RELAXATION EFFECTS IN C1S PHOTOIONIZATION OF CO - A HIGH-RESOLUTION PHOTOELECTRON STUDY IN THE NEAR-THRESHOLD REGION

The angle-resolved Cls photoelectron spectrum of CO has been measured with high resolution in the threshold region. An analysis of the vibra tional fine structure on the main Cls photoelectron line indicates tha t the vibrational energy of the CO+ ion is 302 +/- 2 meV and the lifet ime broadening is 78 +/- 10 meV. The form of the vibrational fine stru cture changes dramatically with photon energy due to the influence of the sigma shape resonance and the double excitations. On a more quant itative level, this is reflected in the corresponding partial cross-se ctions and asymmetry parameters. A meaningful comparison of these data with theory awaits, however, the inclusion of dynamic effects in the calculations. In addition to the direct shake-up satellites observed a t high photon energies several additional, conjugate shake-up satellit es are found in the near-threshold region. The lowest of these overlap partly with the triplet-coupled, direct shakeup satellite S-1 but it is still possible to determine the cross-section of the latter to with in a few electronvolts of threshold.