# A possible answer to the mysterious non-detection of hydroxylamine in space: the thermal desorption mechanism

Mindaugas Jonusas, Lahouari Krim

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Mindaugas Jonusas, Lahouari Krim. A possible answer to the mysterious non-detection of hydroxylamine in space: the thermal desorption mechanism. Monthly Notices of the Royal Astronomical Society, 2016, 459 (2), pp.1977-1984. 10.1093/mnras/stw808. hal-01480748

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# A possible answer to the mysterious non-detection of hydroxylamine in space: the thermal desorption mechanism

Mindaugas Jonusas<fnref n="1" />,<fnref n="2" /> and Lahouari Krim<fnref n="1" />,<fnref n="2" />*

Accepted 2016 April 5. Received 2016 April 1; in original form 2016 February 3

## ABSTRACT

The presence of NH₂OH, one of the main precursors in the formation of amino-acids, on dust grain mantles, may be the most obvious elucidation for the creation of large pre-biotic molecules in the interstellar medium. However, while many laboratory experimental studies, to simulate the icy grain chemistry in space, found that NH₂OH molecules may be easily formed in solid phase with high abundances and then they should desorb, through a temperature-induced desorption into the gas phase, with the same high abundances; all the spatial observations conclude that NH₂OH is not detected in gas phase within any of the explored astronomical sources. Such inconsistencies between laboratory experiment simulations and spatial observations lead our investigations towards this experimental study to see if there is any chemical transformation of NH₂OH, occurring in the solid phase before the desorption processes of NH₂OH from the mantle of interstellar icy grains. Our experimental results show that the heating of NH₂OH–H₂O ices lead to a decomposition of NH₂OH into HNO, NH₃ and O₂, even before reaching its desorption temperature. We show through this work that the NH₂OH non-detection from previous examined astronomical sources could mainly due to its high reactivity in solid phase on the icy interstellar grains.

**Key words:** astrochemistry – methods: laboratory – techniques: spectroscopic – ISM: clouds – ISM: molecules.

## 1 INTRODUCTION

Although various complex molecules have already been identified in space, a simple species constantly considered as the main precursor of amino acids, NH₂OH is still not detected. To solve this mystery many laboratory investigation have focused their theoretical as well as experimental investigations into the formation of NH₂OH under extreme conditions similar to those found in the interstellar medium. From an experimental point of view, most of the studies have been devoted into the synthesis processes of hydroxylamine in solid phase at cryogenic temperatures. Nishi, Shinohara and Okuyama (1984) found that the UV irradiation of ammonia containing ices may be one of the efficient reaction pathways to produce hydroxylamine through radical recombination involving, NH₂, H, OH and H₂O:

$$NH_2 + OH \rightarrow NH_2OH$$

$$NH_2 + H + H_2O \rightarrow NH_2OH + H_2$$

Using electron irradiations of ammonia–water ices between 10 and 50 K, Zheng & Kaiser (2010) investigated the NH₂OH for-

mation, where the synthesis of molecular hydroxylamine has been accompanied by many secondary reaction products such as H₂, H₂O₂ and N₂H₄. A few years ago Strazzulla’s group (Strazzulla & Palumbo 1998) proposed such similar studies, suggesting that the irradiated NH₃–H₂O–CH₄ mixture on Saturnian satellite surfaces can be partially converted into pre-biotic molecules. In recent years various experiments involving atom or radical bombardment of interstellar ice analogues have been proposed to justify the formation of complex organic molecules in space. Within this context, Congiu et al. (2012) illustrated that the NH₂OH formation on interstellar ices analogues might arise from successive hydrogenation of solid nitric oxide at 10 K, while He et al. (2015) reported another hydroxylamine synthesis method on an amorphous silicate surface through the oxidation of ammonia at 70 K.

Concurrently with these experimental studies, many theoretical investigations have been devoted to diverse mechanisms of reactions leading to the formation of NH₂OH as an intermediate or a final reaction product. The radical recombination NH₂ + OH reaction has been theoretically investigated on the lowest singlet and triplet surfaces by Mousavipour et al. (Mousavipour et al. 2009, 2014). They underline the formation of two hydroxylamine isomers NH₂OH and NH₃O which may decompose into small fragments such as HON + H₂, HNO + H₂ and NH₃ + O. Similar study has been also proposed

<footnote n="1">¹Sorbonne Universités, UPMC Univ. Paris 06, MONARIS, UMR 8233, Université Pierre et Marie Curie, 4 Place Jussieu, case courrier 49, F-75252 Paris Cedex 05, France</footnote>

<footnote n="2">²CNRS, MONARIS, UMR 8233, Université Pierre et Marie Curie, 4 Place Jussieu, case courrier 49, F-75252 Paris Cedex 05, France</footnote>

<footnote>* E-mail: Lahouari.krim@upmc.fr</footnote>
by Wang et al. (2004) by investigating the branching product for O + NH₃ reaction which was found to be occurring through an insertion mechanism leading to a long-lived chemically activated NH₂OH as intermediate before it dissociated into NH₂ and OH.

