Beyond the Chemical Shift: Vibrationally Resolved Core-Level Photoelectron Spectra of Adsorbed CO

Vibrational fine structure has been observed in the $\mathrm{C}1s$ and $\mathrm{O}1s$ photoelectron lines of CO adsorbed on Ni(100). This allows a detailed separation of vibrational and electronic contributions to the line profile. The main line is found to represent the fully screened final state. The width of the photoelectron lines is dominated by the intramolecular stretch mode, which strongly depends on the chemical state of the adsorbate. The identification of the adiabatic transition makes the determination of chemical shifts more precise. The equivalent core approximation is found to be highly accurate and electronic and geometric equilibrium properties of the core-ionized adsorbate are extracted.