Picosecond time resolved Raman scattering study of hydrogen abstraction by triplet excited benzoquinone.

Picosecond time resolved Raman spectroscopy is a newly developed technique in chemical kinetics. It offers structural information about short lived chemical species, in contrast to the traditional methods of electronic absorption and emission spectroscopy. In the present experiment, we investigate a reaction that has been previously studied on the nanosecond time scale, in order to search for possible reaction during the vibrational relaxation of an excited electronic state species. p-Benzoquinone in its excited triplet state is a strong oxidizing agent and is observed to abstract an H atom from alcohols. Single 416 nm optical pulses are used to excite p-Benzoquinone, and to generate Raman spectra of the excited triplet and the reaction product (semiquinone radical). The data show that vibrational relaxation at room temperature is complete within ~=20 psec, and that no reaction occurs during relaxation. Reaction occurs from the relaxed triplet on a longer time scale. There is a clear kinetic isotope effect observed upon deuteration of the alcohol.