Refutation of a propensity rule in low-energy electron scattering by neon atoms

Since the work of Kohmoto and Fano [1] there has been considerable interest in the sign of the `orientation' parameter L$_{perp}$, which describes the angular momentum of an excited electronic state perpendicular to the scattering plane imparted by the projectile electron. In a polarization-correlation experiment, L$_{perp}$ can be related to the measurable Stokes parameter P$_{3}$ (L$_{perp}$ = -P$_{3})$, which describes the circular polarization of the photon emitted perpendicular to the scattering plane. Of particular interest is the empirical observation that, for S to P transitions, P$_{3}$ universally trends to negative values for small scattering angles, regardless of the target or incident energy. A number of studies (e.g. [1-3]) have therefore considered the generality of this `propensity rule' and its theoretical basis. Here, a recent joint experimental and theoretical study of electronic excitation of the resonant transition in neon by 25eV electrons is presented. In both the theoretical and experimental data it is observed that P$_{3}$ is positive at small scattering angles, demonstrating a refutation of this propensity rule, in disagreement with the classical arguments of Kohomoto and Fano [1]. \\[4pt] [1] M. Kohmoto and U. Fano, J. Phys. B \textbf{14}, L447 (1981)\\[0pt] [2] D. H. Madison and K. H. Winters, Phys. Rev. Lett. \textbf{47}, 1885 (1981)\\[0pt] [3] K. Bartschat, N. Andersen and D. Loveall, Phys. Rev. Lett. \textbf{83}, 5254 (1999)