Semiclassical analysis of polarization effects in collision-induced intramultiplet mixing for Ne/emph>{(2p)5(3p)}+ He

We have performed new crossed-beam measurements and quantum-mechanical calculations on transitions between the short-lived Ne**{(2p)5(3p)}k={a}k states with k=4,5,6,7 (Paschen numbering), induced by collisions with ground-state He atoms at energies between 70 and 140 meV. The {a}4,5,6,7 multiplet is distinguished by the presence of several avoided crossings between the adiabatic potentials VkO(R)—a sign of strong, localized radial coupling. This has inspired a simple, semiclassical model for the Ne**-He collision process, which has the following ingredients: (i) straight-line trajectories with hard-sphere scattering at the classical turning point RT; (ii) rotational coupling for R>RL and ‘‘locking’’ of the electronic angular momentum J to the internuclear axis for R=RL, with RL the locking radius; (iii) Landau-Zener–type curve-crossing transitions near the crossing radius RC. This model goes a long way toward explaining the experimental polarization effects, i.e., differences between polarized cross sections Ql¿k¿Mk¿ for the {a}k¿{a}l transition depending on the asymptotic orientation of the total electronic angular momentum J, as specified by the magnetic quantum number Mk. These polarization effects are at times very large. For example, at a center-of-mass collision energy E¿100 meV, we find Ql¿5¿0¿/Ql¿5¿1¿=0.5 and 3.5 for l=6 and 7, respectively, Ql¿6¿0¿/Ql¿6¿1¿=0.41 and 1.3 for l=5 and 7, and Ql¿7¿0¿/Ql¿7¿1¿=0.08 and 8.9 for l=4 and 5.