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 suppressed as compared to the dipole matrix elements by the parameter ωR/c <<1, where  ω is the photon energy, R is the ionized shell radius and c is the speed of light.

This shortcoming is absent in suggested studies of angular distributions of electrons emitted in fast charged particle-atom collisions.

We present not only general formulas, but also concrete results of calculations for several noble gas atoms He, Ar and Xe. We have investigated their outer shells. In appeared that even for the case of very small transferred momentum, i.e. in the so-called optical limit the deviation from photoionization case is prominent.