Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Figure 1 shows the band structure of Ag(111) and 3PP spectra for resonant and near-resonant
excitation conditions. For the near-resonant condition, the energy-momentum image of
photoemitted electrons shows dispersive SS and IP states indicated by red and blue curves. At
the resonant condition, however, the dispersive surface bands collapse into a non-dispersive
feature. Such non-dispersive spectra are expected for photoemission from excitonic states,
because in a correlated e-h pair, the electron can take on any momentum spanned by the
occupied part of the hole band, as long as the exciton momentum is conserved. Similar
experiments on Cu(111) surface do not show the non-dispersive feature, consistent with faster
screening suppressing the e-h correlation.
The spectra in Fig. 1 show that MPP spectra can probe the fundamental many-body response of
metals in response to perturbation by an external field. Because excitonic states are the primary
manifestation of interaction between light and-solid state materials, the ability to follow the
formation and decay of excitons is fundamental to solar energy conversion processes.
2D superatom states. In the spatial domain, we have investigated the formation of a molecular
quantum-well with single-molecule resolution. Several years ago 2PP spectroscopy provided the
evidence that a monolayer of flat lying C6F6 molecules on Cu(111) surface forms a molecular
quantum well with an effective mass of 2me for its LUMO state of σ* character. How flat lying
π-conjugated molecules can hybridize into a delocalized band has been until now a mystery.
We have performed low temperature STM measurements on C6F6 on Cu and Au surfaces from
single molecule to multilayer coverages. By dz/dV spectroscopy we could measure the σ* state
energy as a function of the number of nearest neighbors, and found that the energy is stabilized
consistent with an intermolecular hopping integral β=0.026 eV. This behavior could be
reproduced nearly quantitatively by a DFT calculation of the electronic structure of unsupported
C6F6 from a single molecule to a monolayer film. The calculations show that the band formation
is an intrinsic property of C6F6 molecules, which raises the question of the origin of the
intermolecular interactions that enable the dispersive band formation.
We have therefore investigated the electronic properties of the σ* state by low temperature STM
and theory. As required for the strong intermolecular interaction, the σ* state has a diffuse wave
function with a long tail extending past the F atom periphery (Fig. 2). This behavior is
reminiscent of the superatom molecular orbitals (SAMOs) of hollow molecules, which we
discovered in C60 and related materials. SAMOs of hollow molecules are bound to the hollow
Figure 2. STM image of C6F6 monolayer and second monolayer islands on Cu(111) surface.
The calculated probability density of the σ* state for a single molecule and σ* band for a
monolayer quantum well. The density projecting beyond F atoms forms the delocalized state.
The radial wave function cross sections for the π HOMO, σ* LUMO, and π* LUMO+1 orbitals.
The SAMO characteristics of σ*, i.e. the nonnuclear density in the molecular center and beyond
the F atom periphery, enable strong wave function hybridization into a delocalized band.
144