Ultrafast dynamics of electrons in image-potential states on clean and Xe-covered Cu(111)

Lifetimes of electrons in the n=1 and n=2 image states on Cu(111) are studied with femtosecond time-resolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the image-state lifetime, which for the n=1 state changes from 18+/-5 fs at clean Cu(111) to 75+/-15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process.