Browsing by Author "Li, Hui"
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Item CONTROL AND TRANSPORT OF INTENSE ELECTRON BEAMS(2004-08-06) Li, Hui; O'Shea, Patrick G; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The transport of intense beams for advanced accelerator applications with high-intensity beams such as heavy-ion inertial fusion, spallation neutron sources, and intense light sources requires tight control of beam characteristics over long distances. The University of Maryland Electron Ring (UMER), which uses low energy, high current electron beams to model the transport physics of intense space-charge-dominated beams, employs real-time beam characterization and control in order to optimize beam quality throughout the strong focusing lattice. We describe in this dissertation the main beam control techniques used in UMER, which include optimal beam steering by quadrupole scans, beam rotation correction using a skew corrector, rms envelope matching and optimization, empirical envelope matching, beam injection, and phase space reconstruction using a tomographic method. Using these control techniques, we achieved the design goals for UMER. The procedure is not only indispensable for optimum beam transport over long distances, but also provides important insights into the beam physics involved.Item N-HETEROAROMATIC ACID ADLAYER STRUCTURES UNDER HYDROGEN BONDING INFLUENCE:(2007-02-20) Li, Hui; Reutt-Robey, Janice; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, I investigate the adsorption and growth of thin films of N-heteroaromatic acids and related molecules on Ag(111) surface. The N-heteroaromatic acids have ring-nitrogen and carboxylic groups. Hence, two primary hydrogen bond motifs are observed in their bulk crystal structures: head-to-tail hydrogen bond and tail-to-tail hydrogen bond. Furthermore, the high electron density of the aromatic rings increases the complexity of their electrostatic properties. The surface structure thus reflects the delicate balance between the intermolecular interactions and adsorbate-substrate interactions. The surface adsorption model of isonicotinic acid (INA) is suggested based upon the STM and XPS study. The monolayer structure is stabilized by the head-to-tail hydrogen bonds along direction and weak side link hydrogen bonds. 12 rotational/reflection domains c0-exist on the surface. Using reflection absorption infrared spectroscopy (RAIRS), the layer-dependent orientation of the isonicotinic acid is determined. In the first monolayer, INA molecules are qseudo-parallel to the metal surface. The second and third layers possesses a combined twist ( ~ 20°) and tilt ( ~ 10°) orientation. This incline angle is contributed to the slight distortion ( 0.2%) along side link hydrogen bond direction upon the registration to the substrate. With two additional aromatic rings, 9-acridine carboxylic acid (ACA) possesses a greater quadrupole. The surface structure is thus affected. The main interaction is still head-to-tail hydrogen bonding. However, electrostatic interactions are the main interactions to connect the head-to-tail chains. RAIR spectra reveal that at low coverage, the mobile species are head-to-tail linked. A proposed orientation of the monolayer films are given based upon the characteristic vibrational frequencies. The vibrational spectra of Anthroic acid (AA) is also done for the comparison reason. The orientation difference between the ACA monolayer and the AA monolayer are contribute to their electronic properties.