ULTRAVIOLET PRERESONANCE RAMAN-SCATTERING OF BENZENE-DERIVATIVES .1. EXCITATION PROFILES FOR FUNDAMENTALS

The excitation frequency dependence of the Raman scattered intensities of several fundamentals of three benzene derivatives (toluene, m-xylene, and fluorobenzene) has been measured in the ultraviolet spectral region approaching the lowest lying absorption band in each of these molecules. As in benzene, the most preresonance active fundamentals are the C-H out-of-plane bending modes. The scattering cross section of these modes is found to result from the vibronic coupling of the 1Bab states (∼ 190 nm) and weak nearby out-of-plane polarized transitions. Most of the preresonance scattered intensity for the other modes reported here (the ring-breathing, ring-substituent, and internal methyl vibrations) have Raman active electronic origins in the far UV. With excitation very close to resonance with the lowest lying 1Lb transition, this source of scattering cross section becomes conspicuous for those modes seen prominently in this transition (the ring breathing and ring substituent vibrations). In addition the ring-substituent fundamentals are found to derive significant scattering cross section from the region of the 1Ba,b transitions. © 1979 American Institute of Physics.