VELOCITY DEPENDENCE OF QUASI-RESONANT VIBROTATIONAL TRANSFER IN LI-2-ASTERISK-RARE GAS COLLISIONS

We report measurements of velocity-dependent cross sections for vibrot ationally inelastic scattering in the system Li-2 A(1) Sigma(u)(+)(v(i ),j(i))) + X --> Li-2 A(1) Sigma(u)(+)(v(f),j(f)) + X, with v(i) = 9, j(i) = 42, and X = Xe, Ar, and Ne. The measurements range over a facto r of 30 in energy. Quantum levels were chosen to elucidate the quasi-r esonant vibrotational transfer process studied previously. A reduction in collision velocity results in both an increase in total vibrationa lly inelastic cross section and an enhancement of the quasi-resonant e ffect, with final rotational state distributions as narrow as 2.5HBAR (fwhm) observed. The largest cross section for Delta v < 0 is given by the formula Delta j(peak) = -4 Delta v and possesses a roughly 1/v(re l) velocity dependence. For Delta v = +1, energy thresholds shift the peak cross section at low velocity to the nearly energy resonant value of Delta j(peak) = -6. The similarity of the final state distribution s for different target gases observed in previously measured rate cons tants does not hold in the velocity-dependent data; this results in pa rt from the appearance of low-velocity dynamical thresholds for exoerg ic cross sections in the Ar and Ne systems. We compare the experimenta l results for X = Ne with cross sections calculated from quasi-classic al trajectories on an ab initio potential energy surface; for j(f) gre ater than or equal to 46, agreement is quantitative, while for j(f) < 46, the calculation overestimates the cross sections.