Photodissociation dynamics of the reaction CF2Br2+h nu -> CF2+2Br. Energetics, threshold and nascent CF2 energy distributions for lambda=223-260 nm

The dissociation dynamics of the reaction CF2Br2+h nu --> CF2+2 Br have bee n studied for a variety of dissociation energies, E-diss=460-535 kJ mol(-1) (corresponding to lambda=260-223 nm). The laser induced fluorescence spect rum of nascent CF2 products was measured for various dissociation energies within this range. Analysis of the spectra yielded the CF2 vibrational dist ribution and average rotational energy. The translational energy of CF2 was measured via the Doppler broadening of various fully resolved rovibronic t ransitions. The most detailed analysis of energy disposal in the CF2 fragme nts was carried out at E-diss=486 kJ mol(-1) (or lambda=246 nm). At this en ergy each degree of freedom of CF2 had an average energy of E-vib=0.4 +/- 0 .2 kJ mol(-1), E-rot=2.5 +/- 0.5 kJ mol(-1), and E-trans=24 +/- 3 kJ mol(-1 ). These CF2 energies, coupled with the available thermochemical data, allo w us to determine unambiguously that CF2 production must be accompanied by the production of two atomic Br fragments. A photofragment excitation spect rum of CF2Br2, probing for the production of CF2 fragments, provided a reac tion threshold of 460 +/- 3 kJ mol(-1) (corresponding to 260 +/- 1.5 nm). T he range of previously published reaction enthalpies varies from 392 to 438 kJ mol(-1), all of which are substantially below the observed threshold. A dditionally, at E-diss=486 kJ mol(-1), the energy of the CF2 fragment was 2 7 kJ mol(-1) on average, already in excess of the available 26 kJ mol(-1), and without considering the kinetic energy of the recoiling Br atoms. We ra tionalise these data by proposing that the reaction might have a small barr ier in the exit channel. The observed threshold corresponds to the top of t he barrier (460 kJ mol(-1)), while the final energy in the fragments is det ermined by the asymptotic reaction energy (similar to 424 kJ mol(-1)). Simp le dynamical models are presented to show that the proposed mechanism is re asonable. Key future experiments and calculations are identified that would enable a clearer picture of the dynamics of this reaction.