The vacuum UV photoabsorption spectrum of methyl bromide (CH3Br) and its perdeuterated isotopomer CD3Br: a vibrational analysis

The fine structure of the vacuum UV photoabsorption spectrum of CH3Br and CD3Br has been analyzed in the 6.9-10.2 eV photon energy range. A large number of lines have been observed, classified and assigned to the vibrational excitation accompanying a series of Rydberg transitions. The effects of the Jahn-Teller distortion and of the spin-orbit splitting of the ground electronic state of the ion have been considered. The former effect has been evaluated by ab initio calculations, showing that the 2E state (in the C3v symmetry group) splits into 2A′ and 2A″ states in the CS symmetry group. Even though the energy difference of about 1 meV is extremely small, the 2A′ state is energetically the lowest component whereas the 2A″ is found to be a transition state. The Jahn-Teller stabilization energy and the wavenumbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described in terms of three vibrational modes, i.e., hcω4 = 146 ± 6 meV (1178 ± 48 cm−1), hcω5 = 107 ± 6 meV (863 ± 48 cm−1) and hcω6 = 71 ± 4 meV (572 ± 32 cm−1), respectively, as resulting from an average over all analyzed Rydberg states. In CD3Br the corresponding energies are hcω4 = 104 ± 3 meV (839 ± 24 cm−1), hcω5=82 ± 4 meV (661 ± 32 cm−1) and hcω6 = 62 ± 4 meV (500 ± 32 cm−1). These values are in fairly good agreement with those predicted by the present ab initio calculations for the ionic ground state. The experimental isotopic ratio ρi = [ω/ωisot]i is 1.15 ± 0.14, 1.31 ± 0.14 and 1.39 ± 1.0 and is predicted to be 1.04, 1.34 and 1.36, respectively, for ω6, ω5 and ω4. On the basis of the present study an alternative assignment of the CH3Br+(X˜2E) photoelectron band structure is proposed.