Ultraviolet photodissociation dynamics of formyl fluoride Part 2 Energy disposal in the H + FCO product channel

Abstract

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to investigate the total kinetic energy release (TKER) into the H + FCO fragments resulting from near UV photolysis of jet-cooled formyl fluoride, HC(O)F, molecules at numerous wavelengths in the range 248.2–218.4 nm. Analysis of the TKER spectra yield a precise value for the C–H bond dissociation energy D ₀ [H–C(O)F] = 34950 ± 20 cm ⁻¹ , and allow an estimate of the height of the energy barrier in the alternative, and previously unobserved, C–F bond fission channel. Photolysis at the longest wavelengths within this range results in FCO() products carrying only modest rotational and vibrational excitation (the former concentrated in the form of a-axis rotation); the bulk of the available energy {i.e., E _phot − D ₀ [H–C (O)F]} appears as product translation. The rotational energy disposal is deduced to be almost invariant to excitation wavelength, but the extent of product vibration is found to increase near linearly with increasing E _phot . These observations are all explicable in terms of a fragmentation mechanism in which the photo-excited HC(O)F(Ã) molecules undergo intersystem crossing (ISC) to the neighbouring ã ³ A″ surface and then evolve over (or through) an energy barrier in the H–C(O)F dissociation coordinate. A simple impact parameter model suffices to show that the rotational energy disposal is determined largely by the geometry and the forces acting as the molecule traverses this barrier region. The observed energy partitioning between FCO vibration and product recoil is explained in terms of a statistical fragmentation process occurring above a potential-energy barrier. The available energy, E _avl , is viewed in terms of two energy reservoirs: the first of these, corresponding to the exit channel barrier itself, is released impulsively (mainly into product translation), whilst the other, containing the remainder of E _avl , is partitioned statistically amongst the various energetically accessible vibrational states of the FCO fragment. H atom tunnelling is explicitly incorporated in the model and serves to blur the sub-division between these two energy reservoirs. Its inclusion allows good replication of all measured TKER spectra within the range of photolysis wavelengths investigated.