Even though all those laboratory investigations suggest the ef-
ficiency of hydroxylamine formation under conditions similar to
those found in the interstellar medium and some planetary atmo-
spheres, till now the question of the presence of NH₂OH with rela-
tively high abundance in space remains unresolved. And many ex-
plorations of hydroxylamine in several astronomical sources carried
out by McGuire et al. (2015) and Pulliam, McGuire and Remijan
(2012) result in a pure non-detection of this simple molecule. An
obvious and possible explanation is that the NH₂OH abundances in
different regions in space might be lower than estimated from lab-
oratory studies. If all the laboratory experimental studies showed
several reaction pathways to form NH₂OH in solid phase to simulate
the icy grain chemistry and all the space observations conclude that
NH₂OH is not detected, the chemical transformation of NH₂OH in
solid phase during the desorption processes of NH₂OH containing
interstellar grains could result in its very low abundance in space.
Paradoxically, even though NH₂OH is a stable and commercially
available molecule, all the laboratory investigations have focused on
the formation of NH₂OH in ‘cold-solid-phase’ interstellar medium
conditions but no experiments have ever been realized to simulate
the desorption NH₂OH trapped in water ices to monitor the NH₂OH
evolution during the solid–gas transitions and then to simulate the
desorption of interstellar icy grain mantle containing water and hy-
droxylamine.

In this paper, we are investigating the evolution NH₂OH molecule via a thermal chemistry during the desorption of NH₂OH–H₂O ices in the temperature range of 10–200 K. This is the first study devoted to the formation and evolution of NH₂OH–H₂O ices at low temperature. Do NH₂OH molecules really desorb from NH₂OH–H₂O ices? Are there any thermal reactions transforming NH₂OH before its desorption? To answer those questions, we have explored such an experimental study to characterize NH₂OH stability between 10 and 200 K, by coupling infrared spectroscopic analysis and temperature-programmed desorption. Knowing that the NH₂OH desorption temperatures range from 160 to 190 K, we show, through this study, there is a competition between NH₂OH desorption into the gas phase and its chemical transformation in the solid sample occurring at relatively lower temperature and leading to the formation of HNO, NH₃ and O₂. This is the first infrared spectroscopic experimental study probing at different temperatures an ice formed directly by condensing NH₂OH molecules. From our experimental results, we prove that the chemical transformation of NH₂OH starts occurring in solid phase at 130 K, a temperature far lower than that of NH₂OH desorption and this may reduce considerably the abundance of NH₂OH molecules releasing in the gas phase. We also show that this NH₂OH chemical transformation is due mainly to the presence of high amount of H₂O aggregates. Finally, those results might be the primary explanation to the non-detectability of hydroxylamine in space.

**2 EXPERIMENTAL SECTION**

The experiments have been performed in ultrahigh vacuum chamber
maintained at 10⁻⁹ mbar. A closed-cycle helium cryogenerator was
used (Sumitomo cryogenics F-70) connected to a rotary platform
attached in the vacuum chamber holding highly polished, Rh-plated
copper mirror at low temperatures. Using a programmable tempera-
ture controller (Lakeshore 336), the temperature of the mirror sam-

ple holder can be regulated in a wide range, between 3 K and room
temperature. Infrared spectra were recorded in the transmission–
reflection mode between 5000 and 500 cm⁻¹ using a Bruker Vertex
80v fourier transform infrared (FTIR) spectrometer equipped with
KBr beam splitter and liquid N₂ cooled HgCdTe detector. A res-
olution of 0.5 cm⁻¹ was used and 500 scans were recorded for
each spectrum. Infrared spectra of bare mirror are recorded be-
fore the sample deposition and used as references in processing
the sample spectra. Absorption spectra in the mid-infrared region
were collected from the samples through a KBr window separat-
ing the interferometer vacuum (10⁻² mbar) from the cryostatic cell
(10⁻⁹ mbar). Hydroxylamine was produced by heating hydroxy-
lamine phosphate (NH₂OH)₃H₃PO₄ salt (Sigma-Aldrich) at tem-
peratures lower than 40°C in a glass tube. Two methods have been
adopted to create a solid NH₂OH.

