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

• Published in: Chemical physics Vol. 317; no. 1; pp. 87 - 102
• Main Authors: Locht, R., Leyh, B., Dehareng, D., Jochims, H.W., Baumgärtel, H.
• Format: Journal Article Web Resource
• Language: English
• Published: Elsevier Science Bv 10.10.2005
• Subjects: Ab initio calculations; CD3Br; CH3Br; Chemistry; Chimie; Jahn-Teller effect; photoabsorption; photoelectron spectrum; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; Rydberg series; vacuum UV; vibrational analysis
• ISSN: 0301-0104
• DOI: 10.1016/j.chemphys.2005.06.008

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 E-2 state (in the C-3v symmetry group) splits into (2)A' and (2)A" states in the C-s symmetry group. Even though the energy difference of about 1 meV is extremely small, the (2)A' state is energetically the lowest component whereas the (2)A" 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., hc omega(4) = 146 +/- 6 meV (1178 +/- 48 cm(-1)), hc omega(5) = 107 +/- 6 meV (863 +/- 48 cm(-1)) and hc omega(6) = 71 +/- 4 meV (572 +/- 32 cm(-1)), respectively, as resulting from an average over all analyzed Rydberg states. In CD3Br the corresponding energies are hc omega(4) = 104 +/- 3 meV (839 +/- 24 cm(-1)), hc omega(5)=82 +/- 4 meV (661 +/- 32 cm(-1)) and hc omega(6) = 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 rho(i) = [omega/omega(isot)](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 omega(6), omega(5) and omega(4). On the basis of the present study an alternative assignment of the CH3Br+((X) over tilde E-2) photoelectron band structure is proposed. (C) 2005 Elsevier B.V. All rights reserved.