Molecular Collisions and the Depolarization of Fluorescence in Gases

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Abstract
The polarization of fluorescence is expressed in general as the Laplace transform of a correlation function for molecular reorientation. In a dilute gas of linear molecules, the polarization is shown to be determined by the reorientation angle α through which the molecular angular momentum is rotated by collisions. This reorientation angle appears through a collision cross section proportional to sin2α, integrated over all collision impact parameters. The predicted density dependence of the polarization of total fluorescence is in good agreement with previous measurements of iodine fluorescence. If the fluorescence emission is also resolved into rotation—vibration lines, then polarization measurements allow detailed study of the reorientation by collisions which also produce specific changes in the rotational and vibrational quantum numbers.