FEMTOSECOND INTRAMOLECULAR PROTON-TRANSFER OF VIBRATIONALLY HOT MOLECULES IN THE ELECTRONIC GROUND-STATE

Intramolecular proton transfer and vibrational relaxation in 2-(2'-hydroxy-5'-methylphenyl)-benzotriazole are directly monitored in spectrally and temporally resolved experiments with femtosecond ultraviolet pulses. Within 500 fs after excitation of the original enol species, excited-state proton transfer and subsequent internal conversion create keto-type tautomers in the electronic ground state. The keto --> enol back-transfer of the proton occurs with a time constant of 700 +/- 100 fs as is obvious from the kinetics of repopulation of the enol ground state. The fast reaction produces vibrationally hot enol molecules with a strongly broadened S0 --> S1 absorption spectrum. The reshaping of this band at later times gives evidence of the cooling of hot molecules within 30 ps.