Nonequilibrium Band Mapping of Unoccupied Bulk States below the Vacuum Level by Two-Photon Photoemission

We demonstrate angle-resolved, tunable, two-photon photoemission (2PPE) to map a bulk unoccupied band, viz. the Cu sp band 0 to 1 eV below the vacuum level, in the vicinity of the L point. This short-lived bulk band is seen due to the strong optical pump rate, and the observed transition energies and their dispersion with photon energy (ℎ/2π)ω, are in excellent agreement with tight-binding band-structure calculations. The variation of the final-state energy with (ℎ/2π)ω has a measured slope of ∼1.64 in contrast to values of 1 or 2 observed for 2PPE from two-dimensional states. This unique variation illustrates the significant role of the perpendicular momentum (ℎ/2π)kperpendicular in 2PPE.