Hydrogen formation in laser-induced unimolecular reactions of substituted benzenes

The formation of hydrogen atoms following laser excitation of substituted benzenes at 193 nm was investigated in a molecular beam experiment. The H atom products were detected by a two-colour resonance-enhanced two-photon ionization process. For ethylbenzene C6H5CH2CH3, t-butylbenzene C6H5C(CH3)3 and benzotrifluoride C6H5CF3 specific rate constants were extracted from the time dependence of H atom formation. The results agree very well with former results obtained by detecting CH3/CF3 radicals. By measurement of Doppler profiles of the H atoms we find small mean kinetic energies. Both results indicate that the reactions start from vibrationally excited levels of the electronic ground state. Reaction yields between 2.6% and 13% show that hydrogen formation is a minor but not neglectable reaction channel.