FEMTOSECOND PHOTOISOMERIZATION OF CIS-AZOBENZENE

Femtosecond transient absorption measurements of the photoisomerizatio n 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 c omponent. During the decay of the excited state some oscillatory featu res are found. Vibrational cooling of the hot photoproducts in the gro und state occurs on a time scale of 10 ps. A slower initial photoreact ion, 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 photoi somerization via inversion at one of the nitrogen atoms. (C) 1997 Else vier Science B.V.