Inverse photoemission

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Abstract
Inverse photoemission spectroscopy (IPES) is the new name for bremsstrahlung isochromat spectroscopy. It is used to study the unoccupied electronic states in solids and at their surfaces. The author surveys accomplishments in ultraviolet IPES particularly in its angle-resolved form. The author starts with an outline of the phenomenology, history and the theories of inverse photoemission. Signal levels in ultraviolet IPES are about five orders of magnitude weaker than in forward photoemission. Large-aperture photon detector/selectors and high-perveance electron guns have been developed to overcome this difficulty. The author continues with some case studies. A section on bulk bandstructure includes a pedagogical section on band mapping of unoccupied states in metals and offers examples of spin polarised studies of ferromagnets and bandstructure determinations of semiconductors and layer materials. A section on clean surfaces surveys the various kinds of unoccupied surface states which have been observed-Shockley states, image states and d-like surface states. The systematics of Shockley-state and image-state formation can be described with the use of a heuristic multiple-reflection model. The way in which these systematics could lead to a determination of the surface potential barrier is discussed.