EXCITED-STATE KINETICS OF NEUTRAL TRANSITION-METAL ATOMS BY STIMULATED-EMISSION PUMPING - V-ASTERISK(3D(4)4S, A(4)D)+ HYDROCARBONS

Stimulated emission pumping (SEP) by two tunable, ns dye lasers select ively creates metastable V(3d(4)4s,a(4)D(j)*) excited states near 850 0 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 mu s but do not quench the 3d(4)4s, a(4)D multiplet on a time scale of at least 50 mu s. A third probe laser pla ced 5 mm downstream of the SEP lasers monitors the V population at fi xed reaction time t = 40 mu s. The exponential decay of V vs calibrat ed hydrocarbon flaw yields total removal rate constants that include b oth chemical reaction and electronic quenching of the V excited state . Coupling of diffusion with reaction/quenching evidently does not cau se the slight positive curvature observed in the kinetics plots. We de monstrate this by solving two simplified models of the reacting, diffu sing, flowing gas for our experimental geometry, with SEP and probe la ser beams transverse to the flow tube axis. Instead, the curvature is probably caused by the delayed, collisional cascade of a small populat ion 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 collis ions. This is remarkably inefficient reaction/quenching, given that th e low-spin 3d(4)4s state is ideally configured for CH or CC bond inser tion in alkanes. We interpret these new results and earlier work of Ho nma and co-workers on the sextet 3d(4)4s excited state in terms of avo ided intersections between repulsive and attractive diabatic potential surfaces (conserving electron spin and configuration). (C) 1997 Ameri can Institute of Physics.