Early time dynamics of trans-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 (2¹A_g←1¹A_g) transition. This is attributed to interference due to preresonant scattering from the strongly allowed (1¹A_u←1¹A_g) 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.