Introduction

This chapter begins our exploration of the physics of dynamical processes at solid surfaces – adsorption, diffusion, reaction and desorption. To do so, we must leave the ground state problem and concentrate on the excited states of adsorbed atoms and molecules. One way to proceed focuses on the excitation spectrum. As we know, this spectrum comes in two parts: single particle excitations and collective excitations. Our earlier discussion for clean surfaces (Chapter 5) dwelt primarily with the latter and it is possible to duplicate that effort here. For example, Fig. 13.1 illustrates the calculated and measured (by EELS) dispersion of two types of collective excitations for two vastly different adsorbate/substrate combinations. The left panel pertains to vibrations localized in an oxygen adlayer on Ni(100), i.e. overlayer phonons. Theory and experiment are in good accord for this system. Both exhibit three dispersive branches. The low frequency acoustic excitation is the (oxygen-modified) Rayleigh mode of the nickel substrate.

A metallized adsorbate layer can support collective excitations of its charge density in addition to the more familiar phonon modes. The right panel of Fig. 13.1 compares theory and experiment for the dispersion of two-dimensional plasmons in an ordered potassium overlayer adsorbed on a dimerized Si(100)2 × 1 surface.