Image states on bimetallic flat and stepped surfaces; ultrafast two-photon-photoemission spectroscopy

The lateral motion of image-state electrons on two different bimetallic surfaces was investigated by using angle-resolved 2-photon photoemission. To provide a clear, benchmarked system, measurements were first made on flat Ag/Pt (111) and Ag/Ni(111). Two stepped metallic surfaces Ag/Pt(997) and Ag/Ni(977) were then studied; a comparison of the results on the two surfaces allowed a study of the behavior of image electrons on the Ag regions in the presence of different degrees of step regularity. In both systems a measurement of energy dispersion for image electrons moving in a direction perpendicular to steps showed that the energy shifts down and the effective mass decreases, as the Ag coverage is increased. These results were compared to that expected using a Kronig-Penney-like surface potential.; Angle-resolved ultrafast two-photon-photoemission was then used to investigate image-state-electron lifetimes on two bimetallic surfaces: Ag/Pt(111) and Ag/Pt(997). This photoemission probe measures the relative dependence of image-electron dynamics on parallel electron momentum, k∥, for flat and stepped surfaces. The n = 1 and n = 2 image states were identified by their k∥-dependent energy values on the flat surface and for directions parallel and perpendicular to the step array. A new weakly dispersive feature was observed in the energy distribution curves in the direction across steps. The ratio of the n = 2/n = 1 state lifetimes on the flat surface at k∥ = 0 is increased over its value for single-crystal Ag(111) due to the Pt substrate. The k∥-dependence of scattering time of the n = 1 state on both flat and stepped surfaces 2 could be fitted to a function T1 - Sk2∥; on the flat surface S is equal to 205 +/- 45 fsA2, on the stepped surface the measured values for S are 110 +/- 30 fsA2 and 40 +/- 30 fsA2 in the direction parallel and perpendicular to the step edges, respectively. These results were compared to those on flat and stepped Cu(100) surfaces.; Finally, preliminary results for the occupied states of the Ag/Pt(111) and Ag/Pt(997) surfaces and for molecules on metal surfaces are presented.