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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Characterization of multilayer GaAs/AlGaAs transistor structures by variable angle spectroscopic ellipsometry

Variable angle of incidence spectroscopic ellipsometry (VASE) has been implemented as a means of determining layer thickness, alloy composition, and growth quality of GaAs/AlGaAs samples composed of relatively thick layers as well as superlattices. The structures studied in this work contained GaAs/AlGaAs multilayers with a superlattice 'barrier' and were grown for later formation of modulation-doped field effect transistors (MODFETs). Sample modeling was performed by treating the superlattice as a bulk AlGaAs layer of unknown composition. Extremely good data fits were realized when five layer thicknesses and two alloy ratios were allowed to vary in a regression analysis. Room temperature excitonic effects associated with the e-hh(1), e-lh(1) and e-hh(2) transitions were observed in the VASE data.

Merkel, Kenneth G.↗
NASA NTRS1990

Variable magnification variable dispersion glancing incidence imaging x ray spectroscopic telescope

A variable magnification variable dispersion glancing incidence x ray spectroscopic telescope capable of multiple high spatial revolution imaging at precise spectral lines of solar and stellar x ray and extreme ultraviolet radiation sources includes a primary optical system which focuses the incoming radiation to a primary focus. Two or more rotatable carriers each providing a different magnification are positioned behind the primary focus at an inclination to the optical axis, each carrier carrying a series of ellipsoidal diffraction grating mirrors each having a concave surface on which the gratings are ruled and coated with a multilayer coating to reflect by diffraction a different desired wavelength. The diffraction grating mirrors of both carriers are segments of ellipsoids having a common first focus coincident with the primary focus. A contoured detector such as an x ray sensitive photographic film is positioned at the second respective focus of each diffraction grating so that each grating may reflect the image at the first focus to the detector at the second focus. The carriers are selectively rotated to position a selected mirror for receiving radiation from the primary optical system, and at least the first carrier may be withdrawn from the path of the radiation to permit a selected grating on the second carrier to receive radiation.

Hoover, Richard↗
NASA NTRS1988

Spectroscopic observations of X-ray selected late type stars

A spectroscopic survey of nine X-ray selected late type stars was conducted. These stars are serendipitously discovered EINSTEIN X-ray sources, selected from two large x-ray surveys: the Columbia Astrophysical Laboratory survey (five stars) and the CFA Medium Sensitivity survey (four stars). Four of the Columbia survey stars were found to be short period binaries. The fifth was found to be an active single G dwarf. None of the Medium Sensitivity survey stars were found to be either binaries or active stars. Activity was measured by comparing the H-alpha and the CaII infrared triplet (8498, 8542) lines in these stars to the lines in inactive stars of similar spectral type. A correlation was found between the excess H-alpha lime emission and V sin(i) and between the excess H-alpha line emission and X-ray luminosity. No correlation was found between the infrared line emission and any other measured quantity.

Takalo, L. O.↗
NASA NTRS1990

Photometric and spectroscopic modeling of starspots on the RS Canum Venaticorum binary HD 26337

Results are presented on photometric and spectroscopic observations made in 1987-1988 on the rapidly rotating G5 subgiant in the single-lined RS CVn binary HD 26337. Results show the existence of a cool polar spot on HD 26337, which is rather long lived, persisting at least the nine years of the photometry observations. The polar spot contributes only insignificantly to the light curve amplitude, but is crucial for the overall brightness of the system. Evidence is also found for extremely rapid light curve shape variations on a time scale of only one or two rotation cycles. These changes could be modeled by rapidly decreasing a spot area.

Strassmeier, Klaus G.↗
NASA NTRS1989

Particle background effects for Hubble Space Telescope (HST) and the Lyman Far Ultraviolet Spectroscopic Explorer

Requirements on orbital background counting rates are described for the UV and visible detectors on the planned second generation instrument Space Telescope Imaging Spectrograph (STIS) and for the proposed Lyman Far Ultraviolet Spectroscopic Explorer. Expected background rates and shielding requirements are analyzed for detector types in both instruments for low earth orbits, and for high orbits in the case of Lyman.

Woodgate, B. E.↗
NASA NTRS1991

Laser Spectroscopic Measurement Of Temperature And Density

Report discusses research on use of laser-induced fluorescence in oxygen and Raman scattering in air for simultaneous measurement of temperature and density of air. Major application of laser spectroscopic techniques, measurement of fluctuations of temperature and density in hypersonic flows in wind tunnels.

