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.

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. Here we explore the transition from the weak- to the strong-field regimes of laser control for the dissociation of a polyatomic molecule, methyl iodide. The control over the yield of the photodissociation reaction proceeds through the creation of a light-induced conical intersection. The control of the velocity of the product fragments requires external fields with both high intensities and short durations. This is because the mechanism by which control is exerted involves modulating the potentials around the light-induced conical intersection, that is, creating light-induced potentials.

A femtosecond pump-probe experiment, coupled with velocity map ion imaging, is reported on the second absorption band (B-band) of CH(3)I. The measurements provide a detailed picture of real-time B-band predissociation in the band origin at 201.2 nm. Several new data are reported. (i) A value of 1.5+/-0.1 ps has been obtained for the lifetime of the excited state, consistent within errors with the only other direct measurement of this quantity [A. P. Baronavski and J. C. Owrutsky, J. Chem. Phys. 108, 3445 (1998)]. (ii) It has been possible to measure the angular character of the transition directly through the observation of fragments appearing early with respect to both predissociation lifetime and molecular rotation. (iii) Vibrational activity in CH(3) has been found, both in the umbrella (nu(2)) and the symmetric stretch (nu(1)) modes, with estimates of relative populations. All these findings constitute a challenge and a test for much-wanted high level ab initio and dynamics calculations in this energy region.

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