The real time photodissociation dynamics of CH 3 I 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 ͑clocking͒ of the different ͑nonadiabatic͒ 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 modes͒ CH 3 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 CH 3 ͑ =0͒ and I͑ 2 P 3/2 ͒ and I * ͑ 2 P 1/2 ͒ that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I * channels when the CH 3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.

The photodissociation of acetaldehyde in the molecular channel yielding CO and CH(4) at 248 nm has been studied, probing different rotational states of the CO(nu = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm. From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained. One of the mechanisms is consistent with the well-studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism. This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding to a high internal energy in the CH(4) counter-fragment.

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The reaction times of several well-defined channels of the C-I bond rupture of methyl iodide from the A band, which involves nonadiabatic dynamics yielding ground state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments, have been measured by a combination of a femtosecond laser pump-probe scheme and velocity map imaging techniques using resonant detection of ground state CH3 fragments. The reaction times found for the different channels studied are directly related with the nonadiabatic nature of this multidimensional photodissociation reaction.

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