arXiv subjects

L. Hornekær

Publications and source records attributed to L. Hornekær.

4 recordsLinked to original sources

arXiv2026

In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C$_2$H$_2$ ice

Acetylene (C$_2$H$_2$) is commonly observed in star-forming regions, young stellar objects, and the Solar System. Laboratory and theoretical studies have linked this simplest alkyne to volatile hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) through UV- or cosmic-ray-driven chemistry, but it remains unclear whether refractory material can efficiently form through solid-state reactions of C$_2$H$_2$ on dust grains. We experimentally investigate the chemical complexity induced by UV irradiation of pure C$_2$H$_2$ ice and characterize both volatile and nonvolatile photoproducts. Experiments were performed with MATRI$^2$CES under ultra-high-vacuum conditions at 15 K using 7.2--10.2 eV photons. UV-processed ices were monitored in situ by laser desorption post-ionization reflection time-of-flight mass spectrometry (LDPI ReTOF-MS) combined with pulsed ion deflection (PID). Volatile and refractory products were measured in situ at 15 and 300 K, respectively. After a fluence of $3 \times 10^{17}$ photons cm$^{-2}$, corresponding to about $10^6$ years in dense clouds, large saturated and unsaturated hydrocarbons containing up to 13 carbon atoms are formed. After sublimation of the volatile products, the 300 K residue shows a rich and distinct mass spectrum consistent with refractory material containing conjugated triple (-C$\equiv$C-) and double (-C=C-) bonds. These results demonstrate that UV processing of pure C$_2$H$_2$ ice can produce substantial molecular complexity and refractory hydrocarbons under astronomically relevant conditions, with possible implications for unidentified infrared emission bands.

astro-ph.GA↗

arXiv2026

Quantifying the temperature-dependent yields of N$_2$ and N$_2$H$_4$ formation in vacuum-ultraviolet-irradiated NH$_3$ ice

Gas--grain astrochemical models predict that a substantial fraction of elemental nitrogen in dense cores and protoplanetary disks is locked in molecular nitrogen (N$_2$), in the gas or ice phase, although interstellar N$_2$ ice has not been directly identified. Rosetta measurements at comet 67P/Churyumov--Gerasimenko showed strong N$_2$ depletion relative to CO, while NH$_3$ was the dominant detected nitrogen-bearing ice. We experimentally quantify the conversion of NH$_3$ ice into N$_2$ and N$_2$H$_4$ under astronomically relevant conditions by studying its temperature- and fluence-dependent VUV photochemistry. Experiments were performed under ultra-high vacuum using 40-monolayer NH$_3$ ice at 15, 25, and 70 K irradiated at 115--170 nm to a total fluence of $3.8\times10^{18}$ photons cm$^{-2}$. Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was used to monitor N$_2$ and N$_2$H$_4$ simultaneously. VUV photolysis efficiently forms both products, with formation kinetics and product ratios depending on temperature and photon fluence. Initial-growth fits gave the highest apparent ice-retained N$_2$ formation yield at 15 K, approximately an order of magnitude above those at 25 and 70 K. The N$_2$/NH$_3$ column-density ratio was determined as a function of fluence and compared with astronomical constraints. These results suggest that NH$_3$ ice photochemistry may provide an additional pathway to N$_2$ ice in cold outer planetary environments.

astro-ph.IM↗

arXiv2025

Formation of methane and cyclohexane through the hydrogenation of toluene

Methylated polycyclic aromatic hydrocarbons (PAHs) have been hypothesised to be present in the interstellar medium (ISM) through their 3.4 and 6.9 $μ$m absorption bands. To investigate the hydrogenation of these methylated PAHs, the simplest, toluene ($\mathrm{CH_3C_6H_5}$), was exposed to H-atoms. This demonstrated how the presence of a methyl group changes the reactivity towards atomic hydrogen as compared to benzene and other PAHs and how this may alter its chemistry in the ISM. Toluene was deposited onto a graphite surface in an ultrahigh vacuum (UHV) chamber and then exposed to a H-atom beam. Temperature programmed desorption (TPD) measurements were used to investigate the reaction between H-atoms and toluene and the masses of hydrogenation products were measured with a quadrupole mass spectrometer (QMS). H-atom exposure of toluene leads to superhydrogenation of toluene and the formation of methyl-cyclohexane ($\mathrm{CH_3C_6H_{11}}$) at long exposure times. The initial cross-section of H-addition is lower than that of other PAHs. Methyl-cyclohexane can be further hydrogenated, leading to the detachment of the methyl group and production of cyclohexane ($\mathrm{C_6H_{12}}$) and methane ($\mathrm{CH_4}$). Toluene may be fully hydrogenated through its interaction with H-atoms, although it has a smaller initial cross-section for H-atom addition compared to other PAHs. This likely reflects it having a smaller geometric cross-section and the low flexibility of the benzene ring when undergoing sp$^3$ hybridization. The removal of the methyl group at high H-atom fluences provides a top down formation route to smaller molecules with the possibility of the formation of a radical cyclohexane combining with other species in an interstellar environment to form prebiotic molecules.

astro-ph.GA↗

arXiv2025

CO desorption from interstellar icy grains induced by IR excitation of superhydrogenated PAHs

Infrared (IR) radiation dominates dense, interstellar clouds, yet its effect on icy grains remains largely unexplored. Its potential role in driving the photodesorption of volatile species from such grains has recently been demonstrated, providing a crucial link between the solid state reservoir of molecules and the gas phase. In this work, we investigate IR-induced photodesorption of CO for astrophysically relevant ice systems containing perhydropyrene (PHP). This fully superhydrogenated version of pyrene is used as an analogue for large carbonaceous molecules such as polycyclic aromatic hydrocarbons (PAHs) and related species, as well as hydrogenated carbonaceous grains. The abundance and range of strong IR absorption bands of these carbonaceous species make them interesting candidates for IR-induced effects. We present IR spectroscopic and mass spectrometric measurements probing the effects of IR radiation on two ice systems: a layered ice with CO on top of PHP, and a CO:PHP mixed ice. These ices were irradiated with IR radiation from the FELIX IR Free Electron Laser (FEL) FEL-2. In accordance with previous studies, we confirm that direct excitation of CO is not an efficient pathway to CO desorption, indicating that another energy dissipation mechanism exists. We demonstrate that vibrational excitation of the PHP CH stretching modes leads to efficient CO photodesorption. The derived photodesorption yields are an order of magnitude higher for the layered than the mixed system and comparable to those previously obtained for CO photodesorption from CO on amorphous solid water upon excitation of H$_2$O vibrational modes. Our results indicate that IR excitation of carbonaceous molecules and grains in dense clouds could potentially play an important role in the desorption of volatile species such as CO from icy grains.

astro-ph.GA↗