Valence and core excitations of adsorbates on metal surfaces

Abstract

We present a general discussion on the electronic spectroscopy of adsorbates on metal surfaces. We first consider the neutral electronic excitations of weakly adsorbed species, e.g., CO⧸Ag, and establish their relation with photoemission (PE) and inverse photoemission (IPE). We also briefly discuss the origin of the spectral shifts and broadening of the intra-adsorbate excitations and the occurrence of charge-transfer excitations using, as examples, noble gases and aromatic hydrocarbons on metals, respectively. We then concentrate on the empty electronic levels and excitations of chemisorbed CO. We correlate results on the valence and core excitations of CO on Cu and Ni with IPE and cluster calculations. A self-consistent picture regarding the location of the empty 2 π ∗ level thus can be obtained. In the ground state of CO on Cu and Ni this 2π ∗ level is at ~ 3 eV above E F and the change in the electron affinity upon adsorption is larger, the larger the chemisorption energy is. The 5σ → 2π ∗ valence excitation of chemisorbed CO is nearly unshifted. while the O 1s → 2 \\ ̃ gp ∗ core excitation is red-shifted with respect to the corresponding free CO excitations. Simple explanations of the above observations are presented. Finally, we show that intra-adsorbate excitations are not always observable, and that the disappearance of the π → π ∗ excitations of aromatic hydorcarbons on transition metals can be explained in terms of the nature of the chemisorption bond.