PHOTOEMISSION AND THERMAL-DESORPTION STUDIES OF CS-AU AND LI-AU FILMSON RU(001)

The properties of Cs-Au and Li-Au alloy films on Ru(001) have been stu died using temperature-programmed desorption and core- and valence-lev el photoemission. Li-Au alloys have a larger thermal stability than Cs -Au alloys. For CsAu alloy films, thermal decomposition occurs between 400 and 500 K. In contrast, LiAu films are stable up to 600 K In the decomposition process of the alkali-gold alloys, Li-Au or Cs-Au bonds are replaced by Au-Au bonds, and the alkali segregates to the surface of the film before desorbing, with Au remaining on top of the Ru subst rate up to about 1050 K when desorption of the Au multilayer starts. T he results of valence-band photoemission indicate that LiAu has metall ic properties, showing no band gap in the region close to the Fermi le vel. On the other hand, the valence spectra for CsAu show the typical band gap expected for a semiconductor. The formation of Li-Au and Cs-A u bonds produces a large shift toward higher binding energy in the cen troid of the Au 5d band and a reduction in the Au(5d3/2)-Au(5d5/2) sep aration. Alloys saturated with Cs or Li show a separation of approxima tely 1.5 eV between the Au 5d3/2 and 5d5/2 levels. For CsAu, the bindi ng-energy shifts with respect to the Fermi edge are + 1.3 eV for the A u4f levels and -0.3 eV for the Cs 3d and 4d levels. The corresponding shifts with respect to the vacuum level are approximately -2.0 eV for the Au 4f levels and -0.6 eV for the Cs 3d and 4d levels. The shift in the Au levels is a consequence of electron transfer from Cs to Au, wh ereas the shifts in the Cs levels are a result of a large contraction in the atomic volume of Cs. In LiAu, the binding-energy shifts with re spect to the Fermi edge are +0.4 eV for the Au 4f levels and approxima tely 0 eV for the Li Is level.