Controlled Modification of Surface State Lifetimes by Physisorbed Adsorbates
A. Hotzel, E. Knoesel, M. Wolf and G. Ertl
Proceedings SPIE 3272, (1998) 228.
Femtosecond time-resolved two-photon photoemission (2PPE) is used to study the influence of physisorbed xenon and oxygen adlayers on the lifetime of image potential states and interfacial quantum well states on Cu(111). Adsorption of 0 to 3 layers of Xe leads to a pronounced increase of the n = 1 image state lifetime from 22 fs to 300 fs, respectively. However, for adsorbate heterostructures consisting of one monolayer (ML) oxygen on top of Xe spacer layers with variable thickness it is found that the lifetime of an oxygen induced quantum well state (0.35 eV below Evac) decreases from 650 fs to 90 fs when the number of spacer layers is raised from 1 ML to 5 ML. The results can be semiquantitatively reproduced by a model calculation which accounts for the modified image potential due to the Xe and O2 adlayers. The changes of the lifetimes are explained by the differences in the penetration of the excited state wave function into the Cu substrate.