary UV photons to form N–bearing species in
molecular clouds. Furthermore, a recent astronomical observation of CH2CHNH2,
CH3CHNH, and (tentatively) CH3CH2NH2 has been reported for the first time
toward a quiescent giant molecular cloud, G+0.693–0.027 (Zeng et al. 2021,
Rivilla et al., private communication). The simultaneous detections of C2HnN
species suggest that they may share the same formation history and are
chemically connected. Moreover, the astronomically observed CH2CHNH2 abundance
is higher than that of CH3CH2NH2, that is, N(CH2CHNH2)/N(CH3CH2NH2)
=1.7$\pm$0.5, which is consistent with the laboratory results showing the
abundant formation of CH2CHNH2 over other species.

The experimental findings from this study of the UV irradiation of C2H2:NH3
ice mixtures at 10 K are summarized below:

  1. 1.

The interactions of C2H2 with NH2 radicals and H atoms, which are produced by
UV photolysis of NH3, lead to the formation of several N–bearing complex
molecules described by the formula C2HnN, such as vinylamine (CH2CHNH2),
acetaldimine (CH3CHNH), acetonitrile (CH3CN), ketenimine (CH2CNH), and
ethylamine (CH3CH2NH2), in interstellar ice analogs at 10 K.

  2. 2.

The simultaneous detection of CH2CHNH2 and CH3CHNH indicates that
isomerization takes place in the studied chemical system. It is speculated
that intermolecular H–atom exchange with surrounding H—bearing species occurs
as proposed previously for the O–bearing analogs, namely CH2CHOH and CH3CHO,
in the C2H2:H2O ice mixture.

  3. 3.

The kinetic evolution of these newly formed products shows that CH2CHNH2 is
the first-generation product in the UV photolyzed C2H2:NH3 ice mixture at 10
K, followed by the formation of CH3CHNH. Both can be converted into chemically
relevant species through direct dissociation and hydrogenation forming
CH2CNH/CH3CN and CH3CH2NH2, respectively.

This present work verifies and extends our previous studies on the solid-state
chemistry of the unsaturated hydrocarbon C2H2 as a prevalent formation pathway
toward COMs by reactions with different functional groups, including
astronomically relevant OH and NH2 species, under molecular cloud conditions.
It is important to note that fast H–atom addition reactions to C2H2 forming
C2H3, thanks to quantum tunneling at low temperatures, could provide an
alternative barrierless pathway to allow for association with some radicals
that require a high activation energy to attach to C2H2. Following the studies
on O– and N–bearing COMs, possible reactions of C2H2 with SH radicals and H
atoms forming S–bearing COMs are expected and currently under investigation
(Santos et al., in preparation). The proposed chemistry of the simplest
alkyne, a common product of the so-called top-down mechanism based on PAHs and
hydrogenated amorphous carbonaceous grains, provides an efficient reaction
network forming complex organics in interstellar environments. The
contribution of different (non-)energetic processing will need further
laboratory work and astrochemical models.

###### Acknowledgements.

K.J.C. is grateful for support from the Dutch Research Council (NWO) via a
VENI fellowship (VI.Veni.212.296). J.C.S. thanks the Danish National Research
Foundation for the support through the Center of Excellence “InterCat”
(DNRF150). Th.H. acknowledges support from the European Research Council (ERC)
under the Horizon 2020 Framework Programme via the ERC Advanced Grant Origins
83 24 28.

