Femtosecond time- and angle-resolved two-photon photoemission spectroscopy has been used to study fundamental aspects of excited electron dynamics at metal-dielectric interfaces, including layer-by-layer evolution of electronic structure and two-dimensional electron localization. On bare Ag(111), the lifetimes of image states are dominated by their position with respect to the projected bulk band structure. The n = 2 state has a shorter lifetime than the n = 1 state due to degeneracy with the bulk conduction band. As the parallel momentum of the n = 1 image electron increases, the lifetime decreases. With decreasing temperatures, the n = 1 image electrons, with zero or nonzero parallel momentum, all become longer lived. Adsorption of one to three layers of n-heptane results in an approximately exponential increase in lifetime as a function of layer thickness. This results from the formation of a tunneling barrier through which the interfacial electrons must decay, consistent with the repulsive bulk electron affinity of n-alkanes. The lifetimes of the higher quantum states indicate that the presence of the monolayer significantly reduces coupling of the image states to the bulk band structure. These results are compared with predictions of a dielectric continuum model. The study of electron lateral motion shows that optical excitation creates interfacial electrons in quasifree states for motion parallel to the n-heptane/Ag(111) interface. These initially delocalized electrons decay into a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal by tunneling through the adlayer potential barrier. The localization time depends strongly on the electron's initial parallel momentum and exhibits a non-Arrhenius temperature dependence. The experimental findings are consistent with a 2-D self-trapping process in which electrons become localized by interacting with the topmost plane of the alkane layer. The energy dependence of the self-trapping rate has been modeled with an electron transfer theory. This analysis shows that self-trapping involves inter- and intramolecular vibrational modes of the overlayer and the non-Arrhenius temperature dependence is a result of a strong quantum contribution from the intramolecular modes. These results for a model interface contribute to the fundamental understanding of electron behavior at the interface between metals and molecular solids.

Femtosecond, angle resolved two photon photoemission spectroscopy is used as a probe of the metal-molecule interface. In four related investigations, monolayer or submonolayer coverages of a molecule are adsorbed onto an Ag(111) surface under ultrahigh vacuum conditions. Studies use a an optically excited electronic probe to investigate to probe the energetic landscape experienced and the response of a molecular adsorbate to an excess charge. Energy levels, electronic population, and band structure reveal molecular structure and mechanisms of dynamic response to an injected charge. Room temperature ionic liquids (RTIL), consisting of charge separated anions and cations, represent a new and poorly understood class of electrochemical solvents. The mechanism of electron solvation has been proposed to be vastly different at the electrode interface than has been observed in bulk studies. Optical excitation into the RTIL conduction band results in a localized excess charge, as measured by band dispersions. Electron solvation is measured directly as the photoemitted kinetic energy. Solvation by 200-540 meV is found to occur rapidly on the timescale of 350 fs, supporting previous predictions of fast charge solvation at the interface. Further, a previously proposed phase transition is observed between a low temperature ordered phase and high temperature disordered phase. This phase transition, which occurs at 250 K, is quantified by both the change in workfunction and a change in the energy of solvation between the two phases. The image state is often used as a sensitive probe of molecular films, however, the electronic potential experienced by the image state electron is poorly understood. Theoretical predictions of image state energy in molecular films with a positive electron affinity are often wrong by 1-3 eV, and descriptions of the band structure typically remain limited to a measurement of effective mass. An image state is investigated within a film of metallated phthalocyanines. Phthalocyanines crystallize laying flat on the substrate, with a nearly square unit cell of 14 - 15 Å on a side. Angle resolved measurements into the second Brillouin zone reveal folding of the image state due to interactions with the screened potential energy surface within the molecule. The bandgap, measured to be 150 meV, can be used to estimate the corrugation of the energy landscape. Further, a Kronig Penney model is quantitatively compared to the first several backfolded image bands. Comparision between theory and experiment reveals the effects of the fourfold symmetry of lattice on the image state bandstructure. The morphology and crystal structure of thin film molecular semiconductors is well known to directly influence the band structure and electronic properties of the material. Further, several morphologies and crystal structures can result from epitaxial growth of a single molecule on a metal surface. Although phase transitions in 2D-atomic systems have been well studied for over 80 years, molecular systems are less well understood due to their larger size and the complex interplay of relatively weak forces that govern the crystalline packing. Three phases of metallated phthalocyanines are studied as a function of substrate temperature and submonolayer coverage. Three phases, the 2D-gas phase, the low temperature commensurate phase, and the high temperature incommensurate phase, are each studied using TPPE and low energy electron diffraction. The image state is found to be an excellent probe of the local crystal structure and local workfunction. Preliminary studies focus on the temperature and coverage dependent energies and intensities of the image state peak in domains of each phase. Temperature dependent studies reveal a pseudo-isosbestic point in the transition from the low temperature commensurate to the high temperature incommensurate phases. Coverage dependence reveals a redshift of the image state with increasing molecular density and decreasing workfunction, and low temperature studies reveal long time scale kinetics of reorganization. Preliminary experimental results are interpreted with the aid of kinetic monte carlo simulations. Further studies will aim to obtain quantitative thermodynamics of these submonolayer coverages. Two photon photoemission spectrscopy has proven to be a powerful tool for understanding basic surface physics of atomic and molecular systems. In particular, the image state has proven to be a powerful probe of the first 1-2 ML coverage of a molecular system. In order to improve the applicability of TPPE to answer device relevant questions, the technique will need to resolve, identify, and characterize molecularly derived bands and excitonic states. The basic selection rules for metal-molecule charge transfer excitations and exciton formation at a surface are understood, but it is not agreed upon whether hot-electrons or interfacial electronic coupling allow for excitations that break normal selection rules. Further, classical dipole quenching is expected to play a strong role in the exciton lifetimes in molecules adsorbed on metal or semiconductor surfaces, though no TPPE study up to this point has been able to observe the expected distance dependent lifteimes. In this final chapter, selection rules and classical dipole quenching are briefly discussed, and general trends are derived for molecular systems adsorbed on highly oriented metal surfaces. Our work up to this point suggests that selection rules are followed rigourously in molecular adsorbates on Ag(111).

This two-volume work covers ultrafast structural and electronic dynamics of elementary processes at solid surfaces and interfaces, presenting the current status of photoinduced processes. Providing valuable introductory information for newcomers to this booming field of research, it investigates concepts and experiments, femtosecond and attosecond time-resolved methods, as well as frequency domain techniques. The whole is rounded off by a look at future developments.