17 research outputs found
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Processing and evaluation of the AGS Booster ultra-high vaccum system
The AGS Booster is a synchrotron for the acceleration of both protons and heavy ions. To minimize the beam loss due to charge exchange of the partially stripped, low {Beta} very heavy ions with the residual gas molecules, pressure of low 10{sup {minus}11} Torr is required for the 200 m booster ring. To achieve this ultra high vacuum, chemical cleaning, vacuum furnace degassing and insitu bake were employed for all chambers and beam components. Using these procedures, vacuums of low 10{sup {minus}11} Torr have been routinely achieved during the testing of individual half cells and beam components, and during the commissioning of the vacuum sectors. In this paper, the design and layout of chambers, flanges and bakeout hardware is briefly described. The vacuum processing of different components and the results of bakeout and evaluation are summarized. The experience gained during the construction and commissioning of this ultra-high vacuum system is also given. 3 refs., 3 figs., 1 tab
(Ga<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>)(N<sub>1–<i>x</i></sub>O<sub><i>x</i></sub>) Nanocrystals: Visible Absorbers with Tunable Composition and Absorption Spectra
Bulk oxyÂ(nitride) (Ga<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>)Â(N<sub>1–<i>x</i></sub>O<sub><i>x</i></sub>) is a promising photocatalyst
for
water splitting under visible illumination. To realize its solar harvesting
potential, it is desirable to minimize its band gap through synthetic
control of the value of <i>x</i>. Furthermore, improved
photochemical quantum yields may be achievable with nanocrystalline
forms of this material. We report the synthesis, structural, and optical
characterization of nanocrystals of (Ga<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>)Â(N<sub>1–<i>x</i></sub>O<sub><i>x</i></sub>) with the values of <i>x</i> tunable from 0.30 to 0.87. Band gaps decreased from 2.7
to 2.2 eV over this composition range, which corresponded to a 260%
increase in the fraction of solar photons that could be absorbed by
the material. We achieved nanoscale morphology and compositional control
by employing mixtures of ZnGa<sub>2</sub>O<sub>4</sub> and ZnO nanocrystals
as synthetic precursors that could be converted to (Ga<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>)Â(N<sub>1–<i>x</i></sub>O<sub><i>x</i></sub>) under NH<sub>3</sub>. The high quality of the resulting nanocrystals is encouraging for
achieving photochemical water-splitting rates that are competitive
with internal carrier recombination pathways
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Solvents effects on charge transfer from quantum dots.
To predict and understand the performance of nanodevices in different environments, the influence of the solvent must be explicitly understood. In this Communication, this important but largely unexplored question is addressed through a comparison of quantum dot charge transfer processes occurring in both liquid phase and in vacuum. By comparing solution phase transient absorption spectroscopy and gas-phase photoelectron spectroscopy, we show that hexane, a common nonpolar solvent for quantum dots, has negligible influence on charge transfer dynamics. Our experimental results, supported by insights from theory, indicate that the reorganization energy of nonpolar solvents plays a minimal role in the energy landscape of charge transfer in quantum dot devices. Thus, this study demonstrates that measurements conducted in nonpolar solvents can indeed provide insight into nanodevice performance in a wide variety of environments
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A 0. 5 to 3. 0 MeV monoenergetic positron beam
An adjustable, 0.5--3 MeV monoenergetic positron beam has been constructed at Brookhaven. Currently a /sup 22/Na source with a W(100) foil transmission moderator produces a 1.1 mm FWHN beam with an intensity of 3/times/10/sup 5/ e/sup +//sec at a target located downstream from the accelerator. The divergence of the beam is less than 0.1/degree/ at 2.2 MeV energy. A SOA gun with 2 lens transport system brings the beam to a focus at the entrance of an electrostatic 3 MeV Dynamitron accelerator. The post acceleration beam transport system comprises 3 focusing solenolds, 4 sets of steering magnets and a 90/degree/ double focusing bending magnet. The beam energy spread at the target is <1 keV FWHN deduced from the beam size. Below we describe the positron extraction optics and acceleration, the construction of the beamline and the beam diagnostic devices. The salient beam parameters are listed at the end of this paper. 2 refs., 3 figs., 1 tab
Solvents Effects on Charge Transfer from Quantum Dots
To predict and understand the performance
of nanodevices in different environments, the influence of the solvent
must be explicitly understood. In this Communication, this important
but largely unexplored question is addressed through a comparison
of quantum dot charge transfer processes occurring in both liquid
phase and in vacuum. By comparing solution phase transient absorption
spectroscopy and gas-phase photoelectron spectroscopy, we show that
hexane, a common nonpolar solvent for quantum dots, has negligible
influence on charge transfer dynamics. Our experimental results, supported
by insights from theory, indicate that the reorganization energy of
nonpolar solvents plays a minimal role in the energy landscape of
charge transfer in quantum dot devices. Thus, this study demonstrates
that measurements conducted in nonpolar solvents can indeed provide
insight into nanodevice performance in a wide variety of environments
Mapping Nanoscale Absorption of Femtosecond Laser Pulses Using Plasma Explosion Imaging
We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO<sub>2</sub> aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma