ENERGY-TRANSFER IN COLLISIONS OF VIBRATIONALLY EXCITED DIATOMICS WITH ATOMS AT LOW ENERGIES (1-100 MEV)

Rotational and vibrational energy transfer in low-energy collisions (E(coll) = 1-100 meV) of alkali dimers (Li2, Na2) with atoms (He, Ne) is investigated via calculation of quasiclassical trajectories (QCT) for a wide range of initial vibrational and rotational states (upsilon(i) = 0-35; J(i) = 0-35). For vibrational ground-state molecules at E(coll) = 1 meV a monotonic decline of rotational cross section with increasing J(i) is observed, which turns into a nonmonotonic behavior at high upsilon(i). Both observations are in agreement with experimental data. The cross sections are used to mimic the exponential decrease of population in selected rovibrational levels as function of J(i) as observed in laser-induced fluorescence studies of rotational state distributions following nozzle beam expansions. The QCT-results are analyzed in terms of average rotational energy transfer DELTA-E/E and compared with results from a simple impulsive model. Conclusions are drawn about the importance of kinematic versus rovibrational coupling in collisions of highly vibrationally excited diatomics.