Origin of satellite structure in the valence x-ray photoelectron spectrum of CO: A theoretical study

The satellite structure in the valence shell x-ray photoelectron spectrum (XPS) of CO is studied. The configuration interaction method is used to construct correlated many-electron wave functions for the different final states of C${\mathrm{O}}^{+}$. It is found that (1) the conventional shake-up model cannot explain the satellite structure; (2) a considerable amount of intensity is lost from the main, "single-hole," ${}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ states ($3{\sigma }^{-1\mathrm{}}$, $4{\sigma }^{-1\mathrm{}}$, and $5{\sigma }^{-1\mathrm{}}$) due to electron correlation; and (3) the $3{\sigma }^{-1\mathrm{}}$ peak satellites are at lower, rather than higher, binding energy than the main peak. Franck-Condon broadening is estimated and shown to be a major cause of the large width of the observed structures on the high-binding-energy side of the $4{\sigma }^{-1\mathrm{}}$ peak. Our conclusions are supported by the results of other electron spectroscopies in addition to XPS.