Role of multiple scattering within inverse-photoemission studies of unoccupied molecular adsorbate levels

An inverse-photoemission study (at $\hslash \omega =9.7\mathrm{}$ eV) of the adsorption of carbon monoxide on a Pd(111) surface is compared with a first-principles calculation of the cross section for the emission of photons as a function of electron angle of incidence. It is found that the calculation is able to explain the experimental results in terms of the relative strengths of the $\sigma \to \pi$ and $\pi \to \pi$ channels in the optical transitions.