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

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (n_S)) to the LUMO-derived MO (the antibonding orbital localized on the S-S bond (σ*_SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule-metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes.

Optically and thermally induced molecular switching processes at metal surfaces

Using light to control the switching of functional properties of surface-bound species is an attractive strategy for the development of new technologies with possible applications in molecular electronics and functional surfaces and interfaces. Molecular switches are promising systems for such a route, since they possess the ability to undergo reversible changes between different molecular states and accordingly molecular properties by excitation with light or other external stimuli. In this review, recent experiments on photo- and thermally induced molecular switching processes at noble metal surfaces utilizing two-photon photoemission and surface vibrational spectroscopies are reported. The investigated molecular switches can either undergo a trans-cis isomerization or a ring opening-closure reaction. Two approaches concerning the connection of the switches to the surface are applied: physisorbed switches, i.e. molecules in direct contact with the substrate, and surface-decoupled switches incorporated in self-assembled monolayers. Elementary processes in molecular switches at surfaces, such as excitation mechanisms in photoisomerization and kinetic parameters for thermally driven reactions, which are essential for a microscopic understanding of molecular switching at surfaces, are presented. This in turn is needed for designing an appropriate adsorbate-substrate system with the desired switchable functionality controlled by external stimuli.

Formation of two-dimensional polarons that are absent in three-dimensional crystals

We report the direct time domain observation of a many-body process in a two-dimensional system: small polaron formation from the localization of a conduction band electron in NaCl thin films of unit cell thickness. Contrary to theoretical prediction for bulk NaCl crystal where an electron polaron does not exist, time-resolved two-photon photoemission reveals small polaron formation from delocalized conduction band electrons in crystalline NaCl thin films. The increased deformability and the reduced electronic bandwidth of a crystalline lattice in the thin film format are both responsible for the formation of small polarons that are absent in bulk solids.

Electron Dynamics at Polyacene/Au(111) Interfaces

Two-photon photoemission (2PPE) spectroscopy is used to examine the excited electronic structure and dynamics at polyacene/Au(111) interfaces. Image resonances are observed in all cases (benzene, naphthalene, anthrathene, tetracene, and pentacene), as evidenced by the free-electron like dispersions in the surface plane and the dependences of these resonances on the adsorption of nonane overlayers. The binding energies and lifetimes of these resonances are similar for the five interfaces. Adsorption of nonane on top of these films pushes the electron density in the image resonance away from the metal surface, resulting in a decrease in the binding energy (-0.3 eV) and an increase in the lifetime (from <20 to approximately 110 fs). The insensitivity of the image resonances to the size of polyacene molecules and the absence of photoinduced electron transfer from the metal substrate to molecular states both suggest that the unoccupied molecular orbitals are not strongly coupled to the delocalized metal states or image potential resonances.

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.

Electron Transport Across the Alkanethiol Self-assembled Monolayer/Au(111) Interface: Role of the Chemical Anchor

Alkanethiol self-assembled monolayers (SAMs) on Au(111) are model systems for molecular electronics. We probe the role of the chemisorption bond on electron dynamics at the SAM/Au interface using time-resolved two-photon photoemission. Formation of the Au-S bond is evidenced by a localized sigma resonance, which broadens and shifts upward in energy when the lying-down chemisorbed molecules stand up. The localized chemisorption bond does not affect the electronic coupling between delocalized image resonances and the metal substrate. Instead, lifetimes of image resonances are decreased due to scattering with S atoms within the thiol or thiolate monolayer.