The Vacuum UV Photoabsorption spectrum of Methyl Chloride (CH3Cl) and its perdeuterated Isotopomer CD3Cl. II. A vibrational Analysis.

The fine structure of the vacuum UV photoabsorption spectrum of CH3Cl and CD3Cl has been analyzed in the 7.5-10.5 eV photon energy range. A large number of lines have been observed, classified and assigned to vibrational excitation accompanying a series of Rydberg transitions. The vibronic transitions involve both Jahn-Teller distortion and spin-orbit splitting. 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. The 2A' state is energetically the lowest component whereas the 2A" is found to be a transition state. The Jahn-Teller stabilisation energy and the wavenumbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described by using the vibrational modes, i.e. hcomega5= 104+/-7 meV, hcomega6= 77+/-7 meV and homega=162+/-3 meV respectively, as resulting from an average over all the analyzed Rydberg states. In CD3Cl the corresponding energies are hcomega5= 81+/-4 meV, hcomega6= 66+/-5 meV and hcomega= 124+/-4 meV. These values are in good agreement with those predicted by the present ab initio calculations for the ion ground state. However, the agreement is not so good for the wavenumber omega which could be assigned to the nu3 or nu4 vibrational modes. Though some arguments favour nu4 (the CH3 umbrella mode), within the error limits on the present measurements it is formally not possible to ascribe this wavenumber to one of these two vibrations.