(i) Solid samples were prepared using a direct condensation at 3 or 10 K of the thermal decomposition of (NH₂OH)₃H₃PO₄ salt.

(ii) The thermal decomposition of (NH₂OH)₃H₃PO₄ was co-
injected and condensed with neon gas ('Air liquide'; 99.9995%) at
3 or 10 K to create an neon matrix hosting NH₂OH molecules. This
neon matrix isolation technique, which is frequently used to isolate
specific chemical species in a neutral crystal, in addition to permit
an accurate spectroscopic study of free isolated NH₂OH molecules,
it would allow a better characterization of the purity degree of hy-
droxylamine derived from the heating of (NH₂OH)₃H₃PO₄ salt. By
co-injecting NH₂OH and neon gas simultaneously at 3 or 10 K, a
neutral cage formed by neon atoms, is created where NH₂OH may
be trapped and isolated. By heating, for a while, the neon matrix
at temperatures higher than 15 K, the neon desorption tempera-
ture, all the neon atoms released from the solid sample leading to a
formation of a neon-free solid NH₂OH.

To characterize the gas phase of our experiments and distinguish
all the chemical species desorbing from our samples, a Quadrupole
Mass Spectrometer (QMS – Hidden Analytical) is placed in front of
the rotatable sample holder to monitor the evaporated species from
the heated solid NH2OH.

**3 RESULTS**

The infrared spectrum in Fig. 1 illustrates the result of the conden-
sation, during 2 h at 10 K, of NH₂OH derived from the thermal
decomposition of (NH₂OH)₃H₃PO₄ salt. From this figure, we were
barely able to assign the infrared signals around 3000 cm⁻¹ to NH
and OH stretching modes and those below 1600 cm⁻¹ to HNH and
NOH bending modes of solid hydroxylamine. In order to improve
the specific vibrational mode assignments of NH₂OH, we need to
perform the same condensation experiment of the thermal decom-
position of (NH₂OH)₃H₃PO₄ salt through a neon matrix isolation
technique. In this context, a similar experiment was run using neon
as carrier gas for the period of 30 min.

The results of this experiment is shown through the infrared spec-
trum of Fig. 2(a) where most of the detected absorption peaks for
NH₂OH trapped in neon matrix are found to be similar to those in-
vestigated by other groups (Withnall and Andrews 1988; Luckhaus
1997). To compare our results to earlier matrix isolation hydroxy-
lamine investigations, Table 1 gathers the vibrational mode frequen-
cies of NH₂OH measured in our experiments and those given by
previous studies. Through this experimental method, we can then,
in addition to assign all the specific vibrational modes of NH₂OH
molecule isolated in the rare gas matrix as shown in Fig. 2(a),
better characterize the purity of our hydroxylamine arising from the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Infrared spectrum of NH₂OH–H₂O ice at 10 K in the 750–3700 cm⁻¹ spectral region. Sample formed by thermal decomposition of (NH₂OH)₃H₃PO₄ salt.</figcaption></figure>

thermal decomposition of (NH₂OH)₃H₃PO₄ salt. The detection of H₂O vibrational modes in our IR spectrum as shown in Fig. 2(a), at 3744, 3651 and 1595 cm⁻¹, prove that NH₂OH is found to be mixed with water molecules.

In order to connect the infrared spectrum of NH₂OH ice feathered in Fig. 1 which has been recorded after a direct condensation, during 2 h at 10 K, from the decomposition of (NH₂OH)₃H₃PO₄ salt

<table><caption>**Table 1.** Assignments of NH₂OH molecule vibrational mode frequencies (cm⁻¹) compared to other references.</caption><thead><tr><th>Vibrational mode</th><th>This exp</th><th>REF<sup>a</sup></th><th>REF<sup>b</sup></th></tr></thead><tbody><tr><td>ν₁(OH) str</td><td>3637.9</td><td>3634.7</td><td>3649.89</td></tr><tr><td>ν₂(NH₂) s str</td><td>3297.1</td><td>3289.7</td><td>3294.25</td></tr><tr><td>ν₃(NH₂) sci</td><td>1598.4</td><td>1598.9</td><td>1604.52</td></tr><tr><td>ν₄(NOH) bend</td><td>1351.7</td><td>1350.7</td><td>1353.3</td></tr><tr><td>ν₅(NH₂) wag</td><td>1117</td><td>1117</td><td>1115.47</td></tr><tr><td>ν₆(NO) str</td><td>895.7</td><td>895.9</td><td>895.21</td></tr><tr><td>ν₇(NH₂) a str</td><td>3346.5</td><td>3353.3</td><td>3358.76</td></tr><tr><td>ν₈(NH₂) rock</td><td>—</td><td>—</td><td>1294.5</td></tr><tr><td>ν₉ torsion</td><td>—</td><td>375.9</td><td>385.96</td></tr></tbody></table>

