2020ApJ...898L..52N

Phosphine (PH₃), an important molecule for the chemistry of phosphorus (P)-bearing species in the interstellar medium (ISM) is considered to form primarily on interstellar grains. However, no report exists on the processes of PH₃ formation on grains. Here, we experimentally studied the reactions of hydrogen (H) atoms and PH₃ molecules on compact amorphous solid water, with a particular focus on the chemical desorption of PH₃ at 10-30 K. After exposure to H atoms for 120 minutes, up to 50% of solid PH₃ was lost from the icy surface. On the basis of experiments using deuterium atoms, it was concluded that the loss of PH₃ resulted from chemical desorption through the reactions PH₃ + H - PH₂ + H₂ and/or PH₂ + H - PH₃. The effective desorption cross-section was ∼5 x 10^–17 cm², which is three times larger than that of hydrogen sulfide measured under similar experimental conditions. The present results suggest that the formation of PH₃, and possibly PH₂ and PH, followed by their desorption from icy grains, may contribute to the formation of PN and PO in the gas phase, and thus may play a role in the P chemistry of the ISM.