Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0906679

1. Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0906679 Experiments utilizing a beam of low energy positrons were combined with first principles theoretical modeling of annihilation probabilities to study important questions in catalysis and photo-catalysis including charge transfer at surface nano-structures and the nature of active sites in catalytic and photo-catalytic systems. Recent Discovery: New mechanism for sticking of charged particles to surfaces Analysis of annihilation induced Auger measurements made with very low energy positrons beams has pointed to a new, highly efficient, mechanism, Auger mediated sticking (AMS) in which positrons transition directly into surface localized states through a one step quantum sticking mechanism Our measurements demonstrate that the high efficiency of the AMS mechanism makes it possible to use sub eV energy positron beams to selectively probe the surfaces of fragile systems such as nano particles and bio-materials. Schematic of AMS process. (a) A low energy positron is directly trapped into a surface state. (b) Band diagram of AMS process- the energy released is coupled to a valence electron in an Auger like process. http://dx.doi.org/10.1103/PhysRevLett.104.247403

2. Positron Spectroscopy of Surfaces Alex H. Weiss, University of Texas at Arlington, DMR 0906679 This project has provided training at the forefront of research in surface and interface science for 4 Ph.D. students, 3 undergraduate students and 3 11th-12th high school students. Outreach Activities: Public outreach activities have included laboratory tours for local high school students as part of an ongoing partnership with the AP Physics program at Irving High School a Title I campus and lectures to minority high school students through the UTA Upward Bound program. Martin High School students Austin Baty, Stephanie Diong, and Andrew Brown, participated in research on this project through a collaboration between the PI’s and AP Physics teacher J. Atman. The design and construction of advanced positron beam system for surface analysis provided training opportunities Top: L. Lim, U-grad, Center left, Ph.D. students P. Joglekar, S. Satyal Ph.D. students, Center right: M. Nadesalingam, Ph.D., Bottom left: S. Satyal, Bottom right: K. Shastry (at the NSLS).