2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/19585The unequivocal observation in the early 1980s of a new class of unnocupied surface states that are bound to the vacuum level of a variety of metal surfaces has led over the last two decades to an active area of research in condensed-matter and surface physics. We discuss the key ingredients of the theory of these so-called image states, which have their origin in the long range nature of the image potential outside a solid.11 pages, 3 figures, to appear in J. Electron SpectroscMaterials Sciencetext