Valence-Electron Resonances in Alkali-Metal Overlayers Observed via Photoemission Line-Shape Changes.

Alkali metals adsorbed on metal surfaces attract interest as prototype examples of simple chemisorption [1] and simple metal quantum well systems [2]. In spite of the numerous studies made the electronic structure is still rather poorly characterized. So far, observations of the filled states have been limited to the discrete states that may be formed by alkali metal valence electrons in the energy range of substrate band gaps [3]. These states are observed in a photoemission spectrum as narrow peaks which are easily resolved from the low background characteristic of the substrate energy gap. More typical of metal overlayers on a metal is the formation of resonances with an energy spread due to overlap with a continuum of states in the substrate. As a result of the weak optical transition probability of a free-electron-like metal, the alkali-metal overlayer resonances are, in general, difficult to detect by photoemission. Here we show that, at least in a few cases, alkali-metal overlayer resonances can still be observed by this method, although not in the usual manner via the appearance of characteristic emission peaks. Instead, a resonance is revealed by overlayer induced line-shape changes for the emission out of substrate bulk states. For the cases discussed below, two and three atomic layers of Na on Ag(111) or Cu(111), the line-shape change is drastic but only in a narrow range of photon energies when the optical k-conservation rule selects substrate states in the energy range of the resonance. The results can be understood, we believe, as an interference in the photoemission matrix element between contributions from different spatial parts of the initial state. The bulk of the substrate gives a contribution which is characterized by a narrow emission line due to k-conserving optical transitions between s, p bands while a broader spectrum, reflecting the width of the resonance, is generated at the interfaces and in the interior of the film. The present observations are made by recording angle resolved photoemission spectra along the surface normal for Ag(111) and Cu(111) surfaces covered with different amounts of Na by evaporation of the alkali metal from a heated glass ampoule broken in situ. The sample is kept at 170 K by cooling its holder with liquid N2. Previous