# Ion induced formation of complex organic nitrogen molecules in solid-phase adenine

Filip Matuszewski, Véronique Vuitton, Julia Shouse, Thibault Launois, Naïla Chaouche-Mechidal, Elsa Hénault, Laurène Flandinet, Éric Quirico, Rosario Brunetto, Philippe Boduch, et al.

► To cite this version:

Filip Matuszewski, Véronique Vuitton, Julia Shouse, Thibault Launois, Naïla Chaouche-Mechidal, et al. Ion induced formation of complex organic nitrogen molecules in solid-phase adenine. Icarus, 2025, 445, pp.116865. 10.1016/j.icarus.2025.116865. hal-05357879

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Research Paper

# Ion induced formation of complex organic nitrogen molecules in solid-phase adenine

<figure><img src="image_3.png" /></figure>

Filip Matuszewski<sup>a,†</sup>, Véronique Vuitton<sup>a</sup>, Julia Shouse<sup>a</sup>, Thibault Launois<sup>a</sup>, Naïla Chaouche-Mechidal<sup>b</sup>, Elsa Hénault<sup>c</sup>, Laurène Flandinet<sup>a</sup>, Èric Quirico<sup>a</sup>, Rosario Brunetto<sup>c</sup>, Philippe Boduch<sup>d</sup>, Alicja Domaracka<sup>d</sup>, Hermann Rothard<sup>d</sup>, Fabien Stalport<sup>e</sup>, Francois Regis Orthous Daunay<sup>a</sup>, Hervé Cottin<sup>b</sup>, Sarah Hörst<sup>f</sup>, Roland Thissen<sup>g</sup>

## ARTICLE INFO

### Keywords:

- Adenine
- keV ion irradiation
- Ultra-high-resolution mass spectrometry
- HCN-polymers
- Complex organic chemistry
- Tholins
- Infrared spectroscopy

## ABSTRACT

Complex organic molecules have been found in many space environments like meteorites and comets. There, they are subjected to various forms of radiation (photons, electrons, ions), opening the question of their behavior and chemical evolution, which may have played a crucial role in chemical processes of astrobiological interest. In this study, we investigate the irradiation of adenine ($C_5H_5N_5$) with oxygen and neon ions. The free parameters of the experiments include varying energies (30-70 keV), temperatures (150, 300 K), sample thicknesses (138-554 nm), and ion fluences ($0.69-5 \times 10^{15}$ ions cm$^{-2}$). In situ IR spectroscopy reveals the appearance of $C \equiv N$ and $N=C=N$ bands indicating the formation of new species. Ex situ ultra-high-resolution mass spectrometry shows the formation of new complex organic molecules that far exceed the molecular mass of adenine. These macromolecules show great chemical diversity and can be expressed as (HCN)$_x$R families, where z can reach 14 and R can be $C_xH_xN_x$, $N_xC_xH_x$, $C_xH_x$ or $C_xN_y$. In total, nearly 100 individual families have been identified, 28 of which can be found in every irradiated sample. Their aromaticity equivalent is higher than that in other N-rich samples such as Titan tholins and HCN-polymers, corresponding to polycyclic aromatic nitrogen-bearing hydrocarbons. The high amount of nitrogen in these molecules indicates a very efficient incorporation of nitrogen in the solid phase during the irradiation of adenine. The ease with which complex organic matter forms through irradiation highlights the relevance of these species in space environments.

## 1. Introduction

Complex organic molecules play an important role in the chemical evolution of the universe and the origins of life. To date, more than 200 different molecular species have been found in space (McCarthy and McGuire, 2021; Guélin and Cernicharo, 2022). It has been suggested that these complex molecules and their chemical precursors did not develop on but rather reached early Earth through comets and meteorites (Chyba and Sagan, 1992; Ehrenfreund and Charnley, 2000), although this remains a subject of ongoing debate. One group of molecules is of particular interest: polycyclic aromatic nitrogen-bearing hydrocarbons (PANH). These molecules have been known to be present

in meteorites (Stoks and Schwartz, 1981, 1982) and more recently in the atmosphere of Titan (López-Puertas et al., 2013; Haythorn-thwaite et al., 2021). In the interstellar medium (ISM), polycyclic aromatic hydrocarbons (PAH) without nitrogen are confirmed to be common (Onaka et al., 1996; Mattila et al., 1996; Flagey et al., 2006; Tielens, 2008; Salama et al., 2011). PANHs have not been observed in the ISM yet, however, modeling efforts (Devi et al., 2022) as well as laboratory experiments (Yang et al., 2020; Barik et al., 2023) show that the existence of PANHs in the ISM is possible. Wang et al. (2024) propose interstellar formation pathways based on laboratory experiments

<footnote><sup>a</sup> Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, 38000, France</footnote>

<footnote><sup>b</sup> Univ Paris Est Créteil and Université Paris Cité, CNRS, LISA, Créteil, F-94010, France</footnote>

<footnote><sup>c</sup> Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, Orsay, 91405, France</footnote>

<footnote><sup>d</sup> Centre de Recherche sur les Ions, les Matériaux et la Photonique CIMAP Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, Caen, 14000, France</footnote>

<footnote><sup>e</sup> Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, Paris, 75013, France</footnote>

<footnote><sup>f</sup> Johns Hopkins University, Department of Earth and Planetary Science, Baltimore, MD 21218, USA</footnote>

<footnote><sup>g</sup> Université Paris-Saclay, CNRS, Institut de Chimie Physique, UMR8000, Orsay, 91405, France</footnote>

<footnote>* Corresponding author.
E-mail address: filip.matuszewski@univ-grenoble-alpes.fr (F. Matuszewski).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic chemical structure of an adenine molecule.</figcaption></figure>

for simple PANHs like indole (C<sub>8</sub>H<sub>7</sub>N) and their building blocks pyrrole (C<sub>4</sub>H<sub>5</sub>N) and aniline (C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>) representing important precursor molecules for nucleic bases.

The nucleic base adenine (C<sub>5</sub>H<sub>5</sub>N<sub>5</sub>, see Fig. 1) is not only an essential component of DNA and thus a key molecule for life as we know it, but it can also serve as a representative proxy for the broader class of PANHs. Studying adenine under astrophysical conditions can therefore yield insights into the behavior of both biomolecular building blocks and PANHs in space. Adenine has been detected in the Orgeuil (Hayatsu, 1964) and Murchison (Hayatsu et al., 1975). Apart from that, adenine has also been found in synthetic analogs of Titan's aerosols, commonly referred to as tholins (Pilling et al., 2009; Hörst et al., 2012; Poch et al., 2012; He and Smith, 2014; Kawai et al., 2019).

Wherever in space, molecules are subjected to various forms of radiation (i.e. photons, electrons, ions). This raises the question of the chemical behavior and evolution of PANHs when exposed to radiation and what possible photo-products might emerge from that. Previous studies have examined the effect of irradiating solid adenine with radiation sources like ions (Huang et al., 2014; Vignoli Muniz et al., 2017), UV-light (Peeters et al., 2003; Guan et al., 2010; Poch et al., 2014), X-rays (Pilling et al., 2011) and electrons (Evans et al., 2011), investigating the survivability of this nucleic base in space conditions. These studies monitored the irradiation of adenine with infrared (IR) spectroscopy. The spectrum of adenine exhibits two major broad absorption features between ~1200–1700 cm<sup>-1</sup> and ~2500–3500 cm<sup>-1</sup> (Saïagh et al., 2014). Several studies have shown the photodecomposition of adenine with the decrease of absorbance over irradiation time (Peeters et al., 2003; Pilling et al., 2011; Evans et al., 2011; Huang et al., 2014; Vignoli Muniz et al., 2017). Studies using low energy UV-radiation unanimously conclude that adenine possesses a greater photostability than other organic compounds like uracil and glycine. Poch et al. (2014) reports the formation of new appearing bands at ~2200 cm<sup>-1</sup> identifying them as nitriles and hinting at the formation of macromolecules. These would serve as a protective layer that is likely to be the cause for the increase of adenine's photostability. Similar spectral features representative of nitriles could also be observed by Evans et al. (2011) while irradiating adenine with 5 keV electrons. However, Peeters et al. (2003) and Guan et al. (2010) detect no such bands. Studies that used energies > 150 eV report the destruction of adenine and fragmentation into smaller molecules (HCN, C<sub>2</sub>H<sub>4</sub>N<sub>4</sub>) or radicals (·CN) in solid phase (Vignoli Muniz et al., 2017). At higher energies corresponding to cosmic ray analogs, the spectrum shows absorption bands corresponding to HCN, CN<sup>-</sup> and C<sub>2</sub>H<sub>4</sub>N<sub>4</sub>. Furthermore, destruction cross sections have been reported that are 10<sup>9</sup>, 10<sup>5</sup> and 10<sup>3</sup> times higher than UV photons, soft X-rays

and 5 keV electrons, respectively (Vignoli Muniz et al., 2017). Table 1 shows a summary of previous studies irradiating adenine with different irradiation sources and the change in chemistry they could observe. To summarize, these previous studies have mainly relied on the analysis of the spectral properties of adenine by interpreting IR spectra. Some decomposition mechanisms of adenine and formation pathways of new species have been proposed, but not thoroughly investigated, as the focus was put on the destruction of adenine.

