Electronic structure and electron dynamics at an organic molecule/metal interface: interface states of tetra-tert-butyl-imine/Au(111)

Sebastian Hagen,Martin Wolf,Petra Tegeder,Rainer Haag,Ying Luo

doi:10.1088/1367-2630/12/12/125022

Abstract

Time- and angle-resolved two-photon photoemission (2PPE) spectroscopies have been used to investigated the electronic structure, electron dynamics and localization at the interface between tetra-tert-butyl imine (TBI) and Au(111). At a TBI coverage of one monolayer (ML), the two highest occupied molecular orbitals, HOMO and HOMO-1, are observed at an energy of −1.9 and −2.6 eV below the Fermi level (EF), respectively, and coincide with the d-band features of the Au substrate. In the unoccupied electronic structure, the lowest unoccupied molecular orbital (LUMO) has been observed at 1.6 eV with respect to EF. In addition, two delocalized states that arise from the modified image potential at the TBI/metal interface have been identified. Their binding energies depend strongly on the adsorption structure of the TBI adlayer, which is coverage dependent in the submonolayer (⩽1 ML) regime. Thus the binding energy of the lower interface state (IS) shifts from 3.5 eV at 1.0 ML to 4.0 eV at 0.5 ML, which is accompanied by a pronounced decrease in its lifetime from 100 fs to below 10 fs. This is a result of differences in the wave function overlap with electronic states of the Au(111) substrate at different binding energies. This study shows that in order to fully understand the electronic structure of organic adsorbates at metal surfaces, not only adsorbate- and substrate-induced electronic states have to be considered but also ISs, which are the result of a potential formed by the interaction between the adsorbate and the substrate.

Highlights

Time- and angle-resolved two-photon photoemission (2PPE) spectroscopies have been used to investigated the electronic structure, electron dynamics and localization at the interface between tetra-tert-butyl imine (TBI) and Au(111)
Our results demonstrate that for a complete understanding of the electronic structure of organic molecules adsorbed on a metal surface, adsorbate- and substrate-derived electronic states have to be considered and electronic states that are created at the molecule/metal interface
The adsorption of TBI molecules leads to a reduction in the work function as can be directly witnessed in the broadening of the spectra when going from ≈ 0.15 to 1.1 ML

Summary

Experimental details

For the 2PPE measurements, an ultrahigh vacuum chamber for photoemission spectroscopy and surface science techniques in combination with a tuneable femtosecond laser system was used. For samples prepared at substrate temperatures T < 200 K, annealing effects were observed, i.e. changes in the photoemission signals as a function of substrate temperature These effects are most likely attributed to a structural rearrangement of TBI, e.g. disorder/order effects. In the case of an unoccupied electronic state (e.g. LUMO), only the probing photon influences the kinetic energy of the electron; Ekin = hν2 Note that this analysis is generally not applicable to transitions between bulk bands due to their strong perpendicular dispersion, but holds in the case of surface and adsorbate-derived states [52, 53]. Recording the respective 2PPE signal of unoccupied states as a function of the delay time yields the corresponding lifetime of the electronically excited states

Results and discussions

Peak C

Coverage-dependent behavior of unoccupied electronic states: interface states

Conclusions