Femtosecond real-time probing of reactions. III. Inversion to the potential from femtosecond transition-state spectroscopy experiments

Abstract

Femtosecond transition‐state spectroscopy (FTS) of elementary reactions [M. Dantus, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 8 7, 2395 (1987)] provides real‐time observations of photofragments in the process of formation. A classical mechanical description of the time‐dependent absorption of fragments during photodissociation [R. Bersohn and A. H. Zewail, Ber. Bunsenges. Phys. Chem. 9 2, 373 (1988)] forms the basis for the present scheme for relating observations to the potential energy surface. A direct inversion scheme is presented that allows the difference in the two relevant excited‐state potential curves to be deduced from observed transients at different probe wavelength tunings. In addition, from the shape and dependence of the transients on pump wavelength, information on the lower of the two potential curves (i.e., that of the dissociating molecule) is obtained. The methodology is applied to the experimental FTS data (Dantus et al.) on the CN photofragment from the ICN photodissociation.