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Electrons in image-potential states on the surface of bulk helium represent aunique model system of a two-dimensional electron gas. Here, we investigatetheir properties in the extreme case of reduced film thickness: a monolayer ofhelium physisorbed on a single-crystalline (111)-oriented Cu surface. For thispurpose we have utilized a customized setup for time-resolved two-photonphotoemission (2PPE) at very low temperatures under ultra-high vacuumconditions. We demonstrate that the highly polarizable metal substrateincreases the binding energy of the first (n = 1) image-potential state by morethan two orders of magnitude as compared to the surface of liquid helium. Anelectron in this state is still strongly decoupled from the metal surface dueto the large negative electron affinity of helium and we find that even onemonolayer of helium increases its lifetime by one order of magnitude comparedto the bare Cu(111) surface.

Spectroscopy and Dynamics of a Two-Dimensional Electron Gas on top of Ultrathin Helium Films on Cu(111)