Simple analytic form for the velocity-angular-momentum distribution function of drifting linear ions

A simple analytic form is presented for the full velocity-angular-momentum distribution function for gasphase linear ions drifting in an atomic bath gas under a constant external electric field. Predictions of temperatures, drift velocities, and alignment parameters from this form are compared in detail against molecular-dynamics calculations for NO+ drifting in helium. The form is accurate, compact, and based upon a physically motivated expansion. In essence, simple Maxwellian-like functions are generalized by allowing appropriate temperatures to become functions of velocity and rotational ang;lar momentum. Over a wide range of reduced field strengths, including the equilibrium state, this form is able to account accurately for many properties at a microscopic level with only a few adjustable parameters.