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

The properties of Cs-Au and Li-Au alloy films on Ru(001) have been studied using temperature-programmed desorption and core- and valence-level 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 substrate up to about 1050 K when desorption of the Au multilayer starts. The results of valence-band photoemission indicate that LiAu has metallic properties, showing no band gap in the region close to the Fermi level. 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-Au bonds produces a large shift toward higher binding energy in the centroid of the Au 5d band and a reduction in the Au(5d3/2)-Au(5d5/2) separation. Alloys saturated with Cs or Li show a separation of approximately 1.5 eV between the Au 5d3/2 and 5d5/2 levels. For CsAu, the binding-energy shifts with respect to the Fermi edge are + 1.3 eV for the Au4f 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, whereas 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 respect to the Fermi edge are +0.4 eV for the Au 4f levels and approximately 0 eV for the Li Is level.