Fluorescence excitation and emission spectroscopy of the A˜1A"-X1A' system of CHBr (original) (raw)

We report fluorescence excitation and emission spectra of CHBr in the 450-750 nm region. A total of 30 cold bands involving the pure bending levels 2 0 n with n =2-8 and combination bands 2 0 n 3 0 1 ͑n =1-8͒, 2 0 n 3 0 2 ͑n =1-6͒, 2 0 n 3 0 3 ͑n =1-2͒, 1 0 1 2 0 n ͑n =5-7͒, 1 0 1 2 0 n 3 0 1 ͑n =4-6͒, and 1 0 1 2 0 n 3 0 2 ͑n =5͒ in the à 1 AЉ ← X 1 AЈ system were observed, in addition to a number of hot bands. The majority of these are reported and/or rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis yielded band origins and rotational constants for both bromine isotopomers ͑CH 79 Br, CH 81 Br͒. The derived à 1 AЉ vibrational intervals are combined with results of ͓Yu et al. J. Chem. Phys. 115, 5433 ͑2001͔͒ to derive barriers to linearity for the 2 n , 2 n 3 1 , and 2 n 3 2 progressions. The à 1 AЉ state C-H stretching frequency is determined here for the first time, and the observed 3 dependence of the 79 Br-81 Br isotope splitting in the à 1 AЉ state is in good agreement with theoretical expectations. Our dispersed fluorescence spectra probe the vibrational structure of the X 1 AЈ state up to ϳ9000 cm −1 above the vibrationless level; the total number of levels observed is more than twice that previously reported. As first reported by ͓Chen et al. J. Mol. Spectrosc. 209, 254 ͑2001͔͒, these spectra reveal numerous perturbations due to spin-orbit interaction with the low-lying ã 3 AЉ state. The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with previous experimental and theoretical studies, are reported. Our results lead to several revised assignments, including the X 1 AЈ C-H stretching fundamental. Globally, the vibrational frequencies of X 1 AЈ, ã 3 AЉ, and à 1 AЉ are in excellent agreement with theoretical predictions.