ONE-ELECTRON VERSUS MULTIELECTRON EFFECTS IN THE NEAR-THRESHOLD C-1S PHOTOIONIZATION OF ACETYLENE

The C 1s partial photoionization cross section and asymmetry parameter beta of the C2H2 molecule have been determined with high-energy resol ution between threshold and 360 eV. The C 1s shake-up satellite spectr um is richly structured; the cross section of several satellites incre ases strongly near threshold, which indicates that there is a conjugat e contribution to their intensity. It is shown that the large enhancem ent in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a sigma shape resonance, is largely due to this photon energy dependence of the shake-up transitions. (C) 1997 Am erican Institute of Physics.