Abstract

Intramolecular proton transfer is the prototype of an elementary photoreaction that often occurs on a subpicosecond time-scale [1], The kinetics of the reaction cycle of 2-(2′-hydroxy-5′methylphenyl)benzotriazole (commercial name: TINUVIN P, TIN) which is explained in the insert of Fig. 1 (a) has been derived from femtosecond measurements at several fixed wavelengths [2], Excitation to the first electronic state leads to formation of a keto-type reaction product within 100 fs which is followed by radiationless deactivation of the reaction product within 150 fs and a final proton back-transfer to the enol electronic ground state (600 fs). In this paper, we report on vibrationally hot molecules produced by photo-induced intramolecular proton transfer which are studied by femtosecond experiments over a wide spectral range providing the transient spectra of the keto-type reaction product [3], The kinetics of gain and absorption is shown in Fig. 1 (a) displaying the transient change of absorption for two different wavelengths of the probe pulses. In the range of spectral overlap between absorption and emission, the transients consist of an initial decrease of absorption due to stimulated emission (negative sign) that is followed by the transient absorption (positive sign).

© 1996 Optical Society of America