INTRAMULTIPLET MIXING IN THE ELECTRON-VOLT ENERGY-RANGE FOR NE-ASTERISK-ASTERISK((2P)(5)(3P))+HE

We present measurements on fine-structure-changing Ne**{(2p) 5(3p)}+He collisions at superthermal energies with an average value of E=470 meV. For an extensive set of transitions absolute values of polarized-atom cross sections are given. A wide range of cross section magnitudes and polarization effects is observed. Even higher order polarization effects are evidently present. A suitable set of anisotropy parameters is introduced which has large advantages for a direct comparison of experimental and calculated cross sections. Although the experimentally observed polarization effects are reproduced by quantum calculations, the calculated magnitude of the cross section is, in general, larger than the experimental value. Moreover, in contrast with the thermal energy range, the current semiclassical model fails in describing the most prominent cross sections in terms of the ''visible'' avoided crossings in the adiabatic potentials. In the quantum calculations, clear indications are found for an additional transition mechanism of radial matching. In this mechanism a change in the rotational quantum number of the nuclear motion matches the asymptotic energy gap ΔEkl between the initial and final state k and l, respectively. This process is most efficient at large internuclear distances, thus enhancing the magnitude of the cross section considerably. Radial matching is most effective when E/ΔEkl1. The calculated magnitude of the cross section is thus sensitive to details of the long-range part of the potentials used as input. At thermal energies this sensitivity is masked, which explains the good agreement of the calculations with the model potentials of Hennecart and Masnou-Seeuws [J. Phys. B 18, 657 (1985)] with the experiments in the thermal range. © 1995 The American Physical Society.