Quantum coherence and lifetimes of surface-state electrons

We discuss a novel approach to measure the electron phase-relaxation length and femtosecond lifetimes at surfaces. It relies on the study of the spati al decay of quantum interference patterns in the local density of states (L DOS) with the STM. The method has been applied to s-p derived surface-state electrons on Cu(111) and Ag(111). The characteristic decay length of the L DOS oscillations is influenced by the finite lifetime, and thus reveals inf ormation about inelastic scattering in the two-dimensional (2D) electron ga s. After an introduction in Section 1, we present a model describing the de cay of Friedel oscillations off from straight steps in Section 2. Energy de pendent lifetime measurements of hot electrons are presented in Section 3 a nd interpreted in terms of electron-electron scattering. The temperature de pendent lifetime measurements of low-energy quasiparticles discussed in Sec tion 4 give insight into the interaction of these 2D electrons with phonons . Our results on inelastic lifetimes are discussed in comparison with high- resolution angle-resolved photoemission and femtosecond two-photon photoemi ssion measurements. (C) 2000 Elsevier Science B.V. All rights reserved.