In this study, we investigate the effects of irradiating adenine thin films with 30 to 70 keV O<sup>q+</sup>- and Ne<sup>q+</sup>-ions at two different irradiation facilities: Grand Accélérateur National d'Ions Lourds (GANIL) and Laboratoire de Physique des 2 Infinis Irène Joliot Curie (IJCLab). This energy range has only been investigated at the lower end with 30 keV Ar<sup>q+</sup>-ions (Huang et al., 2014). Oxygen ions in this energy range have been observed in various locations of the Solar System like by the Advanced Composition Explorer (ACE) at Lagrange point 1 originating from solar winds (Mewaldt et al., 2007), by Phobos 2 around Mars (Cravens et al., 2002), from cometary coma (Cravens, 1989) or by Cassini in the Saturnian System (Johnson et al., 2005; Sittler et al., 2006). Furthermore, oxygen is a highly reactive species that can induce oxidation, fragmentation, and structural modifications, potentially leading to the formation of novel functional groups or molecules with astrobiological implications.

This study will shed light on the chemical evolution of adenine, when submitted to ionizing radiation present in the Solar System. Here, in-situ IR spectroscopy and ex-situ ultra high resolution mass spectrometry (HRMS) will give insight into the chemical properties of irradiated adenine. The nature of the macromolecules formed during irradiation of adenine will be answered here for the first time. Additionally, these newly appearing macromolecules will be compared to Tholins and HCN-polymers to explore potential similarities and differences in their chemical structures and properties.

## 2. Methods

The experiments carried out in this study can be divided into three parts. Part one concentrates on the preparation of the samples describing the different techniques to produce adenine samples. Next, the irradiation experiments are presented where the samples are subjected to different ion beams and characterized in situ by IR spectroscopy. The last part focuses on the ex-situ mass spectrometry performed on the samples.

### 2.1. Sample preparation

Two distinct methods were used to prepare the adenine samples. For the first method, done at the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), 1 mg of adenine (Aldrich ≥ 99%) was dissolved in 1 ml of ethanol-water solution (60.8% ethanol in water, v/v (Krzaczkowska et al., 2004; Vignoli Muniz et al., 2017)) with the help of an ultrasonic bath. This solution was then deposited drop-wise onto 20 mm diameter MgF<sub>2</sub>-windows. The solvent was evaporated in an oven at 100 °C leaving an adenine film on the windows. As the resulting films from this method show an inhomogeneous thickness throughout the sample, they will be referred to as “grainy” from here on Muniz (2017). In the second method, adenine samples were prepared at the Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA). Here, a vacuum sublimation system, developed by Guan et al. (2010), was used to produce homogeneous films. In this vacuum chamber, adenine powder (Aldrich ≥ 99%) is placed in an oven and heated over its sublimation temperature of 130 °C at a pressure of ~1 × 10<sup>-4</sup> mbar. Recondensation onto a 20 mm diameter MgF<sub>2</sub> window follows. We call these homogeneous samples “thin-films”.

The thickness of the thin-films was chosen as the penetration depth of the ions should not be larger than the thickness itself. This ensures that the ions are implanted in the thin-films rather than traverse them
<caption>**Table 1**</caption>

Summary table of previous studies irradiating adenine; methods are exclusively referring to the analysis of the solid phase with Fourier transform infrared spectrometer (FTIR).

<table><thead><tr><th>Study</th><th>Irradiation type</th><th>Energy</th><th>Change in chemistry</th></tr></thead><tbody><tr><td>Peeters et al. (2003)</td><td>UV</td><td>–</td><td>Destruction</td></tr><tr><td>Pilling et al. (2011)</td><td>X-rays</td><td>150 eV</td><td>Destruction, photo-product: HCN</td></tr><tr><td>Evans et al. (2011)</td><td>electrons</td><td>5 keV</td><td>Destruction, formation of nitriles</td></tr><tr><td>Huang et al. (2014)</td><td>Ar<sup>+</sup></td><td>30 keV</td><td>Destruction</td></tr><tr><td>Poch et al. (2014)</td><td>UV</td><td>–</td><td>Destruction, formation of nitriles</td></tr><tr><td rowspan="6">Vignoli Muniz et al. (2017)</td><td>Xe<sup>23+</sup></td><td>92 MeV</td><td rowspan="6">Destruction, photo-products<br>HCN,<br>C<sub>2</sub>H<sub>4</sub>N<sub>4</sub></td></tr><tr><td>Kr<sup>31+</sup></td><td>820 MeV</td></tr><tr><td>Ca<sup>10+</sup></td><td>190 MeV</td></tr><tr><td>Ca<sup>10+</sup></td><td>190 MeV</td></tr><tr><td>C<sup>4+</sup></td><td>12 MeV</td></tr><tr><td>C<sup>4+</sup></td><td>12 MeV</td></tr></tbody></table>

and be implanted in the underlying MgF₂-window. The penetration depth of the ions was calculated with the software SRIM (Ziegler et al., 2010) assuming a density of 1.47 g/cm³ (Kilday and M.V, 1978). For the interested reader, we have included calculations of the nuclear and electronic stopping powers and doses in the supplementary information. The thickness *z* of the sample was monitored during the recondensation using interferometry and confirmed at a later stage by analyzing IR spectra, where the Beer-Lambert law states that the absorption area is proportional to the column density. It can be calculated with

$$z = \ln(10) \frac{NM}{\rho N_a} \qquad (1)$$

where *M* is the molar mass of adenine (135.13 g/mol), *ρ* its density and *N*<sub>a</sub> the Avogadro constant. The column density is determined by

$$N = \frac{a}{A} \qquad (2)$$

where *a* is the area under the 1970–3600 cm⁻¹ of the IR spectrum, called α-band by Saïagh et al. (2014). The value *A* is taken from the same study and is equivalent to 7.9 × 10⁻¹⁶ cm molecule⁻¹. The error for the thicknesses includes a 1% error in the density (Kilday and M.V, 1978), 4% for *A* (Saïagh et al., 2014) and a 10% uncertainty, which we estimated for *a* as this value strongly depends on the implemented baseline correction. Due to the uneven surface of the grainy samples, we were not able to obtain suitable IR-spectra and therefore reliable values for the thicknesses.

## 2.2. Irradiation experiments

The irradiation experiments were performed at two different facilities in 2021 and 2022. The first one is the Grand Accélérateur National d'Ions Lourds (GANIL) in Caen, France. Here, the ARIBE beam line was used together with the IGLIAS setup (Irradiation de GLaces d'Intérêt Astrophysique) (Augé et al., 2018). The second is the Laboratoire de Physique des 2 Infinis Irène Joliet Curie (IJCLab) with the SIDONIE ion seperator and the Institut d'Astrophysique Spatiale (IAS) in Orsay, France, using the INGMAR chamber set-up. In total, 17 adenine samples were irradiated with ¹⁸Oq⁺, ¹⁷Oq⁺ and ²⁰Neq⁺-ion beams with energies ranging from 30 to 70 keV (see Table 2). The neon ions, being chemically inert, serve as a reference point for comparison against the reactive oxygen ions. The maximum total fluence was varied depending on the thickness of the sample with the highest reaching 5 × 10¹⁵ ions cm⁻². The sample holder was cooled down to 150 K for a selected few samples to have a cold-temperature comparison to the majority of the samples that were kept at 300 K. This was done as there was the possibility of residual water vapor condensing to ice inside the chamber while cooling, despite reaching an ultra-high vacuum of down

to 9 × 10⁻⁹ mbar (Augé et al., 2018). Table 2 shows the sample names which are a composition of thin-film type (G or T), year (21 or 22), ion (Oxygen or Neon) and energy (30, 35, 50, 60 or 70 keV). During the irradiation, Infrared (IR) Spectra of the samples were captured using a Fourier-Transform infrared spectrometer (FTIR, Bruker Vertex 70V at GANIL, Bruker TENSOR 37 at IJCLab). The resolution was set to 1 cm⁻¹ and 2 cm⁻¹ at GANIL and IJCLab, respectively. At IJCLab, the irradiations were paused for acquiring the spectra, resulting in a total number of up to 8 spectra being taken for these samples. At GANIL, the spectra were taken simultaneously with the irradiation in 6.5 s intervals, capturing between 29 and 60 spectra depending on the sample. This was possible as the ion flux can be tracked in real time at GANIL, which was not done at IJCLab. The ion flux reached ~0.3–1.8 × 10¹² ions cm⁻² s⁻¹ and ~0.25–1 × 10¹² ions cm⁻² s⁻¹ at GANIL and IJCLab, respectively. Background measurements were done on a clean MgF₂-window without an adenine thin-film. For the interested reader, we added calculations of the electronic and nuclear stopping powers and doses in the Supplementary.

