Threshold phenomena in high-resolution core-level photoelectron spectroscopy: the ethene molecule

The creation of a core hole in a molecule leads to vibrationally and electronically excited states of the resulting ion as the system adjusts to the new molecular potential. These can be observed in the corresponding photoelectron spectrum as vibrational fine structure on the main line and as satellite excitations, respectively. In the region near the photoelectron threshold, however, other phenomena occur, such as shape resonances, multi-electron excitations and conjugate satellite processes, which affect the partial cross-sections of the various channels. As an example, we present C 1s photoelectron spectra of ethene (C 2 H 4 ) measured with high energy resolution in the near-threshold region. The vibrational fine structure on the C 1s −1 line can be fitted with two progressions with energies of 185(±5) meV and 424(±10) meV, which can be assigned to totally symmetric modes. It is expected that vibronic coupling occurs, as in the case of the corresponding photoabsorption spectrum, and that an antisymmetric stretching mode contributes to the higher energy progression. This cannot be determined from the fit, however. There is no definitive evidence for a shape resonance in the C 1s single-hole cross-section. In addition, the C 1s shake-up satellite spectrum was also measured with high resolution in the near-threshold region. Apart from the well-resolved π–π* satellites, S 0 and S 1 , the measurements revealed several other peaks. Their partial cross-sections and asymmetry parameters indicate the existence of conjugate processes.