Near ultraviolet photolysis of HFCO: The H+FCO channel

Abstract

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to study the process HFCO(S1)→H+FCO(X̃), near its appearance threshold, at excitation wavelengths ca. 247 nm. Analyses of the resulting total kinetic energy release spectra lead to an accurate determination of the C–H bond strength: D0(H–FCO)=34950±20 cm−1. The resulting FCO fragments are observed to be formed with little internal energy, distributed mainly in the form of a-axis rotation. Fragmentation is shown to involve S1–T1 intersystem crossing, followed by rapid passage along the minimum energy path to the eventual H+FCO products. This minimum energy path involves passage over (or H atom tunnelling through) a saddle point, the height of which is ⩾4740 cm−1 above the dissociation asymptote. The observed propensity for a-axis rotation in the FCO product reflects changes in the parent geometry as it evolves along the C–H dissociation coordinate on the T1 surface; past the saddle point, the barrier energy is released largely in the form of product recoil.