Number 9 (Story #5), November 20, 1990 by Phillip F. Schewe and Ben Stein THE TRAJECTORIES OF PARTICLES UNDERGOING A CHEMICAL REACTION can be mapped with a femtosecond time resolution. Scientists at Caltech (contact: Ahmed H. Zewail, 213-356-6536) use a femtosecond pulsed laser to follow the dissociation of sodium iodide. The reaction is described in terms of the time evolution of the internuclear separation, a coordinate which is measured with an experimental resolution of half an angstrom. (Nature, Nov. 15.)