Quantifying non-thermal desorption from NH3 ices: a comparative study of Photon and Electron Irradiation in the Valence–and Core–Shell energy ranges D. Torres-Díaz 1,2 , R. Basalgète 2 , L. Sala 1 , A. Hacquard 2 , D. Toulouse 2 , L. Janin 1 , J.A. Noble 3 , S. Del Fré 4 , X. Michaut 2 , L. Philippe 2 , G. Feraud 2 , M. Bertin 2 , J.-H. Fillion 2 , L. Amiaud 1 , A. Lafosse 1 1 ISMO, CNRS, Université Paris-Saclay, France, 2 LERMA, CNRS, Sorbonne Université, Observatoire de Paris, Université PSL, France, 3 PIIM, CNRS, Aix-Marseille Université, Marseille, France, 4 PhLAM, Université de Lille, Lille, France The relative abundances of gaseous and solid nitrogen in the interstellar medium remain controversial .[1,2] Besides the difficulty in observing N and N 2 , the quantities of nitrogen returning to the gas phase by photo-processing of the ice mantles are not well constrained. In star-forming regions, NH 3 containing ices are exposed to UV and X-Ray photons. [3,4] This leads to non-thermal photodesorption induced by the primary radiation or by exothermic photochemistry (recombination of the secondary species created in the ice mantles). In order to quantify the non-thermal desorption of N containing molecules and to have a deeper understanding of the complex surface chemistry in ice mantles, we have conducted comparative measurements of photon- and electron-induced desorption (PSD and ESD) from condensed NH 3 . Two setups were used for these experiments, the SPICES setup of the LERMA group, and the E/SOLID setup housed at ISMO. In both systems NH 3 ices are formed by cryogenic deposition (50-100 molecular layers), and the desorption of neutral species is measured by mass spectrometry. Thermal desorption experiments (TPD) were used to calibrate the desorption signal, the ice thickness and for the analysis the processed ice layers. The SPICES setup was coupled to two SOLEIL synchrotron beamlines: (i) DESIRS, valence-shell range (6-12eV) and (ii) SEXTANTS, core-shell range (395- 420eV, N-Kedge). The twin electron irradiation experiments were performed using the E/SOLID setup, in the low-energy range as well as at 385eV, in order to mimic the secondary Auger electrons known to be major vectors of chemical changes and desorption in X-ray irradiations at the N-K edge. [5,6] Desorption yields are compared for photon and electron irradiation either as a function of the incident energy in the low-dose regime (spectroscopic study), or as a function of the accumulated dose at a fixed irradiation energy (kinetic study). In the valence-shell range, NH 3 and N 2 are the major desorbing species, with an apparent energy threshold at ~6eV, showing that both direct and indirect desorption occurs. Different mechanisms at play will be discussed. In the core-shell range, again NH 3 and N 2 are the major desorption products. The remarkable matching between the photodesorption yields in the ionization continuum and the ESD yields at 385 eV confirms the key role of the Auger electrons in the photodesorption process. References 1. R. Le Gal et al. , A&A 2014 , 562, A83. 2. O. Sipilä et al. , MNRAS 2019 , 487, 1269–1282. 3. M. Bertin et al. , ApJ 2016 , 817, L12. 4. R. Dupuy et al. , Nat Astron 2018 , 2, 796–801.
5. R. Basalgète et al. , J. Chem. Phys. 2022 , 157, 084308. 6. D. Torres-Díaz et al. , ChemPhysChem 2023 , e202200912.
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