One-electron versus multielectron effects in the near-threshold C 1s photoionization of acetylene

The C 1s partial photoionization cross section and asymmetry parameter ? of the C2H2 molecule have been determined with high-energy resolution between threshold and 360 eV. The C 1s shake-up satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a ?* shape resonance, is largely due to this photon energy dependence of the shake-up transitions.