More Than Skin Deep

摘要

electrons ate normally not supposed to wander very far from metal surfaces. Yet on page 1480 of this issue, Hofer et al.(l) report an interdisciplinary investigation that catches surface electrons at their most adventurous. By combining laser techniques used so successfully in atomic, molecular, and semiconductor physics (2-4) with angle-resolved photpelectron spectroscopy, the workhorse of surface science (5), Hofer et al. give us a glimpse of the time evolution of electronic states at a metal surface. For the first time, quantum beats between coherently excited states at a clean metal surface have been observed, allowing the authors to resolve high-order states, inaccessible with standard photoelectron spectroscopy. Coherent excitation of higher order Rydberg-like states, selected by the energy in the photoelectron spectrometer, display dramatic time evolution as, a consequence of the motion of the wave packet, which travels about 200. A away from the surface into the vacuum and then returns with a period of 800 fs.