Vibrational motion, electrical anharmonicity

We are interested here in the linear and nonlinear optical properties that determine the response of a chemical system to spatially uniform electric fields. The vibrational contribution to this response, which arises from vibronic coupling, can often be as important as tlie pure electronic contribution or even more important [1-14]. In addition, it is often inadequate in this context to treat the effect of vibrational motions at tlie harmonic level of approximation. The purpose of this review, then, is to show how the vibrational contribution to linear and nonlinear optical properties can be evaluated with both harmonic and anharmonic effects included. 

Molecular motions (rotation, translation, and vibration) of a water molecule also turn out to be quite different from those of other common liquids. Here all the six unique features of an individual water molecule outlined in Chapter 1 manifest themselves in diverse ways. As we discuss below, not only is the mechanism of displacements of individual water molecules different, but the collective dynamics and dynamical response of bulk water are also different. For example, the rotational motion of an individual water molecule contains a surprising jump component and vibrational energy relaxation of the O—H mode involves a cascading effect mediated by anharmonicity of the bond. These motions are reflected in many important processes such as electrical conductivity, solvation dynamics, and chemical reactions in aqueous medium. 

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