TDS Study of Potassium Adsorption on Al(111) and fivefold Al-Pd-Mn
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TDS Study of Potassium Adsorption on Al(111) and fivefold Al-Pd-Mn Barbara I. Wehner1, James W. Anderegg1, Cynthia J. Jenks1, Amy R. Ross1, Thomas A. Lograsso1, Renee D. Diehl2, Patricia A. Thiel1 1 Ames Laboratory, Iowa State University, Ames, IA 50011. 2 Penn State University, 104 Davey, University Park, PA 16802. ABSTRACT We have used thermal desorption spectroscopy to carry out a comparative study of potassium adsorption on Al(111) and on the fivefold Al-Pd-Mn surface. Potassium adsorption on the quasicrystal was found to be different than on Al(111). The potassium monolayer desorbed from fivefold Al-Pd-Mn at lower temperatures than from Al(111). Potassium is known to form a dense monolayer on Al(111), with an ideal coverage of 0.33, but for the monolayer on fivefold Al Pd Mn we find that the saturation coverage is only one twelfth.
INTRODUCTION Alkali metal adsorption has been studied during the last twenty years on a large variety of surfaces, especially metal and semiconductor surfaces1,2. Known to decrease the work function, alkali metals are able to influence the surface electronic structure, even leading to adsorption induced reconstruction of the structurally less-stable surfaces3. Potassium is known as a promotor in the field of catalysis, increasing the sticking coefficient and dissociation of small molecules, catalyzing the oxidation of semiconductors, for example4. There is an ongoing discussion about the surface electronic structure of quasicrystals. A particular issue is whether the surface density of states has a pseudogap at the Fermi level as is the case for the bulk density of states5,6,7 and whether this pseudogap can affect chemical interactions with other materials, such as wetting. For this reason, we chose to study the interaction between a strong electron donor--the alkali metal potassium--and the quasicrystal surface, for which a low electron density is postulated. The Al(111) surface was chosen for comparison because in the topmost layer of the fivefold (5f) Al-Pd-Mn surface, only Al atoms are found, and a surface structure model shows that the lateral density of the two topmost layers taken together is similar to that of the Al(111) surface8.
EXPERIMENTAL DETAILS A single grain Al-Pd-Mn Bridgman sample was oriented by Laue X-ray diffraction along the fivefold axis. The surface was diamond polished down to 1 µm. Inductively coupled plasma atomic emission spectroscopy of an adjacent piece showed the bulk composition to be Al72.2Pd19.1Mn8.7. A single grain Al sample oriented along the [111]-axis was prepared in the same manner. The sample dimensions were about 7 × 6 × 1.5 mm3. The experiments were carried out in an ultrahigh vacuum chamber with a base pressure of 8 × 10-11 Torr. A 5 × 5 mm2 sample area was cleaned by sputtering and annealing as described in detail elsewhere9. The upper K11.4.1
limits of the annealing temperature were 800 K for the Al-Pd-Mn sample and 750 K for the Al sample. In order to accommodate W/5%Re-W/26%Re thermocouples, holes were drilled into the side of the s
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