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

TL;DR

This study explores the properties of a two-dimensional electron gas formed on ultrathin helium films on Cu(111), revealing significant increases in binding energy and electron lifetime due to the helium layer, using time-resolved photoemission.

Contribution

It provides new insights into electron behavior on ultrathin helium films on metals, demonstrating enhanced binding energy and lifetime effects with monolayer coverage.

Findings

Binding energy of the first image-potential state increases by over two orders of magnitude.

Electron lifetime is extended by an order of magnitude with one monolayer of helium.

Helium layer maintains electron decoupling from the metal surface despite proximity.

Abstract

Electrons in image-potential states on the surface of bulk helium represent a unique model system of a two-dimensional electron gas. Here, we investigate their properties in the extreme case of reduced film thickness: a monolayer of helium physisorbed on a single-crystalline (111)-oriented Cu surface. For this purpose we have utilized a customized setup for time-resolved two-photon photoemission (2PPE) at very low temperatures under ultra-high vacuum conditions. We demonstrate that the highly polarizable metal substrate increases the binding energy of the first (n = 1) image-potential state by more than two orders of magnitude as compared to the surface of liquid helium. An electron in this state is still strongly decoupled from the metal surface due to the large negative electron affinity of helium and we find that even one monolayer of helium increases its lifetime by one order of magnitude compared to the bare Cu(111) surface.