One- and two-photon photodissociation of Fe(CO)5 at 248 nm. Application of an accurate method for calculating angle resolved velocity distributions for multiple sequential bond rupture processes

Abstract

A method is presented for computing the effective center-of-mass velocity distribution for photofragments produced by sequential bond ruptures using Fourier transforms. The method has the advantage that energy and linear momentum conservation are correctly accounted for while remaining computationally feasible. To illustrate the method, the one- and two-photon photodissociation of Fe(CO)5 at 248 nm has been experimentally investigated using the crossed laser-molecular beam method and the measured velocity distributions compared to the prediction of various statistical models for the photodissociation process calculated by the Fourier transform method. The strength of the Fourier transform method is illustrated by the two-photon channel which involves five sequential bond ruptures. The main conclusion regarding the photodissociation mechanism is that a modified form of the separate statistical ensemble theory developed by Wittig and co-workers can quantitatively explain the observed velocity distributions for the one-photon process. Fluxional interchange of the CO ligands is shown not to be important on the CO elimination time scale. The two-photon products have a faster than statistical translational energy distribution which is rationalized by both a dynamical constraint on CO rotational excitation and on the change in orbital– and spin–coupling configuration of the Fe atom as the last two CO ligands are removed.