Locking of the intrinsic angular momentum in the capture of quadrupole diatoms by ions

The capture dynamics of rotationally polarised homonuclear diatoms in low-energy collisions with ions is studied theoretically. The interaction potential includes charge-quadrupole and charge-induced dipole terms. The former, taken in the perturbed rotor approximation, exhibits an R-3 anisotropic dependence and causes an only gradual locking of the intrinsic angular momentum of the diatom to the collision axis. As a result, the capture occurs in a regime of incomplete locking, and the passage over centrifugal barriers is accompanied by considerable gyroscopic effects. The j-specific capture cross-sections for rotationally-aligned diatoms show a marked dependence on the polarisation state which is not described by the conventional adiabatic channel model. The latter, however, provides a fair approximation for the cross-sections of unpolarised diatoms. These features are due to a considerable angle of rotation of the collision axis before the barrier is reached and to a small angle of rotation during the passage across the barrier. The numerically determined, scaled, capture cross-sections are represented by an approximate analytical expression that interpolates between two limiting cases, very small (adiabatic channel model) and very large (fly-wheel model) gyroscopic effects of the rotating diatom.