Collision-induced intramultiplet mixing for Ne/emph>{(2p)5 (3p)}+He : experiments and quantum calculations

We have investigated fine-structure–changing collisions of short-lived (20-ns) Ne** atoms in the {a}={(2p)5(3p)} multiplet with ground-state He atoms. A newly designed cross-beam apparatus allows the measurement of accurate polarized cross sections Ql¿k¿Mk¿ for the {a}k¿{a}l transition. In the experiment, the initial {a}k state is prepared with a well-defined asymptotic orientation Mk of its electronic angular momentum J, through excitation of metastable Ne* atoms with a polarized laser. The reported transitions are mainly between states in the {a}4,5,6,7 group (Paschen numbering), at approximately 100 meV center-of-mass energy. Some of these exhibit very strong polarization effects, with differences between Ql¿k¿Mk¿ and Ql¿k¿Mk’¿ of up to a factor 4. Fully quantum-mechanical coupled-channel calculations on a diabatic basis, with the Ne**-He model potentials of Hennecart and Masnou-Seeuws as input, prove successful in reproducing experimental results. Cross-section behavior may be qualitatively understood from the presence of avoided crossings between the adiabatic potentials, indicative of strong radial coupling. The restraint of reflection symmetry is strongly felt here. In addition, the effects of rotational coupling can be readily identified.