Direct femtosecond mapping of trajectories in a chemical reaction

Abstract

IN chemical reactions, the dynamics of the transition from reagents to products can be described by the trajectories of particles (or rigorously, of quantum mechanical wave packets) moving on a potential-energy surface. Here we use femtosecond pulsed laser techniques to follow directly the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. The bond breakage can be described in terms of the time evolution of a single reaction coordinate, the internuclear separation. As the velocities of the separating fragments are typically of the order of a kilometre per second, a time resolution of a few tens of femtoseconds is required^1,2 to view the motions on a molecular distance scale of less than an ångstrom. The resolution obtained here permits the direct visualization of the wave packet's motion and provides snapshots of the trajectories along the reaction coordinate.