X.-Y. Zhu

Delocalized electron resonance at the alkanethiolate self-assembled monolayer∕Au(111) interface

We probe the electronic structure of alkanethiolate self-assembled monolayers (SAMs) on Au(111) using two-photon photoemission spectroscopy. We observe a dispersive unoccupied resonance close to the vacuum level with a lifetime shorter than 30 fs. The short lifetime and the insensitivity of the energy level and dispersion to molecular length (and thus layer thickness) suggest that the probability density of the electron wave function is concentrated inside the molecular layer close to the SAM/Au interface. Such an interfacial resonance results from the image like potential at the SAM/Au interface.

Using image resonances to probe molecular conduction at the n-heptane∕Au(111) interface

The binding energies and lifetimes of the n=1 image resonance on Au(111) are measured as a function of n-heptane layer thickness by femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy. The lifetime of the image resonance dramatically increases from approximately 4 fs on clean Au(111) to 1.6 ps with three layers of n-heptane. Because the image resonance is above the L1 band edge of Au, this increase in lifetime is attributed to the tunneling barrier presented by the sigma-sigma* band gap of the n-heptane film. We use the one-dimensional dielectric continuum model (DCM) to approximate the surface potential and to determine the binding energies and the lifetimes of the image resonances. The exact solution of the DCM potential is determined in two ways: the first by wave-packet propagation and the second by using a tight-binding Greens function approach. The first approach allows band-edge effects to be treated. The latter approach is particularly useful in illustrating the similarity between TR-2PPE and conductance measurements.

Lateral confinement of image electron wave function by an interfacial dipole lattice

Image-potential states on Cu(111) surfaces covered by thin films of C60 fullerene have been characterized by angle-resolved two-photon photoemission spectroscopy. Metal-to-molecule electron transfer within the first layer creates a 4×4 superlattice of surface dipoles. We show that such a surface dipole lattice provides lateral confinement of image-electron wave functions. Measurements of parallel dispersion indicate that the n=1 image state is localized in the presence of one monolayer of C60 but becomes delocalized by the addition of a second layer. Quantum mechanical calculations explain this in terms of the screening of the dipole potential, thus, restoring the free-electron behavior parallel to the surface. These results show that a surface dipole lattice can effectively control the interfacial electronic structure.