Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0907679

1. Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0907679 1. Background Free Measurement of the Spectra of Low Energy Electrons Emitted as a Result of Auger Transitions in Metals Positron annihilation induced Auger electron spectroscopy was used to obtain Cu and Au Auger spectra that are free of primary beam induced background by impinging the positrons at energy below the secondary electron emission threshold (Fig.1). The spectrum indicates that there is an intense low energy tail (LET) associated with the Auger peak that extends all the way to 0 eV. The LET has been interpreted as being due to processes in which the filling of the core by a valence electron results in the ejection of two or more valence electrons which share the energy of the otherwise Auger electron. Our results, the first of their kind, have important implications in quantitative analysis of Auger spectra. 2. Oxidation of the Cu(100) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES) PAES measurements (Fig. 2) show a large increase in the intensity of the positron annihilation induced Cu M2,3VV Auger peak as the oxidized Cu(100) surface is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ~550 °C. The large changes in the intensity have been theoretically modeled at high coverage (Fig. 3) by assuming a missing row structures on the oxidized Cu(100) surface that are characterized by a significant charge redistribution within the first two layers caused by occupation of subsurface sites by oxygen atoms. The Cu 3p core annihilation probability has been found to decrease from 6.990% for clean Cu(100) surface to 0.39% at these coverages, in agreement with the experiment. FIG. 1. First Auger spectra from Au and Cu completely free of primary beam induced secondary background obtained using a positron beam energy below the threshold for secondary electron emission. FIG. 2. Normalized intensities of the positron annihilation induced Cu M2,3VV Auger signals as a function of annealing temperature on the surface of the previously oxidized Cu(100) single crystal. Fig. 3. Positron wave function at the missing row reconstructed Cu(100) surface with 0.75 ML oxygen coverage.

2. Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0907679 This project has employed experiments utilizing a beam of low energy positrons in combination with first principles theoretical modeling of annihilation probabilities on nano-structured surfaces to study important questions in catalysis and photo-catalysis. This project has provided training at the forefront of research in surface and interface science for 5 Ph.D. and 2 undergraduate students. Public outreach activities have included laboratory tours for local high school students and lectures to minority high school students (through Upward Bound) and at-risk high school students. PIs have been directly involved in K-12 education through participation in the SCIE teacher training program, which is a joint effort between the College of Science at UTA and the UTA College of Education. The construction of positron beam system for surface analysis provided training for Ph.D. students S. Satyal (top right); P. Joglekar, S. Satyal (center left), S. Satyal, P. Joglekar (bottom left); . K. Shastry (bottom right) and U.Grad student L. Lim (center right);