S. Schuppler

Coverage-dependent electronic structure of Na on Cu(111)

Abstract The electronic states of Na on Cu(111) have been studied by high-resolution two-photon photoemission. Image states which are well separated from alkali states are observed at all coverages. An abrupt change of the occupancy of the alkali state is observed at coverages between 0.40 and 0.45 ML. We interpret this as the transition from a more ionic bonding of alkali atoms on the surface to the metallic bond in alkali islands. Other alkali metals on close-packed surfaces show a very similar electronic structure as a function of alkali coverage.

Bichromatic two-photon photoemission spectroscopy of image potential states on Ag(100)

Bichromatic two-photon photoemission spectroscopy (Bi2PPES) leads to an increased signal-to-noise ratio compared to conventional two-photon photoemission spectroscopy and therefore allows the observation of the first four image potential states as well as a lineshape analysis with improved accuracy on Ag(100). The n=2 image state with a measured linewidth of 37 meV FWHM is the narrowest unoccupied structure measured on any solid surface so far. The intrinsic linewidths of the first two image states were determined as 21±4 meV and 5±5 meV, respectively, in reasonable agreement with theoretical calculations. Disordered adsorption of oxygen on Ag(100) leads to a linewidth broadening of the first and — to a lesser extent — of the second image potential state. A quantitative analysis of the broadening suggests that the underlying mechanism is lifetime shortening due to scattering of the image state electrons by the adsorbate atoms.