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Investigation of the Quantrum Structure of Surfaces with Far UV Excitation Spectrosscopies

Description: Clean surfaces and those with adsorbates have been investigated to obtain electronic and atomic structure data. The various modes of synchrotron radiation (SR) photoemission spectroscopy (PES) have been performed mostly beam line 7.0.1 at the Advanced Light Source (ALS) in Berkeley, CA and some at the Synchrotron Radiation Center (SRC) in Madison, WI. These were done on silicon, carbon, and group III nitrides.
Date: October 17, 2008
Creator: Lapeyre, Gerlad
Partner: UNT Libraries Government Documents Department

Chemical structure of vanadium-based contact formation on n-AlN

Description: We have investigated the chemical interaction between a Au/V/Al/V layer structure and n-type AlN epilayers using soft x-ray photoemission, x-ray emission spectroscopy, and atomic force microscopy. To understand the complex processes involved in this multicomponent system, we have studied the interface before and after a rapid thermal annealing step. We find the formation of a number of chemical phases at the interface, including VN, metallic vanadium, aluminum oxide, and metallic gold. An interaction mechanism for metal contact formation on the entire n-(Al,Ga)N system is proposed.
Date: May 17, 2010
Creator: Pookpanratana, S.; France, R.; Blum, M.; Bell, A.; Bar, M.; Weinhardt, L. et al.
Partner: UNT Libraries Government Documents Department

Space charge effect and mirror charge effect in photoemission spectroscopy

Description: We report the observation and systematic investigation of the space charge effect and mirror charge effect in photoemission spectroscopy. When pulsed light is incident on a sample, the photoemitted electrons experience energy redistribution after escaping from the surface because of the Coulomb interaction between them (space charge effect) and between photoemitted electrons and the distribution of mirror charges in the sample (mirror charge effect). These combined Coulomb interaction effects give rise to an energy shift and a broadening which can be on the order of 10 meV for a typical third-generation synchrotron light source. This value is comparable to many fundamental physical parameters actively studied by photoemission spectroscopy and should be taken seriously in interpreting photoemission data and in designing next generation experiments.
Date: August 17, 2004
Creator: Zhou, X.J.; Wannberg, B.; Yang, W.L.; Brouet, V.; Sun, S.; Douglas, J.F. et al.
Partner: UNT Libraries Government Documents Department

Final report for grant: DE-FG02-93ER45481 [Polymers at engineered interfaces]

Description: Studies have been made of polymer interfaces, self-assembly of micelles at surfaces, phase separation in blends, diffusion and dewetting at and near interfaces, and nanomechanical properties of thin films. The main projects are summarized under the following topics: dislocations in lamellar diblock structures, effects of surface tension; compliance measurements and profiles of end-grafted polystyrene in solution observed by atomic force microscopy and neutron reflectivity; self-assembly of diblock polymer micelles from solution; dewetting dynamics; polymers on patterned surfaces; Flory-Huggins interaction parameter for polystyrene/poly-2-vinylpyridine (PS/P{sub 4}VP) blends; phase separation-induced patterns in dPS/PVME and dPEP/PEP blends; and high-resolution lateral imaging studies of phase separation and dewetting by scanning transmission X-ray microscopy (STXM) and photoemission electron microscopy (PEEM).
Date: January 17, 2000
Creator: Rafailovich, Miriam & Sokolov, Jonathan
Partner: UNT Libraries Government Documents Department

Mechanism of the optogalvanic effect in a hollow-cathode discharge

Description: There are two significantly different mechanisms proposed for the origin of the optogalvanic effect in a hollow-cathode discharge: (1) laser excitation of atoms to higher electronic states leads to an increased cross section for electron impact ionization, with the result that the excited atom becomes ionized and the conductivity of the discharge increases; and (2) laser excitation of atoms to higher electronic states perturbs the equilibrium established between the electron temperature and the atomic excitation temperature. Superelastic collisions between the electrons and the laser-excited atoms restore the equilibrium, with the excess energy ending up in an increased electron temperature and therefore an increased conductivity of the discharge. Both mechanisms undoubtedly proceed simultaneously and what needs to be determined is their relative importance at different discharge conditions and different excitation conditions. This is important because laser isotope enrichment schemes have been proposed using selective excitation in a hollow-cathode discharge. In order for these schemes to work, (1) must be the predominant mechanism. We have measured the optogalvanic signal, concentration of uranium atoms, impedance of the discharge, and electron temperature as a function of the discharge current in a neon-filled uranium hollow-cathode discharge. The hollow cathode operating characteristics are used as input parameters in a simple discharge model. Predictions of electron density, changes in electron temperature, and discharge impedance compare well with experimental observations. Our model and experimental observations yield a qualitative understanding of the optogalvanic effect in a hollow-cathode discharge and estimate the relative importance of the two optogalvanic mechanisms.
Date: June 17, 1983
Creator: Keller, R.A.; Warner, B.E.; Zalewski, E.F.; Dyer, P.; Engleman, R. Jr. & Palmer, B.A.
Partner: UNT Libraries Government Documents Department