The determination of the lifetime of hot electrons in metals by time-resolved two-photon photoemission: the role of transport effects, virtual states, and transient excitons

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