Excimer Laser Photofragmentation of Organophosphonates. 2. State Distributions, Quenching, and Reaction of PO Radicals Generated from Dimethyl Methylphosphonate

摘要

Ultraviolet lasers show promise for specific and sensitive detection of chemical warfare agents by providing the means for photofragmentation of the parent molecule followed by efficient fragment excitation and emission detection. Results are provided in this report concerning a number of parameters which are important in assessing the practical viability of this approach. 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. Probing the PO radical by laser-induced fluorescence, utilizing the A2 sigma + -X2 Pi 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 2 Pi 1/2, 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 1.8 + or - .0000000005 cu cm/molecule/sec. Ground state PO reacts bimolecularly with molecular oxygen with a rate constant of 1.2 + or - 0.2 x 10 to the -11th cu cm/molecule/sec, equivalent to about 1/40 the hard-sphere collision rate.