Photon energy dependence of the high resolution C 1sphotoelectron spectrum of CO in the threshold region

The C 1s photoelectron spectrum of CO has been measured with high resolution in the threshold region. The data exhibit hitherto unresolved vibrational structure on the C 1s main photoelectron line as well as new features in the satellite region. The vibrational energy of the ${\mathrm{CO}}^{+}$ ion is 301±3 meV. The vibrational structure changes dramatically with photon energy due to the influence of the ${\mathrm{\sigma }}^{\mathrm{*}}$ shape resonance. The new satellite lines which are not present at high photon energies are attributed to conjugate shakeup processes.