Imaging transient species in the femtosecond A-band photodissociation of CH3I

A nonresonant femtosecond laser pulse centered at 802 nm is used to probe the real time photodissociation dynamics of CH3I in the A-band at 267 nm. Using multiphoton ionization with this probe laser pulse and velocity map ion imaging of CH3+, we have followed the time evolution of the translational energy and spatial anisotropy of the CH3 fragment, which in turn has permitted to image the C-I bond breaking from the initial Franck-Condon region up to the final products along the reaction coordinate. Given the temporal width of our pump and probe laser pulses (similar to 80 fs), a mechanism is proposed by which transient species are probed by simultaneous absorption of pump and probe laser pulses through intermediate Rydberg and ionic states of CH3I while the pump and probe pulses overlap in time. This study shows how the combination of femtosecond multiphoton ionization and ion imaging techniques provides an ideal tool to resolve in time the different stages of the bond breaking event in a polyatomic molecule. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236808]