33 research outputs found
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Fundamental sputtering studies: Nonresonant ionization of sputtered neutrals
Because of the practical importance of sputtering, numerous theories and computer simulations are used for predicting many aspects of the sputtering process. Unfortunately, many of the calculated sputtering results are untested by experiment. Until recently, most sputtering experiments required either very high ion fluences or the detection of only minor constituents of the sputtered flux, i.e., ions. These techniques may miss the subtleties involved in the sputtering process. High-detection-efficiency mass spectrometry, coupled with the laser ionization of neutral atoms, allows the detection of the major sputtered species with very low incident ion fluences. The depth-of-origin of sputtered atoms is one example of an important but poorly understood aspect of the sputtering process. By following the sputtering yield of a substrate atom with various coverages of an adsorbed overlayer, the depth of origin of sputtered atoms has been determined. Our results indicate that two-thirds of the sputtered flux originates in the topmost atomic layer. The ion-dose dependence of sputtering yields has long been assumed to be quite minor for low- to-moderate primary ion fluences. We have observed a two-fold decrease in the sputtering yield of the Ru(0001) surface for very low primary ion fluences. Data analysis results in a cross section for damage of 2.7 {plus minus} 1.0 {times} 10{sup {minus}15}cm{sup 2}. 40 refs., 3 figs., 2 tabs
Abundances of the elements in the solar system
A review of the abundances and condensation temperatures of the elements and
their nuclides in the solar nebula and in chondritic meteorites. Abundances of
the elements in some neighboring stars are also discussed.Comment: 42 pages, 11 tables, 8 figures, chapter, In Landolt- B\"ornstein, New
Series, Vol. VI/4B, Chap. 4.4, J.E. Tr\"umper (ed.), Berlin, Heidelberg, New
York: Springer-Verlag, p. 560-63
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Waste volume reduction using surface characterization and decontamination by laser ablation. 1998 annual progress report
'Much of the contaminated concrete from nuclear facilities contains radionuclides only in the near surface region. Removal of the contaminated layer would greatly reduce the volume of waste requiring storage. The objectives of this research are to understand the depth-dependent concentration and chemistry of radionuclide-contaminated concrete surfaces, to determine the mechanism and efficacy of laser ablation in removing contaminated surface layers, and to chemically and physically characterize the captured ablation effluent which would become the stored waste. This report summarizes work from the first eight months of a three-year project. Samples consisted of specimens of non-contaminated high density concrete from the Experimental Boiling Water Reactor at Argonne National Laboratory (sectioned into small coupons) and samples cast in the laboratory using Type 1 Portland cement and either silica, alumina, or sand. Some samples of concrete and cement were doped with Cs-133. Ablation experiments were done with a 1.6 kW pulsed Nd:YAG laser. The beam was delivered via a fiber optic cable, focused to a 0.5 \265m spot, and rastered across the surface. The ablated material was collected with a vacuum shroud system and captured on a 0.2 micron filter. Virgin and ablated surfaces were chemically analyzed with laser desorption surface mass spectrometry.
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Femtosecond laser postionization of sputtered and laser desorbed atoms
This paper examines the photoionization efficiency of a femtosecond laser ionization source on several atomic species. Use of femtosecond laser ionization pulses to photoionize the desorbing flux from a sample surface is examined. Example of mass spectra produced is given using 248 nm pulse on sputtered Au. Al and Mo have widely different ionization potentials and show that 248 nm pulses are more efficient at photoionization than longer wavelengths; this is enhanced for Mo with high ionization potential. It is concluded that efficient photoionization occurs for atoms with ionization potentials low enough for two-photon ionization to occur
Modulation of the Primary Electron Transfer Rate in Photosynthetic Reaction Centers by Reduction of a Secondary Acceptor
Velocity distribution of sputtered Zr atoms as determined by laser induced fluorescence spectroscopy
A second harmonic generation study of the iron electrode surface using a picosecond laser
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Velocity and electronic state distributions of sputtered Fe atoms by laser-induced fluorescence spectroscopy
Velocity distributions and relative populations in the fine-structure levels of the a/sup 5/D/sub J/ ground state of Fe atoms, produced by sputtering with 3 keV argon ions, have been investigated by Doppler shifted laser induced fluorescence. The laser system employs a single-mode, scanning ring dye laser, amplified by a sequence of three excimer-pumped flowing-dye cells. Frequency doubling in a KD*P crystal was used to produce high energy (> .5 mJ) pulses of narrowband tunable UV output near 300 nm. Laser power influence on effective velocity bandwidth was investigated. Favorable light-collection geometry minimized distortion of the velocity spectra from apparatus-averaging effects. In impurity flux diagnostic applications in fusion devices, substantial spatial averaging may occur. In the latter case, the narrow velocity bandwidth (70 m/s, transform limit) of the present laser system is particularly useful
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Second harmonic generation and sum frequency generation
Second harmonic generation and sum frequency generation are increasingly being used as in situ surface probes. These techniques are coherent and inherently surface sensitive by the nature of the mediums response to intense laser light. Here we will review these two techniques using aqueous corrosion as an example problem. Aqueous corrosion of technologically important materials such as Fe, Ni and Cr proceeds from a reduced metal surface with layer by layer growth of oxide films mitigated by compositional changes in the chemical makeup of the growing film. Passivation of the metal surface is achieved after growth of only a few tens of atomic layers of metal oxide. Surface Second Harmonic Generation and a related nonlinear laser technique, Sum Frequency Generation have demonstrated an ability to probe the surface composition of growing films even in the presence of aqueous solutions. 96 refs., 4 figs