## 2.3. Ex-situ analysis

The chemical composition of the samples was determined using an ultra high-resolution LTQ Orbitrap XL™ mass spectrometer (Thermo Fisher Scientific). An IonMax Electrospray Ionization System (ESI) allows the analysis of the soluble part of the samples. This soft ionization adds (positive ion mode) or removes (negative ion mode) one or several protons without fracturing the rest of the molecule. For preparation, the samples were solubilized in a solvent made out of 1/3 parts toluene and 2/3 methanol by volume. The sample windows were immersed in 2 ml of solvent overnight and then left in an ultrasonic bath for 20 min. Centrifuging the solution for 10 min at 10000 rpm ensures that the insoluble components of the sample are separated from the solution. In addition to the irradiated adenine samples, which we will call IrAS from now on, a grainy, non-irradiated adenine sample was also dissolved, to serve as a blank sample. Mass spectra were captured at different m/z ranges in negative ion mode. The m/z ranges 50–130 m/z and 150–500 m/z were selected to obtain a clear picture of molecules around the mass of adenine, excluding it to ensure that the prominent adenine peak is not overshadowing smaller peaks in the spectrum. The voltage of the ionization source was kept at 3.5 kV with the tube lens at 72 V. The Capillary voltage maintained at 50 V with a temperature of 275 °C. The solvent was injected into the ESI at a rate of 3 µl/min while the maximum ion injection time was set to not exceed 500 ms. With the resolving power of the Orbitrap set to 100 000, the mass spectra contain up to 4000 peaks each after filtering for noise. The interpretation of the mass spectra was done using a program called
<table><caption>**Table 2**

List of adenine samples (part 1). Samples are marked with a * for 150K and ' for ¹⁷O-ions.</caption><thead><tr><th>Sample</th><th>Film</th><th>t [nm]</th><th>Irradiation facility - date</th><th>Ion</th><th>E [keV]</th><th>T [K]</th><th>F [10<sup>15</sup> ions cm<sup>-2</sup>]</th></tr></thead><tbody><tr><td>G21Ne70</td><td>grainy</td><td>-</td><td>GANIL - 23.06.2021</td><td><sup>20</sup>Ne<sup>4+</sup></td><td>70</td><td>300</td><td>1.29</td></tr><tr><td>G21Ox70</td><td>grainy</td><td>-</td><td>GANIL - 22.06.2021</td><td><sup>18</sup>O<sup>5+</sup></td><td>70</td><td>300</td><td>1.71</td></tr><tr><td>G21Ox70*</td><td>grainy</td><td>-</td><td>GANIL - 22.06.2021</td><td><sup>18</sup>O<sup>5+</sup></td><td>70</td><td>150</td><td>1.63</td></tr><tr><td>G21Ox70'</td><td>grainy</td><td>-</td><td>GANIL - 23.06.2021</td><td><sup>17</sup>O<sup>4+</sup></td><td>70</td><td>300</td><td>4.96</td></tr><tr><td>T22Ox70</td><td>thin</td><td>487 ± 53</td><td>GANIL - 28.09.2022</td><td><sup>18</sup>O<sup>4+</sup></td><td>70</td><td>300</td><td>1.00</td></tr><tr><td>G22Ox60</td><td>grainy</td><td>-</td><td>IJCLab - 08.09.2022</td><td><sup>18</sup>O<sup>2+</sup></td><td>60</td><td>300</td><td>1.60</td></tr><tr><td>T22Ox60</td><td>thin</td><td>408 ± 44</td><td>IJCLab - 08.09.2022</td><td><sup>18</sup>O<sup>2+</sup></td><td>60</td><td>300</td><td>0.80</td></tr><tr><td>T22Ne50</td><td>thin</td><td>554 ± 60</td><td>GANIL - 26.09.2022</td><td><sup>20</sup>Ne<sup>4+</sup></td><td>50</td><td>290</td><td>1.00</td></tr><tr><td>T22Ox50</td><td>thin</td><td>549 ± 59</td><td>GANIL - 27.09.2022</td><td><sup>18</sup>O<sup>4+</sup></td><td>50</td><td>290</td><td>1.00</td></tr><tr><td>T22Ox50*</td><td>thin</td><td>463 ± 50</td><td>GANIL - 27.09.2022</td><td><sup>18</sup>O<sup>4+</sup></td><td>50</td><td>150</td><td>1.00</td></tr><tr><td>T21Ne35</td><td>thin</td><td>138 ± 15</td><td>GANIL - 23.06.2021</td><td><sup>20</sup>Ne<sup>3+</sup></td><td>35</td><td>300</td><td>2.75</td></tr><tr><td>T21Ox35</td><td>thin</td><td>210 ± 23</td><td>GANIL - 21.06.2021</td><td><sup>18</sup>O<sup>5+</sup></td><td>35</td><td>300</td><td>0.93</td></tr><tr><td>T21Ox35*</td><td>thin</td><td>168 ± 18</td><td>GANIL - 21.06.2021</td><td><sup>18</sup>O<sup>5+</sup></td><td>35</td><td>150</td><td>3.04</td></tr><tr><td>G21Ox35'</td><td>grainy</td><td>-</td><td>GANIL - 24.06.2021</td><td><sup>17</sup>O<sup>4+</sup></td><td>35</td><td>300</td><td>5.00</td></tr><tr><td>G22Ne30</td><td>grainy</td><td>-</td><td>IJCLab - 07.09.2022</td><td><sup>20</sup>Ne<sup>+</sup></td><td>30</td><td>300</td><td>1.60</td></tr><tr><td>T22Ne30</td><td>thin</td><td>485 ± 53</td><td>IJCLab - 07.09.2022</td><td><sup>20</sup>Ne<sup>+</sup></td><td>30</td><td>300</td><td>1.60</td></tr><tr><td>T22Ox30</td><td>thin</td><td>389 ± 42</td><td>IJCLab - 09.09.2022</td><td><sup>18</sup>O<sup>+</sup></td><td>30</td><td>300</td><td>0.69</td></tr></tbody></table>

t: thickness. E: total energy of the ion. T: temperature. F: maximum fluence.

