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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.

Summary by J.L. Herek. Nobel prize-winner: Professor Ahmed Zewail. In the spring of 1990, I was a young liberal arts student soon to graduate from Lawrence University in Wisconsin, dreaming of a career in chemical physics. Thanks to an elective course on applications of lasers in chemistry and physics, which also required a literature study and report, I had found that a revolution in chemical physics was underway, with many groups clamouring to conquer new territory in the study of chemical reactions: the elusive and fleeting transition state. My interest in this emerging field dictated my choices of potential graduate schools, with Caltech and the group of Ahmed Zewail at the top of my list.