QUANTITATIVE STUDY OF EXCITATION PROFILES OF TOTALLY SYMMETRIC AND NONTOTALLY SYMMETRIC VIBRATIONS OP PYRAZINE WITH TUNABLE UV LASERS--A CRITICAL EXAMINATION OF VARIOUS THEORIES OF RAMAN INTENSITIES

Abstract

The resonant Raman effect of pyrazine was studied quantitatively with tunable UV lasers at liquid helium temperature. Solid solutions of pyrazine in benzene with various concentrations (3% to 0.1%) were used. Using the 990 $cm^{-1}$ benzene vibration as reference, the excitation profiles of Raman intensities for $\nu _{1}, \nu_{9}, \nu_{6a},$ and $\nu _{10a}$ were determined. It was found that both $\nu_{6a}$ and $\nu_{10a}$ are enhanced by a factor of $10^{6}$. The functional dependence is quite different for different vibrations, e.g., $\nu _{6a}$ shows a $(\Delta E)^{-2}$ dependence $(\Delta E $ is the difference between the excitation energy and the resonance), whereas, $\nu_{10a}$ shows a $(\Delta E)^{-4}$ dependence off resonance and a $(\Delta E)^{-1}$ dependence near resonance. Existing theories by Albrecht, Peticolas, etc., would predict $(\Delta E)^{-4}$ off resonance and $(\Delta E)^{-2}$ near resonance for $\nu_{10a}$ and $(\Delta E)^{-4}$ for $\nu_{6a}$ throughout.