About the vacuum UV photoabsorption spectrum of methyl fluoride (CH3F): the fine structure and its vibrational analysis Original Research Article

The vacuum UV photoabsorption spectrum of CH3F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e−1, 5a−1 and 1e−1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10–13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e→3p and 2e→6s/5d configurations. These states are split by a Jahn–Teller distortion. Ab initio calculations lead to a Jahn–Teller stabilization energy of about 0.84 eV. The main features observed in the two bands are assigned to one single long vibrational progression of hcω≃0.160 eV (1 290 cm−1) related to the C–F Jahn–Teller inactive stretching vibration. The remaining structure is assigned to the low excitation of the Jahn–Teller active vibrational modes, i.e. the bending modes ν5 and ν6 characterized by hcω5≃0.120 eV (970 cm−1) and hcω6≃0.082 eV (660 cm−1). Leaning on the present results and assignments previously reported photoelectron spectroscopic data on CH3F+ [L. Karlsson, R. Jadrny, L. Mattsson, F.T. Chau, K. Siegbahn, Phys. Scripta 16 (1977) 225] have been reconsidered and reinterpreted.