## References

  * Abdulgalil et al. (2012) Abdulgalil, A. G., Marchione, D., Rosu-Finsen, A., et al. 2012, Journal of Vacuum Science & Technology A, 30
  * Abplanalp & Kaiser (2020) Abplanalp, M. J., & Kaiser, R. I. 2020, ApJ, 889, 3
  * Allain et al. (1996) Allain, T., Leach, S., & Sedlmayr, E. 1996, A&A, 305, 616
  * Álvarez-Barcia et al. (2018) Álvarez-Barcia, S., Russ, P., Kästner, J., & Lamberts, T. 2018, Monthly Notices of the Royal Astronomical Society, 479, 2007
  * Anderson et al. (2017) Anderson, D. E., Bergin, E. A., Blake, G. A., et al. 2017, The Astrophysical Journal, 845, 13
  * Andres et al. (1998) Andres, J., Domingo, L., Picher, M., & Safont, V. 1998, International journal of quantum chemistry, 66, 9
  * Apeloig (1990) Apeloig, Y. 1990, The Chemistry of Enols, ed. Z. Rappoport (Wiley, Chichester:John Wiley & Sons)
  * Bacmann et al. (2012) Bacmann, A., Taquet, V., Faure, A., Kahane, C., & Ceccarelli, C. 2012, A&A, 541, L12, doi: 10.1051/0004-6361/201219207
  * Bisschop et al. (2008) Bisschop, S. E., Jørgensen, J. K., Bourke, T. L., Bottinelli, S., & van Dishoeck, E. F. 2008, A&A, 488, 959, doi: 10.1051/0004-6361:200809673
  * Boogert et al. (2015) Boogert, A. C. A., Gerakines, P. A., & Whittet, D. C. B. 2015, ARA&A, 53, 541, doi: 10.1146/annurev-astro-082214-122348
  * Bosco et al. (1984) Bosco, S., Nava, D., Brobst, W., & Stief, L. 1984, The Journal of chemical physics, 81, 3505
  * Boudin et al. (1998) Boudin, N., Schutte, W. A., Greenberg, J. M., et al. 1998, A&A, 331, 749
  * Brooke et al. (1996) Brooke, T., Tokunaga, A., Weaver, H., et al. 1996, Nature, 383, 606
  * Brunning & Stief (1985) Brunning, J., & Stief, L. 1985, The Journal of chemical physics, 83, 1005
  * Butscher et al. (2015) Butscher, T., Duvernay, F., Theule, P., et al. 2015, MNRAS, 453, 1587, doi: 10.1093/mnras/stv1706
  * Canta et al. (2023) Canta, A., Öberg, K. I., & Rajappan, M. 2023, ApJ, 953, 81, doi: 10.3847/1538-4357/acda99
  * Carr & Najita (2008) Carr, J. S., & Najita, J. R. 2008, Science, 319, 1504
  * Cecchi-Pestellini & Aiello (1992) Cecchi-Pestellini, C., & Aiello, S. 1992, Monthly Notices of the Royal Astronomical Society, 258, 125
  * Cernicharo et al. (2012) Cernicharo, J., Marcelino, N., Roueff, E., et al. 2012, ApJL, 759, L43, doi: 10.1088/2041-8205/759/2/L43
  * Cernicharo et al. (1999) Cernicharo, J., Yamamura, I., González-Alfonso, E., et al. 1999, ApJL, 526, L41
  * Charnley (2004) Charnley, S. 2004, Advances in Space Research, 33, 23
  * Charnley et al. (2001) Charnley, S. B., Rodgers, S. D., & Ehrenfreund, P. 2001, A&A, 378, 1024, doi: 10.1051/0004-6361:20011193
  * Chevance et al. (2020) Chevance, M., Kruijssen, J. D., Vazquez-Semadeni, E., et al. 2020, Space Science Reviews, 216
  * Chuang et al. (2020) Chuang, K., Fedoseev, G., Qasim, D., et al. 2020, Astronomy & Astrophysics, 635, A199
  * Chuang (2018) Chuang, K. J. 2018, PhD thesis, Leiden University
  * Chuang et al. (2016) Chuang, K.-J., Fedoseev, G., Ioppolo, S., van Dishoeck, E. F., & Linnartz, H. 2016, MNRAS, 455, 1702, doi: 10.1093/mnras/stv2288
  * Chuang et al. (2017) Chuang, K.-J., Fedoseev, G., Qasim, D., et al. 2017, MNRAS, 467, 2552, doi: 10.1093/mnras/stx222
  * Chuang et al. (2021) Chuang, K.-J., Fedoseev, G., Scirè, C., et al. 2021, Astronomy & Astrophysics, 650, A85
