Electronic spectroscopy, lifetimes, and barrier to linearity in the A˜1B1←X˜1A1 system of dibromocarbene

Using an improved discharge recipe for the production of dibromocarbene (CBr2), we recently reassigned the electronic origin of the A˜1B1←X˜1A1 system, based on an extensive data set of isotope shifts for the pure bending transitions [C. Tao, C. Mukarakate, D. Brusse, Y. Mishchenko, S. A. Reid, J. Mol. Spectrosc. 240 (2006) 139-140]. In this study, we report the complete analysis of the fluorescence excitation spectrum of the A˜1B1←X˜1A1 system in the 525-650 nm region, obtained at a rotational temperature of ∼10 K. A total of 32 cold bands involving the pure bending levels 20n with n = 2-19 and combination bands 10120n (n = 1-16) in the A˜1B1←X˜1A1 system were observed; a number of these are reported here for the first time. Rotational analysis typically yielded A rotational constants and band origins for all three bromine isotopomers (C79Br2, C79Br81Br, C81Br2), and Dunham expansion fits yielded an extensive set of vibrational parameters for each. The isotope shifts are in good agreement with the product rule, and, when plotted vs. bending quantum number, the measured A constants follow a linear trend except for the highest bending levels, where an abrupt increase is observed, indicative of the approach to linearity. This is mirrored in the vibrational intervals, which also change abruptly in this region, and the estimated barrier height of ∼18 800 cm−1 above the vibrationless level of the X˜1A1 state is in excellent agreement with the ab initio prediction of Sendt and Bacskay [K. Sendt, G.B. Bacskay, J. Chem. Phys. 112 (2000) 2227-2238]. We also report fluorescence lifetimes as a function of vibrational level and Ka′; the lifetimes decrease rapidly with energy, but display no dependence on Ka′ over the measured range. The implications of these results for understanding the excited state structure of this prototypical carbene are emphasized.