Attosecond interferometry unravels complex delays in photoemission from solids

Recent progress in ultrafast spectroscopy has allowed us to directly study the dynamics of electrons, which is essential to understand underlying processes since electrons are the carriers of energy and information and form the chemical bonds between atoms. These electronic processes naturally occur on an attosecond timescale as a result of the characteristic electron velocities and length scales. Small relative delays between photoemission from different electronic states were observed in noble gas atoms and from metal surfaces. Despite these advances, our understanding of the temporal characteristics of the photoemission process in condensed matter systems remains limited. We extend gas phase methodology based on quantum path interference to noble metal surfaces and we demonstrate that we can derive energy-dependent photoemission delays using attosecond pulse trains. Taking photoemission from Ar 3p as temporal reference enables us to extract absolute delays since the case of argon atoms has been extensively studied and is well understood from a theoretical point of view. Our experimental photoemission delays from Ag(111) and Au(111) are in the same time range as those obtained from simulations based on scattering theory and ballistic transport, but vary strongly with small changes in energy. This interesting behaviour cannot be reproduced with our theoretical model and it must be attributed to effects beyond transport. Complex interactions of the outgoing electron with the photohole, the remaining electrons and the crystal lattice are imprinted on the temporal characteristics and accessible in our experiment. Our approach does not require single attosecond pulses and directly delivers photoemission delays at different electron energies.