and those recorded using rare gas matrix isolation experiments, the neon matrix samples formed at 3 K have been heated progressively between 3 and 20 K to release the neon atoms from the sample and generate a NH₂OH neon-free ice. The heating evolution of the neon matrix sample is shown in Figs 2(b)–(d). As shown in Fig. 2(c), at 15 K almost all the neon atoms leave the solid sample and a pure NH₂OH–H₂O ice mixture can be obtained at 20 K, characterized by wide absorption bands related to solid H₂O and NH₂OH. Once all the neon atoms are desorbed at 20 K from the sample, the resulting ice has been cooled down at 10 K, to compare the two sample deposition methods adopted to create a solid NH₂OH.

The comparison between Figs 1 and 2 shows that the thermal deposition of NH₂OH, under our experimental conditions with and

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Left-hand panel: infrared spectrum NH₂OH and H₂O trapped in neon matrix. Right-hand panel: NH₂OH–H₂O ice deposition at 10 K versus neon matrix deposition. (a) Neon matrix deposition at 3 K. (b) Neon matrix heated at 10 K. (c) Neon matrix heated at 15 K and cooled down to 10 K. (d) Neon matrix heated at 20 K and cooled down to 10 K. (e) Direct NH₂OH–H₂O ice deposition at 10 K.</figcaption></figure>

<figure><img src="image_5.png" /></figure>

<footnote><sup>a</sup>Withnall & Andrews (1988).</footnote>

<footnote><sup>b</sup>Luckhaus (1997).</footnote>
<figure><img src="image_6.png" /><figcaption>Figure 3. Heating of NH₂OH–H₂O ice. (a) Deposition at 10 K. Heating at (b) 100 K, (c) 130 K, (d) 140 K, (e) 150 K, (f) 160 K, (g) at 170 K.</figcaption></figure>

without a carrier gas, lead mainly to the formation of samples con-
taining NH₂OH and H₂O species. The right-hand panel of Fig. 2
allows to show the evolution of the vibrational modes of NH₂OH
and H₂O at 10 K from a NH₂OH–H₂O–neon matrix to NH₂OH–
H₂O ice. Fig. 2(b) shows the absorption bands of the trapped species
in neon matrix at 10 K, while Fig. 2(d) shows the progress of those
bands after heating of the sample, at 20 K for a while, to remove
all neon atoms, followed by a cooling down of the sample to 10 K.
The similarity between the spectrum of Fig. 2(d) and that recorded
directly after a direct condensation during 2 h at 10 K of hydroxy-
lamine in Fig. 2e, shows that even though the experimental methods
are different but they may lead to the formation of the same solid
sample, namely a NH₂OH–H₂O ice.

After the vibrational characterization of NH₂OH–H₂O ices at 10 K, our main interest was to study the thermal stability of NH₂OH in contact with water molecules at low temperatures. A scenario that might be used, to characterize the NH₂OH molecule evolutions in some icy grains of the interstellar medium and various planetary atmospheres. Our NH₂OH–H₂O ice samples obtained at 10 K have been gradually heated at higher temperatures until a total desorp- tion of the sample. By increasing the temperature from 10 to 100 K, there were no visible changes in our infrared spectra, Figs 3(a) and (b). This phenomenon was not very surprising as most of the many previous experimental works related the chemistry of solid NH₂OH show that the desorption temperature of NH₂OH is higher than 160 K. Using the mass spectroscopy detection, where a mass signal at m/z = 33 corresponding to NH₂OH molecules has been detected at temperatures ranged between 160 and 190 K, Congiu et al. (2012) and He et al. (2015) have already reported that NH₂OH may be formed through NO hydrogenation or ammonia oxidation in solid phase at very low temperature and might desorb into the gas phase when the solid sample is heated. In our case, by heating a NH₂OH–H₂O ice mixture between 100 and 200 K and as it is exposed in Figs 3(c)–(g), the recorded IR spectra show decreases of NH₂OH absorption signals and the appearance of new IR features, while the mass spectra, recorded during the sample heating show a very low signal at m/z = 33 (NH₂OH) when the desorption temper- ature of hydroxylamine is reached around 180 K. Such behaviours

suggest that, instead of NH₂OH desorption, chemical transforma-
tions of NH₂OH molecules into water–hydroxylamine ice might
occur between 100 and 170 K.

