Low energy ion scattering spectroscopy studies of metallic and bimetallic surfaces.

摘要

This dissertation describes the results of several investigations using low energy ion scattering spectroscopy (LEISS) of metallic and bimetallic surfaces. In the first part, LEISS was used to measure the desorption cross sections for one monolayer chemisorbed carbon on a Mo(100) surface induced by sputtering with noble gas ions (Ne+, Ar+, Xe+) at different ion energies, incident angles, and substrate temperatures under ultrahigh vacuum (UHV) conditions. The carbon desorption cross section increases with increasing mass and energy of the impinging ions. There is a maximum in the desorption cross section at an incident ion angle of 30° from the surface plane and at a substrate temperature of 300° C. In particular, the carbon desorption cross sections for carbon adatoms on Mo(100) by Xe+ ion sputtering are on the order of 10--15 cm2. This is two orders of magnitude higher than that required to artificially reduce the Mo erosion rate in ion-engine lifetime measurements in ground tests, and so these tests can be used with confidence to predict ion-engine wear in space.;In the second part, a form of LEISS called low-energy alkali ion scattering spectroscopy (ALISS) was used to determine atomic surface structures of Ge/Pt(111), Zn/Pt(111), and Cu/Pt(111) bimetallic systems. These surfaces were also investigated by using X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). An ordered, two-domain (√19x√19)R23.4°-Ge/Pt(111) surface alloy was created following Ge deposition on Pt(111). In the Zn/Pt(111) and Cu/Pt(111) bimetallic alloys, no long-range, ordered surface structures were observed. The Ge, Zn, and Cu atoms alloy with Pt(111) to form single layer, bilayer, and triplelayer alloys, respectively. All three added metallic atoms are substitutionally incorporated with the topmost layer of Pt atoms and are located very close to the same positions as those of the replaced Pt atoms without significant vertical buckling. Temperature programmed desorption (TPD) measurements reveal both that CO and NO bond more weakly on these three alloyed surfaces compared to that on the clean Pt(111) surface.