  * Chuang et al. (2022) Chuang, K.-J., Jäger, C., Krasnokutski, S., Fulvio, D., & Henning, T. 2022, The Astrophysical Journal, 933, 107
  * Collin & Delwiche (1967) Collin, J. E., & Delwiche, J. 1967, Canadian Journal of Chemistry, 45, 1883
  * Compagnini et al. (2009) Compagnini, G., D’Urso, L., Puglisi, O., Baratta, G., & Strazzulla, G. 2009, Carbon, 47, 1605
  * Dickerson et al. (2018) Dickerson, C. E., Bera, P. P., & Lee, T. J. 2018, The Journal of Physical Chemistry A, 122, 8898
  * Drozdovskaya et al. (2019) Drozdovskaya, M. N., van Dishoeck, E. F., Rubin, M., Jørgensen, J. K., & Altwegg, K. 2019, Monthly Notices of the Royal Astronomical Society, 490, 50
  * Evans et al. (2012) Evans, N. L., Yu, H., Roberts, G. M., Stavros, V. G., & Ullrich, S. 2012, Physical Chemistry Chemical Physics, 14, 10401
  * Fedoseev et al. (2017) Fedoseev, G., Chuang, K.-J., Ioppolo, S., et al. 2017, ApJ, 842, 52, doi: 10.3847/1538-4357/aa74dc
  * Fedoseev et al. (2015) Fedoseev, G., Ioppolo, S., & Linnartz, H. 2015, MNRAS, 446, 449, doi: 10.1093/mnras/stu1852
  * Ferris & Ishikawa (1988) Ferris, J. P., & Ishikawa, Y. 1988, Journal of the American Chemical Society, 110, 4306
  * Georgieva & Velcheva (2006) Georgieva, M. K., & Velcheva, E. A. 2006, International journal of quantum chemistry, 106, 1316
  * Gerakines et al. (2004) Gerakines, P., Moore, M., & Hudson, R. 2004, Icarus, 170, 202
  * Gerakines et al. (2022) Gerakines, P. A., Yarnall, Y. Y., & Hudson, R. L. 2022, Monthly Notices of the Royal Astronomical Society, 509, 3515
  * Hama & Watanabe (2013) Hama, T., & Watanabe, N. 2013, Chemical Reviews, 113, 8783, doi: 10.1021/cr4000978
  * Hamada et al. (1984) Hamada, Y., Hashiguchi, K., Tsuboi, M., Koga, Y., & Kondo, S. 1984, Journal of Molecular Spectroscopy, 105, 93
  * Hashiguchi et al. (1984) Hashiguchi, K., Hamada, Y., Tsuboi, M., Koga, Y., & Kondo, S. 1984, Journal of Molecular Spectroscopy, 105, 81
  * Hatch et al. (1992) Hatch, S., Polizzotti, R., Dougal, S., & Rabinowitz, P. 1992, Chemical physics letters, 196, 97
  * Hattori et al. (2005) Hattori, R., Suzuki, E., & Shimizu, K. 2005, Journal of molecular structure, 750, 123
  * He et al. (2022) He, J., Simons, M., Fedoseev, G., et al. 2022, Astronomy & Astrophysics, 659, A65
  * Hennig & Wagner (1995) Hennig, G., & Wagner, H. 1995, Berichte der Bunsengesellschaft für physikalische Chemie, 99, 989
  * Herbst & van Dishoeck (2009) Herbst, E., & van Dishoeck, E. F. 2009, ARA&A, 47, 427, doi: 10.1146/annurev-astro-082708-101654
  * Hiraoka et al. (2000) Hiraoka, K., Takayama, T., Euchi, A., Hand a, H., & Sato, T. 2000, ApJ, 532, 1029, doi: 10.1086/308612
  * Hudson et al. (2014) Hudson, R., Ferrante, R., & Moore, M. 2014, Icarus, 228, 276
  * Hudson & Moore (1997) Hudson, R., & Moore, M. 1997, Icarus, 126, 233
  * Hudson & Moore (2004) —. 2004, Icarus, 172, 466
  * Hudson et al. (2022) Hudson, R. L., Gerakines, P. A., & Yarnall, Y. Y. 2022, The Astrophysical Journal, 925, 156
  * Ito & Nakanaga (2010) Ito, F., & Nakanaga, T. 2010, Journal of Molecular Spectroscopy, 264, 100
  * Jacox (1979) Jacox, M. E. 1979, Chemical Physics, 43, 157
  * Jochims et al. (1994) Jochims, H., Ruhl, E., Baumgartel, H., Tobita, S., & Leach, S. 1994, ApJ, 420, 307
  * Jørgensen et al. (2012) Jørgensen, J. K., Favre, C., Bisschop, S. E., et al. 2012, ApJL, 757, L4, doi: 10.1088/2041-8205/757/1/L4