From our infrared spectra, we show that the new IR feathers are
appearing clearly during the heating of NH₂OH–H₂O ices, before
reaching the desorption temperature of NH₂OH but only at temper-
ature above 130 K. These results reveal that the NH₂OH desorption
from the NH₂OH–H₂O ice may be very limited and most of NH₂OH
molecules might be transformed between 130 and 170 K into new
reaction products embedded into the sample ice.

Most of the detected new signals due to the formation of new
species overlap with the NH₂OH absorption bands. However, one
of these new chemical species shows an isolated spectral absorption
band around 980 cm⁻¹, far from the NH₂OH infrared signals. To
quantify such a thermal chemical transformation of NH₂OH instead
of its desorption, the right-hand panel of Fig. 3 shows a zoom of two
distinguishing IR absorption bands at 1194 cm⁻¹ and 980 cm⁻¹, cor-
responding to NH₂OH and one of the thermal reaction product from
NH₂OH decomposition in water–hydroxylamine ice, respectively.

The 980 cm⁻¹ band appears in the NH₃ vibration spectral re-
gion and it may be easily attributed to NH₃ molecules trapped in
NH₂OH–H₂O ice. Fedoseev et al. (2015) have already reported the
absorption band signal of ammonia in solid phase at 975 cm⁻¹. The
appearance in our infrared spectra of a band at 980 cm⁻¹ is con-
sistent with the presence of NH₃ diluted in ice, and not as pure
ammonia ice which has a characteristic band at 1070 cm⁻¹ (Richey
& Gerakines 2012). In fact, when ammonia is present in solid phase
as a minor species trapped in ice, its spectral position is shifted
to lower frequency, which is also reliable with Fedoseev’s work
who detected ammonia as a minor reaction product in solid phase,
around 975 cm⁻¹. Comparison between Figs 3(a)–(c), to explore
the decrease of the hydroxylamine absorption bands, shows that the
NH₂OH transformations start occurring only at temperatures higher
than 100 K and at 130 K only 15% of NH₂OH is transformed. The
whole transformation takes place between 130 and 170 K.

The closeup of two infrared spectra recorded for two extreme
temperatures, 10 and 170 K, associated to a subtraction spec-
trum, are shown in Fig. 4. The detection of ammonia under our
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Infrared spectra in the 800–1400 cm⁻¹ region of NH₂OH ice. (a) After deposition at 10 K. (b) At 170 K. (c) Subtraction spectrum between 170 and 10 K.</figcaption></figure>

experimental condition just by inducing a thermal reaction between water and hydroxylamine proves that NH₂OH molecules desorbing from NH₂OH–H₂O ice undergoes decomposition leading to the formation of new chemical species which may be formed and trapped into the NH₂OH–H₂O ice host between 100 and 170 K without any desorption (Collings et al. 2004; Zheng, Jewitt and Kaiser 2006; Brown & Bolina 2007; Martín-Doménech et al. 2014).

From the results exposed in Fig. 4 we can determine the column densities of our reactants and products N (molec cm⁻²), using the band strengths A (cm molec⁻¹) of the detected species, in order to get an order of magnitude of the composition of our samples before and after the reaction occurs at 10 and 170 K, respectively. Table 2 gathers, the characteristic vibrational frequencies, the band strengths and the column densities of the three detected species which show specifically characteristic isolated band absorptions, namely, H₂O, NH₃ and NH₂OH.

We show through those column density calculations that just after the deposition of the sample, the amount of NH₂OH is 3 times higher than that of H₂O, and while the concentration of H₂O decreases by only 20%, approximately 60% of NH₂OH have been decomposed from 10 to 170 K to form new reaction products. Among others reaction products, NH₃, which may be simply identified in our IR spectra, could be associated to the formation O₂, a species, easier to distinguish with a mass spectroscopy detection. In fact, although the nature of all the formed species in the NH₂OH thermal transformation will be argued in the discussion part, from the mass spectroscopy point of view, the observation of a very low mass signal of the desorbed NH₂OH at 180 K, in comparison with other desorbed species such as NH₃, HNO, H₂O and O₂, might also

be a prove of hydroxylamine decomposition instead of its simple desorption. As an example, Fig. 5 shows the evolution of desorbed NH₂OH and O₂ abundances.

