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 partl y ascribed to vibrationless Rydberg transitions. In the high photon energy range of 12-25 eV, very weak diffuse bands are mostly assigned to transitio ns from the 3a(1), le and 2a(1) to 3s orbitals. In the 6-12 eV photon energ y range, numerous weak to strong bands are observed. The sharpness is very variable over the entire spectral region. In a first step, the interpretati on of the spectrum and the assignment of the Rydberg transitions is based o n the simple Rydberg formula. The observed features are classified in two g roups of four series, each converging to one of the two spin-orbit componen ts of the (X) over tilde E-2 state of CH3Cl+. Rydberg series of nsa(1) (6 = 1.069 and 1.064), npa(1) (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 phot on energy range. The same Rydberg transitions are observed. Analogous serie s are characterized by about the same delta values: nsa(1) (delta = 1.038 a nd 0.968), npa(1) (delta = 0.65 and 0.61), npe (delta = 0.458 and 0.462) an d nd (delta = -0.004 and -0.082). Ionization energies for CD3Cl (X) over ti lde E-2(3/2) at 11.320 eV and (X) over tilde E-2(1/2) at 11.346 eV are dedu ced. In a second step, we fitted the experimental data for the nsa(1) and n pa(1) 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). (C) 2001 Elsevier Scienc e B.V. All rights reserved.