  We present a spectroscopic method which utilizes virtual photons to
selectively measure the electronic structure of the top-most atomic layer.
These virtual photons are created when incident positrons transition from
vacuum states to bound surface states on the sample surface and can transfer
sufficient energy to excite electrons into the vacuum. The short interaction
range of the virtual photons restricts the penetration depth to approximately
the Thomas-Fermi screening length. Measurements and analysis of the kinetic
energies of the emitted electrons made on a single-layer of graphene deposited
on Cu and on the clean Cu substrate shows that the ejected electrons originate
exclusively from the top-most atomic layer. Moreover, we find that the kinetic
energies of the emitted electrons reflect the density of states at the surface.
These results demonstrate that this technique will be a complementary tool to
existing spectroscopic techniques in determining the electronic structure of 2D
materials and fragile systems due to the absence of subsurface contributions
and probe-induced surface damage.