Mckenzie, Robert L.↗
NASA NTRS1991

Study of InGaAs based MODFET structures using variable angle spectroscopic ellipsometry

Variable angle spectroscopic ellipsometry was used to estimate the thicknesses of all layers within the optical penetration depth of InGaAs based MODFET structures. Strained and unstrained InGaAs channels were made by MBE on InP substrates and by MOCVD on GaAs substrates. In most cases, ellipsometrically determined thicknesses were within 10 percent of the growth calibration results. The MBE made InGaAs strained layers showed large strain effects, indicating a probable shift in the critical points of their dielectric function toward the InP lattice matched concentration.

Alterovitz, S. A.↗
NASA NTRS1990

Planetary atmospheres: Microwave spectroscopic and space physics

Ground-based spectroscopic observations of isotopes of CO in the atmosphere of Mars, Venus, and Titan were collected over the 1982-1990 period. These observations were analyzed to obtain information on the photochemistry, dynamics, and thermal profiles of these planetary atmospheres. In the cases of the mesosphere (80-100 km altitude) of Venus and the lower atmosphere (0-70 km altitude) of Mars, the primary conclusion of this research is that significant interannual variation in the global, thermal, and compositional structures of these atmospheres occur over 10 year periods. The Titan studies have focussed on pinning down the true atmosphere CO abundance. A more detailed summary of the results for each of these planetary atmospheres is provided.

Clancy, R. Todd↗
NASA NTRS1990

The envelopes of RV Tauri stars. I - Linear polarimetric and spectroscopic observations

Simultaneous spectroscopic and polarimetric observations are presented for five RV Tauri stars and one RV Tauri-like semiregular variable. Time-dependent variations are found in the polarizations for four of the objects, as are wavelength-dependent variations in two and possibly three stars. In general, the largest polarizations and strongest wavelength dependences in the polarization occur immediately following primary light minima.

Nook, Mark A.↗
NASA NTRS1991

Absolute magnitude calibration using trigonometric parallax - Incomplete, spectroscopic samples

A new numerical algorithm is used to calibrate the absolute magnitude of spectroscopically selected stars from their observed trigonometric parallax. This procedure, based on maximum-likelihood estimation, can retrieve unbiased estimates of the intrinsic absolute magnitude and its dispersion even from incomplete samples suffering from selection biases in apparent magnitude and color. It can also make full use of low accuracy and negative parallaxes and incorporate censorship on reported parallax values. Accurate error estimates are derived for each of the fitted parameters. The algorithm allows an a posteriori check of whether the fitted model gives a good representation of the observations. The procedure is described in general and applied to both real and simulated data.

Ratnatunga, Kavan U.↗
NASA NTRS1991

Spectroscopic observations of bright and dark emission features on the night side of Venus

Near-infrared spectra of a bright and a dark thermal emission feature on the night side of Venus have been obtained from 2.2 to 2.5 microns at a spectral resolution of 1200 to 1500. Both bright and dark features show numerous weak absorption bands produced by CO2, CO, water vapor, and other gases. The bright feature emits more radiation than the dark feature throughout this spectral region, but the largest contrasts occur between 2.21 s 2.32 microns, where H2SO4 clouds and a weak CO2 band provide the only known sources of extinction. The contrast decreases by 55 to 65 percent at wavelengths longer than 2.34 microns, where CO, clouds, and water vapor also absorb and scatter upwelling radiation. This contrast reduction may provide direct spectroscopic evidence for horizontal variations in the water vapor concentrations in the Venus atmosphere at levels below the cloud tops.

Bell, J. F., III↗
NASA NTRS1991

Comparison of thermal emission spectroscopic measurements of the lunar surface - 1968-1990

Four sets of spectra of resolved locations on the lunar surface obtained by Earth-based telescopes since 1968 are compared. The results obtained show that there is good consistency among the datasets within limits of signal-to-noise, and that ranges of overlap in wavelength and major spectral differences due to both the Christiansen emission peak and reststrahlen bands can be detected in remote measurements of the moon in the thermal infrared wavelength region. The conclusion of Goetz (1968) that only minor differences exist on the lunar surface in the region of thermal emission is found to be incorrect. Direct comparison of the four highest quality spectroscopic experiments in the region of thermal emission from 7 to 14 microns, which collected spectra of spatially resolved locations on the lunar surface, shows that this wavelength region reveals major spectral differences among lunar locations.

Lucey, P. G.↗
NASA NTRS1991

Tomographic separation of composite spectra - The components of the O-star spectroscopic binary AO Cassiopeiae

The UV photospheric lines of the short-period, double-lined O-star spectroscopic binary AO Cas are analyzed. Archival data from IUE (16 spectra uniformly distributed in orbital phase) were analyzed with a tomography algorithm to produce the separate spectra of the two stars in six spectral regions. The spectral classifications of the primary and secondary, O9.5 III and O8 V, respectively, were estimated through a comparison of UV line ratios with those in spectral standard stars. An intensity ratio of 0.5-0.7 (primary brighter) at 1600 A is compatible with the data.

