Interpretation of satellite structure in the x-ray photoelectron spectra of CO adsorbed on Cu(100)

Abstract

By employing the $X\ensuremath{\alpha}$ scattered-wave method with a ${\mathrm{Cu}}_{9}$CO cluster to model the chemisorption of CO on a onefold site of a Cu(100) surface, a simple interpretation of the satellite structure observed in the x-ray photoelectron spectrum in the C $1s$ and O $1s$ regions has been obtained. The physical model obtained by analyzing the results of the ${\mathrm{Cu}}_{9}$CO cluster calculations is qualitatively the same as that obtained in a previous study of a ${\mathrm{Cu}}_{5}$CO cluster with the CO in a fourfold site [Solid State Commun. 36, 265 (1980)]. The qualitative differences suggest that the present ${\mathrm{Cu}}_{9}$CO cluster is the better model, however. Experimentally, a three-peak structure is observed in both the O $1s$ and C $1s$ hole spectra. The "first" peak, at lowest binding energy, is followed by a second peak at 2-3 eV higher binding energy and the third peak is at 7-8 eV higher binding energy with respect to the first peak. The theoretical model derived here suggests that the unoccupied $2\ensuremath{\pi}$ level of isolated CO is split into two levels $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}$ and $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}$ on interaction with the Cu metal. In the neutral ground state neither of these levels is occupied. On the introduction of a core hole in the chemisorbed CO (e.g., the C $1s$ hole) the $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}$ and $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}$ orbitals change their character quite significantly to become $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}^{\ensuremath{'}}$ and $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}^{\ensuremath{'}}$. The former is now partially occupied and closely resembles the isolated $2\ensuremath{\pi}$ orbital of CO, and the latter is unoccupied with significant metal character and less CO content. The character of the $1\ensuremath{\pi}$ level of isolated CO is basically the same for the chemisorbed ground state (where it is labeled $1\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}$). However, it changes rather dramatically (labeled $1{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}^{\ensuremath{'}}$) after the removal of the core electron, as it shifts to screen the core hole. A description of the final states which give rise to the three peaks observed in the experimental spectrum can be given in terms of the occupancies of the three orbitals $1{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}^{\ensuremath{'}}$, $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}^{\ensuremath{'}}$, and $2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}^{\ensuremath{'}}$; there is of course a $1s$ hole in each of the final states. The assignment of the final-state configuration corresponding to the three observed peaks (in order of increasing binding energy) is as follows: (1) ${(1{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}^{\ensuremath{'}})}^{4}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}^{\ensuremath{'}})}^{1}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}^{\ensuremath{'}})}^{0}$, (2) ${(1{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}^{\ensuremath{'}})}^{4}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}^{\ensuremath{'}})}^{0}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}^{\ensuremath{'}})}^{1}$, and (3) ${(1{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}^{\ensuremath{'}})}^{3}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{b}^{\ensuremath{'}})}^{2}{(2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\pi}}}_{a}^{\ensuremath{'}})}^{0}$. The last final state corresponds to the final-state configuration found in the isolated CO molecule due to a $1{\ensuremath{\pi}}^{\ensuremath{'}}\ensuremath{\rightarrow}2{\ensuremath{\pi}}^{\ensuremath{'}}$ shake up.