State distributions, quenching, and reaction of the PO radical generated in excimer laser photofragmentation of dimethyl methylphosphonate

Abstract

Focused KrF and ArF excimer laser radiation acting on dimethyl methylphosphonate (DMMP) produces ground electronic state PO radicals via an initial two-photon absorption by the DMMP parent followed by a sequence of daughter photo-fragmentations. Probing the PO radical by laser-induced fluorescence, utilizing the A2Σ+-X2Π transition near 247 nm, reveals that the nascent PO rotational population has a distribution characterized by a temperature considerably greater than 300 K, while at least 95% of the PO radicals are formed in the lowest vibrational state. The nascent spin-orbit distribution in the 2Π1/2,3/2 state is near that characteristic of 300 K. Quenching of the A state of PO by both nitrogen and oxygen occurs with a rate constant of about 1.8(±0.5) × 10-10 cm3 molecule-1 s-1. Ground-state PO reacts bimolecularly with molecular oxygen with a rate constant of 1.2(±0.2) × 10-11 cm3 molecule-1 s-1, equivalent to about 1/40 the hard-sphere collision rate.