Field Emission Spectroscopy of Singly Adsorbed Atoms

Abstract

Duke and Alferieff (1967) were the first to point out that a resonance in the local density of states, corresponding to a broadened atomic energy level, may lead to specific structure, e.g., a peak, in the energy distribution of the electrons which are field-emitted from the neighborhood of a singly adsorbed (on a metal surface) atom. In their calculation the adatom was represented by a one-dimensional potential well, superimposed on the surface potential barrier of a jellium (free-electron) metal substrate, as shown schematically in Fig. 7.1. In the same figure we show schematically the energy distribution of the emitted electrons from the neighborhood of the adatom, when a resonance, a broadened atomic level centered at E S lies in the energy region (within 2 eV or so from the Fermi level) which is acccessible through field emission. On the experimental side a very promising beginning has been made by Clark and Young (1968) and by Plummer and Young (1970). The latter measured the relative change in the total energy distribution of the emitted electrons upon adsorption of single Ba, Sr, and Ca atoms on various crystal planes of tungsten (we describe some of their results in Section 7.3). Unfortunately, no further work has been done along these lines during the last ten years. On the theory side a number of papers have been published (Gadzuk, 1970; Penn, Gomer, and Cohen, 1972; Modinos and Nicolaou, 1971) following the pioneering work of Duke and Alferieff. In all these papers the adatom is represented by a more or less realistic three-dimensional potential, but in none of these theories is the electronic structure of the metal substrate properly taken into account. [For a summary of these theories the reader is referred to the review article of Gadzuk and Plummer (1973).]