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

By employing the $X\alpha$ 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 $2\pi$ level of isolated CO is split into two levels $2{\tilde{\pi }}_a$ and $2{\tilde{\pi }}_b$ 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 $2{\tilde{\pi }}_b$ and $2{\tilde{\pi }}_a$ orbitals change their character quite significantly to become $2{\tilde{\pi }}_b^{\prime }$ and $2{\tilde{\pi }}_a^{\prime }$. The former is now partially occupied and closely resembles the isolated $2\pi$ orbital of CO, and the latter is unoccupied with significant metal character and less CO content. The character of the $1\pi$ level of isolated CO is basically the same for the chemisorbed ground state (where it is labeled $1\tilde{\pi }$). However, it changes rather dramatically (labeled $1{\tilde{\pi }}^{\prime }$) 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 $1{\tilde{\pi }}^{\prime }$, $2{\tilde{\pi }}_b^{\prime }$, and $2{\tilde{\pi }}_a^{\prime }$; 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) ${\left(1\mathrm{}{\tilde{\pi }}^{\prime }\right)}^4{\left(2\mathrm{}{\tilde{\pi }}_b^{\prime }\right)}^1{\left(2\mathrm{}{\tilde{\pi }}_a^{\prime }\right)}^0$, (2) ${\left(1\mathrm{}{\tilde{\pi }}^{\prime }\right)}^4{\left(2\mathrm{}{\tilde{\pi }}_b^{\prime }\right)}^0{\left(2\mathrm{}{\tilde{\pi }}_a^{\prime }\right)}^1$, and (3) ${\left(1\mathrm{}{\tilde{\pi }}^{\prime }\right)}^3{\left(2\mathrm{}{\tilde{\pi }}_b^{\prime }\right)}^2{\left(2\mathrm{}{\tilde{\pi }}_a^{\prime }\right)}^0$. The last final state corresponds to the final-state configuration found in the isolated CO molecule due to a $1{\pi }^{\prime }\to 2{\pi }^{\prime }$ shake up.