High-resolution C 1s photoelectron spectra of methane

The C 1s partial photoionization cross section and photoelectron angular distribution of methane (CH4) have been measured with high-energy resolution between threshold and 385 eV photon energy. From the analysis of the vibrational fine structure on the C 1s(-1) photoelectron line a vibrational energy of 396+/-2 meV and an equilibrium bond length of 1.039(+/-0.001) Angstrom for the CH4+ ion have been determined. The lifetime broadening was found to be 83(+/-10) meV. The weak feature in the photoabsorption cross section just above threshold does not influence the vibrational fine structure in a way typical for a shape resonance. We therefore suggest that it is due to doubly excited states of the type C (1s)(-1)(Val)(-1)(Ryd) (1)(a)(Ryd)(b)(1), an assignment which is supported by recent Auger decay studies. Measurements of the shakeup structure revealed six satellite lines, one of which increases strongly in intensity at threshold, thus pointing to the existence of a conjugate shakeup process.