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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Photoemission in the second positive system of molecular nitrogen in the earth's dayglow

The results are presented of analyses of 3371-A profiles measured by the visible airglow experiment reported by Hayes et al. (1973). The measured emission rate profiles are presented as functions of the O and N2 densities determined simultaneously by means of an open source mass spectrometer. The experimental methods used are discussed along with the obtained results. The experimental data are compared with theoretical predictions. It is found that the 3371-A emission depends greatly on the O/N2 concentration ratio.

Kopp, J. P.↗
NASA NTRS1981

X-ray photoemission studies of superficially oxidized cesium antimonide photoemitters

Superficially oxidized cesium antimonide photoemitting surfaces prepared in ultrahigh vacuum were studied by X-ray photoelectron spectroscopy. Oxidation of Cs3Sb to produce a surface with enhanced photosensitivity converts part of the antimony to elemental antimony and part of the cesium to cesium suboxide. The latter is identified on the basis of an O1 s peak at 531.3 eV, characteristic of Cs11O3. The production of Cs2O is not ruled out in this process since its signature at 527.5 eV is masked by an antimony shake-up peak at 527 eV.

Bates, C. W., Jr.↗
NASA NTRS1984

Surface sensitive photoemission EXAFS: A new structural tool

A new technique for surface EXAFS has been developed. In this technique the absorption fine structure is observed by monitoring the photoelectrons emitted from core levels as a function of photon energy. The surface sensitivity is achieved by detecting elastic photoelectrons that escape to the vacuum from a few atomic layers near the surface. EXAFS from Mn 3p and F2s outer core levels in MnF2 were measured, the results were compared with transmission EXAFS from Mn ls core level and good agreement was found. This development is significant because: (1) almost all elements have suitable core levels of low binding energy; (2) since photopeaks from adjacent core levels do not cross as the photon energy is varied, the limitations imposed on other surface EXAFS techniques by this interference is removed. Auger electrons, which have fixed kinetic energies and so can cross photopeaks, will generally have low energies if they originate from outer core levels and be unlikely to appear in the EXAFS region of kinetic energies, i.e., in excess of about 50 eV; (3) VUV light is used, which greatly extends the usable range of photon energies; (4) more than one element in a solid may be studied with the same monochromator. The experiments were carried out on the VUV beam line U14A at NSLS.

Choudhary, K. M.↗
NASA NTRS1990

Role of photocurrent in low-temperature photoemission studies of Schottky-barrier formation

Photoelectron spectroscopy is frequently used to study band bending in semiconductors due to charge stored in surface or interface states. This paper examines how such experimental results are modified by photovoltages generated within the band-bending region not only by ambient light sources, but by the incident X-rays themselves. Recent experiments which have suggested dopant-dependent and reversible temperature-dependent band bending in the initial stages of formation of the metal-GaAs(110) interface are used as an example. It is shown that the reported dependence derives from a photovoltaic effect.

Hecht, M. H.↗
NASA NTRS1990

Oxidation of the Si(100) surface promoted by Sr overlayer - An X-ray photoemission study

The interaction of strontium films with the underlaying Si (100) surface and the Sr-promoted low-temperature oxidation of Si were investigated, using XPS, at three different Sr coverages (theta): theta = 0.55 monolayer (ML), theta = 1 ML, and theta = 1.85 ML. Oxygen adsorption was studied both at room temperature and at 500 C, and at oxygen exposures up to 2 x 10 to the 6th L (1 L = 10 to the -6th torr) and 2 x 10 to the 5th L, respectively. The XPS spectra of the Si2p, O1s, and Sr3d core levels were measured for the atomically clean Si, the Sr-covered Si, and for the Sr-covered Si after each oxygen exposure. Results indicate that Sr interacts with the Si(100) surface forming a strong ionic bond, and that Sr promotes the oxidation of the Si (100) surface.

Mesarwi, A.↗
NASA NTRS1995

Measurement of photoemission and secondary emission from laboratory dust grains

The overall goal of this project is experimentally determine the emission properties of dust grains in order to provide theorists and modelers with an accurate data base to use in codes that predict the charging of grains in various plasma environments encountered in the magnetospheres of the planets. In general these modelers use values which have been measured on planar, bulk samples of the materials in question. The large enhancements expected due to the small size of grains can have a dramatic impact upon the predictions and the ultimate utility of these predictions. The first experimental measurement of energy resolved profiles of the secondary electron emission coefficient, 6, of sub-micron diameter particles has been accomplished. Bismuth particles in the size range of .022 to .165 micrometers were generated in a moderate pressure vacuum oven (average size is a function of oven temperature and pressure) and introduced into a high vacuum chamber where they interacted with a high energy electron beam (0.4 to 20 keV). Large enhancements in emission were observed with a peak value, delta(sub max) = 4. 5 measured for the ensemble of particles with a mean size of .022 micrometers. This is in contrast to the published value, delta(sub max) = 1.2, for bulk bismuth. The observed profiles are in general agreement with recent theoretical predictions made by Chow et al. at UCSD.

Hazelton, Robert C.↗
NASA NTRS1995

Electronic Structure of HgBa2CaCu2O(6+delta) Epitaxial films measured by x-ray Photoemission

The electronic structure and chemical states of HgBa2CaCu20(sub 6 + delta), epitaxial films have been studied with x-ray photelectron spectroscopy. Signals from the superconducting phase dominate all the core-level spectra, and a clear Fermi edge is observed in the valence-band region. The Ba, Ca, Cu, and O core levels are similar to those of Tl2Ba2CaCu208(+)O(sub 6 + delta), but distinct differences are observed in the valence bands which are consistent with differences in the calculated densities of states.

Vasquez, R. P.↗
NASA NTRS1996

Electronic Structure of Tl2Ba2CuO(6+Delta) Epitaxial Films Measured by X-Ray Photoemission

The valence electronic structure and core levels of Tl2Ba2CuO(6 + delta) (Tl-2201) epitaxial films have been measured with X-ray photoelectron spectroscopy and are compared to those of Tl2Ba2CaCu2O(8 + delta) (Tl-2212). Changes in the Tl-2201 core-level binding energies with oxygen doping are consistent with a change in the chemical potential. Differences between the Tl-2201 and Tl-2212 measured densities of states are consistent with the calculated Cu 3d and Tl 6s partial densities of states.

Vasquez, R. P.↗
NASA NTRS1993

Photoresponse Model for Si_(1-x)Ge_x/Si Heterojunction Internal Photoemission Infrared Detector

A photoresponse model has been developed for the Si_(1-x)Ge_x/Si heterojunction internalphotoemission (HIP) infrared detector at wavelengths corresponding to photon energies less than theFermi energy. A Si_(0.7)Ge_(0.3)/Si HIP detector with a cutoff wavelength of 23 micrometers andan emission coefficient of 0.4 eV^(-1) has been demonstrated. The model agrees with the measureddetector response at lambda greater than 8 micrometers. The potential barrier determined by themodel is in close agreement (difference similar to 4 meV) with the potential barrier determined by theRichardson plot, compared to the discrepancies of 20-50 meV usually observed for PtSi Schottkydetectors.

Lin, T.↗