Nascent PO(X 2Π) E,V,R,T excitations from collision-free IR laser photolysis: Specificity toward the PO(X 2Π1/2) spin-orbit state

Abstract

PO (X 2Π) is produced via the collision-free infrared multiple photon dissociation (IRMPD) of volatile organophosphorous molecules, and is detected by two-frequency two-photon ionization, using the B 2Σ+ state to provide a spectral signature from which X 2Π populations are obtained. Sequential dissociations occur during the IR laser photolysis, in which nascent fragments continue to undergo IRMPD, and PO (X 2Π) accrues from a series of bond fission reactions. Nascent vibrational, rotational, and translational excitations are in sensible accord with this mechanism, except for a few rotational states near J=19.5. Unlike the nuclear degrees of freedom, the PO (X 2Π) spin-orbit states are populated quite selectively. The 2Π3/2 state, lying only 224 cm−1 above the 2Π1/2 ground state, contains only ∼11% of the population, compared to 34% for a 300 K sample. This result is unambiguous; it persists with all precursors, laser fluences, etc., and is verified by comparisons to spectra obtained using a microwave discharge, a flame, and when thermalizing nascent excitations with an inert diluent. This result underscores the importance of the separate potential surfaces which correlate to the product spin-orbit states, and the small amount of 2Π3/2 population can be accounted for by nonadiabatic coupling during dissociation, and/or ‘‘freezing’’ the amount of S1 character in an excited precursor in which S0 and S1 are coupled nonradiatively. We note that such electronic specificity should be dealt with in the analogous recombination reactions.