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要旨

Experience has shown that theoretically determined lifetimes of bulk states of hot electrons in real metals agree quantitatively with the experimental ones, if theory fully takes into account the crystal structure and many-body
effects of the investigated metal, i.e., if the Dyson equation is solved at the ab initio level and the effective electron– electron interaction is determined beyond the plasmon-pole
approximation. Therefore the hitherto invoked transport effect [Knoesel et al.: Phys. Rev. B 57, 12 812 (1998)] does not seem to exist. In this paper we show that likewise neither
virtual states [Hertel: et al. Phys. Rev. Lett. 76, 535 (1996)] nor damped band-gap states [Ogawa: et al.: Phys. Rev. B 55, 10 869 (1997)] exist, but that the hitherto unexplained d-band
catastrophe in Cu [Cu(111), Cu(110)] can be naturally resolved by the concept of the transient exciton. This is a new quasiparticle in metals, which owes its existence to the dynamical
character of dielectric screening at the microscopic level. This means that excitons, though they do not exist under stationary conditions, can be observed under ultrafast experimental conditions.