Orientational contributions to the vibrational dephasing rate

Abstract

The dependence of the vibrational dephasing rate of a dense molecular fluid on reorientational motions and on the nature of the observation process, whether isotropic or anisotropic vibrational Raman scattering or vibrational absorption, is investigated from first principles. The spectral lineshape is calculated directly as the power lost from a quantized incident radiation field interacting with the electronic states of the molecules. Intramolecular vibronic coupling then produces vibrational excitation. The spectral lineshape is determined by the detailed way in which intermolecular forces and reorientational motions dephase this vibrational excitation process. The spectral lineshape is found to be a generalized Lorentzian with frequency dependent linewidth which is the sum of the conventional rotational relaxation rate and a generalized vibrational dephasing rate. This vibrational dephasing rate is shown to depend explicitly on reorientational correlation functions and on the nature of the radiation interaction. It is concluded that the orientational and vibrational contributions to the vibrational Raman spectrum are not simply factorizable. Comparison is made with the previous results of mode coupling theory.

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Notes on contributors

B.P. Hills

On sabbatical leave from Universitas Kristen Satya Wacana, Salatiga, Java, Indonesia.