When a chemical bond is broken in a direct dissociationreaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10^(−13) s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as "transition states." Femtosecond transition‐state spectroscopy (FTS) is a real‐time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real‐time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s)