<table><caption>**Table 3**

Assignments of adenine samples major bands for this work (irradiated) and studies from Mohamed et al. (2009), Saïagh et al. (2014) and Vignoli Muniz et al. (2017). Wavenumbers marked with * are new appearing bands as a result of irradiation. The value in parentheses corresponds to the new appearing band only visible for samples at 150K.</caption><thead><tr><th rowspan="2">Assignments</th><th colspan="4">Wavenumber [cm<sup>-1</sup>]</th></tr><tr><th>This work</th><th>Mohamed et al. (2009)</th><th>Saïagh et al. (2014)</th><th>Vignoli Muniz et al. (2017)</th></tr></thead><tbody><tr><td>NH<sub>2</sub></td><td>1018</td><td>1025</td><td>1023</td><td>1022</td></tr><tr><td>N<sub>(9)</sub>H</td><td>-</td><td>1065</td><td>-</td><td>-</td></tr><tr><td>CN R(5)</td><td>1125</td><td>1126</td><td>1120</td><td>1126</td></tr><tr><td>CN R(5)</td><td>1252</td><td>1252</td><td>1251</td><td>1252</td></tr><tr><td>C<sub>(8)</sub>H</td><td>1252</td><td>1252</td><td>1251</td><td>1252</td></tr><tr><td>CN R(6)</td><td>1308</td><td>1309</td><td>1307</td><td>1308</td></tr><tr><td>CN R(5)</td><td>1332</td><td>1335</td><td>1330</td><td>1332</td></tr><tr><td>C<sub>(6)</sub>N<sub>(10)</sub></td><td>1367</td><td>1368</td><td>1366</td><td>1367</td></tr><tr><td>C<sub>(12)</sub>H</td><td>1418</td><td>1420</td><td>1417</td><td>1418</td></tr><tr><td>CC R(6)</td><td>1450</td><td>1452</td><td>-</td><td>1451</td></tr><tr><td>CN R(6)</td><td>1485</td><td>1483</td><td>-</td><td>-</td></tr><tr><td>CN R(5)</td><td>1505</td><td>1506</td><td>-</td><td>1508</td></tr><tr><td>CC R(6)</td><td>1603</td><td>1603</td><td>1600</td><td>1600</td></tr><tr><td>CN R(6)</td><td>1603</td><td>1603</td><td>1600</td><td>1600</td></tr><tr><td>NH<sub>2</sub></td><td>1667</td><td>1673</td><td>1665</td><td>1668</td></tr><tr><td>HCN</td><td>(2088*)</td><td>-</td><td>-</td><td>(2100*&amp;2090*)</td></tr><tr><td>N=C=N</td><td>2139*</td><td>-</td><td>-</td><td>-</td></tr><tr><td>C≡N</td><td>2220*</td><td>-</td><td>-</td><td>2212*&amp;2174*</td></tr><tr><td>C<sub>(12)</sub>H</td><td>2984</td><td>2980</td><td>2984</td><td>2975</td></tr><tr><td>C<sub>(8)</sub>H</td><td>2688</td><td>-</td><td>2689</td><td>2685</td></tr><tr><td></td><td>2798</td><td>-</td><td>2798</td><td>2790</td></tr><tr><td></td><td>2984</td><td>2980</td><td>2984</td><td>2975</td></tr><tr><td></td><td>3127</td><td>3119</td><td>3122</td><td>3115</td></tr><tr><td>NH<sub>2</sub></td><td>3317</td><td>3296</td><td>3314</td><td>3293</td></tr><tr><td>N<sub>(9)</sub>H</td><td>-</td><td>3347</td><td>-</td><td>3347</td></tr><tr><td>NH<sub>2</sub></td><td>3430</td><td>3426</td><td>-</td><td>-</td></tr></tbody></table>

ATTRIBUTOR, developed in house for the software IGOR Pro (Orthous-Daunay et al., 2019). This program is able to attribute molecules (or ions) to individual mass peaks from a set number of hydrogen, carbon, nitrogen and oxygen atoms as constraints. Oxygen (¹⁶O) is put as a constraint as we expect contamination, either by residual oxygen containing gases in the irradiation chamber reacting with the adenine and its fragments (Vignoli Muniz et al., 2017) or by contaminants found in the solvent blank like fatty acids. As the amount of possibly implanted isotopic O is too small, we exclude ¹⁸O and ¹⁷O in this assignment process. An internal calibration of the peaks allows the precision of the attribution to not exceed 1.7 ppm. For a detailed description of the assignment process see the dissertation of Wolters (2021). After the assignment process, we obtain a list of molecular ions and their masses for every adenine sample. Figure A.1 shows an example of the 1957 assigned molecular ions of T22Ox50*. Here, the number of H, C, N and O-atoms is plotted over m/z.

### 3. Results

The results highlight the spectral features and chemical structure of the irradiated adenine samples. A brief description of the IR spectra recorded during the irradiation experiments is followed up by an in-depth characterization of the samples chemical composition with the presentation of new complex organic nitrogen molecules.

#### 3.1. IR spectra

The IR spectra of all presented samples exhibit similar spectral features with matching main peaks, characteristic for the chemical bonds found in adenine. Table 3 shows the main spectral features of samples which lie in agreement with the results of other studies. Fig. 2 presents two IR spectra of the sample T22Ox50 before (brown solid line) and after irradiation (blue dashed line). Fig. 4 shows the 29 IR spectra of T22Ox50 taken over the course of the irradiation. The
<figure><img src="image_5.png" /><figcaption>Fig. 2. IR-spectra for T22Ox50 before and after irradiation.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Decrease of four major peaks from the IR-spectra of two adenine samples shown as the absorbance as a function of fluence. The absorbance values correspond to the maxima of the most intense IR bands with high signal-to-noise ratios.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Cumulation of 29 IR-spectra for T22Ox50 at different fluences for the wavenumber range 2300–2075 cm⁻¹; new appearing peaks at 2139 and 2220 cm⁻¹ correspond to the N=C=N bond and nitriles.</figcaption></figure>

progress of time is represented here with the fluence, which increases over time. Throughout the irradiation we see a gradual decrease in absorbance and a reduction of all the main peaks presented in Table 3. This occurs, as the sample is progressively destroyed by either being sputtered or chemically transformed into more complex species. During the irradiation, the decrease in absorption is initially fast and then slows down over time (see Fig. 3). At several points in the spectra, this decrease is either interrupted or reversed with the emergence of new bands indicating the formation of new chemical bonds. In particular, new bands appear at one distinct area highlighted in Fig. 4 in the area of 2100–2250 cm⁻¹. These bands at 2139 and 2220 cm⁻¹ correspond to the N=C=N and C=N bonds, respectively. Here, the absorbance first increases rapidly, which indicates the emergence of new material being greater than the destruction/sputtering of the sample. At a certain stage during irradiation, the rate of formation and destruction reach a steady-state, subsequently followed by a phase where the rate of destruction predominates, indicated by a decrease of absorbance. Vignoli Muniz et al. (2017) report new appearing bands during irradiation of adenine in this area as well. An unexpected peak at 2088 cm⁻¹ is only visible in samples irradiated at the lower temperatures of 150 K (s. Fig. 5). This peak corresponds to the vibrational mode $v_3$ of HCN, indicating the formation of HCN-ice (Moore and Hudson, 2003; Noble et al., 2013; Gerakines et al., 2022).

## 3.2. Sample fragments

As mentioned above, the adenine samples are continuously destroyed during the irradiation as shown by the decrease in absorbance of the IR spectra. This “destruction” can be explained by a variety of phenomena. Adenine could be sputtered away from the sample into the gas phase while either being (i) chemically intact or (ii) broken into smaller fragments like HCN. Here, however, we examine the solid

<figure><img src="image_8.png" /><figcaption>Fig. 5. Cumulation of 35 IR-spectra for T22Ox50* at different fluences for the wavenumber range 2025–2175 cm⁻¹ with the new appearing band at 2088 cm⁻¹ corresponding to the formation of HCN-ice at 150 K.</figcaption></figure>

sample itself and disregard the sputtered material that has been lost. HRMS of the IrAS (50–130 m/z) gives us insight into the chemical diversity of the adenine fragments in the solid phase. The most intense mass peaks were attributed and then compared among the samples. Four N-containing molecules that can be found in the IrAS are not present in the non-irradiated sample. The masses of their negative ions are 65.015, 117.021, 119.036 and 120.031 u, which can be assigned to the molecules $C_3H_2N_2$, $C_5H_2N_4$, $C_5H_4N_4$ and $C_4H_3N_5$, respectively (see Fig. 6).

## 3.3. Chemical composition

The following results on the chemical composition of the samples are based on the spectra with a high mass range (150–500 m/z) and the assignment of the peaks present. Each sample was assigned a molecular list of up to 2000 individual molecular ions. We were unable
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Mass spectra of the close-up mass range 115–121 m/z for pure non-irradiated adenine (top, green), T22Ne50 (middle, blue) and T22Ox50 (bottom, red). The masses 117.021, 119.036, 120.031 u correspond to C<sub>5</sub>H<sub>2</sub>N<sub>4</sub>, C<sub>5</sub>H<sub>4</sub>N<sub>4</sub> and C<sub>4</sub>H<sub>3</sub>N<sub>5</sub>, respectively. C<sub>2</sub>H<sub>3</sub>N<sub>4</sub><sup>-</sup> has a mass that is close to that of C<sub>8</sub>H<sub>7</sub>O<sup>-</sup> (119.050 u) which can only be found in the non-irradiated sample and is considered HCO-contamination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

to obtain satisfactory mass spectra and hence molecular assignments for G21Ox70', G21Ox35' and G22Ox60. It is likely that these grainy samples of unknown thickness were sputtered away.

