Energetics and dynamics of unoccupied electronic states at the h-BN/Ni(111) interface

Adsorbed on the nickel (111) surface, hexagonal boron nitride (h-BN) forms a commensurate monolayer, which retains much of the band structure of layered bulk h-BN, specifically the wide band gap. This article explores its unoccupied band structure with femtosecond time-resolved two-photon photoelectron spectroscopy in a two-color setup, and presents spectra, dispersion, and cross-correlation measurements. Two distinct, dispersive intermediate states are observed at the origin of the surface Brillouin zone (Gamma) over bar: a pi(star)-related interface state at 1.51 eV above the Fermi level, which forms the conduction band, and an image-potential state at 0.65 eV below the vacuum level. With 260 fs for the image state and 110 fs for the interface state the lifetimes of the two unoccupied states are remarkably high due to the very small overlap of the corresponding wave functions with the nickel bulk states.