Real-time observation of adsorbate atom motion above a metal surface

The dynamics of cesium atom motion above the copper(lll) surface following electronic excitation with light was studied with femtosecond (10(-15) seco nds) time resolution. Unusual changes in the surface electronic structure w ithin 160 femtoseconds after excitation, observed by time-resolved two-phot on photo-emission spectroscopy, are attributed to atomic motion in a copper -cesium bond-breaking process. Describing the change in energy of the cesiu m antibonding state with a simple classical model provides information on t he mechanical forces acting on cesium atoms that are "turned on" by photoex citation. Within 160 femtoseconds, the copper-cesium bond extends by 0.35 a ngstrom from its equilibrium value.