Observation of an anomalous increase in total cross sections with high vibrational excitation in the Li2 (v=0–21)+Na reactive system

The effect of initial vibrational excitation on Li2(v,J)+Na collisions was studied experimentally and theoretically. The relative integral cross sections σ(v,J) were measured in a crossed beam experiment for vibrational states v populated thermally or by optical pumping, and for final states detected by laser induced fluorescence. It is found that σ(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 as that found for the Li2–rare gas systems for the same amount of vibrational excitation. The effect is about an order of magnitude larger than predicted by our quasiclassical trajectory calculations on a LEPS potential energy surface, thus suggesting that additional long-range potential terms ought to be included. We also attempted to measure state-selectively the LiNa (v′,J′) products from the thermal reaction and found an upper limit of the reactive cross section of 1 Å2. This 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.