Femtosecond photoisomerization of cis-azobenzene

Femtosecond transient absorption measurements of the photoisomerization of azobenzene excited at 435 nm in the long-wavelength (n pi*) band are presented. For cis-azobenzene the experiments reveal an extremely fast photoproduct formation with a dominating 170 fs and a weak 2 ps component. During the decay of the excited state some oscillatory features are found. Vibrational cooling of the hot photoproducts in the ground state occurs on a time scale of 10 ps. A slower initial photoreaction, which can be described by two time constants, 0.32 and 2.1 ps, is found for the trans-isomer. The experimental results support the idea, that excitation of the S-1 (nn*) bands of azobenzene leads to photoisomerization via inversion at one of the nitrogen atoms. (C) 1997 Elsevier Science B.V.