Abstract

Spectroscopic observation in real time of the "transition states" of a chemical reaction has recently become possible with advances in ultrafast laser technology. A.H. Zewail et al. have con ducted femtosecond transition-state spectroscopy (FTS) experiments’ on several gas-phase photodissociation reactions. In these FTS experiments, a femtosecond UV pump pulse first excites a molecule to a repulsive electronic state. A tunable femtosecond probe pulse, delayed by a variable time from the pump pulse, then detects one of the photofragments via laser induced fluorescence (LIF) or multiphoton ionization. Spectral changes that arc observed in the (LIF) excitation spectrum at small pump-probe separations thus provide information about variations in the separation between the potential curves involved in the probe transition, occurring as the interatomic spacing is varied, with the photofragment still in the force field of the remaining parts of the molecule, i.e., with the whole photodissociating complex still in the process of passing through a continuous sequence of "transition states".

© 1990 Optical Society of America