An Experimental Study of Chemical Desorption for Phosphine in Interstellar Ice

Abstract

Phosphine (PH 3 ), 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 PH 3 formation on grains. Here, we experimentally studied the reactions of hydrogen (H) atoms and PH 3 molecules on compact amorphous solid water, with a particular focus on the chemical desorption of PH 3 at 10–30 K. After exposure to H atoms for 120 minutes, up to 50% of solid PH 3 was lost from the icy surface. On the basis of experiments using deuterium atoms, it was concluded that the loss of PH 3 resulted from chemical desorption through the reactions PH 3 + H → PH 2  + H 2 and/or PH 2 + H → PH 3 . The effective desorption cross-section was ∼5 × 10 −17 cm 2 , which is three times larger than that of hydrogen sulfide measured under similar experimental conditions. The present results suggest that the formation of PH 3 , and possibly PH 2 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.