Investigation of Short-Time Isomerization Dynamics inp-Nitroazobenzene from Resonance Raman Intensity Analysis

Resonance Raman (RR) spectra are presented for p -nitroazobenzene dissolved in chloroform using 18 excitation wavelengths, covering the region of 1 (n → π*) electronic transition. Raman intensities are observed for various totally symmetric fundamentals, namely, C−C, C−N, N N, and N−O stretching vibrations, indicating that upon photoexcitation the excited-state evolution occurs along all of these vibrational coordinates. For a few fundamentals, interestingly, in p -nitroazobenzene, it is observed that the RR intensities decrease near the maxima of the resonant electronic 1 (n → π*) transition. This is attributed to the interference from preresonant scattering due to the strongly allowed 1 (π → π*) electronic transition. The electronic absorption spectrum and the absolute Raman cross section for the nine Franck−Condon active fundamentals of p -nitroazobenzene have been successfully modeled using Heller's time-dependent formalism for Raman scattering. This employs harmonic description of the lowest energy 1 (n → π*) potential energy surface. The short-time isomerization dynamics is then examined from a priori knowledge of the ground-state normal mode descriptions of p -nitroazobenzene to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 20 fs after photoexcitation in p -nitroazobenzene, the N N and C−N stretching vibrations undergo significant changes and the unsubstituted phenyl ring and the nitro stretching vibrations are also distorted considerably.