Electronic excitations in metals and at metal surfaces

Extensive theoretical study of electron and hole dynamics in bulk metals and at metal surfaces has been conducted since the middle of the 1990s. The developed models and first-principle calculations allowed one to study in detail the decay mechanisms of excited electrons (holes). In general, a good agreement with measurements by different spectroscopies has been obtained. However, despite great success achieved during the past decade in the study of electron and hole dynamics in bulk metals and at clean surfaces, as well as at surfaces with adatoms and adlayers, very much remains to be done in this very active research field. In particular, the decay mechanisms have been mostly explored in paramagnetic and nonrelativistic (nonheavy) metals. In the future, one can expect more calculations for bulk ferromagnetics, for clean ferromagnetic metal surfaces, and for surfaces covered with magnetic adatoms and adlayers. In these systems, charge, spin, and phonon degrees of freedom are interrelated, thus giving rise to new decay mechanisms of excited electrons. The theoretical challenge for the future will be the extension of manybody calculations of quasiparticle dynamics to heavy metal and semimetal surfaces and ultrathin films. In these materials, a very strong spin-orbit splitting in surface bands can have drastic consequences for electron and hole dynamics in surface states. In particular, the splitting should influence the surface response function via the inclusion of all the spinflip processes between the split surface bands with different spin directions. This splitting should also lead to different hole (electron) lifetimes in surface states compared to that for the nonsplit surface state. Of particular interest may be the study of laterally confined surface states. While they have been identified clearly, e.g., on noble metal surfaces, not much is known about their dynamic properties. More complex systems, such as quantum well states confined to a substrate surface, molecular-induced states on metal substrates, and spin-dependent quantum states on ferromagnetic surfaces, covered with molecules should be studied too. This extension should be accompanied by analysis of the role of different approximations used in the theory, for instance, the role of nonlinear effects in screening, the importance of vertex corrections in many-body calculations of the electron - electron contribution, and the role of short-range strong correlations in quasiparticle dynamics. © 2006 American Chemical Society.