Image-potential-induced states at metal surfaces
  The 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. 
P. M. Echenique, J. M. Pitarke, E. V. Chulkov, and V. M. Silkin