  * Jørgensen et al. (2016) Jørgensen, J. K., van der Wiel, M. H. D., Coutens, A., et al. 2016, A&A, 595, A117, doi: 10.1051/0004-6361/201628648
  * Kameneva et al. (2017) Kameneva, S. V., Volosatova, A. D., & Feldman, V. I. 2017, Radiation Physics and Chemistry, 141, 363
  * Kamp et al. (2023) Kamp, I., Henning, T., Arabhavi, A. M., et al. 2023, Faraday discussions, 245, 112
  * Kaye & Strobel (1983) Kaye, J. A., & Strobel, D. F. 1983, Icarus, 54, 417
  * Knez et al. (2012) Knez, C., Moore, M., Ferrante, R., & Hudson, R. 2012, ApJ, 748, 95
  * Knez et al. (2008) Knez, C., Moore, M., Travis, S., et al. 2008, Proceedings of the International Astronomical Union, 4, 47
  * Kobayashi et al. (2017) Kobayashi, H., Hidaka, H., Lamberts, T., et al. 2017, ApJ, 837, 155
  * Kress et al. (2010) Kress, M. E., Tielens, A. G., & Frenklach, M. 2010, Advances in Space Research, 46, 44
  * Lacy et al. (1989) Lacy, J., Evans, N. J., Achtermann, J., et al. 1989, ApJ, 342, L43
  * Lahuis & van Dishoeck (2000) Lahuis, F., & van Dishoeck, E. 2000, A&A, 355, 699
  * Lammertsma & Prasad (1994) Lammertsma, K., & Prasad, B. V. 1994, Journal of the American Chemical Society, 116, 642
  * Lesclaux et al. (1985) Lesclaux, R., Veyret, B., & Roussel, P. 1985, Berichte der Bunsengesellschaft für physikalische Chemie, 89, 330
  * Lin et al. (1995) Lin, J.-F., Wu, C.-C., & Lien, M.-H. 1995, The Journal of Physical Chemistry, 99, 16903
  * Linnartz et al. (2015) Linnartz, H., Ioppolo, S., & Fedoseev, G. 2015, International Reviews in Physical Chemistry, 34, 205
  * Lo et al. (2020) Lo, J.-I., Peng, Y.-C., Chou, S.-L., Lu, H.-C., & Cheng, B.-M. 2020, MNRAS, 499, 543
  * Locht et al. (1991) Locht, R., Leyh, B., Denzer, W., Hagenow, G., & Baumgärtel, H. 1991, Chemical physics, 155, 407
  * Loomis et al. (2013) Loomis, R. A., Zaleski, D. P., Steber, A. L., et al. 2013, The Astrophysical Journal Letters, 765, L9
  * Lovas et al. (2006) Lovas, F. J., Hollis, J., Remijan, A. J., & Jewell, P. 2006, The Astrophysical Journal, 645, L137
  * Martín-Doménech et al. (2020) Martín-Doménech, R., Öberg, K. I., & Rajappan, M. 2020, ApJ, 894, 98
  * McGuire (2018) McGuire, B. A. 2018, The Astrophysical Journal Supplement Series, 239, 17
  * McNaughton & Evans (1999) McNaughton, D., & Evans, C. J. 1999, Journal of molecular spectroscopy, 196, 274
  * Mencos & Krim (2016) Mencos, A., & Krim, L. 2016, Monthly Notices of the Royal Astronomical Society, 460, 1990
  * Miller & Klippenstein (2004) Miller, J. A., & Klippenstein, S. J. 2004, Physical Chemistry Chemical Physics, 6, 1192
  * Molpeceres & Rivilla (2022) Molpeceres, G., & Rivilla, V. M. 2022, Astronomy & Astrophysics, 665, A27
  * Moskaleva & Lin (1998) Moskaleva, L., & Lin, M.-C. 1998, The Journal of Physical Chemistry A, 102, 4687
  * Mumma et al. (2003) Mumma, M., DiSanti, M., Russo, N. D., et al. 2003, Advances in Space Research, 31, 2563
  * Mungan et al. (1991) Mungan, C., Spitzer, R., Sethna, J., & Sievers, A. 1991, Physical Review B, 43, 43
  * Noble et al. (2013) Noble, J. A., Theule, P., Borget, F., et al. 2013, Monthly Notices of the Royal Astronomical Society, 428, 3262
  * Öberg et al. (2010) Öberg, K. I., Bottinelli, S., Jørgensen, J. K., & van Dishoeck, E. F. 2010, ApJ, 716, 825, doi: 10.1088/0004-637X/716/1/825
