Influence of a finite scattering volume on the determination of electron-impact coherence parameters

Abstract

Superelastic electron scattering from laser-excited '"Ba(. . .6s6p 'P) is investigated both theoretically and experimentally. The theoretical framework is developed and incorporated into a model of a realistic scattering geometry of finite spatial extent. Model calculations predict that the dependence of the superelastic scattering intensity on the linear polarization direction of the laser beam can be severely distorted due to the existence of a finite scattering volume. Our measurements confirm the presence of this distortion and are well described by the model calculations. Conclusions are drawn concerning the inAuence of this effect on the extraction of electron-impact coherence parameters.

Elastic electron scattering by laser-excited138Ba( ... 6s6p1P1) atoms

“…This agreement indicates that extended scattering volume effects (see Zetner et al 1990) are not important in the present measurements and that the CCC calculational scheme used here is applicable to elastic scattering by Ba 1 P 1 atoms. The rate of convergence and the importance of the ionization channels in our calculations were investigated by also performing calculations with 55 discrete states in the expansion.…”

“…During the measurements the fluorescence signal from the laserexcited section of the Ba beam was also continuously recorded in order to monitor the laser pumping conditions. Details concerning the electron gun and detectors, the Ba and laser beam sources and other experimental aspects have been described previously (Register et al 1983, Trajmar and Register 1984, Zetner et al 1990.…”

“…Later, we will define the relations between the two sets of laser polar angles. For a detailed description of the coordinate systems, see Zetner et al (1990). The laser beam is linearly polarized, and the angle of polarization with respect to the scattering plane is denoted as ψ.…”

“…Shortly thereafter similar studies on Ba (see, e.g., Register et al 1978Register et al , 1983 were initiated in our laboratory. Measurements have been reported subsequently on Na (Herman and Hertel 1982, McClelland et al 1992, Sang et al 1994, Hall et al 1996, Ba (Zetner et al 1990, Li and Zetner 1994a, Ca Teubner 1995, Teubner et al 1996), Li (Karagonov et al 1996, Rb (Hall et al 1996), Cr (Hanne et al 1993) and Yb (Li and Zetner 1994b). In these studies the superelastic scattering signal corresponding to the electron-impact de-excitation of the laser-prepared state to the ground state was measured as a function of the laser geometry and polarization (with respect to the collision frame).…”

Low energy super-elastic scattering studies of calcium over the complete angular range using a magnetic angle changing device

“…The reduction in P lin from unity around e = 180° is probably due to the 'finite volume' effect [40] that all measurements suffer from (since the scattering plane in no longer well defined at this angle), however this does not explain the difference between theory and experiment seen from e = 150° to e = 180°. Since both L e ( ) and e ( ) are in good agreement with theory at these angles, the discrepancy in P lin e ( ) is considered to be a shortcoming of the RDWA model, rather than a systematic problem with experiment.…”