NH₂OH and O₂ species start desorbing from the NH₂OH–H₂O refractory ice and reach highest desorption signals at 180 and 170 K, respectively. We show through this mass spectrum evolution that the signal due to the desorbed O₂ starts increasing at low temperature but not lower than 120 K which is consistent with the infrared measurements exposed in Fig. 3. The analysis of the hydroxylamine IR absorption bands illustrate that 15% of NH₂OH is transformed between 100 and 130 K and a clear IR signature of NH₃, one of the reaction products in the alteration of NH₂OH during the heating of the solid sample, is obviously detected only at temperatures above 130 K. Similarly, the mass spectrum evolution of desorbed O₂ reveals that oxygen may be another reaction product in the thermal transformation of NH₂OH. It is important to note that, the mass signal of desorbed O₂ shown in Fig. 5 is almost 60 times higher than that of NH₂OH, while the NH₂OH–H₂O solid sample formed at 10 K contained simply NH₂OH and H₂O as primary reactants, with an amount of NH₂OH, 3 times higher than that of H₂O. As a reference, we have added the mass spectrum of pure NH₂OH recorded under our experimental conditions, during the sample deposition (Fig. 6) where the main mass signal situated at m/z = 33 is due to NH₂OH molecules in addition to some low signals at m/z = 32, 31 and 30 due to the fragmentation patterns of hydroxylamine in the mass spectrometer.

## 4 DISCUSSION

The presence of NH₂OH, the main precursor molecule in the formation of amino acids, on dust grain mantles is probable even if all the space observations conclude that NH₂OH is not detected in gas phase. This work is to see if there is any chemical transformation of NH₂OH occurring in the solid phase before its desorption from the mantle of interstellar icy grains. Such a chemical conversion of hydroxylamine, taking place in the solid phase, could be a consequence of its very low abundance in space and then it might explain its non-detection. In this perspective our investigations have been devoted to describe the behaviours of NH₂OH–H₂O ices between 10 and 200 K; to simulate the desorption NH₂OH in presence of water molecules and monitor the NH₂OH evolution during the solid–gas transitions.

Our experimental results prove that by inducing progressively the evaporation of NH₂OH–H₂O ices; a reaction between water and hydroxylamine, the only main chemical species present in our samples, takes place even before the NH₂OH molecules start desorbing. By observing a decrease of NH₂OH infrared absorption signals and an appearance of new absorption bands, we conclude that a chemical transformation, leading to a decomposition of NH₂OH and a formation of some new chemical species, might be thermally induced. However some of the new detected absorption signals

<table><caption>**Table 2.** Characteristic vibrational frequencies, band strengths and column densities of H₂O, NH₃ and NH₂OH.</caption><thead><tr><th>Characteristic vibrational frequency (cm<sup>-1</sup>)</th><th>Molecule</th><th>Band strength<sup>a</sup></th><th>Vibrational mode</th><th>Column densities (molec cm<sup>-2</sup>), at 10 K</th><th>Column densities (molec cm<sup>-2</sup>), at 170 K</th></tr></thead><tbody><tr><td>760</td><td>H<sub>2</sub>O</td><td>3.1 &times; 10<sup>-17</sup></td><td>libration</td><td>0.18 &times; 10<sup>17</sup></td><td>0.14 &times; 10<sup>17</sup></td></tr><tr><td>980</td><td>NH<sub>3</sub></td><td>1.2 &times; 10<sup>-17</sup></td><td>NH wag</td><td>0</td><td>0.12 &times; 10<sup>17</sup></td></tr><tr><td>1194</td><td>NH<sub>2</sub>OH</td><td>1.2 &times; 10<sup>-17b</sup></td><td>NH wag</td><td>0.59 &times; 10<sup>17</sup></td><td>0.26 &times; 10<sup>17</sup></td></tr></tbody></table>

<footnote><sup>a</sup>Gerakines et al. (1995); Richey & Gerakines (2012).</footnote>

<footnote><sup>b</sup>Adopted from the value of NH₃.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Mass evolution of desorbed O₂ and NH₂OH during the heating of NH₂OH–H₂O ice. Desorbed O₂ signal reaches a maximum at 170 K while that of NH₂OH at 180 K.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Mass spectrum NH₂OH recorded during the sample deposition: fragmentation patterns of NH₂OH in the mass spectrometer.</figcaption></figure>

overlap with those of NH₂OH, as shown in Fig. 3 in 2800 and 1600 cm⁻¹ spectral regions, and this makes it impossible to separate the reactants, namely water and hydroxylamine from the new products. Thankfully, one of these new chemical products shows a strong and isolated spectral absorption band around 980 cm⁻¹, in the NH₃ vibration spectral region and it might be easily attributed to NH₃ molecules, trapped in NH₂OH–H₂O ice, or coupled to other chemical species in the solid sample.

