The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond ...time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on different time scales, from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for a long time after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride.
At fluences well below the threshold for plasma formation, we have characterized the direct desorption of atomic ions from fused silica surfaces during 157
nm irradiation by time-resolved mass ...spectroscopy. The principal ions are Si
+ and O
+. The emission intensities are dramatically increased by treatments that increase the density of surface defects. Molecular dynamics simulations of the silica surface suggest that silicon ions bound at surface oxygen vacancies (analogous to E′ centers) provide suitable configurations for the emission of Si
+. We propose that emission is best understood in terms of a hybrid mechanism involving both antibonding chemical forces (Menzel–Gomer–Redhead model) and repulsive electrostatic forces on the adsorbed ion after laser excitation of the underlying defect.
We describe the response of poly(methyl methacrylate) surfaces to localized mechanical stimulation by the tip of an atomic force microscope (AFM) in water, methanol, ethanol, and aqueous alcohol ...solutions. Simply pressing the AFM tip into the surface with no horizontal motion fails to produce visible features in subsequent low contact force images. A single small-area (40 × 40 nm2) high contact force scan has little effect in air but in water or in alcohol−water mixtures produces soft bumps (local volume increase) adjacent to the scanned area. These bumps typically have lateral dimensions of ∼100 nm and rise tens of nanometers above the surrounding surface. Larger, micron-scale scans produce approximately parallel, raised ridges 50−150 nm apart. These structures are stable over time periods of hours or more in air and in solvent. We present evidence that these modifications are due primarily to stress-enhanced solvent uptake in material surrounding the area of tip−polymer contact.
We report measurements of photostimulated electron emission (PSE) from single-crystalline aluminum (99.995%) and high-purity polycrystalline aluminum (>99.9%) during uniaxial tensile deformation. ...Photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and slip band formation. In the single crystalline material, the PSE intensity increases linearly with strain. In the polycrystalline material, the PSE intensity increases exponentially with strain. In both materials, time-resolved PSE measurements show step-like increases in intensity consistent with the heterogeneous nucleation and growth of slip bands during tensile deformation. In this sense, we have “observed” dislocation motion by this technique. Slip bands on the surfaces of deformed samples were subsequently imaged by atomic-force microscopy (AFM). Photoelectron measurements can provide reliable, quantitative information for dislocation dynamics.
The PROSPECT reactor antineutrino experiment Ashenfelter, J.; Balantekin, A.B.; Baldenegro, C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2019, Volume:
922
Journal Article
Peer reviewed
Open access
The Precision Reactor Oscillation and Spectrum Experiment, PROSPECT, is designed to make both a precise measurement of the antineutrino spectrum from a highly-enriched uranium reactor and to probe ...eV-scale sterile neutrinos by searching for neutrino oscillations over meter-long baselines. PROSPECT utilizes a segmented6Li-doped liquid scintillator detector for both efficient detection of reactor antineutrinos through the inverse beta decay reaction and excellent background discrimination. PROSPECT is a movable 4-ton antineutrino detector covering distances of 7m to 13m from the High Flux Isotope Reactor core. It will probe the best-fit point of the ν̄e disappearance experiments at 4σ in 1 year and the favored regions of the sterile neutrino parameter space at more than 3σ in 3 years. PROSPECT will test the origin of spectral deviations observed in recent θ13 experiments, search for sterile neutrinos, and address the hypothesis of sterile neutrinos as an explanation of the reactor anomaly. This paper describes the design, construction, and commissioning of PROSPECT and reports first data characterizing the performance of the PROSPECT antineutrino detector.
We report observations of localized growth on the (101̄4) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact ...forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.
Virtual environments (VEs) are becoming popular as media for training, modelling and entertainment. Little is known, however, about the factors that affect efficient and rapid acquisition of ...knowledge using this technology. Five experiments examined the influence of gender, passive/active navigation, cognitive style, hemispheric activation measured by electroencephalography and display information on the acquisition of two types of navigational knowledge using a VE: route and survey knowledge. Males acquired route knowledge from landmarks faster than females. In situations where survey knowledge must be used, proficiency in visual-spatial cognition is associated with better performance. The right cerebral hemisphere appears to be more activated than the left during navigational learning in a VE. In identifying cognitive factors that influence VE navigation, these results have a number of implications in the use of VEs for training purposes and may assist in linking processes involved in navigation to a more general framework of visual-spatial processing and mental imagery.
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