Experimental study of the removal of excited state phosphorus atoms by H${}_2$O and H${}_2$: implications for the formation of PO in stellar winds

Abstract

The reactions of the low-lying metastable states of atomic phosphorus, P(${}^2$D) and P(${}^2$P), with H${}_2$O and H${}_2$ were studied by the pulsed laser photolysis at 248 nm of PCl${}_3$ , combined with laser induced fluorescence detection of P(${}^2$D), P(${}^2$P) and PO. Rate coefficients between 291 and 740 K were measured, along with a yield for the production of PO from P(${}^2$D or ${}^2$P) + H${}_2$O of (35$\pm$15)%. H${}_2$ 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(${}^4$S) to P(${}^2$D), followed by reaction with H${}_2$O, is a significant pathway for producing PO (in addition to the reaction between P(${}^4$S) and OH). The model also demonstrates that the PN fractional abundance in a steady (non-pulsating) outflow is under-predicted by about 2 orders of magnitude. However, under shocked conditions where sufficient thermal dissociation of N${}_2$ occurs at temperatures above 4000 K, the resulting N atoms convert a substantial fraction of PO to PN.

24 pages, 7 figures, accepted for publication in MNRAS

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Cited by in corpus (3)

• ATOMIUM: Molecular inventory of 17 oxygen-rich evolved stars observed with ALMA
• Bottom-up dust nucleation theory in oxygen-rich evolved stars II. Magnesium and calcium aluminate clusters
• New routes for PN destruction and formation in the ISM via neutral-neutral gas-phase reactions and an extended database for reactions involving phosphorus