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

The vacuum UV photoabsorption spectrum of CH3Br has been recorded between 6 and 25 eV. A large number of vibronic bands are observed. They were partly ascribed to vibrationless Rydberg transitions. In the high photon energy range of 12-25 eV, very weak diffuse bands are mostly assigned to transitions from the 3a1, 1e and (2a1 + 1a1) to 3s orbitals. In the 6-12 eV photon energy range, numerous weak to strong bands are observed. The sharpness is very variable over the entire spectral region. In a first step, the interpretation of the spectrum and the assignment of the Rydberg transitions is based on the simple Rydberg formula. The observed features are classified in two groups of four series, each converging to one of the two spin-orbit components of the X˜2E state of CH3Br+. Rydberg series of nsa1, npa1, npe, nd and possibly nf characters are observed. The same measurements have been made for the first time on CD3Br in the 6-12 eV photon energy range. The observed features are classified into the same Rydberg series characterized by nearly the same δ values. Ionization energies for CD3Br X˜2E3/2 at 10.565 eV and X˜2E1/2 at 10.902 eV are deduced. In a second step, we fitted the experimental data to an energy expression taking into account both the exchange interaction and the spin-orbit coupling. Already states with n = 6 are found to correspond to Hund's case (c). Constant values of the quantum defects are deduced.