Early days and later development of resonance Raman spectroscopy

The development of the simplest variant of the semi-classical theory of the Raman effect (RE) resulted in the establishment of two main mechanisms of this phenomenon, favoured the formation of a new direction-Raman line excitation spectroscopy, and stimulated the search for conditions of excitation of Raman scattering (RS) by molecules near and in absorption bands. The change in the traditional experimental conditions made it possible to obtain distinct Raman lines of p-nitroaniline in the region of a very intense band of the electronic vibrational absorption spectrum, i.e. under the conditions of resonance between the incident light wave and electronic oscillator. Owing to the improvements in experimental techniques, it became possible to obtain resonance Raman spectra of almost any classes of chemical compounds and to observe any forms of resonance RE. Further development of the quantum and classical theories made it possible to describe the main types of resonance secondary radiation in spectral and time scanning, to establish the interrelations between electronic vibrational absorption spectra, light scattering (RS), fluorescence and phenomena of intermediate types, to estimate more correctly the possibilities of distinguishing such notions as one- or two-photon processes, virtual and real transitions, light scattering and fluorescence and to compare more correctly the notions of the transformation of one photon and the statistical ensemble of photons. Problems relating to further investigations according to the possibilities of modern experimental and theoretical physics are discussed. (C) 1997 by John Wiley & Sons, Ltd.