Velocity dependence of rotationally inelastic collisions: 7Li2( A  1Σ)+Ne, Ar, and Xe

We present velocity-selected measurements of the cross sections for rotationally inelastic collisions of 7Li*2(A 1Σ) with Xe, Ar, and Ne, with various pairs of initial and final rotational levels, ji and jf: Li*2(vi=9, ji) +X→Li*2(vf=9, jf=ji+Δ)+X. For each target gas measurements were made with ji=8, 22, and 42 for approximately eight values of Δ with an overall range of −20 to +24; a total of 69 cross sections were measured. Velocity selection is accomplished by a Doppler-based laser-selection technique combined with further numerical inversion methods. We achieve a measurable relative velocity range of ≂0.5→3×105 cm/s, corresponding to kinetic energies of 120 to 3500 K. In contrast to the few previous similar studies, our data show a dramatic and varied velocity dependence. We have observed a few cross sections which increase substantially with velocity, some which rise faster than v−1r at low velocity, some that fall like v−2r at high velocity, and numerous examples of peaks in endoergic cross sections near threshold. We have seen two distinct types of rotationally adiabatic reduction in certain cross sections: ‖Δ‖=2 in Li*2–Xe and ji=42, Δ≲−16, in all three systems, particularly in Li*2–Ne. The velocity dependent behavior of these cross sections is sensitive to the long-range and short-range interaction potential, respectively. Adiabatic effects in the latter case result in the existence of classical ‘‘dynamical threshold velocities’’ for these large −Δ collisions. Some analytical results of a quantitative investigation into these rotationally adiabatic effects are also given.