Electronic spectroscopy of jet-cooled CFCl: Laser-induced fluorescence, dispersed fluorescence, lifetimes, and C–Cl dissociation barrier

Abstract

The Ã(1A″)–X̃(1A′) transition of jet-cooled chlorofluorocarbene (CFCl) has been measured by laser-induced fluorescence (LIF) excitation and dispersed fluorescence spectroscopy. Over 170 vibronic transitions were measured in the LIF spectrum, consisting of cold bands and hot bands of carbenes containing both Cl35 and Cl37 isotopes. Dispersed fluorescence spectroscopy was used both to map the ground-state vibrational levels and to provide confirmation of the vibronic identity of the emitting level. A predictor–corrector method was used to progressively assign almost all of the vibronic transitions, resulting in the positive assignment and measurement of almost every bound vibrational state within the Ã-state manifold. The vibrational structure is modeled well by a Morse potential with frequencies ν1′=1229 cm−1, ω2′=399.2 cm−1, and ω3′=748.0 cm−1 for CF 35Cl and 1235 cm−1, 397.0 cm−1, and 744.5 cm−1 for the same three vibrations in CF 37Cl. The standard diagonal and cross-anharmonicity constants for a three-coordinate Morse potential were also measured for each isotopic species. Dispersed fluorescence spectroscopy provided a map of ground-state vibrational levels up to about 4000 cm−1. Franck–Condon factors were modeled well by a simple, one-dimensional harmonic potential, and these were also used to confirm assignment of many transitions. The fluorescence lifetime of the excited vibronic states decreased markedly from a consistent 650 ns for most states, to <20 ns for the highest lying observed state. In addition, the Franck–Condon analysis indicates that higher lying members of progressions were missing in the LIF spectrum. This strongly indicated the presence of a nonradiative pathway that opens for energies above T00+4073 cm−1. Analysis of the rotational structure of many transitions indicated that the molecule was not reaching the Renner–Teller intersection, where the à and X̃ states are degenerate. We attribute the nonradiative channel to cleavage of the C–Cl bond directly on the à state, in exact analogy with the observed process in CFBr. The height of the barrier, and the vibrational frequencies are all in reasonable agreement with recent ab initio values.