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.

Time-resolved two-photon photoemission was used to investigate the two-dimensional electron solvation by methanol, 1-propanol, 1-butanol, and 1-pentanol overlayers on a Ag(111) surface. For each system at coverages higher than one monolayer, several image potential state series with time-dependent energies were observed by using two-photon photoemission indicating that multiple time-dependent local work functions can originate from multiple coverages. The time-dependent energy relaxation seen is attributed to the rotation of the adsorbate molecular dipoles to solvate the electron. This rotation lowers the electron-layer interaction energy, causing a dynamic reduction of the local work function which is a signature of the solvation of the electron by the adsorbate layer. A classical electron-disk dipole model supports the conclusion that the dynamic variation in the image potential state energies results from a local work function effect and indicates that the evolution of the energy results from the time-dependent projection of the molecular dipole onto the surface normal on the femtosecond time scale.

Two-photon photoemission is used to investigate the interfacial band structure of the methanethiolate/Ag(111) interface. Two adsorbate-induced electronic states are observed:  one occupied, the other an unoccupied excited state. The initially occupied and unoccupied electronic states have effective masses of −2m e and 0.5m e, respectively. At a low coverage of thiolate molecules, the unoccupied state is measured to be nondispersive. At intermediate coverages both nondispersive and dispersive unoccupied electronic states are observed. This coverage dependent evolution can be interpreted as a phase transition from a less dense to a more dense layer configuration.