  * Okabe (1975) Okabe, H. 1975, The Journal of Chemical Physics, 62, 2782
  * Okabe & Lenzi (1967) Okabe, H., & Lenzi, M. 1967, The Journal of Chemical Physics, 47, 5241
  * Padovani et al. (2024) Padovani, M., Galli, D., Scarlett, L. H., et al. 2024, Astronomy & Astrophysics, 682, A131
  * Pereira et al. (2020) Pereira, R., de Barros, A., da Costa, C., et al. 2020, MNRAS, 495, 40
  * Perrero et al. (2022) Perrero, J., Enrique-Romero, J., Martínez-Bachs, B., et al. 2022, ACS Earth and Space Chemistry, 6, 496
  * Potapov et al. (2019) Potapov, A., Jäger, C., & Henning, T. 2019, ApJ, 880, 12
  * Prasad & Tarafdar (1983) Prasad, S. S., & Tarafdar, S. P. 1983, ApJ, 267, 603, doi: 10.1086/160896
  * Raczyńska et al. (2005) Raczyńska, E. D., Kosińska, W., Ośmiałowski, B., & Gawinecki, R. 2005, Chemical reviews, 105, 3561
  * Shen et al. (2004) Shen, C. J., Greenberg, J. M., Schutte, W. A., & van Dishoeck, E. F. 2004, A&A, 415, 203, doi: 10.1051/0004-6361:20031669
  * Simons et al. (2020) Simons, M., Lamberts, T., & Cuppen, H. 2020, A&A, 634, A52
  * Stolkin et al. (1977) Stolkin, I., Ha, T.-K., & Günthard, H. H. 1977, Chemical physics, 21, 327
  * Tabone et al. (2023) Tabone, B., Bettoni, G., van Dishoeck, E., et al. 2023, Nature Astronomy, 1
  * Taquet et al. (2017) Taquet, V., Wirström, E., Charnley, S. B., et al. 2017, A&A, 607, A20
  * Thiel et al. (2017) Thiel, V., Belloche, A., Menten, K., Garrod, R., & Müller, H. 2017, Astronomy & Astrophysics, 605, L6
  * Tielens (2013) Tielens, A. 2013, Reviews of Modern Physics, 85, 1021
  * Tielens (1992) Tielens, A. G. G. M. 1992, in Tokyo: Univ. Tokyo Press, Vol. 251, Chemistry and Spectroscopy of Interstellar Molecules, ed. N. Kaifu, 237
  * Tsuge & Watanabe (2023) Tsuge, M., & Watanabe, N. 2023, Proceedings of the Japan Academy, Series B, 99, 103
  * Turner et al. (2021) Turner, A. M., Chandra, S., Fortenberry, R. C., & Kaiser, R. I. 2021, ChemPhysChem, 22, 985
  * van Dishoeck et al. (2023) van Dishoeck, E., Grant, S., Tabone, B., et al. 2023, Faraday Discussions
  * van Gelder et al. (2020) van Gelder, M., Tabone, B., van Dishoeck, E., et al. 2020, A&A, 639, A87
  * Vazart et al. (2020) Vazart, F., Ceccarelli, C., Balucani, N., Bianchi, E., & Skouteris, D. 2020, Monthly Notices of the Royal Astronomical Society, 499, 5547
  * Vinogradoff et al. (2012) Vinogradoff, V., Duvernay, F., Farabet, M., et al. 2012, The Journal of Physical Chemistry A, 116, 2225
  * Volosatova et al. (2022) Volosatova, A. D., Zasimov, P. V., & Feldman, V. I. 2022, The Journal of Chemical Physics, 157
  * West et al. (2018) West, B., Castillo, S. R., Sit, A., et al. 2018, Physical Chemistry Chemical Physics, 20, 7195
  * Wu et al. (2002) Wu, C. R., Judge, D., Cheng, B.-M., et al. 2002, Icarus, 156, 456
  * Xia et al. (1991) Xia, T., Chien, T., Wu, C. R., & Judge, D. 1991, Journal of Quantitative Spectroscopy and Radiative Transfer, 45, 77
  * Zeng et al. (2021) Zeng, S., Jiménez-Serra, I., Rivilla, V. M., et al. 2021, The Astrophysical Journal Letters, 920, L27
  * Zhang et al. (2023) Zhang, C., Wang, J., Turner, A. M., et al. 2023, The Astrophysical Journal, 952, 132
  * Zhen et al. (2014) Zhen, J., Castellanos, P., Paardekooper, D. M., Linnartz, H., & Tielens, A. G. 2014, ApJL, 797, L30

