Inner shell excitation spectroscopy of the tetrahedral molecules CX4 (X = H, F, Cl)

The near edge x-ray absorption spectra Of CX4 (X = H, F, Q have been measured with high resolution at the Is edges of carbon and fluorine, and the 2p edge of chlorine. The absolute photoabsorption cross section has been determined at the resonances and in the nearby continuum. In accord with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C Is edge. We report the first detailed study of the CCl4 Rydberg states. The vibrational frequency of the CF4 Rydberg states is anomalously high, which we attribute to a Fermi resonance. The C Is line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionization. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 > CH4 > CCl4.