Velocity Dependence of Quasi-Resonant Vibrotational Transfer in Li2*−Rare Gas Collisions

Abstract

We report measurements of velocity-dependent cross sections for vibrotationally inelastic scattering in the system Li2 + X → Li2 + X, with vi = 9, ji = 42, and X = Xe, Ar, and Ne. The measurements range over a factor of 30 in energy. Quantum levels were chosen to elucidate the quasi-resonant vibrotational transfer process studied previously. A reduction in collision velocity results in both an increase in total vibrationally inelastic cross section and an enhancement of the quasi-resonant effect, with final rotational state distributions as narrow as 2.5ℏ (fwhm) observed. The largest cross section for Δv < 0 is given by the formula Δjpeak = −4Δv and possesses a roughly 1/vrel velocity dependence. For Δv = +1, energy thresholds shift the peak cross section at low velocity to the nearly energy resonant value of Δjpeak = −6. The similarity of the final state distributions for different target gases observed in previously measured rate constants does not hold in the velocity-dependent data; this results in part from the appearance of low-velocity dynamical thresholds for exoergic cross sections in the Ar and Ne systems. We compare the experimental results for X = Ne with cross sections calculated from quasi-classical trajectories on an ab initio potential energy surface; for jf ≥ 46, agreement is quantitative, while for jf < 46, the calculation overestimates the cross sections.