Femtosecond Studies of Electron Dynamics at Dielectric-Metal Interfaces

Abstract

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.

Quantum-well states with image state character for Pb overlayers on Cu(111)

“…50,51,[55][56][57][58][59][60] However, in the case of dielectric spacer layers, the lifetimes of excited states show the dependence on the overlayer thickness distinct from the present results. Thus, for Xe/Ag(111) the decay rates of the n = 2 and n = 3 ISs hybridized with QWSs show an oscillating structure with increasing Xe coverage.…”

“…Thus, for Xe/Ag(111) the decay rates of the n = 2 and n = 3 ISs hybridized with QWSs show an oscillating structure with increasing Xe coverage. 51,[55][56][57] Similar results were reported for Kr/Cu(100) and Xe/Cu(100). 51 This is in contrast to the smooth increase of many-body decay rates with overlayer thickness as reported here for Pb/Cu(111).…”

“…This has been experimentally confirmed in TR-2PPE studies of the ISs at surfaces of rare-gas adlayers on metal surfaces. 50,51,[55][56][57][58][59][60] In these systems, strong quantum size effects on the lifetimes of the ISs have been found and successfully interpreted as due to the hybridization between the ISs and the QWSs localized in the conduction band of the dielectric adlayer. The purpose of the present contribution is to show how the QWSs in metallic Pb overlayers on Cu(111) surfaces are modified close to the vacuum level by the hybridization with the ISs.…”

Decay of electronic excitations at metal surfaces

“…In the presence of adsorbates that do not introduce unoccupied electronic levels in the energy range of the image-potential states, the wave functions of the latter will retain their simple hydrogenic character, but will be repelled from the metal. Systems with such properties that have been studied by time-resolved 2PPE include Xe/Ag(1 1 1) [258,259], alkanes/Ag(1 1 1) [260,261], Xe/Cu(1 1 1) [262,263], O 2 /Xe/ Cu(1 1 1) [264], N 2 /Cu(1 1 1) [263], N 2 /Xe/Cu(1 1 1) [263], naphthalene/Cu(1 1 1) [265], Xe/ Ru(0 0 0 1) [266,267] and Ar/Cu(1 0 0) [268,269]. Data obtained for the two latter systems are shown as examples in Figs.…”

“…This is attributed to the larger density of states around E F [195]. The influence of rare-gas layers on the properties of image-potential states have been investigated for Xe/Ag(1 1 1) [259,272], Xe/Cu (1 1 1) [262,263,318], Ar, Kr and Xe on Cu(1 0 0) [268,269,273] and Xe/Ru(0 0 0 1) [266,267]. Experimental results for the lifetime of the states are collected in Table 10 for various systems together with predictions from different models.…”