**Fig. 7** presents six plots of the mass defect as a function of m/z. **Fig. 7a** and **b** show the solvent blank sample and T22Ox50, respectively. The solvent blank displays a loosely bound collection of peaks with overall higher mass defects while the sample T22Ox50 peaks are accumulated more densely between a mass defect of 0 and 0.15. The **Figs. 7c-f** show the most prominent groups of molecules attributed to the peaks of T22Ox50. **Figs. 7c** and **d** highlight molecular ions that are attributed to HCN- (blue) and HCO-molecules (red), respectively. **Figs. 7e** and **f** both show HCNO molecules with differing amounts of nitrogen where the atomic N/C>0.3 and N/C≤0.3, respectively. The molecule groups HCN and HCNO (where N/C>0.3) are used to determine the chemical composition of the samples in terms of H, C, N and O by calculating the average of the weight distribution over each assigned molecule and then weighing each composition by the intensity of the molecule in the mass spectrum. The other molecules groups can be considered background contamination from, the solvent. In general, apart from a few outliers, all samples have a similar elemental composition that does not follow a trend (see Figure A.2). The average hydrogen content of the samples is 2.81 ± 0.30 wt% while carbon and nitrogen show contents of 49.18 ± 1.80 and 46.49 ± 1.63 wt%, respectively with the rest being oxygen with 1.52 ± 0.45 wt%. The uncertainties of these values are the standard deviation of all the individual elemental composition for each sample shown in Figure A.2. The contamination of the adenine samples with oxygen is further discussed in the supplementary. Since the HCN-molecules are free from oxygen contamination, the focus will now shift to them from here on out.

### 3.3.1. *New complex organic molecules*

The main difference between non- and irradiated samples is the presence of complex organic molecules in the latter. In order to avoid confusion, we will call these molecules that are present in our irradiated samples “New Complex Organic Molecules” (NCOMs). **Fig. 8** shows an illustration highlighting the terminology we use for the NCOMs. NCOMs are expressed as (HCN)<sub>z</sub>R, where z is a number between 1 and 17 and R is a residual of C<sub>x</sub>, H<sub>y</sub>, N<sub>x</sub>, N<sub>x</sub>H<sub>y</sub>, C<sub>x</sub>H<sub>y</sub> or C<sub>x</sub>N<sub>y</sub>. Each R-group contains a set of families which itself is composed of a number of individual molecules n with differing numbers of HCN defined as z.

**Table 4** shows a selection of the highest peak assignments and their m/z. The order of peak intensities vary from sample to sample except for the ion H<sub>3</sub>C<sub>6</sub>N<sub>6</sub><sup>-</sup> which is the most intense in all IrAS. What these most intense peaks have in common is that they all belong to the

<caption>**Table 4**</caption>

Molecular ions attributed to the most intense peaks together with their masses. The order does not follow most intense to least as every sample has a unique order except for the molecular ion H<sub>3</sub>C<sub>6</sub>N<sub>6</sub><sup>-</sup> which is the most intense in every presented sample.

<table><thead><tr><th>Molecular Ion</th><th>m/z [u]</th></tr></thead><tbody><tr><td>H<sub>3</sub>C<sub>6</sub>N<sub>6</sub><sup>-</sup></td><td>159.042</td></tr><tr><td>H<sub>3</sub>C<sub>8</sub>N<sub>6</sub><sup>-</sup></td><td>183.042</td></tr><tr><td>H<sub>5</sub>C<sub>7</sub>N<sub>6</sub><sup>-</sup></td><td>173.058</td></tr><tr><td>H<sub>5</sub>C<sub>9</sub>N<sub>8</sub><sup>-</sup></td><td>225.064</td></tr><tr><td>H<sub>4</sub>C<sub>8</sub>N<sub>7</sub><sup>-</sup></td><td>198.053</td></tr><tr><td>H<sub>7</sub>C<sub>10</sub>N<sub>10</sub><sup>-</sup></td><td>267.086</td></tr><tr><td>H<sub>4</sub>C<sub>7</sub>N<sub>5</sub><sup>-</sup></td><td>158.047</td></tr><tr><td>H<sub>4</sub>C<sub>9</sub>N<sub>7</sub><sup>-</sup></td><td>210.053</td></tr><tr><td>H<sub>6</sub>C<sub>10</sub>N<sub>9</sub><sup>-</sup></td><td>252.075</td></tr><tr><td>H<sub>6</sub>C<sub>8</sub>N<sub>7</sub><sup>-</sup></td><td>200.069</td></tr></tbody></table>

R = C<sub>x</sub>N<sub>y</sub> families. The C<sub>x</sub>N<sub>y</sub> families are closely followed by molecules with R = C<sub>x</sub>, which possess the largest z-values. Families with R = C<sub>x</sub>H<sub>y</sub> are less abundant and display lower intensities in comparison, while R = NH, H<sub>2</sub>, N<sub>2</sub> show the lowest abundance. R = H<sub>2</sub>, N<sub>2</sub> are not found in T22Ox60 due to the overall low intensity of those families paired with a low number of attributions in the molecule list compared to the other samples. The reason for this is an unknown contamination with high intensities in this particular sample that consequently reduces the intensity of the NCOMs. Through the molecules attribution method we were able to separate the contamination from the NCOMs leaving us with lower intensities of the latter.

In total, we identify 94 individual families from which 28 can be found in every irradiated samples. None of them can be found in the non-irradiated sample. **Table 5** shows the 28 families together with their average z and n. The averaged normalized intensity is calculated by normalizing the intensity of each molecule to the highest peak for each sample individually and averaging these values over all samples. NCOMs with R = C<sub>x</sub>N<sub>y</sub> represent the most diverse group as well as the group with the strongest intensities.

**Fig. 9** shows the NCOMs as normalized intensity as a function of z for a few selected samples, irradiated under varying free parameters, including energy, temperature, and ion type. Each column of plots represents a distinct irradiation parameter, while each line depicts the most intense family of each R-group, allowing for a direct comparison of how these factors influence the intensity and therefore the abundance of NCOMs in the IrAS. The first column shows the behavior of the NCOMs for differing energies for the samples T22Ox70, T22Ox60, T22Ox50 and T22Ox30 with their corresponding energies, 70, 60, 50 and 30 keV, respectively. A significant trend with rising or decreasing energies cannot be observed for the different R-groups. The second row depicts the NCOMS for T22Ne50 and T22Ox50 which were irradiated with different ions (O and Ne) while all other experimental parameters stayed the same. No difference can be seen here either. The last column shows the NCOMs for different temperatures (150 K, 300 K) in samples T22Ox50* and T22Ox50. The only difference, in this figure, lies in the shorter molecules of R=C<sub>2</sub>H<sub>2</sub>, where z is in the range of 5 to 8, which are less intense for higher temperature. This difference cannot be observed in the other R-groups. Aside from the free parameters, an interesting observation that can be made from this figure is that there is a peak in intensities at different z for the groups R=C<sub>2</sub>N<sub>2</sub> and R=C. These peaks can be observed at z=8 and z=10 for R=C<sub>2</sub>N<sub>2</sub> and R=C, respectively. This phenomenon is further analyzed and discussed in detail in Section 4.1.

Focusing on the different distributions of x over z for different R-groups for certain samples, we do observe a slight difference for samples irradiated at 150 K. **Fig. 10** shows two sets of plots illustrating the NCOMS for two sets of samples (G21:top, T22:bottom, "*" marks
<figure><img src="image_10.png" /><figcaption>Fig. 7. Molecular mass defect versus the mass/charge of the solvent blank sample (a), T22Ox50 (b) with marked attributed molecular ions of HCN (c, blue), HCO (d, red), HCNO where N/C > 0.3 (e, orange), and HCNO where N/C < 0.3 (f, purple). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Schematic representation illustrating the significance of R in (HCN)₂R, families and the number of individual molecules n.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Normalized intensity as a function of the number of HCN for R = C₂N₂, C and C₂H₂ and E, Ion and T used in the experiment; The samples used the free parameters are for E: T22Ox70, T22Ox60, T22Ox50, T22Ox30; for Ion: T22Ne50, T22Ox50, and for T: T22Ox50*, T22Ox50.</figcaption></figure>

<caption>**Table 5**</caption>

List of families detected in all IrAS with average z, average n (number of assignments in the molecule lists) and average normalized intensity in the mass spectra. Families marked with a † are missing in T22Ox60.

