Dynamic processes in the liquid state can be examined from different points of view; the rapid development of highly sophisticated time-resolved experiments and very complex computer simulation techniques have certainly improved the accuracy of some labratory results, but they are often not followed by an adequate attempt at interpretation. In any case, the extent of the subject and the enormous spread of experimental data do not help systematic analysis.This review attempts to contribute to a critical examination of a particular aspect of the dynamics of the liquid phase. Through a study of vibrational relaxation in isotropic molecular liquids it is possible to gain information about the molecular environment and the main intermolecular forces operative in this state of matter. It is therefore possible to formulate hypotheses about the force potentials and to describe the dynamic regime of the liquid system under consideration.(1) diatomic and/or highly symmetric molecules (e.g. HCl, 0, , CH,);(2) derivatives of hydrocarbons, especially methane (e.g. CH,NO, , CHCl, , CH,CI,);(3) aromatic and cyclic aliphatic molecules (e.g. benzene and its derivatives, cyclohexane, pyridine); (4) hydrogen bonded (with OH and/or HCO groups).This paper concerns the comparison between vibrational motions that have similar normal coordinate descrip tions; for this reason, we shall limit our considerations to the second and third groups in the above list. Hydrogenbonded and highly symmetrical systems are excluded because they need specific considerations.The currently used basic nomenclature of this field will be illustrated, in addition to the physical meaning of the dynamic parameters usually obtained by steady-state (continuous Raman and infrared), time-resolved spectroscopy and computer simulation techniques; these different experimental data will be compared, when available. We shall also outline and discuss current theoretical models, and attempt to emphasize their potential and limitations.At present, we may single out four groups of molecules that have been investigated by these methods:
VIBRATIONAL RELAXATION IN LIQUIDS: THE PROBLEMSince the early 1960s, the study of vibrational relaxation in condensed phases has been considered a way of investigating dynamic processes,' but only with the advent of lasers as spectroscopic sources have frequency-and time-resolved experiments been performed; lineshape and coherent picosecond excitation studies allowed accurate information to be obtained on the dynamics in the solid and liquid states. Since that time, many papers2-14 have proved useful in the development of theoretical models and experimental results; for a detailed analysis of the whole question, one may refer to the above publications and references therein. For the purpose of this paper, the basic definitions of the main physical entities and variables involved in the subject must be clarified, avoiding the conflicts and ambiguities often present in the literature.
DEPHASING: DEFINITIONSDephasing is important as probe of the...
