VIBRATIONAL-RELAXATION PROCESSES IN ISOTROPIC MOLECULAR LIQUIDS - A CRITICAL COMPARISON

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 laboratory 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. At present, we may single out four groups of molecules that have been investigated by these methods: (1) diatomic and/or highly symmetric molecules (e.g. HCl, O-2, CH4); (2) derivatives of hydrocarbons, especially methane (e.g. CH3NO2, CHCl3, CH2Cl2); (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. Hydrogen-bonded 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.