Analysis of physical observables and approximate distribution functions for drifting linear ions

The accuracy of two recently proposed approximate distribution functions for drifting linear ions [R. Baranowski and M. Thachuk, Phys. Rev. A 63, 032503 (2001) is tested at a number of levels of microscopic detail. Through formal mathematical manipulations. low-order expressions are derived for a number of physical observables, including the angular momentum dependence of the drift velocity, and parallel and perpendicular translational temperatures. as well as the dependence of the quadrupolar alignment parameter on the parallel and perpendicular velocity components. Finally, comparisons are made between the approximate forms and basis set expansions of the distribution function utilized in formal solutions of the Boltzmann equation, with the goal of suggesting general features of these basis sets that might help improve convergence.