Collision-induced rotational transitions of dye laser excited Li2(A1Σu+) molecules: cross sections and their +ΔJ/-ΔJ asymmetry

Abstract

Li2 molecules in a vapour cell were excited by dye laser irradiation into the J'=11, 13, 15, 17, 19, 21, 23, 27, 29 rotational levels of the A1 Sigma u+, nu '=15 vibronic state. Collision-induced rotational transitions out of these levels occurring in the presence of a foreign gas (He through Xe) were monitored by means of the fluorescence spectrum. Effective cross sections for transitions up to Delta J=20 were measured. They exhibit a strong + Delta J/- Delta J asymmetry, the cross sections for upward and downward Delta J transitions from the same level differing by up to a factor of 2.5. For Delta J=+or-2 this effect can be treated by time-dependent perturbation theory. Cross sections were calculated by adapting formulas given by Verter and Rabitz (1974) to the present case and using realistic potential parameters. It was found that the relative magnitudes of the measured cross sections, including the asymmetry, are well reproduced by the calculation. Application of the surprisal theory to the measured Delta J dependence of cross sections shows that the 'exponential energy gap law' is not an adequate description.