Early time dynamics of <i>trans</i>-azobenzene isomerization in solution from resonance Raman intensity analysis

Abstract

Resonance Raman spectra have been recorded for trans-azobenzene in carbon tetrachloride using 16 excitation wavelengths in the region from 355–600 nm. It has been observed that for many totally symmetric fundamentals viz. C–N, N=N stretch, etc., the resonance Raman intensities decrease near the maxima of the resonant electronic (2 1Ag←1 1Ag) transition. This is attributed to interference due to preresonant scattering from the strongly allowed (1 1Au←1 1Ag) electronic transition. The Raman excitation profiles (REPs) for the ten Franck–Condon active fundamentals have been successfully modeled using Heller’s time-dependent approach with the inclusion of interference effect from higher electronic state. The short time isomerization dynamics is then examined from a priori knowledge of ground-state normal mode descriptions to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 5–30 fs of photoexcitation, the major changes experienced by trans-azobenzene are on N=N and C–N stretching vibrations, while N=N suffers reduction, C–N bond elongates, and with time the ring C atoms distort relatively out of the plane.