UNIMOLECULAR DECOMPOSITION OF METHYLSUBSTITUTED BENZENES INTO BENZYL RADICALS AND HYDROGEN-ATOMS

A homologous series of methylsubstituted benzenes and pyrazines are excited with an energy E by a short laser pulse following which, typically on a time scale of several hundred nanoseconds, hydrogen atoms are released from the methyl groups. The rate constants k(E) for the formation of the H or D atoms were determined from the time dependence of the laser-induced fluorescence (LIF) signals, and their velocity distributions were measured from the shapes of their LIF excitation curves. The velocity distributions were Maxwellian with translational temperatures remarkably close to the vibrational temperatures, typically 2000-3000 K, of the excited molecules assuming internal equilibration of the large energy E among the large number of internal modes. The logarithm of the rate constants obtained with 193 nm excitation plotted against 1/T v yields a straight line with an activation energy of 97 kcal/mol. A Rice-Ramsberger-Kassel-Marcus model invoking a transition state with several soft modes accounts very well for the rates of dissociation and for most but not all of the average kinetic energy of the released atoms. © 1990 American Institute of Physics.