Transition from alignment to orientation under drift of excited ions in a low-temperature plasma

This study considers the alignment conversion of the angular momenta of excited ions into orientation occurring in collisions of ions with neutral particles in a low-temperature plasma when anisotropy of the collisional relaxation is caused by the ion drift. Anisotropy characteristics of the distribution function of the colliding particles' relative velocities are analyzed as functions of the ratio of the ion drift velocity to the particles' average thermal relative velocity. Dynamic multipole moments of the distribution function controlling the processes of the anisotropic collisional relaxation are introduced and calculated. The selection rules for contributions of these multipole moments to processes of interconversion of the polarization moments are stated. The signal of orientation (circular light polarization) generated by the anisotropic collisions from the alignment inclined with respect to the drift direction is calculated for the excited states of the ions with angular momenta J = 1,3/2, and 2 as a function of the ion drift velocity, temperature, and pressure. It is found that studies of this signal permit one to determine the ion drift velocity as well as the rate constants of diverse processes of the anisotropic collisional interconversion of polarization moments of the excited ions in the plasma. 16 refs., 3 figs