Excitons

Abstract

The general procedure to account for many-body effects in optical and energy-loss spectra consists of three steps, (i) preparation of a starting electronic structure, (ii) its improvement due to quasiparticle effects, and (iii) the inclusion of electron-hole attraction and exchange. Their combination gives rise to novel quasiparticles, the excitons. Singlet and triplet states differ by twice the electron-hole exchange. Instructive exciton models are derived studying only pairs of conduction and valence bands. For large distances of electron and hole in real space the effective mass approximation and a constant screening can be employed. The resulting pairs are hydrogen-like Wannier-Mott excitons, which give rise to Rydberg series below the ionization edge and a Coulomb enhancement, the Sommerfeld factor, of the pair density of states above the edge. Localized excitons such as Frenkel and charge-transfer excitons possess larger binding energies and electron-hole distances of the order of atomic or molecular distances. Exciton binding is increased by spatial confinement as demonstrated for two-dimensional systems.