Single vibronic level emission spectroscopy of the A˜1B1→X˜1A1 system of dibromocarbene

Following the development of an improved discharge recipe for production of dibromocarbene (CBr2), and an extensive spectroscopic survey of the A˜1B1←X˜1A1 system that has led to a revision in the position of the origin, we have recorded single vibronic level (SVL) emission spectra of C79Br81Br and C81Br2 which probe the vibrational structure of the X˜1A1 state up to ∼7000 cm−1 above the vibrationless level. These spectra reveal many previously unassigned levels. In the region between ∼3600 and 5900 cm−1, or between the singlet-triplet gap proposed by Hsu et al. [H.-J. Hsu, W.-Z. Chang, B.-C. Chang, Phys. Chem. Chem. Phys. 2005, 7, 2468-2473] to that predicted by theory, we find that ∼90% of the lines can be assigned to X˜1A1 levels within three standard deviations of our Dunham expansion fit, suggesting that the vast majority of emission lines terminate on levels of the singlet state. A nearly complete set of vibrational parameters was determined for the X˜1A1 state; the derived parameters are in good agreement with recent ab initio predictions.