Vibronic dephasing of anharmonic molecules. I. Theory and its application to the separability of intra- and intermolecular processes

A quantum-mechanical theory of vibronic dephasing of impurity molecules in condensed media is presented. An expression for the dephasing time is derived that exhibits explicit dependences upon microscopic properties of both impurity and medium. The expression, which contains the temperature dependence and the cross section for dephasing, is used to predict qualitatively and semiquantitatively some features of vibronic spectral lines in solids. Of particular interest are systems in which the influence of intramolecular properties (of the impurity) upon dephasing is separable from that of intermolecular properties (of the medium). The distinction between vibrational and purely electronic effects on vibronic dephasing is also emphasized.