Spectroscopy and population decay of a van der Waals gap state in layered TiSe₂

Two-photon photoemission (2PPE) spectroscopy is used to map the momentum-dependent energy distribution of unoccupied bands in the layered 1T -TiSe2 transition-metal dichalcogenide compound. A comparison of the experimental results with previous calculations based on the local-density functional approach enables us to identify the second Ti3d conduction band and a localized unoccupied state that we assign to the presence of excess titanium atoms in the van der Waals gap of the crystal. Time-resolved 2PPE measurements show clear differences in the lifetime between the two states, indicative for the decoupling of the Ti excess atoms from the bulk electronic structure.