OBSERVATION OF AN ANOMALOUS INCREASE IN TOTAL CROSS-SECTIONS WITH HIGH VIBRATIONAL-EXCITATION IN THE LI-2 (V=0-21)+NA REACTIVE SYSTEM

The effect of initial vibrational excitation on Li-2(v,J)+ Na collisio ns was studied experimentally and theoretically. The relative integral cross sections sigma(v,J) were measured in a crossed beam experiment for vibrational states v populated thermally or by optical pumping, an d for final states detected by laser induced fluorescence. It is found that sigma(v,J) increases by 35+/-7% between v=0 and v=20, while the dependence on the initial rotational state J is insignificant, within +/-5%. The effect of vibrational enhancement is about twice as large a s that found for the Li-2-rare gas systems for the same amount of vibr ational excitation. The effect is about an order of magnitude larger t han predicted by our quasiclassical trajectory calculations on a LEPS potential energy surface, thus suggesting that additional long-range p otential terms ought to be included. We also attempted to measure stat e-selectively the LiNa (v',J') products from the thermal reaction and found an upper limit of the reactive cross section of 1 Angstrom(2), T his contrasts with the trajectory calculations that predict a 20 times larger cross section; we thus conclude that the reactive part of the potential also needs to be re-evaluated.