Steady state model for the collision induced rotational alignment of molecular ions in electric drift fields

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Abstract
A relationship is derived between the state multipole moments which characterize the deviation in an ensemble of rotors from an isotropic MJ distribution and the microscopic state multipole cross sections for rotational energy transfer in an atom-diatom system. The result is obtained for cylindrical collision symmetry under steady state conditions. We allow for the possibility that the velocity distribution, which is used to calculate the rates, is dependent on the rotational angular momentum quantum numbers J and MJ . Using an approximate velocity distribution we show how the rotational alignment of molecular ions colliding with a buffer gas in an electric drift field can be related to the zeroth and second order tensor cross sections. The theory is discussed in terms of the rotational alignment resulting when N+ 2 ions are drifted in He.