Resonance Raman and femtosecond absorption studies of vibrational relaxation initiated by ultrafast intramolecular proton transfer

Abstract

The subpicosecond photoinduced proton-transfer cycle of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole creates ground state molecules with a highly excited vibrational system. As a result, the S 0-S 1 absorption shows a drastic broadening in the picosecond regime that is directly monitored in temporally and spectrally resolved experiments. The vibronic structure of these spectra is quantitatively analyzed with the help of resonance Raman measurements and theoretical calculations. At delay times longer than 10 ps, the intramolecular vibrational distribution is well characterized by a vibrational temperature that decreases within 50 ps from 1200 to 300 K via collisional energy transfer to the solvent. In contrast, transient spectra observed within the first 5 ps suggest nonthermal vibrational populations of the Franck-Condon active modes.