Real-Time Observation of Adsorbate Atom Motion Above a Metal Surface

Abstract

The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10 ^–15 seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photoemission spectroscopy, are attributed to atomic motion in a copper–cesium bond-breaking process. Describing the change in energy of the cesium antibonding state with a simple classical model provides information on the mechanical forces acting on cesium atoms that are “turned on” by photoexcitation. Within 160 femtoseconds, the copper–cesium bond extends by 0.35 angstrom from its equilibrium value.