To have a clear description of NH₂OH–H₂O reaction in solid phase at cryogenic temperatures, lower than that of NH₂OH desorption, we have first to discuss the reaction pathways leading to a decomposition of hydroxylamine. Regarding such a molecule with N–H, N–O and O–H chemical bonds, NH₂OH may decompose into several fragments, depending on involved reaction mechanisms and on the nature of the chemical bonds engaged in such a decomposition. Several theoretical investigations have been interested in the formation and decomposition reactions of hydroxylamine. According

to Feller & Dixon (2003) NH₂OH may react with water to form NH₃ and H₂O₂:

$$H_2O + NH_2OH \rightarrow NH_3 + H_2O_2$$

This reaction is found to be exothermic, showing a heat of formation ΔH = -11 kcal mol⁻¹. Such a reaction may take place under our experimental conditions, as we have detected a very specific signal characteristic of ammonia, and then it might explain the low stability of hydroxylamine in contact with water at temperatures lower than that of hydroxylamine desorption. In this situation H₂O₂ may be considered as the other reaction product formed during the thermal decomposition of NH₂OH–H₂O ices. However, an accurate analysis of our infrared spectra show no characteristic signals related to H₂O₂ which can be understandable, as many investigations have already shown that H₂O₂ might decompose into H₂O + O (Giguère & Liu 1957).

The association of our experimental results and Feller's group calculations allows asserting that the thermal decomposition of NH₂OH–H₂O ices leads to the formation of ammonia and probably atomic oxygen as new reaction products. As, the reaction is occurring in solid phase at cryogenic temperatures, those oxygen atoms may complex with ammonia molecules to form NH₃O or recombine to form molecular oxygen. In this case the heated NH₂OH–H₂O ice may decompose into NH₃ + O₂ + H₂O instead of simple desorption of NH₂OH and H₂O.

Similar calculations have been carried out to study the decomposition of NH₂OH aggregates and NH₂OH in contact with water molecules. Wang et al. (2010) investigated a thermal decomposition of NH₂OH more deeply by varying the amount of the reactants, namely NH₂OH and H₂O. They proposed a decomposition mechanism involving dimmers or aggregates of hydroxylamine. They show that the association of two hydroxylamine molecules lead to the formation of H₂O, NH₃ and HNO as reaction products:

$$2NH_2OH \rightarrow H_2O + NH_3 + HNO$$

HNO, molecules is characterized by three IR absorption signals at 2685, 1565 and 1500 cm⁻¹ corresponding to NH stretching, NO
stretching and HNO bending vibrational modes (Clough et al. 1973). The formation of HNO as a new reaction product, in addition to ammonia, in the thermal decomposition of NH₂OH–H₂O ices may be possible as the analysis of our infrared spectra, exposed in Fig. 3, shows an increase of a signal around 2700 cm⁻¹ which may be due to the NH stretching mode of HNO molecules, while that of NH₂OH is decreasing. Also, in the spectral region between 1500 and 1600 cm⁻¹, even though partially overlap with NH₂OH signals, new appearing signal may be due to the two other vibrational modes of HNO.

Wang's group calculations show that NH₂OH dimmer lead to a decomposition of NH₂OH to form ammonia, which is consistent with our experimental condition where solid samples were prepared using a direct condensation of pure NH₂OH favouring the formation of NH₂OH aggregates and also NH₂OH–H₂O clusters. The role of those aggregates can be underlined by analysing the reaction of NH₂OH decomposition from formation and activation energies point of view. Starting from one NH₂OH molecule, Wang et al. (2010) showed that hydroxylamine molecule undergoes isomerization into ammonia oxide:

$$NH_2OH \xrightarrow{50 \text{ kcal mol}^{-1}} NH_3O \quad \Delta H = +25 \text{ kcal mol}^{-1}$$

This reaction is highly endothermic, showing a very high activation energy and it probably needs an initiating energy to convert NH₂OH into NH₃O. However by passing from monomolecular mechanism to bimolecular mechanism, a situation more favourable in solid phase, the energy barrier decreases from 50 kcal mol⁻¹ to 25 kcal mol⁻¹, and the energy formation decreases from +25 kcal mol⁻¹ to +15 kcal mol⁻¹.

$$2NH_2OH \xrightarrow{25 \text{ kcal mol}^{-1}} NH_3O + NH_2OH \quad \Delta H = +15 \text{ kcal mol}^{-1}$$

Wang et al. limited its calculations to the isomeric conversion reaction for only three NH₂OH molecules.

$$2NH_2OH + NH_2OH \xrightarrow{20 \text{ kcal mol}^{-1}} NH_3O + 2NH_2OH$$

$$\Delta H = +10 \text{ kcal mol}^{-1}$$

Similarly, with three NH₂OH molecules the conversion mechanism leading to the formation of NH₃O confirm a decrease of both activation and formation energies showing that the reaction is more favourable in solid phase by increasing the NH₂OH aggregate size, which is consistent with our experimental conditions. From these calculations, we can imagine that by increasing the number of NH₂OH molecules, the activation energy might decrease to zero and the NH₂OH conversion reaction would become highly exothermic. In such a situation, calculations are required to validate or invalidate this conclusion.

