Non-Franck-Condon behavior in the C 1s photoelectron spectrum of the methane molecule

Vibrationally resolved carbon 1s photoelectron spectra of methane are measured over a photon energy range between 300 and 480 eV. The intensity ratios of the vibrational components vary significantly, exhibiting a minimum at about 330 eV. The observed non-FC distribution is tentatively explained as a result of the internal inelastic scattering of the photoelectron on valence electrons, which leads to the loss of the photoelectron current that depends on the vibrational excitation of the residual ion.