Electron Dynamics in Image Potential States at Metal Surfaces

Abstract

Electron dynamics describes the temporal evolution of an electronic state due to interactions with particles or quasiparticles. The ground state of the system is characterized by the stationary states whose time dependence is determined by the energy of the state. In the present context, we consider an excited electron at the surface, for example, in a surface or adsorbate state. On its relaxation path to the ground state (or thermal equilibrium) it is going to be scattered and the possible processes can be divided into the following categories:. Electron-electron scattering: This most important process for energy relaxation leads to decay into bulk or surface states at lower energy with simultaneous creation of an electron-hole pair. The coupling to the electronic system can also include other quasiparticles such as excitons, plasmons, and magnons that are not covered in this chapter. . Electron-phonon scattering: This scattering process mainly changes the direction of the electron motion and embraces both scattering to bulk bands and scattering within the surface state band. The energy loss is usually rather small compared to electron-electron scattering. Temperature is a convenient control of electron-phonon scattering. . Electron defect scattering: Real surfaces contain always a nonnegligible amount of defects, such as steps or impurity atoms. The associated electron defect scattering mainly changes the electron momentum leaving the energy almost unchanged. In many aspects, it is similar to electron-phonon scattering. Electron defect scattering can be identified by controlling the defect density.Image potential states [1] are a special class of surface states that exist on many metal surfaces (see chapter 3 of volume 2). The small binding energies relative to the vacuum level point to a weak coupling to the metal. Correspondingly, image potential states can have relatively long lifetimes (>10 fs) and can conveniently be studied with