The electronic spectroscopy of jet-cooled difluorocarbene (CF2): The missing Ã-state stretching frequencies

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Abstract
The Ã(1B1)←X̃(1A1) electronic transition of difluorocarbene (CF2) has been studied by laser-induced fluorescence spectroscopy in a supersonic free jet and by ab initio theoretical methods. The radical was formed by pyrolysis of C2F4 at ∼1000 °C with a heating chamber at the tip of the nozzle just prior to the gas expansion. Fairly complete rotational cooling, but incomplete vibrational cooling allowed the identification of several new hot band transitions. Ab initio calculations for the X̃(1A1), ã(3B1), and Ã(1B1) electronic states were performed using the CASSCF method in conjunction with Dunning’s cc-pVTZ basis set, as well as two smaller sets. The calculations allow us to distinguish between several possible assignments of the observed transitions, and hence determine the vibrational frequencies of the two previously unassigned Ã-state stretching frequencies: ν3′=1180±2 cm−1 and ν1′=1011±2 cm−1. The ordering of these modes is different from that in the ground state (ν1″≳ν3″) and mechanisms for the switching are discussed.