2022MNRAS.515...99D

The reactions of the low-lying metastable states of atomic phosphorus, P(²D) and P(²P), with H₂O and H₂ were studied by the pulsed laser photolysis at 248 nm of PCl₃, combined with laser-induced fluorescence detection of P(²D), P(²P), and PO. Rate coefficients between 291 and 740 K were measured, along with a yield for the production of PO from P(²D or ²P) + H₂O of (35 ± 15) %. H₂ reacts with both excited P states relatively efficiently; physical (i.e. collisional) quenching, rather than chemical reaction to produced PH + H, is shown to be the more likely pathway. A comprehensive phosphorus chemistry network is then developed using a combination of electronic structure theory calculations and a Master Equation treatment of reactions taking place over complex potential energy surfaces. The resulting model shows that at the high temperatures within two stellar radii of a MIRA variable AGB star in oxygen-rich conditions, collisional excitation of ground-state P(⁴S) to P(²D), followed by reaction with H₂O, is a significant pathway for producing PO (in addition to the reaction between P(⁴S) and OH). The model also demonstrates that the PN fractional abundance in a steady (non-pulsating) outflow is underpredicted by about 2 orders of magnitude. However, under shocked conditions where sufficient thermal dissociation of N₂ occurs at temperatures above 4000 K, the resulting N atoms convert a substantial fraction of PO into PN.