Resonance Raman Spectroscopy as a Probe of the Mixed-Valence State of Linear-Chain Complexes
A Raman spectrum recorded using exciting radiation whose frequency coincides with that of an electronic transition of a molecule is known as a resonance Raman (r.R.) spectrum. Such a spectrum differs from a normal Raman spectrum in that the intensities of certain bands are enhanced in the resonance case, and overtone and combination tone progressions may be observed. The value of r.R. spectroscopy lies in its close connection with electronic absorption spectroscopy.1 Those vibrations which are ‘active’ in an electronic transition, i.e. provide intensity in an absorption band, are also the ones which give rise to the enhanced bands and progressions in the r.R. spectrum. Therefore, since the widths of Raman bands are normally very much less than those of electronic absorption bands, it is possible, by monitoring the changes in the Raman spectrum as the frequency of the exciting radiation is changed, to deduce the modes which are active in the resonant electronic transition. This is especially valuable in the case of mixed-valence compounds for which the absorption bands are typically thousands of wavenumbers in width even at low temperatures.2 A plot of the variation of intensity of a Raman band with excitation frequency is known as an excitation profile.
KeywordsRaman Band Exciting Radiation Resonance Raman Spectrum Resonance RAMAN Spectroscopy Excitation Profile
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