Rovibrational energy transfer in Ne-Li2(A1Sigmau+,v=0): Comparison of experimental data and results from classical...

Absolute rate constants for rotational and rovibrational energy transfer in the system Ne-Li2(A1Sigmau+) were measured by a dispersed fluorescence technique following excitation of the (v = 0,j = 18) initial level of Li2(A1Sigmau+). The rate coefficients for Deltav = 0 processes decline monotonically with increasing |Deltaj|. The Deltav = 1 rate coefficients are also peaked near Deltaj = 0 but show a broad shoulder extending to approximately Deltaj = 30. Classical trajectory calculations and accurate quantum mechanical close-coupled calculations were used to compute theoretical rate constants from an ab initio potential surface. The agreement between the classical and quantum calculations is very good. The calculations slightly overestimate the measured rate constants for Deltav = 0, Deltaj or= 20, implying that the anisotropy of the ab initio surface is too small at short range and too large at long range. For Deltav = 1 collisions, the calculations agree well with experiment for Deltaj 0 collisions, disagreeing with experiment by a factor of 2 for Deltaj approximately 20 but agreeing better at higher and lower Deltaj. Analysis of classical trajectories indicates that the vibrationally inelastic collisions fall into two groups corresponding to equatorial and near-end impacts; the former generally produce small Deltaj while the latter produce large Deltaj. Studies of a simple model potential show that this dual mechanism may be a general phenomenon not limited to the particular potential surface employed here. Criteria controlling the relative importance of the two vibrational excitation routes are enumerated.