Femtosecond control of an elementary unimolecular reaction from the transition-state region

We present a simple example of temporal control of the evolving dynamics in a reactive system — a unimolecular reaction with two channels. Using three femtosecond pulses, a wave packet is prepared, probed, and intercepted in the transition-state region. The femtosecond control pulse intercepts the predissociative system at a fixed time and internuclear separation, both determined by the 'window' of the laser pulse. Experiments are performed on the NaI system which is a prototype for studies of elementary nuclear motion and reaction dynamics along two potentials, the covalent (Na + I) and the ionic (Na^+ + I^−) channels.