Polarization properties of ion-excitation mechanisms in high-voltage gaps

We investigate the excitation of atomic levels in a high-voltage discharge gap by studying the spectrum of the 2p-3d transitions of Li I atoms flowing in the gap. The Stark splitting of this transition, due to the gap electric field, enables resolving the lines originating from different magnetic sublevels of the 3d level. We show that the atomic level populations are dominated by excitations due to ions accelerated in the gap. The directionality of the ion beam induces magnetic sublevel populations that are remarkably different from the statistical distribution, where the major role is played by the relatively low-energy ions accelerated near the ion-emitting surface. By accounting for the polarization properties of the beam-induced level excitations we were able to obtain good agreement with observed populations of atomic levels with different m_l values.