The Vacuum UV Photoabsorption Spectrum of Methyl Chloride (CH3Cl) and its perdeuterated Isotopomer CD3Cl. I. A Rydberg Series Analysis.

The vacuum UV photoabsorption spectrum of CH3Cl has been recorded between 6 and 25 eV. A large number of vibronic bands are observed. These were partly ascribed to vibrationless Rydberg transitions. In the higher energy range of 12-25 eV very weak diffuse bands are mostly assigned to transitions from the 3a1, 1e and 2a1 to 3s orbitals. In the 6-12 eV photon energy range, numerous weak to strong bands are observed. The sharpness of the features 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 X2E state of CH3Cl+. Rydberg series of nsa1 (delta=1.069 and 1.064), npa1 (delta= 0.68 and 0.66), npe (delta= 0.438 and 0.427) and nd (delta= -0.040 and -0.092) characters are observed. The same measurements have been made for the first time on CD3Cl in the 6-12 eV photon energy range. The same Rydberg transitions are observed. Analogous series are characterized by about the same delta values: nsa1 (delta= 1.038 and 0.968), npa1 (delta= 0.65 and 0.61), npe (delta= 0.458 and 0.462) and nd (delta= -0.004 and -0.082). Ionization energies for CD3Cl X2E3/2 at 11.320 eV and X2E1/2 at 11.346 eV are deduced. In a second step, we fitted the experimental data for the nsa1 and npa1 states to an energy expression taking into account both the exchange interaction and the spin-orbit coupling. This accounts for the progressive switching from Hund's case (a) to Hund's case (c).