Low-Velocity Ion-Molecule Collisions with Quadrupole Interaction

Low-velocity collision between an ion and a nonpolar molecule with a large quadrupole moment is theoretically studied. The mathematical formulation is based on the expansion of the molecular rotational wave function in terms of the PRS (Perturbed Rotational States). A straight classical trajectory and a constant velocity are assumed for the relative motion of the colliding pair. A simple scaling law is derived for the cross section of the rotational excitation. The cross section for the hitting collision, where the colliding pair come into a close contact, is also calculated approximately for some lower states of the molecular rotation. Numerical calculations are given for the ion-CO 2 collisions. Because of the scaling law, the results are applicable, at least approximately, to other system with an appreciable quadrupole interaction.