SOLVATION STATE-SELECTIVE EXCITATION IN RESONANCE RAMAN-SPECTROSCOPY - I - EXPERIMENTAL-STUDY ON THE C=N AND THE C=O STRETCHING MODES OF PHENOL BLUE

We have measured the excitation photon energy dependence of the resona nce Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluid s of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C=N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, n o dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C=O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlate d with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR abso rption spectra of PB are measured in chloroform and carbon tetrachlori de, and compared with the Raman spectra in the same solvents. The exci tation energy dependence is explained by the solvation state selective excitation in the resonance Raman spectroscopy, that is, the inhomoge neity that appears in the absorption spectra of PB also appears in the resonance Raman process. (C) 1998 American Institute of Physics. [S00 21-9606(98)00444-9].