Laser-induced desorption of Ar from Cu(1 0 0) probed by two-photon photoemission spectroscopy of image-potential states

Comparing the intensities of the well separated two-photon photoemission lines from image-potential states on clean and Ar covered Cu(1 0 0) surfaces we show that Ar atoms are desorbed by photons of only 4.43 eV energy. The crosssection of this desorption by electronic transition process is very low, only 1.3 barn (1.3 · 10 24 cm). Experimentally, it is accessible due to the high light intensity attainable within the laser focus and the sensitivity of our technique to detect very small islands of bare metal which are created by the desorption process. By varying the thickness of the Ar layer we show that only atoms from the layer directly physisorbed onto the metal surface are excited. Within the range of our laser intensities, the yield is linear with the photon flux, indicating a one-photon process. Possible mechanisms for this photon-stimulated reaction are discussed. 2003 Elsevier B.V. All rights reserved.