First-principles calculations of optical gaps and exciton binding energies in molecules adsorbed on metal surfaces

Physical Review Letters (2010)

First-principles calculations of optical gaps and exciton binding energies in molecules adsorbed on metal surfaces

Juan Maria Garcia-Lastra, Kristian S. Thygesen

The electronic structure and lowest optical excitations of benzene and a set of donor-TCNE charge transfer complexes is calculated in the gas-phase and on Al(111) using the GW approximation and the Bethe-Salpeter equation (BSE). The calculated gas-phase quasiparticle (QP) and optical gaps are in good agreement with experiments. The optical gap of benzene is unchanged upon adsorption while that of benzene-TCNE is reduced by almost 1 eV when placed a distance of z = 4.0 °A above the surface. The reduction of the optical gap is directly related to the size of the excited state dipole moment and can be described by a classical image potential model. Close to the surface the repulsive exchange part of the electron-hole interaction can dominate the screened attractive part leading to an optical gap exceeding the QP gap. Time-dependent density functional theory calculations do not capture the substrate polarization effects.

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