THE ELECTRONIC SPECTROSCOPY OF JET-COOLED DIFLUOROCARBENE (CF2) - THEMISSING (A)OVER-TILDE-STATE STRETCHING FREQUENCIES

The (A) over tilde(B-1(1))<--(X) over tilde((1)A(1)) electronic transi tion of difluorocarbene (CF2) has been studied by laser-induced fluore scence spectroscopy in a supersonic free jet and by ab initio theoreti cal methods. The radical was formed by pyrolysis of C2F4 at similar to 1000 degrees C with a heating chamber at the tip of the nozzle just p rior to the gas expansion. Fairly complete rotational cooling, but inc omplete vibrational cooling allowed the identification of several new hot band transitions. Ab initio calculations for the (X) over tilde((1 )A(1)), (a) over tilde(B-3(1)), and (A) over tilde(B-1(1)) electronic states were performed using the CASSCF method in conjunction with Dunn ing's cc-pVTZ basis set, as well as two smaller sets. The calculations allow us to distinguish between several possible assignments of the o bserved transitions, and hence determine the vibrational frequencies o f the two previously unassigned (A) over tilde-state stretching freque ncies: nu'(3) = 1180 +/- 2 cm(-1) and nu'(1) = 1011 +/- 2 cm(-1). The ordering of these modes is different from that in the ground state (nu ''(1) > nu''(3)) and mechanisms for the switching are discussed. (C) 1 995 American Institute of Physics.