Electronic spectroscopy and ab initio quantum chemical study of the Ã(1A″)−X(1A′) transition of CFBr

The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the à state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and à states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the à state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.