Fluorescence polarization as a probe of the rotational dynamics of isolated highly excited molecules

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Abstract
There is now much evidence that the vibrational modes of isolated highly excited molecules are strongly mixed, but little is known about the rotational motion of such molecules. The basic question is whether rotation is approximately separable from vibration or is strongly coupled with vibration by Coriolis and centrifugal forces. In an effort to develop experimental methods to answer this question, we present computations of the polarization of fluorescence from electronically excited symmetric top molecules undergoing two extreme types of rotational motion: Regular rotation, for which the rotational motion is that of a rigid body, and statistical motion, in which the nuclear energy is partitioned statistically between rotational and vibrational motion within the constraints of total angular momentum and energy conservation. This statistical assumption yields a particular distribution for the angle between the angular momentum and molecular symmetry axis, which in turn determines the fluorescence polarization. Both quantum and classical theories for the fluorescence polarization are given, and we present extensive numerical calculations of the polarization in the classical limit. In these calculations, the fluorescence wavelength is not resolved but we allow for several degrees of resolution of the excitation spectrum. We find that the fluorescence polarization is in general much different in the regular and statistical limits, and that the statistical polarization is generally much less than the regular polarization. For statistical rotors, fluorescence polarization decreases as the rovibrational energy of the excited molecule is increased, while the polarization from regular rotors is unchanged. We find that polarization experiments can be interpreted even if no rotational structure is resolved in absorption. This technique thus does not require nozzle cooling to simplify spectra, and it can be applied to rotationally hot molecules. It is shown that electronic radiationless transitions can act either to enhance or reduce fluorescence polarization. We conclude that polarization experiments are excellently suited to probing the rotational motion of highly excited molecules.