<table><thead><tr><th>Family</th><th>Avg. z</th><th>Avg. n</th><th>Avg. normalized intensity [%]</th></tr></thead><tbody><tr><td>(HCN)₂</td><td>8.79</td><td>6.57</td><td>14</td></tr><tr><td>(HCN)₂H₂†</td><td>7.04</td><td>3.43</td><td>1</td></tr><tr><td>(HCN)₂N₂†</td><td>6.31</td><td>3.64</td><td>1</td></tr><tr><td>(HCN)₂C</td><td>9.54</td><td>8.07</td><td>29</td></tr><tr><td>(HCN)₂C₂</td><td>9.06</td><td>9.07</td><td>26</td></tr><tr><td>(HCN)₂C₃</td><td>9.43</td><td>9.86</td><td>11</td></tr><tr><td>(HCN)₂C₄</td><td>8.89</td><td>10.64</td><td>8</td></tr><tr><td>(HCN)₂C₅</td><td>8.83</td><td>10.64</td><td>4</td></tr><tr><td>(HCN)₂C₆</td><td>8.76</td><td>9.14</td><td>2</td></tr><tr><td>(HCN)₂CH₂</td><td>8.11</td><td>5.07</td><td>2</td></tr><tr><td>(HCN)₂C₃H₂</td><td>8.08</td><td>7.14</td><td>3</td></tr><tr><td>(HCN)₂C₄H₂</td><td>8.45</td><td>7.86</td><td>3</td></tr><tr><td>(HCN)₂C₄H₂</td><td>7.97</td><td>8.93</td><td>2</td></tr><tr><td>(HCN)₂C₃H₂</td><td>8.04</td><td>8.71</td><td>2</td></tr><tr><td>(HCN)₂C₆H₂</td><td>7.64</td><td>8.43</td><td>1</td></tr><tr><td>(HCN)₂NH</td><td>7.54</td><td>4.07</td><td>3</td></tr><tr><td>(HCN)₂CN₂</td><td>7.59</td><td>6.14</td><td>13</td></tr><tr><td>(HCN)₂C₂N₂</td><td>7.79</td><td>8.57</td><td>100</td></tr><tr><td>(HCN)₂C₃N₄</td><td>4.41</td><td>3.57</td><td>2</td></tr><tr><td>(HCN)₂C₄N₂</td><td>8.38</td><td>9.71</td><td>36</td></tr><tr><td>(HCN)₂C₄N₄</td><td>5.89</td><td>6.79</td><td>6</td></tr><tr><td>(HCN)₂C₄N₂</td><td>7.94</td><td>10.86</td><td>34</td></tr><tr><td>(HCN)₂C₃N₄</td><td>6.16</td><td>9.29</td><td>15</td></tr><tr><td>(HCN)₂C₄N₂</td><td>8.01</td><td>11.00</td><td>15</td></tr><tr><td>(HCN)₂C₄N₄</td><td>6.51</td><td>10.00</td><td>20</td></tr><tr><td>(HCN)₂C₆N₂</td><td>7.57</td><td>11.00</td><td>8</td></tr><tr><td>(HCN)₂C₆N₄</td><td>6.32</td><td>10.64</td><td>12</td></tr><tr><td>(HCN)₂C₇N₂</td><td>7.24</td><td>10.64</td><td>6</td></tr><tr><td>(HCN)₂C₇N₄</td><td>6.14</td><td>10.79</td><td>14</td></tr><tr><td>(HCN)₂C₈N₄</td><td>6.39</td><td>8.93</td><td>4</td></tr></tbody></table>

150 K samples) that differ in the temperature at which they were irradiated. Here, z is shown as a function x for different R. In both sets, we observe a shift at R=CₓHᵧ to greater y. The 150 K samples show a relatively large collection of molecules at R=R=CₓH₈ which cannot be observed at 300 K. A similar behavior is seen with G21Ox70* but for

R=CₓN₈. At 300 K, the relative proportions of H, C, and N are more similar, while at 150 K, the composition is more skewed, with certain elements being more dominant than others.

### 3.4. Comparison with other samples

In addition to the IrAS, there exists other samples that are N-rich and relevant to understand the formation of complex N-bearing compounds. The first type of samples are Titan aerosol analogs (tholins). These were produced in the PHAZER reactor at the John Hopkins University (JHU), where different concentrations of N₂ and CH₄ (10, 5 and 2.5% CH₄) in the gas phase were exposed to a cold plasma discharge. These methane mixing ratios correspond to varying methane abundances at different altitudes in Titan's atmosphere (Müller-Wodarg et al., 2008; Magee et al., 2009; Cui et al., 2009; Niemann et al., 2010). These tholin samples where produced and then analyzed using the same Orbitrap as in this study in 2018. The second type of samples are HCN-polymers which are believed to be present in various icy bodies in the solar system (Cruikshank et al., 1991; Matthews, 1997; Strazzulla, 1999). They were prepared through the reaction of sodium cyanide (NaCN, Aldrich 98%) with stearic acid (CH₃(CH₂)₁₆COOH, Merck, 97%) (Bonnet et al., 2013). For a detailed description of these synthesis processes see He et al. (2017, 2022), Hörst et al. (2018) for the tholins and Vuitton et al. (2010), Bonnet et al. (2013) for the HCN-polymers. Both the tholins and HCN-polymers were dissolved in methanol to create a solution of 1 mg/ml in order to be analyzed with the same Orbitrap equipped with an ESI-source. Mass spectra were captured at different times for mass ranges of 200–500 m/z in 2018 and 150–1000 m/z in 2013 for the tholins and HCN-polymers, respectively. The results showed that the method of tholin synthesis would rather create PANHs rich in nitrogen as opposed to pure PAHs making tholins richer in nitrogen than the molecules observed in Titan's atmosphere. The HCN-polymers analysis showed that the molecules are build up of HCN monomers with chains that possess amine groups. Furthermore aromatic rings could be included in them (Bonnet et al., 2013). After reanalyzing the HRMS data of tholins and HCN-polymers using the same attribution procedure applied to our adenine samples, we obtained a similar list of molecules. The chemical composition of all IrAS, tholins and HCN-polymers can be seen in Fig. 11. Here, all
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Illustration of NCOMs with *z* as a function *x* for G21 and T22 samples irradiated with energies of 70 and 50 keV, respectively, where *R* represents Cₓ, CₓHᵧ and CₓNᵧ.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Elemental composition of the IrAS, Tholins and HCN-polymers for H, C and N in wt%.</figcaption></figure>

molecules containing O were removed leaving only HCN-molecules. HCN-polymers show the highest amount of nitrogen and hydrogen, while carbon is most abundant in the tholins with a high CH₄ content in the initial gas mixture.

**Fig. 12** shows the number of families of each R-group for 14 IrAS (no mass spectra for 3 of the 17 IrAS), three tholin and one HCN polymer sample, all analyzed in negative mode. The most dominant R-group in the tholins is R=CₓHᵧ, while all other groups play a minor role. This is also shown in **Fig. 13** where in R=CₓHᵧ, *y* can reach up to 18. The number of these CₓHᵧ families as well as *y* increase when the CH₄ content of the initial gas mixture is increased. Families with R=CₓNᵧ, which are most abundant in IrAS, are present in all tholins and HCN-polymer samples, albeit in smaller numbers. Tholins and HCN-polymers have a higher number of families with R=NₓHᵧ compared to IrAS. Families with R=H₂ and R=N₂ are present in all samples with the exception of T22Ox60. The tholins are also the only samples in which the family R = H₄ occurs. Overall, the NCOMs in the IrAS are much more evenly distributed across all R-groups, in contrast to the tholins with their dominant CₓHᵧ families. While The HCN-polymers seem to lie in between when it comes to the amount of different families, the total number of molecules identified within these families is significantly lower (see **Fig. 13**). This observation is expected, as most molecules follow the (HCN)₂ structural pattern characteristic of polymers, without R.

