Resonant coherent three-photon photoemission from Cu(001)

We demonstrate the contribution of coherent excitation pathways in three-photon photoemission from Cu(001). We identify separate peaks in the photoelectron spectra as originating from two specific Cu d bands that are excited at different k points via the same unoccupied image-potential state. By model calculations we show that our experimental observations impose lower limits on the relevant Cu d-hole dephasing time on the order of 10-20 fs. Our findings suggest extensions to future studies in electronic structure mapping with tunable laboratory and free-electron laser sources, exploiting possible multiphoton resonances between occupied bulk and unoccupied surface states.