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.