From Table 2, we show that even if 60% of NH₂OH decompose between 10 and 170 K to form new reaction products, concentration of H₂O also decreases by 20%, showing that water molecules may also play a role in NH₂OH transformations. Wang et al. investigated the influence of the number of water molecules into the NH₂OH alteration reaction:

$$2NH_2OH + H_2O \xrightarrow{21 \text{ kcal mol}^{-1}} NH_3O + NH_2OH + H_2O$$

$$\Delta H = +13 \text{ kcal mol}^{-1}$$

By comparing reactions involving three NH₂OH molecules to that obtained by substituting one of the three NH₂OH molecules by one H₂O molecule, we notice that the NH₂OH conversion into

NH₃O occurs slightly under the same energetic condition. Both of the reactions are endothermic with almost the same activation energy, around 20 kcal mol⁻¹. However by increasing the number of water molecules, the NH₂OH conversion into NH₃O become more favourable showing a considerable decrease of the energy barrier.

$$2NH_2OH + 2H_2O \xrightarrow{18 \text{ kcal mol}^{-1}} NH_3O + NH_2OH + 2H_2O$$

$$\Delta H = +7 \text{ kcal mol}^{-1}$$

$$2NH_2OH + 3H_2O \xrightarrow{16 \text{ kcal mol}^{-1}} NH_3O + NH_2OH + 3H_2O$$

$$\Delta H = +4 \text{ kcal mol}^{-1}$$

Wang et al. limited these calculations to three water molecules and two hydroxylamine molecules. From those calculations we can also conclude that by increasing the number of water molecules in contact with hydroxylamine, we would probably transform the NH₂OH into NH₃O through a barrierless and exothermic process.

Connected to our experimental results, Wang et al. show that, since ammonia oxide is less stable than hydroxylamine, further reactions may occur, leading to a decomposition of NH₃O, in absence of water molecules, into HNO and H₂. They also, showed that, in contact with water, ammonia oxide NH₃O may decompose, leading to the formation of ammonia and hydrogen peroxide as new reaction products in the transformation of NH₂OH.

$$NH_3O \rightarrow HNO + H_2$$

$$NH_3O + H_2O \rightarrow H_2O_2 + NH_3$$

The association of our experimental results and those calculations, enables to affirm that the heating of NH₂OH–H₂O ices may lead to a decomposition of NH₂OH before reaching its desorption temperature. The decomposition of NH₂OH instead of its desorption may be clearly proved though the infrared spectroscopy used as a probe method to show that the decrease of NH₂OH infrared signals is accompanied by an increase new IR signals due to the appearance of new reaction products. We showed that among those new signals, the most important one is that strong and isolated signal located at 980 cm⁻¹ in the NH₃ vibration spectral region and which it may be due to NH₃ molecules as well as to NH₃O molecule. The latter species has never been detected experimentally. However even though the other new signals overlap partially with the NH₂OH absorption bands, we are able to associate the final reaction products predicted by the theory and those detected experimentally, namely, HNO and O₂ which have characteristic absorption signals in the spectral region between 1500 and 1600 cm⁻¹ (Bennett & Kaiser 2005).

## 5 CONCLUSION

Our investigations on the evolution of hydroxylamine via a thermal chemistry during the desorption of NH₂OH–H₂O ices in the temperature range of 10–200 K allow the characterization of NH₂OH stability, by coupling infrared spectroscopic analysis and temperature-programmed desorption. Knowing that the NH₂OH desorption temperatures range from 160 to 190 K, we show, through this study, there is a competition between NH₂OH desorption into the gas phase and its chemical transformation in the solid phase, occurring at low temperature and leading to the formation of into HNO, NH₃ and O₂. This is the first infrared spectroscopic experimental study probing at different temperatures an ice formed
directly by condensing NH₂OH molecules. From our experimental results, we prove that the chemical transformation of NH₂OH starts occurring in solid phase at 120 K, a temperature far lower than that of NH₂OH desorption and this may reduce considerably the abundance of NH₂OH molecules releasing in the gas phase. We also show that the hydroxylamine chemical transformation is due mainly to the presence of high amount H₂O aggregates, thus consistent with the solid phase icy interstellar grain chemistry. Finally, those results might be the primary explanation for the non-detection of hydroxylamine in space.

## ACKNOWLEDGEMENTS

This work was supported in part by the LabEx MiChem 'French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02'.

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