FEMTOSECOND STUDIES OF VIBRATIONALLY HOT MOLECULES PRODUCED BY INTRAMOLECULAR PROTON-TRANSFER IN THE EXCITED-STATE

The keto-type reaction product of intramolecular proton transfer in th e excited state of 2-(2'-hydroxy-5'methylphenyl)benzotriazole is inves tigated by femtosecond experiments in a wide spectral range. Keto-type molecules produced on a 100 fs time scale show an emission band betwe en 600 and 800 nm that decays with a time constant of 150 fs. The stro ng spectral overlap of the emission with the absorption spectrum of th e keto-type ground state extending from 400 to 700 nm gives evidence o f non-equilibrium vibrational excitations of the product molecules. Th e keto-type species decays by proton back-transfer to the enol ground state with a time constant of 600 fs. Measurements with different exci tation wavelengths demonstrate that the rates of this ultrafast reacti on cycle are not affected by the initial amount of vibronic excess ene rgy.