Depolarisation spectra for optical collisions in the alkali-helium series

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Abstract
The spectra of fluorescence depolarisation (orientation, alignment) applying to single collision conditions are analysed for the alkali-helium series. These are derived from a quantum calculation of the optical collision cross sections for non-resonant collision pair excitation and include non-adiabatic couplings and thermal averaging over the collision energy. The authors find strong spectral variations of the polarisation which reflect the potential characteristics of these systems. A numerical test is performed for the classical Lewis model using realistic trajectories and a reasonable choice of the decoupling radius and fair quantitative agreement with the quantum calculation is shown. Large residual alignments in the far wings demonstrate a lack of complete reorientation of the transition dipole in a strong collision.