Bagnuolo, William G., Jr.↗
NASA NTRS1991

Infrared spectroscopic measurements of the vertical column abundance of sulfur hexafluoride, SF6, from the ground

The solar observations involved in the present monitoring evaluation were made at two facilities, using Fourier-transform spectrometers achieving spectral resolutions of 0.005/cm and signal-to-rms noise ratios for individual scans from near 500 to near 1000. The monthly mean total vertical-column abundances of SF6 above both facilities are reported for time intervals from June 1986 to June 1990 and from March 1981 to June 1990. It is found that the vertical-column abundances increased at mean rates of 6.9 +/- 2.8 pct above one station and 6.6 +/- 7.2 pct above the other. Since all the results were retrieved using the same spectroscopic parameters and similar nonlinear least-squares curve-fitting algorithms validated through intercomparison exercises, the large error reported for the second station is attributed to a larger measurement uncertainty due to stronger H2O and C2O interferences, and a greater variability during each month.

Zander, R.↗
NASA NTRS1991

Carbonate stability in the earth's mantle - A vibrational spectroscopic study of aragonite and dolomite at high pressures and temperatures

The structural changes of aragonite and dolomite taking place at high pressures and temperatures were investigated by measuring the Raman spectra of these materials to pressures of 23 and 28 GPa (generated in a diamond anvil cell), respectively; in addition, the IR spectra of aragonite were measured to 40 GPa. The spectroscopic data demonstrated that, at 300 K, dolomite and aragonite samples were stable to pressures of 28 and 41 GPa, respectively. No phase transitions were observed following heating of aragonite and dolomite to temperatures of 2000 K and 800 K, respectively. The mode Grueneisen parameters indicate that the carbonate group in these two minerals is relatively insensitive to pressure, with the dominant compaction mechanism being the compression of the Ca and Mg polyhedra.

Kraft, Susan↗
NASA NTRS1991

Spectroscopic analyses of Fe and water in clays: A Martian surface weathering study

Martian surface morphology suggests the presence of liquid H2O on Mars in the past. Reflectance spectra of the Martian surface include features which correspond to the crystal field transitions of iron, as well as features supporting the presence of ice and minerals containing structural OH and surface water. Researchers initiated further spectroscopic studies of surface iron and water and structural OH in clays in order to determine what remotely obtained spectra can indicate about the presence of clays on Mars based on a clearer understanding of the factors influencing the spectral features. Current technology allows researchers to better correlate the low frequency fundamental stretching and bending vibrations of O-H bonds with the diagnostic near infrared overtone and combination bands used in mineral characterization and identification.

Bishop, J. L.↗
NASA NTRS1991

Spectroscopic planetary detection

One of the most promising methods for the detection of extra-solar planets is the spectroscopic method, where a small Doppler shift (approx. 10 meter/sec) in the spectrum of the parent star reveals the presence of planetary companions. However, solar type stars may show spurious Doppler shifts due to surface activity. If these effects are periodic, as is the solar activity cycle, then they may masquerade as planetary companions. The goal of this study was to determine whether the solar cycle affects the Doppler stability of integrated sunlight. Observations of integrated sunlight were made in the near infrared (approx. 2 micron), using the Kitt Peak McMath Fourier transform spectrometer, with a N2O gas absorption cell for calibration. An accuracy of approx. 5 meter/sec was achieved.

Deming, Drake↗
NASA NTRS1991

Potential approaches to the spectroscopic characterization of high performance polymers exposed to energetic protons and heavy ions

A potential limitation to human activity on the lunar surface or in deep space is the exposure of the crew to unacceptably high levels of penetrating space radiations. The radiations of most concerns for such missions are high-energy protons emitted during solar flares, and galactic cosmic rays which are high-energy ions ranging from protons to iron. The development of materials for effective shielding from energetic space radiations will clearly require a greater understanding of the underlying mechanisms of radiation-induced damage in bulk materials. This can be accomplished in part by the detailed spectroscopic characterization of bulk materials that were exposed to simulated space radiations. An experimental data base thus created can then be used in conjunction with existing radiation transport codes in the design and fabrication of effective radiation shielding materials. Electron Paramagnetic Resonance Spectroscopy was proven very useful in elucidating radiation effects in polymers (high performance polymers are often an important components of structural composites).

Suleman, Naushadalli K.↗