Non-dipole effects in angular distributions of secondary electrons in fast particle-atom scattering

Abstract

We demonstrate that the angular distribution of electrons knocked out from an atom by a fast charge particle is determined not only by dipole but also by quadrupole transitions, the contribution of which can be considerably enhanced as compared to the case of photoionization. To obtain these matrix elements one has to study the angular distribution of electrons emitted by the atom in its collision with a fast charged particle. The distribution has to be measured relative to the momentum q transferred from the projectile to the target atom. The situation is similar, but not identical to the photoionization studies, where the matrix elements of continuous spectrum atomic quadrupole transitions can be determined by measuring the so-called non-dipole angular anisotropy parameters of photoelectrons. However, they are strongly suppressed as compared to the dipole matrix elements by small ratio of atomic size to the photon wavelength. This suppression is controlled in fast electron-atom collisions, where it can be much less than in photoionization since the respective virtual photon wavelength can be much smaller than in photoionization. We present not only general formulas, but also concrete results of calculations for noble gas atoms He, Ar and Xe. We have investigated their outer and subvalent subshells, as well as strongly collectivized 4d10 subshell in Xe. It appeared that even for the case of very small transferred momentum q, i.e. in the so-called optical limit the deviation from photoionization case is prominent.