EARLY TIME DYNAMICS OF TRANS-AZOBENZENE ISOMERIZATION IN SOLUTION FROM RESONANCE RAMAN INTENSITY ANALYSIS

Resonance Raman spectra have been recorded for trans-azobenzene in car bon tetrachloride using 16 excitation wavelengths in the region from 3 55-600 nm. It has been observed that for many totally symmetric fundam entals viz. C-N, N=N stretch, etc., the resonance Raman intensities de crease near the maxima of the resonant electronic (2 (1)A(g) <-- 1 (1) A(g)) transition. This is attributed to interference due to preresonan t scattering from the strongly allowed (1 (1)A(u) <-- 1 (1)A(g)) elect ronic transition. The Raman excitation profiles (REPs) for the ten Fra nck-Condon active fundamentals have been successfully modeled using He ller's time-dependent approach with the inclusion of interference effe ct 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-azobenze ne are on N=N and C-N stretching vibrations, while N=N suffers reducti on, C-N bond elongates, and with time the ring C atoms distort relativ ely out of the plane. (C) 1997 American Institute of Physics.