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Weiss, A. H.

Publications and source records attributed to Weiss, A. H..

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Identification of Auger mechanisms responsible for low energy electron emission from graphene on copper using Auger-gamma coincidence spectroscopy

We have applied positron annihilation induced Auger-gamma coincidence spectroscopy to identify important mechanisms responsible for the emission of low energy electrons following the sudden creation of holes in bilayer graphene on copper substrate. The novel surface spectroscopic method measures the energy of the Doppler shifted annihilation gamma photon in coincidence with the Auger electron emitted following the relaxation of the hole created by the annihilation of a surface electron with a surface trapped positron. By extracting and theoretically modelling the annihilation gamma spectra coincident with low energy electrons, we associate majority of the intensity in the low energy (7 eV–25 eV) region of the Auger spectrum to electron emission following Auger decays of 2s holes in adsorbed oxygen and deep valence holes in graphene. We provide additional support to this conclusion by showing that most of the Auger electrons with energy less than ~ 25 eV are coincident with gamma photons with small Doppler shift (511 keV–512 keV) indicating that the primary electron whose annihilation resulted in low energy Auger electron emission had small momentum parallel to the gamma emission direction. On the other hand, majority of the Auger electrons measured in coincidence with the photons having maximum Doppler shift (515 keV–521 keV) were emitted following the decay of core holes (C 1s and O 1s). Furthermore, by selecting annihilation gamma photons coincident only with the O KVV (from surface adsorbed oxygen), C KVV (graphene), and Cu MVV (copper substrate) Auger electrons, we have also experimentally derived the energy spectrum of Doppler shifted gamma photons resulting from the annihilation of positrons with 1s electrons of O, 1s electrons of C and 3p electrons of Cu respectively. Model Doppler broadened gamma spectra produced using ab-initio calculations agree well with the experimentally derived line shapes. This demonstrates the ability of Auger-gamma coincidence method to experimentally resolve the Doppler broadened annihilation gamma spectrum from surfaces into its veiled electronic level constituents which has, heretofore, relied solely on theoretical analysis. Our results also demonstrate that the line shape of the Doppler broadened annihilation gamma peak reflect the chemical composition of the topmost atomic layer and can thus be used to characterize both external and inaccessible surfaces of porous materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Measurement of the full electron spectrum associated with the Ag N 3 VV Auger transitions: Evidence for the contribution of multi-electron Auger processes

In this paper, we report the first measurement of the full energy spectrum (down to 0 eV) of electrons emitted from the surface of Ag(100) as a result of the N 3 VV Auger transition. A comparison with a Monte-Carlo model was used to obtain an estimate of the contribution of multi-electron Auger processes to the low energy part of the Auger spectrum. The experimental Auger spectrum was obtained in time coincidence with photo-emitted 4p 3/2 core electrons from Ag using Auger-Photoelectron coincidence spectroscopy (APECS). The coincidence condition in APECS can remove most, but not all, spectral contributions from events unrelated to the Auger transition. The remaining contributions to the APECS spectrum from Auger-unrelated events, which are significant, were determined from a series of coincidence measurements of inelastically scattered valence band photoelectrons. The contribution from Auger-unrelated events was subtracted from the APECS Auger spectrum of Ag(100) to obtain the full Auger spectrum down to 0 eV. The resulting spectrum includes, in addition to Auger electrons emitted without energy loss, inelastically scattered Auger electrons. We observe that the spectral weight of the low energy tail (LET) in the APECS spectrum is reduced by more than a factor of two after subtracting the Auger-unrelated contributions. However, the intensity of the LET after this subtraction is still two times the integrated intensity in the Auger peak. A Monte Carlo model that includes the inelastic transport of the N 3 VV Auger electrons through the surface showed that LET produced by the inelastic scattering of the Auger electrons has the same intensity as the integrated intensity in the Auger peak. The excess intensity in the experimental LET provides strong evidence that the Auger related spectrum also includes contribution from multiple electron emissions that are intrinsic to the creation and/or the Auger decay of the core hole. In conclusion, our estimate shows that as low as 20% to as high as 60% of the core hole decay events result in the emission of more than one Auger electron from Ag (100). This result demonstrates the need for a better understanding of the contribution of valence-electron correlations to the entire CVV Auger spectrum down to 0 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Lastly, our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy

We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flight spectrometer. The spectrometer’s new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted from the sample surface at kinetic energies ranging from ∼500 eV to 0 eV. The spectrometer’s unique characteristics made it possible to perform (a) first experiments demonstrating the direct transition of a positron from an unbound scattering state to a bound surface state and (b) the first experiments demonstrating that Auger electron spectra can be obtained down to 0 eV without the beam induced secondary electron background obscuring the low energy part of the spectra. Data are presented which show alternative means of estimating positron surface state binding energy and background-free Auger spectra.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1988

