Excitonic precursor states in ultrafast pump-probe spectroscopies of surface bands

We discuss the formation and evolution of transient coherent excitonic states induced by ultrashort laser irradiation of metal surfaces supporting the surface and image potential bands (typically the low index surfaces of Cu and Ag). These states, which evolve into the image potential states in the course of screening of primary optically excited electron hole pair, may play the role of early intermediate states in pump probe spectroscopies of surfaces (e.g. two-photon-photoemission or sum-frequency generation) if the formation of image charge density proceeds on the time scale of the order of or longer than the pump probe pulse duration and delay. In this regime a pump probe experiment may yield information on the characteristics of such states rather than the states in relaxed image potential bands. Time scales of the various stages of these processes are estimated using an exactly solvable model of surface screening. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim