Excited state kinetics of neutral transition metal atoms by stimulated emission pumping: V*(3d44s,a4D)+hydrocarbons

Abstract

Stimulated emission pumping (SEP) by two tunable, ns dye lasers selectively creates metastable V*(3d44s,a4DJ*) excited states near 8500 cm−1 in a fast flow reactor with 1.2 Torr He buffer gas at 300 K. Subsequent collisions with He equilibrate the spin-orbit levels J*=1/2–7/2 on a time scale of 1 μs but do not quench the 3d44s, a4D multiplet on a time scale of at least 50 μs. A third probe laser placed 5 mm downstream of the SEP lasers monitors the V* population at fixed reaction time t=40 μs. The exponential decay of V* vs calibrated hydrocarbon flow yields total removal rate constants that include both chemical reaction and electronic quenching of the V* excited state. Coupling of diffusion with reaction/quenching evidently does not cause the slight positive curvature observed in the kinetics plots. We demonstrate this by solving two simplified models of the reacting, diffusing, flowing gas for our experimental geometry, with SEP and probe laser beams transverse to the flow tube axis. Instead, the curvature is probably caused by the delayed, collisional cascade of a small population of higher excited states into the V* state. Ethylene removes the V* state on essentially every hard-spheres collision. The alkanes CH4, C2H6, and C3H8 remove V* on one in 75, one in 25, and one in 12 collisions. This is remarkably inefficient reaction/quenching, given that the low-spin 3d44s state is ideally configured for CH or CC bond insertion in alkanes. We interpret these new results and earlier work of Honma and co-workers on the sextet 3d44s excited state in terms of avoided intersections between repulsive and attractive diabatic potential surfaces (conserving electron spin and configuration).