Energy transfer in collisions of vibrationally excited diatomics with atoms at low energies (1–100 meV)

Abstract

Rotational and vibrational energy transfer in low-energy collisions ( E coll=1–100 meV) of alkali dimers (Li 2, Na 2) with atoms (He, Ne) is investigated via calculation of quasiclassical trajectories (QCT) for a wide range of initial vibrational and rotational states ( v 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 v 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 Δ 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.