Nonresonant hyper-Raman and hyper-Rayleigh scattering in benzene and pyridine

Abstract

Nonresonant hyper-Raman and hyper-Rayleigh spectra excited at 1064 nm are reported for neat benzene and pyridine. The theory of Herzberg–Teller vibronic coupling in nonresonant and preresonant hyper-Raman scattering is developed. Nonresonant hyper-Raman scattering is shown to be vibronically induced by modes that efficiently couple strongly allowed one-photon and two-photon transitions. A weak and broad (55 cm−1) hyper-Rayleigh band was observed in benzene and attributed to collective scattering, while in pyridine, a much more intense and much narrower hyper-Rayleigh band was observed. Only the a2u vibration (ν11) was observed in the hyper-Raman spectrum of benzene, while several strong bands were observed in pyridine. Possible vibronic-coupling pathways are discussed for these modes. In addition, the observed hyper-Raman spectrum of pyridine is compared to a recent calculation.