Quantifying the chemical desorption of H$_2$S and PH$_3$ from amorphous water ice surfaces

Abstract

Nonthermal desorption of molecules from icy grain surfaces is required to explain molecular line observations in the cold gas of star-forming regions. Chemical desorption is one of the nonthermal desorption processes and is driven by the energy released by chemical reactions. After an exothermic surface reaction, the excess energy is transferred to products' translational energy in the direction perpendicular to the surface, leading to desorption. The desorption probability of product species, especially that of product species from water ice surfaces, is not well understood. This uncertainty limits our understanding of the interplay between gas-phase and ice surface chemistry. In the present work, we constrain the desorption probability of H 2 S and PH 3 per reaction event on porous amorphous solid water (ASW) by numerically simulating previous laboratory experiments. Adopting the microscopic kinetic Monte Carlo method, we find that the desorption probabilities of H 2 S and PH 3 from porous ASW per hydrogen addition event of the precursor species are 3 ± 1.5% and 4 ± 2%, respectively. These probabilities are consistent with a theoretical model of chemical desorption proposed in the literature if ∼7% of energy released by the reactions is transferred to the translational excitation of the products. As a byproduct, we find that approximately 70% (40%) of adsorption sites for atomic H on porous ASW should have a binding energy lower than ∼300 K (∼200 K). The astrochemical implications of our findings are briefly discussed.