##  Appendix A Infrared spectra of C2H2:NH3 ice mixtures

Figure 8: Infrared spectra of the deposited C2H2:NH3 ice mixture with ratios
of (a) 1:0, (b) 1:0.2, (c) 1:0.5, (d) 1:1.4, (e) 1:2.7, and (f) 0:1 at 10 K.
The spectra are offset for clarity.

Figure 8 shows the IR spectra obtained after the deposition of C2H2:NH3 ice
mixtures with ratios of (a) 1:0, (b) 1:0.2, (c) 1:0.5, (d) 1:1.4, (e) 1:2.7,
and (f) 0:1 at 10 K. For a direct comparison, the absolute C2H2 abundance was
kept the same, namely the averaged N(C2H2)=(8$\pm$2)$\times$1016 molecule
cm-2, in different ratios, while the column density N(NH3) changes from
$\sim$2.3$\times$1016 to $\sim$3.1$\times$1017 molecule cm2. The main features
of pure C2H2 ice are found at 759/789, 1391, 1962, 3225 cm-1 in the spectrum
(a). It is clear that these IR peaks significantly shift and even broaden when
C2H2 ice is mixed with NH3 (i.e., spectra (b)-(e)). The non-negligible changes
of C2H2 spectral characteristics have been reported in Knez et al. (2012) for
CO, CO2, CH4, and H2O ice. Future spectroscopic studies that include NH3 ice
are very much required and astronomically relevant to identifying C2H2 in the
interstellar ice.

##  Appendix B Kinetic evolution of IR absorbance area for the selected
products

Figure 9: Kinetic evolution of the selected products as a function of
temperature during the TPD experiment. The IR peaks used for monitoring the
corresponding species are shown in parentheses. The arrows highlight the
temperature range of desorption.

Figure 9 presents the kinetic evolution of IR peak intensities for the
selected products, including CH2CHNH2, CH3CHNH, CH2CNH, CH3CN, C2H4, and HCN
as a function of temperature during the TPD experiment with a ramping rate of
5 K min-1. Each data point was collected over 5 K due to the IR measuring time
(i.e., 60 seconds); the uncertainty of the recorded temperatures is $\sim$5 K.