Resonant Raman studies of pyrazine with tunable uv lasers: An analysis based on the generalized vibronic theory of Raman intensity

Abstract

The resonant Raman effect of pyrazine is studied with tunable uv lasers. Polycrystalline samples of pure pyrazine and pyrazine in benzene were studied at low temperature to eliminate fluorescence. Genuine Raman spectra were obtained with this cryogenic technique as close as 40 cm−1 below the 0–0 of the 1B3u transition. Using the benzene 992 cm−1 band as the reference, absolute Raman cross sections were obtained from the known absolute Raman cross section of the benzene line. It was found that ν6a and ν10a have an enhancement factor of 106, corresponding to the strong vibronic coupling case. The data were analyzed by using the strong vibronic coupling theory of Raman intensity recently developed by Hong [J. Chem. Phys. 67, 801 (1977)]. In this generalized vibronic theory both the Franck–Condon effect (for ν6a) and the nonadiabatic Herzberg–Teller coupling (for ν10a) are treated in similar fashion through the use of the Green’s function. Vibronic coupling strengths were derived from analyses of the absorption spectra and used to calculate the absolute Raman cross sections. Agreement between theory and experiments is generally good. Comparison between the Green’s function approach and the traditional weak coupling theory is also made. It is emphasized that Raman spectroscopy and electronic spectroscopy are intimately related as is evident from the success of the theory to predict consistently not only the spectral features of absorption but also the absolute Raman cross sections.