Electronic level alignment at the interface between an adsorbed molecular
layer and a semiconducting substrate determines the activity and efficiency of
many photocatalytic materials. We perform G0âW0â calculations to determine
the coverage dependence of the level alignment for a prototypical
photocatalytic interface: 1/2 and 1 monolayer (ML) intact and dissociated
CH3âOH on rutile TiO2â(110). We find changes in the wavefunction's spatial
distribution, and a consequent renormalization of the quasiparticle energy
levels, as a function of CH3âOH coverage and dissociation. Our results
suggest that the occupied molecular levels responsible for hole trapping are
not those observed in the ultraviolet photoemission spectroscopy (UPS)
spectrum. Rather, they are those of isolated CH3âO on the surface. We find
the unoccupied molecular levels have either 2D character with weight above the
surface at 1 ML coverage, or significant hybridization with the surface at 1/2
ML coverage. These results suggest the resonance observed in the two photon
phooemission (2PP) spectrum arises from excitations to unoccupied "Wet
electron" levels with 2D character.Comment: 8 pages, 5 figures, 1 tabl