High variable mixture ratio oxygen/hydrogen engine

The ability of an O2/H2 engine to operate over a range of high-propellant mixture ratios was previously shown to be advantageous in single stage to orbit (SSTO) vehicles. The results are presented for the analysis of high-performance engine power cycles operating over propellant mixture ratio ranges of 12 to 6 and 9 to 6. A requirement to throttle up to 60 percent of nominal thrust was superimposed as a typical throttle range to limit vehicle acceleration as propellant is expended. The object of the analysis was to determine areas of concern relative to component and engine operability or potential hazards resulting from the operating requirements and ranges of conditions that derive from the overall engine requirements. The SSTO mission necessitates a high-performance, lightweight engine. Therefore, staged combustion power cycles employing either dual fuel-rich preburners or dual mixed (fuel-rich and oxygen-rich) preburners were examined. Engine mass flow and power balances were made and major component operating ranges were defined. Component size and arrangement were determined through engine layouts for one of the configurations evaluated. Each component is being examined to determine if there are areas of concern with respect to component efficiency, operability, reliability, or hazard. The effects of reducing the maximum chamber pressure were investigated for one of the cycles.

Erickson, C. M.↗
NASA NTRS1975

Edible carbohydrates from formaldehyde in a spacecraft

The autocatalytic nature of the base catalyzed condensation of formaldehyde to formose sugars is eliminated by using as a cocatalyst, an aldose, or ketose having an alpha-hydrogen. This is more strongly complexed by base than is formaldehyde and the cocatalyst and sugar products accumulate as catalyst complexes instead of formaldehyde. Because of the presence of alpha-hydrogen atoms in cocatalysts and formose sugars, their removal by cross Cannizzaro reaction of complexed sugars does not occur, so the formose reaction behaves autocatalytically due to this accumulation. It is believed that a given catalytic formose complex is not a discrete complexed sugar, but rather, a scrambled dynamic mixture of sugars having weakened structures. The sugar complexes derive from a common salt-like formaldehyde complex, which, because of the absence of alpha-hydrogen, has a greater tendency to undergo Cannizzaro reaction, rather than formose condensation. Because of this, the Cannizzaro reaction can proceed without measurable formose condensation. The reverse is not possible.

Weiss, A. H.↗
NASA NTRS1974

Production of edible carbohydrates from formaldehyde in a spacecraft. pH variations in the calcium hydroxide catalyzed formose reaction

The study of the calcium hydroxide catalyzed condensation of formaldehyde was extended to a batch reactor system. Decreases in pH were observed, often in the acid regime, when using this basic catalyst. This observation was shown to be similar to results obtained by others using less basic catalysts in the batch mode. The relative rates of these reactions are different in a batch reactor than in a continuous stirred tank reactor. This difference in relative rates is due to the fact that at any degree of advancement in the batch system, the products have a history of previous products, pH, and dissolved catalyst. The relative rate differences can be expected to yield a different nature of product sugars for the two types of reactors.

Weiss, A. H.↗
NASA NTRS1973

Fabrication of a glycerol from CO2 reaction system, supplement

The fabrication, installation, and testing of a glycerol hydrogenation and a CO2 hydrogenation - CH4 partial oxidation units are reported. The glycerol system proved to be operational while the CO2 system was installed but not bought on operational steam.

Weiss, A. H.↗
NASA NTRS1973

An interfaced system for production of methane in a spacecraft

The formose reaction, the homogeneously catalyzed condensation of formaldehyde to sugars, proceeds simultaneously with Cannizzaro and crossed Cannizzaro reactions. Reaction studies in a continuous stirred tank reactor have shown that rate instabilities are exhibited. There are temperature instabilities as well as concentration instabilities in calcium hydroxide catalyst, formaldehyde reactant, and hydroxyl ion. It is postulated that Ca(OH)+ is the actual catalytic species for the formose system. A unifying mechanism is developed that postulates that reactions proceed from a common intermediate complexed species, and that the selectivity for each reaction depends on the nature of the catalyst forming the carbohydrate complex. The catalytic mechanism explains the Lobry de Bruyn-van Eckenstein aldose ketose rearrangements and mutarotations of sugars that also proceed in the system.

Weiss, A. H.↗
NASA NTRS1972

Study of the techniques feasible for food synthesis aboard a spacecraft

Synthesis of sugars by Ca(OH)2 catalyzed formaldehyde condensation (the formose reaction) has produced branched carbohydrates that do not occur in nature. The kinetics and mechanisms of the homogeneously catalyzed autocatalytic condensation were studied and analogies between homogeneous and heterogeneous rate laws have been found. Aldol condensations proceed simultaneously with Cannizzaro and crossed-Cannizzaro reactions and Lobry de Bruyn-Van Eckenstein rearrangements. The separate steps as well as the interactions of this highly complex reaction system were elucidated. The system exhibits instabilities, competitive catalytic, mass action, and equilibrium phenomena, complexing, and parallel and consecutive reactions. Specific finding that have been made on the problem will be of interest for synthesizing sugars, both for sustained space flight and for large scale food manufacture. A contribution to methodology for studying complex catalyzed reactions and to understanding control of reaction selectivity was a broad goal of the project.

Weiss, A. H.↗