A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

The real time photodissociation dynamics of CH3I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH3 fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH3(nu=0) and I(P-2(3/2)) and I*(P-2(1/2)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I-* channels when the CH3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model. (c) 2008 American Institute of Physics.