LIF EXCITATION SPECTRUM OF DIBROMOCARBENE IN A SUPERSONIC FREE-JET EXPANSION

CBr2 has been produced by pyrolysis in a supersonic free-jet expansion and investigated by laser-induced fluorescence-excitation spectroscop y. Observations of the vibrational isotope effect for several bending progressions have led to a reassignment of the earlier gas-phase nu2' numbering and have consequently cleared up some earlier discrepancies. For the (BrCBr)-Br-79-Br-81 species, values of several vibrational fr equencies (referred to the lowest vibrational level of each electronic state) are omega2(0)' = 475 cm-1, omega2(0)' = 185 cm-1, and omega1(0 )'' = 599 cm-1; and the origin of the A B-1(1) <-- X 1A1 electronic tr ansition is placed at T00 = 15 091.5 cm-1. Partially resolved rotation al sub-band structure yields some structural results. In particular, t he BrCBr bond angle increases 21-degrees upon excitation from the X to the A state. The electronic origins, T00, and the argon matrix shifts of T00, are shown to satisfy physically reasonable additivity relatio nships for CBr2, CCl2, and CBrCl.