The double bond equivalent (DBE) gives us insight on the structure of the molecules by being the sum of unsaturations plus rings in a
<figure><img src="image_15.png" /><figcaption>Fig. 12. Number of families for every R-group for every IrAS, tholin and HCN-polymer sample, (HCN)₂R where R = Hₓ, Nₓ, Cₓ, CₓHᵧ, NₓHᵧ or CₓNᵧ.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 13. Similar illustration to Fig. 10 but for the tholin and HCN-polymer samples.</figcaption></figure>

molecule. It can be calculated as:

$$DBE = 1 + C\# + \frac{N\#}{2} - \frac{H\#}{2} \quad (3)$$

where C#, H# and N# is the number of carbon, hydrogen and nitrogen atoms of a molecule, respectively. It is an appropriate tool for quantifying aromaticity in pure hydrocarbons, however, it is insufficient when dealing with heteroatoms (Koch and Dittmar, 2006). An adjustment has to be made for more complex organic molecules containing an increased amount of nitrogen, like our NCOMs. Yassine et al. (2014) suggest the aromaticity equivalent (Xc) to be a more fitting indicator for the structure of such molecules. For heteroatoms containing nitrogen they propose:

$$X_c = \frac{C\# - ((H\# - N\#) - C\#)}{DBE} + 1 \quad (4)$$

Yassine et al. (2014) calculate a threshold value of Xc ≥ 2.5 for the presence of aromatic structures and Xc ≥ 2.7143 for condensed aromatic structures or, in our case, PANH with high amounts of nitrogen. Fig. 14 shows Xc as a function of C# for the IrAS, tholins and HCN-polymers. Almost all molecules in the IrAS are situated above the PANH-threshold, indicating a nearly complete lack of aliphatic or single ring organics in our samples. That is not the case for HCN-polymers that show molecules with Xc < 2.5. The tholins exhibit a more broadly spaced distribution of C# for the same Xc. For example, at Xc=2.8 the C# ranges from 6–14 and 6–17 for IrAS and tholins, respectively. Furthermore, the intensities for higher C# molecules are greater in tholins. In contrast to the IrAS and the tholins, the HCN-polymers show significantly more molecules in the region 2.5 ≥ Xc < 2.7143 implying the presence of many single ring aromatic compounds.
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** The aromaticity equivalent (Xc) as a function of number of carbon atoms (C#) for the molecules in the IrAS (green), Tholins (red) and HCN-polymers (blue). The size of the circles corresponds to the normalized intensity of the molecule in the mass spectrum. Dotted and dashed lines indicate the thresholds for aromatic and condensed aromatic structures, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

# 4. Discussion

## 4.1. Emergence of new molecules

We have presented results that show the formation of new molecules by analyzing the spectral and chemical properties of our samples via IR spectroscopy and HRMS, respectively.

The IR spectra show two new appearing bands that can be assigned to N=C=N and C≡N. Several studies have investigated the irradiation of adenine, capturing IR spectra. Evans et al. (2011), Poch et al. (2014), Góbi et al. (2017) and Vignoli Muniz et al. (2017) report one or more new appearing bands which are in agreement with ours, while Guan et al. (2010) and Pilling et al. (2011) show no new appearing bands. Vignoli Muniz et al. (2017) points out that this is likely due to the low abundance of photolyzed adenine in the latter. Poch et al. (2014) suggest the formation of polymer-like nitrogen-bearing molecules similar to the one described in Völker (1960) and shown in Fig. 16a. They support this, by pointing out that these newly formed molecules might protect the adenine from further destruction by radiation, increasing its photostability. This phenomenon could also explain why we can observe an exponential decay of the absorbance over fluence in our samples (s. Fig. 3). It is worth mentioning that the compression of the sample or ice forming on the sample in the moment when the ion beam hits the target may be an additional explanation for the sharp initial decrease in absorbance, which is exceptionally pronounced in the samples irradiated at 150 K (sample name marked with "*"). Such compaction was observed in the past when irradiating astrophysical ice analogs (Dartois et al., 2013; Mejía C. De Barros et al., 2015; Boduch et al., 2015).

None of the previous studies delivers any HRMS of IrAS, proving the formation of molecules with masses greater than adenine. The chemical analysis of the IrAS reveals a great diversity of NCOMs that are lacking

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Schematic molecular structure of a possible isomer for H<sub>4</sub>C<sub>6</sub>N<sub>6</sub>.</figcaption></figure>

in non-irradiated samples. This shows that the irradiation of adenine leads not only to the destruction and fragmentation of adenine but also the formation of heavier and more complex organic molecules and possibly polymers. While the chemical formula follows (HCN)<sub>2</sub>R, the molecules differ in size and may vary substantially in their chemical structure.

The molecule H<sub>4</sub>C<sub>6</sub>N<sub>6</sub> shows the highest intensity in all mass spectra of the IrAS. While there are several different molecules that would fit this formula, a Xc value of 2.75 indicates that it is a PANH consisting of two rings. The molecular structure in Fig. 15 could be a possible candidate. It is a structure that could be derived from adenine in which the loss of a primary amine is accompanied by the opening of the five-membered ring, allowing the incorporation of an additional carbon atom.

When it comes to larger NCOMs with higher z, their structure can be approximated by taking into account the new appearing bands from the IR spectra and their Xc. The appearance of a nitrile band can give us further clues on the molecule structures. From the Xc values for the NCOMs in the IrAS we know that we are dealing with PANHs. We can assume that the CN ring of adenine serves as the nucleus to which new rings form during irradiation. Ricca et al. (2001) state that the existence of a nitrogen atom in an aromatic ring furthers the development of new rings, which we can observe here as well.

Following the (HCN)<sub>2</sub>R scheme, one might assume that the NCOMs could form polymeric structures. The term “HCN-polymer” or “poly-HCN” is often used in the literature for molecules that follow the molecular structure (HCN)<sub>2</sub>. The literature presents us with several suggestions for these kinds of molecules. Fig. 16 shows four examples for the chemical structure of (HCN)<sub>2</sub>. The two polymers from Sandström and Rahm (2021) represent the simplest structures (16c,d) with CN double bond and an H atom, bound to either C or N. Guan et al. (2010) presents a more complex structure (16b) consisting of a chain with an amine and nitrile group attached to it. Völker (1960) presents the most complex configuration (16a) with “Azulminsäure” or azulmic acid where hydrogen cyanide (HCN) is polymerized in a non aqueous solution. It is a possible candidate for a chemical structure emerging from the irradiation of adenine with its repeating nitrogen-bearing carbon rings, primary amine and nitrile groups. The new appearing bands in the IR spectra together with the Xc rule out the occurrence of other structures in the IrAS like the ones in Fig. 16 “b”, “c” and “d”. However, these interpretations only apply to the (HCN)<sub>2</sub> part of the molecules, where z ≥ 8 so that at least two rings can form. Looking at average x in Table 5 and z spanning from 1 to 14, this only applies to some molecules found in our samples. Furthermore, molecules where R > (HCN)<sub>2</sub> in terms of mass, would likely not follow a repeating polymer structure but would nevertheless form aromatic rings with a sufficient number of atoms instead of aliphatic structures.

As mentioned in Section 3.3.1, we could observe a peak of intensity for NCOMs with z=8 and z=10 for R=C<sub>2</sub>N<sub>2</sub> and R=C<sub>2</sub>H<sub>2</sub>, respectively. Possible molecular structure for molecules that correspond to (HCN)<sub>8</sub>C<sub>2</sub>N<sub>2</sub> and (HCN)<sub>10</sub>C<sub>2</sub>H<sub>2</sub>, are presented in Fig. 17 “e” and “f”. These molecules structures represent quasi-dimers of adenine. In structure “e”, two adenine molecules each lose a H-atom at their N<sub>(9)</sub>
<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Schematic chemical structure versions of HCN-polymers from Völker (1960) (a), Guan et al. (2010) (b) and Sandström and Rahm (2021) (c,d).</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Schematic molecular structure of a possible isomers for H<sub>8</sub>C<sub>10</sub>N<sub>10</sub> (e) and H<sub>10</sub>C<sub>11</sub>N<sub>10</sub> (f).</figcaption></figure>

which then bond together forming a N-N bond. Similarly, in structure “f”, hydrogen is lost; however, instead of direct bonding between the two nitrogen atoms, a methylene group serves as a linking piece. Both of these molecular structures demonstrate a simple and efficient process for the synthesis of NCOMs. However, neither of these structures exhibit a repeating HCN sequence characteristic of structure “a”, highlighting the challenges in accurately characterizing the chemical composition of the NCOMs. This necessitates the use of more advanced techniques for definitive identification.

## 4.2. Free parameters

The distinguishing feature of every sample is its unique set of free parameters that were either determined before the irradiation experiments or adapted to changing experimental condition. These include (i) sample parameter like the film type and thickness, (ii) ion beam parameter like used ion, beam energy and fluence, as well as (iii) the temperature inside the irradiation chamber.

