Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule.

Ultrafast relaxation dynamics of electrons at dielectric-metal interfaces reflect the nature of the electronic interaction with both the substrate and the adsorbed layer. The full understanding of macroscopic electrical transport properties across an interface requires the knowledge of the energies, spatial extent of the interfacial electronic states, and the electron scattering length. With the femtosecond two-photon photoemission technique, it is possible to directly observe the dynamics of interfacial electrons with specific energy and parallel momentum. Interband and intraband electron relaxation dynamics in excited surface and quantum well states are determined with momentum and time-resolved two-photon photoemission. The study of charge carrier scattering at interfaces and in ultrathin films of Xe on Ag(111) provides a wealth of information on the energy, parallel momentum, and layer thickness dependence of the electron scattering rate. Adsorption of Xe on metal surfaces modifies the interfacial potential and drastically changes the spatial extent of the interfacial electronic wavefunction. The spatial extent of the electronic wavefunction in the direction perpendicular to the interface determines the interband relaxation rate. Oscillation in the interband relaxation time as a function of layer thickness is attributed to a quantum size effect. The lifetime of the n = 1 surface state shows a strong parallel momentum dependence. This phenomenon is attributed to intraband momentum relaxation. The thickness dependence of intraband relaxation suggests a change in the scattering potential in the direction parallel to the interface for a monolayer and bilayer of Xe. The possibility of scattering due to thermal and structural disorder is discussed.

The layer dependent evolution of the unoccupied electronic structure and electron dynamics at the naphthalene/ Ag(111) and the anthracene/Ag(111) interfaces have been investigated with femtosecond time and angle resolved two photon photoemission. With the exception of the peaks observed for the naphthalene monolayer, all excitations in the two photon photoemission spectra fit a hydrogenic progression, substantiating their assignment as image potential states. The monolayer excitations for naphthalene cannot be assigned as either image potential states or electron affinity (EA) levels, but rather as hybridized EA/image potential states. The binding energies and lifetimes of the image potential states for naphthalene and anthracene exhibit two significant differences that demonstrate the tremendous variation in the coupling between the image potential and the EA levels of naphthalene and those of anthracene. First, the binding energies at the naphthalene/ Ag(111) interface exceed those of the anthracene/Ag(111) interface, even though anthracene has a larger EA than naphthalene. Second, the 1.1 ps lifetime for the n ) 1 image potential state for a bilayer of anthracene exceeds the n ) 1 lifetime for a bilayer of naphthalene by a factor of 30. Theoretical calculations demonstrate that the transition from a near resonant to a nonresonant interaction between the image potential and the adsorbate EA levels causes these significant variations in binding energies and lifetimes.