Velocity dependence of rotationally inelastic collisions: 7Li*2(A 1Σ)+Ne, Ar, and Xe

Abstract

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, j_i and j_f: Li^*₂(v_i=9, j_i) +X→Li^*₂(v_f=9, j_f=j_i+Δ)+X. For each target gas measurements were made with j_i=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⁻¹_r at low velocity, some that fall like v⁻²_r 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 j_i=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.