Femtosecond probing of photodissociation dynamics in acyl cyanides

The photodissociation of two acyl cyanide compounds, R-C(O)-CN, where R=met hyl and tert-butyl groups, has been investigated using femtosecond time-res olved laser-induced fluorescence (LIF) spectroscopy. Both compounds were ex cited by two-photon excitation at a total energy of similar to6.4 eV and th e formation of the free CN(X) radical products was probed in real time by m onitoring the CN X -->B LIF signal. The results revealed that the temporal evolution of the CN(X) formation can be well characterized by delayed biexp onential rise functions with time constants in the picosecond time scale, i ndicating that the dissociation occurs via a complex-mode mechanism. We pro posed a dissociation mechanism involving two discernable stages to account for the observed temporal behaviors as well as previous photofragment trans lational spectroscopic results reported by other groups. Our analyses sugge sted that the selectivity between the C-CN and C-R bond cleavage is determi ned by the competition between the adiabatic and nonadiabatic dynamics of t he S-2 state. The results also indicated that the adiabatic dissociation pr ocess occurring on the S-2 surface is not statistical. We speculate that th is nonstatistical dissociation behavior is due to an initial nonuniform pha se space distribution and a slow intramolecular vibrational energy redistri bution process that prevents the system from sampling the entire phase spac e before the reaction completes. (C) 2001 American Institute of Physics.