Relaxation effects in C1s photoionisation of CO: a high resolution photoelectron study in the near-threshold region

Abstract

The angle-resolved C1s photoelectron spectrum of CO has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure on the main C1s photoelectron line indicates that the vibrational energy of the CO + ion is 302 ± 2 meV and the lifetime broadening is 78 ± 10 meV. The form of the vibrational fine structure changes dramatically with photon energy due to the influence of the σ ∗ shape resonance and the double excitations. On a more quantitative level, this is reflected in the corresponding partial cross-sections 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 at high photon energies several additional, conjugate shake-up satellites are found in the near-threshold region. The lowest of these overlap partly with the triplet-coupled, direct shake-up satellite S 1 but it is still possible to determine the cross-section of the latter to within a few electronvolts of threshold.