The samples G21Ox70* and T22Ox50*, that have been irradiated at 150 K, have the highest number of (HCN)<sub>z</sub>R families with 94 and 81, respectively. In addition they also possess the highest number of unique

nitrogen-bearing molecule attributions in their molecule lists with 605 and 653, respectively. This trend suggests that irradiation at 150 K promotes a greater variety of molecules, potentially influencing the overall chemical complexity of our samples. This pattern is also seen in Fig. 10 with the larger distribution of R=C<sub>x</sub>H<sub>y</sub> and R=C<sub>x</sub>N<sub>y</sub>. The third low temperature sample T21Ox35* ranks in the middle with 64 families and only 437 assignments. Similar behaviors can be observed with the other parameters, where one part of the samples follow a pattern and others do not. Furthermore, with the IrAS being produced at different times and through varying methods, it becomes challenging to identify a clear correlation between the chemical diversity of our NCOMs and the free parameters, making it difficult to draw significant conclusions about the samples' behavior (see Fig. 9). Given these challenges, as well as the complex nature of the NCOMs, any dependence on the free parameters must be interpreted with caution. While trends may exist, their significance remains uncertain due to potential underlying interdependencies that are not yet fully understood. Therefore, further experiments, ideally with a larger sample size and a more controlled variation of individual parameters, are necessary to establish any definitive correlations.

## 4.3. Comparing IrAS to tholins and HCN-polymers

Complex organic nitrogen molecules are found in various astro-physical environments (Imanaka and Smith, 2010). They are likely an important component of the aerosols in Titan's atmosphere (Haythorn-thwaite et al., 2021) and can be found in their laboratory analogs, tholins (Vuitton et al., 2010). The detection of NCOMs in the IrAS begs the question how our results fit into the overall picture.

Comparing the diversity of (HCN)<sub>z</sub>R families between IrAS, tholins and HCN-polymers we see the following differences. The IrAS are richer in families where R=C<sub>x</sub>N<sub>y</sub> or C<sub>x</sub> compared to tholins and HCN-polymers. Tholins have by far the largest amount of molecules where R=C<sub>x</sub>H<sub>y</sub>, while HCN-polymers possess more R=N<sub>x</sub>H<sub>y</sub> than any other sample type (see Fig. 12). The high abundance of R=C<sub>x</sub>N<sub>y</sub> as well as high *N* wt% (see Fig. 11) in IrAS indicates that nitrogen is very efficiently incorporated into them and does not necessarily escape into the gas phase during the irradiation. The opposite is the case for tholins, where only trace amounts of N<sub>2</sub> from the initial gas mixture are incorporated into the end product, most likely due to the strong nitrogen triple bond of the initial N<sub>2</sub> gas, which is difficult to break. This implies that the incorporation of nitrogen in the final product is more efficient in the solid-phase chemistry of the irradiation experiment than in the gas-phase chemistry of the tholin synthesis. Nevertheless, the presence of nitrogen in the initial gas mixture promotes the formation of nitrogen-bearing molecules, which constitute the majority of the identified species. This is supported by Lévêque P. Queffelec et al. (2024), who demonstrate that during the synthesis of chondritic insoluble organic matter (IOM) analogs with varying initial nitrogen content in the gas mixture, nitrogen-bearing molecules remain dominant.

The Xc value gives us insight into the structure of all the samples presented in this study. The IrAS show the highest degree of aromaticity of all the samples. Tholins have a broader distribution of C# due to the higher amounts of carbon, creating molecules with less nitrogen. Although possessing some single-ringed molecules, they are nonetheless filled with PANHs which is in agreement with Imanaka et al. (2004). HCN-polymers show Xc values over the whole spectrum, indicating that they most likely contain every exemplary polymer structure shown in Fig. 16. This confirms the assumptions made by Bonnet et al. (2013) that HCN polymers contain aromatic compounds. Furthermore, the results presented here indicate that the molecules are even more complex, exhibiting a higher degree of aromaticity than previously anticipated.

Even though, tholins and HCN-polymers share similar optical properties in the IR, Vuitton et al. (2010) have revealed the relative molecular simplicity of HCN-polymers to tholins. This is due to similar distribution of R-groups rather than structural similarity (Vuitton et al.,
<figure><img src="image_21.png" /><figcaption>Fig. A.1. Assignments for T22Ox50*, number H (black), C (red), N (green) and O (blue)-atoms as a function of m/z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

2010). We can agree with this statement. The NCOM found in irradiated adenine do not have this distribution of R-groups and differ substantially in the amount of nitrogen in the molecules thus having a different set of R-families. However, they share a similar degree of aromaticity with the tholins.

## 5. Summary and conclusion

In this study, 17 adenine films were irradiated with oxygen and neon ions to simulate the collision of low energy ions with PANHs in an astrophysical context. The experiments were performed at two different sites (IJCLab,Orsay; GANIL,Caen) with varying free parameters like temperature, energy, kind of ion, sample thickness and fluence. In-situ IR and, for the first time, ex-situ HRMS measurements provide information on the change in the optical and chemical properties of adenine during irradiation. The following conclusions can be drawn:

1. New appearing bands in the IR spectra indicate the formation of new $N=C=N$ and $C≡N$ bonds.

2. Low energy ion irradiation of adenine leads not only to its destruction but also to the formation of larger more complex organic molecules that can be characterized as $(HCN)_zR$, where $z≤17$.

3. These HCN-molecules show a great chemical diversity and complexity with R being $C_x$, $H_x$, $N_x$, $N_xH_y$, $C_xH_y$ or $C_xN_y$, with the latter being most abundant. This chemical diversity likely stems from the different fragmentation pathways of adenine.

4. The aromaticity equivalent Xc of these newly created molecules reveal that they are condensed aromatic structures or polycyclic aromatic nitrogen bearing hydrocarbons (PANH).

5. While variations in the free parameters, especially cold temperatures, of the irradiation experiment may influence the chemical complexity of the samples, any clear trends remain challenging to discern due to potential interdependencies

6. Nitrogen is very efficiently incorporated into the new forming macromolecules in solid-phase conditions compared to tholins.

The formation of larger and more complex PANHs from a molecule like adenine, which can be classified as a PANH itself, opens the question of how an aliphatic molecule would react under similar experimental condition. The question of whether polymers with a repeating HCN sequence would form and whether they would exhibit a similar degree of aromaticity remains an open question for future experiments. Overall, we have presented a new pathway for the formation of complex organic matter in the solid-phase, which could occur on various celestial bodies like meteorites, comets or the icy satellites of Jupiter and Saturn.

### CRediT authorship contribution statement

**Filip Matuszewski:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Véronique Vuitton:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization. **Julia Shouse:** Methodology, Data curation. **Thibault Launois:** Methodology. **Naïla Chaouche-Mechidal:** Methodology. **Elsa Hénault:** Methodology. **Laurène Flandinet:** Methodology. **Èric Quirico:** Methodology. **Rosario Brunetto:** Methodology. **Philippe Boduch:** Methodology. **Alicja Domaracka:** Methodology. **Hermann Rothard:** Methodology. **Fabien Stalport:** Methodology. **Francois Regis Orthous Daunay:** Software. **Hervé Cottin:** Methodology. **Sarah Hörst:** Data curation. **Roland Thissen:** Data curation.

### Declaration of competing interest

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

### Acknowledgments

We thank the Programme National de Planétologie (PNP) for supporting this work. This work is supported by the French National Research Agency in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02) and the generic call for proposals (ANR-22-CE49-0017). The experiments were performed at the Grand Accélérateur National d'Ions Lourds (GANIL) by means of the CIRIL Interdisciplinary Platform, part of CIMAP laboratory, Caen, France. We thank the staff of CIMAP-CIRIL and GANIL for their invaluable support. We acknowledge the fundings from ANR IGLIAS grant ANR-13-BS05-0004 of the French Agence Nationale de la Recherche. INGMAR is a IAS-IJCLab facility funded by the French Programme National de Planétologie (PNP), Faculté des Sciences d'Orsay, Université Paris-Sud (Attractivité 2012), P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d'Avenir (ANR-11-IDEX-0003-01). A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the ANR grants' open access conditions.

## Appendix A

See Figs. A.1–A.3.

## Appendix B. Supplementary data

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.icarus.2025.116865.
<figure><img src="image_22.png" /><figcaption>**Fig. A.2.** Elemental composition of the IrAS samples for H, C, N and O in wt%. The oxygen content presented here only includes the contamination of the sample itself and not the oxygen-bearing background contaminants from the solvent.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. A.3.** Molecular mass defect versus the mass/charge of the solvent blank sample with marked molecular ions of HCO (a, red) and HCNO (b, purple). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

## Data availability

Data will be made available on request.

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