Quantifying the chemical desorption of H$_2$S and PH$_3$ from amorphous
  water ice surfaces
  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
$3 \pm 1.5$ % and $4 \pm 2$ %, respectively. These probabilities are consistent
with a theoretical model of chemical desorption proposed in the literature if
$\sim$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 $\sim$300 K ($\sim$200 K). The astrochemical
implications of our findings are briefly discussed.
