Organic Molecules on Wide-Gap Insulators: Electronic Excitations

TL;DR

This study investigates the electronic excitations at a molecule-insulator interface using many-body Green's function methods, revealing charge transfer excitonic effects dominate the lowest excited states.

Contribution

It applies the Bethe-Salpeter equation on top of GW quasiparticle energies to accurately describe interface excitations, highlighting the importance of electron-hole interactions.

Findings

Charge transfer excitonic effects dominate the lowest excited state.

The lowest excitation involves a transition from the surface valence band to the molecule's π-π* state.

Full excitonic Hamiltonian is necessary for accurate interface excitation description.

Abstract

The electronic excitation of a conjugated molecule-insulator interface, as exemplified by the adsorption of benzoic acid and its phenolic derivative on NaCl(001) surface, is addressed by many-body Green's function methods. By solving the two-particle Bethe-Salpeter equation on top of the $GW$ quasiparticle energies, it turns out that instead of the intramolecular $\pi-\pi^\ast$ transition of the adsorbate, the lowest singlet excited state of the adsorbate system, being a charge transfer excitonic effect, is essentially assigned to the transition from the surface valence band maximum to the $\pi-\pi^\ast$ state of the molecule. An accurate description of this lowest electronic excitations confined at the interface requires the knowledge of a full excitonic Hamiltonian due to the sizable electron-hole exchange interaction.