Nondipole parameters of TDCS for electron impact ionization and drag current

• Published in: Central European journal of physics 2011-10, Vol.9 (5), p.1209-1220
• Main Authors: Baltenkov, Arkadiy S., Manson, Steven T., Msezane, Alfred Z.
• Format: Article
• Language: English
• Subjects: back/forward asymmetry; Biological and Medical Physics; Biophysics; drag current; Environmental Physics; Geophysics/Geodesy; Impact ionization; non-dipole parameters; Physical Chemistry; Physics; Physics and Astronomy; Research Article; triple differential cross-section
• ISSN: 1895-1082; 2391-5471; 1644-3608; 2391-5471
• DOI: 10.2478/s11534-011-0046-z

A comprehensive study is undertaken of angular distributions of electron knock-out from atomic targets by fast electrons with a small transfer of momentum. The general expressions for the parameters of the triple differential cross-section of impact ionization in the optical limit are derived. The calculated parameters are compared with those of the angular distribution of electrons ejected from an atom in the process of photoionization. In these processes, when the multipole transitions are involved, the one-to-one correspondence between the photoionization and impact ionization parameters disappears. The nondipole transitions lead to the backward/forward asymmetry of the angular distribution of ejected electrons that is absent in the dipole approximation for ionization by both fast electrons and photons. Using the He atom as an example, the character of the asymmetry for these two processes is qualitatively different and the backward/forward asymmetry results in macroscopic directed motion of secondary electrons accompanying the passing of a fast electron beam through gas or plasma. The general formulas for this drag current are derived and applied to gaseous He.