Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal molecule interface and single-molecule junction transport data We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant X-ray photoemission spectroscopy We compare these results to scanning tunnelling microscope-based break-junction measurements of single molecule conductance and to first-principles calculations We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections On the Au(110) that presents Au atoms with lower-coordination, critical in break-junction conductance measurements, we see chat the HOMO level shifts further from the Fermi level These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data
