Ultrafast Reaction Dynamics

With new laser techniques and with gas phase and molecular beam experiments, it is now possible to determine the ultrafast motion in isolated chemical reactions-chemistry on the I 0 l 3 -second time scale. A decade ago this magazine devoted a special issue to laser chemistry (see PHYSICS TODAY, November 1980). One of the articles emphasized the importance of time scales in chemical reactions and the possible use of ultrashort lasser pulses to induce chemistry. Over the past 10 years new laser techniques, and gas-phase and molecular-beam experiments, have revealed much about the fundamental steps of elementary chemical reactions. These approaches and the tremendous detail they have exposed about the dynamics of chemical reactions are the subject of the present article. The most elementary steps into which any sequence of chemical reactions can be broken have a common time scale dictated by the rapidity of nuclear rearrangement. This fundamental chemical time scale ranges from 10 femtoseconds to 10 picoseconds. Research in gas phasemolecular beam chemical dynamics uses either indirect or direct time-resolved methods to study such chemical motions free of external perturbations, the goal being an understanding of what happens in these 10-lo4 femtoseconds. In general, one can write an elementary chemical reaction proceeding from reactants to products as follows: