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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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NASA NTRS1996

Infrared laser spectroscopy of jet-cooled carbon clusters: the nu 5 band of linear C9

The nu 5 antisymmetric stretching vibration of 1 sigma+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079-2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported nu 6 transitions yielded the following molecular constants for the nu 5 band: nu 0 = 2 079.673 58(17) cm-1, B"= 0.014 321 4(10) cm-1, and B'=0.014 288 9(10) cm-1. The IR intensity of the nu 5 band relative to nu 6 was found to be 0.108 +/- 0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).

Non-NASA Center↗
NASA NTRS2009

Delta-Doped Back-Illuminated CMOS Imaging Arrays: Progress and Prospects

In this paper, we report the latest results on our development of delta-doped, thinned, back-illuminated CMOS imaging arrays. As with charge-coupled devices, thinning and back-illumination are essential to the development of high performance CMOS imaging arrays. Problems with back surface passivation have emerged as critical to the prospects for incorporating CMOS imaging arrays into high performance scientific instruments, just as they did for CCDs over twenty years ago. In the early 1990's, JPL developed delta-doped CCDs, in which low temperature molecular beam epitaxy was used to form an ideal passivation layer on the silicon back surface. Comprising only a few nanometers of highly-doped epitaxial silicon, delta-doping achieves the stability and uniformity that are essential for high performance imaging and spectroscopy. Delta-doped CCDs were shown to have high, stable, and uniform quantum efficiency across the entire spectral range from the extreme ultraviolet through the near infrared. JPL has recently bump-bonded thinned, delta-doped CMOS imaging arrays to a CMOS readout, and demonstrated imaging. Delta-doped CMOS devices exhibit the high quantum efficiency that has become the standard for scientific-grade CCDs. Together with new circuit designs for low-noise readout currently under development, delta-doping expands the potential scientific applications of CMOS imaging arrays, and brings within reach important new capabilities, such as fast, high-sensitivity imaging with parallel readout and real-time signal processing. It remains to demonstrate manufacturability of delta-doped CMOS imaging arrays. To that end, JPL has acquired a new silicon MBE and ancillary equipment for delta-doping wafers up to 200mm in diameter, and is now developing processes for high-throughput, high yield delta-doping of fully-processed wafers with CCD and CMOS imaging devices.

imagers↗
NASA NTRS1979

Mechanical and chemical effects of ion-texturing biomedical polymers

To determine whether sputter etching may provide substantial polymer surface texturing with insignificant changes in chemical and mechanical properties, an 8 cm beam diameter, electron bombardment, argon ion source was used to sputter etch (ion-texture process) nine biomedical polymers. The materials included silicone rubber, 32% carbon impregnated polyolefin, polyoxymethylene, polytetrafluoroethylene, ultrahigh molecular weight (UHMW) polyethylene, UHMW polyethylene with carbon fibers (10%), and several polyurethanes (bioelectric, segmented, and cross linked). Ion textured microtensile specimens of each material except UHMW polyethylene and UHMW polyethylene with 10% carbon fibers were used to determine the effect of ion texturing on tensile properties. Scanning electron microscopy was used to determine surface morphology changes, and electron spectroscopy for chemical analysis was used to analyze the near surface chemical changes that result from ion texturing. Ion energies of 500 eV with beam current densities ranging from 0.08 to 0.19 mA/sq cm were used to ion texture the various materials. Standard microtensile specimens of seven polymers were exposed to a saline environment for 24 hours prior to and during the tensile testing. The surface chemical changes resulting from sputter etching are minimal in spite of the often significant changes in the surface morphology.

Weigand, A. J.↗
NASA NTRS1989

The influence of Sb doping on the growth and electronic properties of GaAs(100) and AlGaAs(100)

Isoelectronic doping using antimony has been shown to reduce traps and improve material properties during epitaxial growth of Si doped GaAs(100) and AlGaAs(100). In this study, the effect of the antimony dopant on the optimal growth temperature is examined with the aim of producing high-quality heterostructures at lower temperatues. High-quality films of GaAs and AlGaAs have been grown by molecular-beam epitaxy at the normal growth temperatures of 610 and 700 C, respectively, and 50-100 C below this temperature using varying small amounts of Sb as a dopant. Electrical properties of the films were then examined using Hall mobility measurements and deep-level transient spectroscopy.

Jamison, K. D.↗
NASA NTRS1993

Visible photoluminescence of porous Si(1-x)Ge(x) obtained by stain etching

We have investigated visible photoluminescence (PL) from thin porous Si(1-x)Ge(x) alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using X-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge-rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the bandgap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.

Ksendzov, A.↗
NASA NTRS1989

Hydrogen-terminated silicon substrates for low-temperature molecular beam epitaxy

The preparation of hydrogen-terminated silicon surfaces for use as starting substrates for low-temperature MBE growth is examined in detail. The procedure involves the ex situ removal under nitrogen of residual oxide from a silicon substrate using a spin-clean with HF in ethanol, followed by the in situ low-temperature desorption (150 C) of physisorbed etch residues. The critical steps and the chemical basis for these steps are examined using X-ray photoelectron spectroscopy. Impurity residues at the epilayer-substrate interface following subsequent homoepitaxial growth are studied using AES, SIMS and TEM. Finally, scanning tunneling microscopy is used to examine the effect of cleaning methods on substrate morphology.

Grunthaner, P. J.↗
NASA NTRS2005

Interstellar PAH in the Laboratory and in Space. What have we Learned from the New Generation of Laboratory and Observational Studies?

Polycyclic Aromatic Hydrocarbons (PAHs) are an important and ubiquitous component of carbon-bearing materials in space. PAHs are the best-known candidates to account for the IR emission bands (UIR bands) and PAH spectral features are now being used as new probes of the ISM. PAHs are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge for laboratory astrophysics is to reproduce (in a realistic way) the physical conditions that exist in the emission and/or absorption interstellar zones. An extensive laboratory program has been developed at NASA Ames to assess the physical and chemical properties of PAHs in such environments and to describe how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments provide measurements of the spectral characteristics of interstellar PAH analogs from the ultraviolet and visible range to the infrared range for comparison with astronomical data. This paper will focus on the recent progress made in the laboratory to measure the direct absorption spectra of neutral and ionized PAHs in the gas phase in the near-UV and visible range in astrophysically relevant environments. These measurements provide data on PAHs and nanometer-sized particles that can now be directly compared to astronomical observations. The harsh physical conditions of the IS medium - characterized by a low temperature, an absence of collisions and strong VUV radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (of the order of 100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase. Intrinsic band profiles and band positions of cold gas-phase PAHs can now be measured with high-sensitivity spectroscopy and directly compared to the astronomical data. The electronic bands measured for ionized PAH are found to be intrinsically broad (about 20 cm(sup -1)) while the bands associated with the neutral precursors are narrower (of the order of 2 - 10 cm(sup -1)). The laboratory data are discussed and compared with recent astronomical spectra of large and narrow DIBs and with the spectra of circumstellar environments of selected carbon stars and the implications for the interstellar PAH population are derived. Preliminary results also show that carbon nanoparticles are formed during the short residence time of the precursors in the plasma.

Salama, Farid↗
NASA NTRS2005

PAHs and the Diffuse Interstellar Bands. What have we Learned from the New Generation of Laboratory and Observational Studies?

Polycyclic Aromatic Hydrocarbons (PAHs) are an important and ubiquitous component of carbon-bearing materials in space. PAHs are the best-known candidates to account for the IR emission bands (UIR bands) and PAH spectral features are now being used as new probes of the ISM. PAHs are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge for laboratory astrophysics is to reproduce (in a realistic way) the physical conditions that exist in the emission and/or absorption interstellar zones, An extensive laboratory program has been developed at NASA Ames to characterize the physical and chemical properties of PAHs in astrophysical environments and to describe how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments provide measurements of the spectral characteristics of interstellar PAH analogs from the ultraviolet and visible range to the infrared range for comparison with astronomical data. This paper will focus on the recent progress made in the laboratory to measure the direct absorption spectra of neutral and ionized PAHs in the gas phase in the near-W and visible range in astrophysically relevant environments. These measurements provide data on PAHs and nanometer-sized particles that can now be directly compared to astronomical observations. The harsh physical conditions of the IS medium - characterized by a low temperature, an absence of collisions and strong V W radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (of the order of 100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase. Intrinsic band profiles and band positions of cold gas-phase PAHs can now be measured with high- sensitivity spectroscopy and directly compared to the astronomical data. The electronic bands measured for ionized PAH are found to be intrinsically broad (about 20/cm) while the bands associated with the neutral precursors are narrower (of the order of 2 - 10/cm).

Salama, Farid↗
NASA NTRS2006

Astrochemistry: Recent Advances in the Study of Carbon Molecules in Space

Carbon molecules and ions play an important role in space. Polycyclic Aromatic Hydrocarbons (PAHs) are the best-known candidates to account for the infrared emission bands (UIR bands) and PAH spectral features are now being used as probes of the interstellar medium in Galactic and extra-galactic environments. PAHs are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge for laboratory Astrochemistry is to reproduce (in a realistic way) the physical conditions that exist in the emission and absorption interstellar zones. An extensive laboratory program has been developed in various laboratories to characterize the physical and chemical properties of PAHs in astrophysical environments and to describe how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments provide measurements of the spectral characteristics of interstellar PAH analogs from the ultraviolet and visible range to the infrared range for comparison with astronomical data. The harsh physical conditions of the interstellar medium - characterized by a low temperature, an absence of collisions and strong ultraviolet radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (of the order of 100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase.

Salama, Farid↗
NASA NTRS2003

Molecular Carbon in the Galaxy: Laboratory and Observational Studies

In a collaboration with the Mats Larsson group from Stockholm, we carried out a new measurement of the rate of dissociative recombination of H(sup *, sub j), using a new pulsed supersonic beam source of rotationally cold H(sup *, sub j). This source was first designed and characterized in our lab by IR cavity ringdown spectroscopy, determining a rotationaYtranslationa1 temperature of 20-60K, depending on conditions. This new source was then taken to Stockholm for the recombination rate studies at the CRYRING storage ring. The recombination rate constant measured against temperature yields values consistent with the most recent calculations, whereas previous experimental measurements varied over a range of 10(exp 4) and were poor agreement with theory. This is a crucial achievement for understanding the ion chemistry of diffuse clouds. Moreover, this result in combination with recent observations implies a greatly enhanced (factor of 40) cosmic ray ionization rate in a diffuse cloud (zeta Persei) relative to previous studies. The implications of this are discussed in our recent Nature paper. An enhanced cosmic-ray flux towards zeta Persei inferred from a laboratory study of the H(sup *, sub j)-e(sup -) recombination rate.

Saykally, Richard James↗
NASA NTRS1987

Method and apparatus for enhancing laser absorption sensitivity

A simple optomechanical method and apparatus is described for substantially reducing the amplitude of unwanted multiple interference fringes which often limit the sensitivities of tunable laser absorption spectrometers. An exterior cavity is defined by partially transmissible surfaces such as a laser exit plate, a detector input, etc. That cavity is spoiled by placing an oscillating plate in the laser beam. For tunable diode laser spectroscopy in the mid-infrared region, a Brewster-plate spoiler allows the harmonic detection of absorptances of less than 10 to the -5 in a single laser scan. Improved operation is achieved without subtraction techniques, without complex laser frequency modulation, and without distortion of the molecular lineshape signal. The technique is applicable to tunable lasers operating from UV to IR wavelengths and in spectrometers which employ either short or long pathlengths, including the use of retroreflectors or multipass cells.

Webster, Christopher R.↗
NASA NTRS1989

BIN Diode For Submillimeter Wavelengths

Diode formed by selective doping during epitaxial growth, starting with semi-insulating substrate. Use of high-mobility semiconductors like GaAs extends cutoff frequency. Either molecular-beam epitaxy (MBE) or organometallic chemical-vapor deposition used to form layers of diode. Planar growth process permits subsequent fabrication of arrays of diodes by standard photolithographic techniques, to achieve quasi-optical coupling of submillimeter radiation. Useful for generation of harmonics or heterodyne mixing in receivers for atmospheric and space spectroscopy operating at millimeter and submillimeter wavelengths. Used as frequency doublers or triplers, diodes of new type extend frequency range of present solid-state oscillators.

Maserjian, J.↗
NASA NTRS1994

Characterization of silicon-carbon clusters by infrared laser spectroscopy: the nu 3(sigma u) band of linear Si2C3

The nu 3(sigma u) fundamental vibration of 1 sigma g+ Si2C3 has been observed using a laser vaporization-supersonic cluster beam-diode laser spectrometer. Forty rovibrational transitions were measured in the range of 1965.8 to 1970.9 cm-1 with a rotational temperature of 10-15 K. A least-squares fit of these transitions yielded the following molecular constants: nu 3(sigma u)=1968.188 31(18) cm-1, B"=0.031 575 1(60) cm-1, and B'=0.031 437 4(57) cm-1. These results are in excellent agreement with recent Fourier transform infrared (FTIR) measurements of Si2C3 trapped in a solid Ar matrix [J. Chem. Phys. 100, 181(1994)] and with ab initio calculations [J. Chem. Phys. 100, 175 (1994)] which suggest cumulenic-like bonding for Si2C3, analogous to the isovalent C5 carbon cluster.

Non-NASA Center↗
NASA NTRS1980

Crosslinking of aromatic polyamides via pendant propargyl groups

Methods for crosslinking N-methyl substituted aromatic polyamides were investigated in an effort to improve the applicability of these polymers as matrix resins for Kavlar trademark fiber composites. High molecular weight polymers were prepared from isophthaloyl dichloride and 4,4'- bis(methylamino)diphenylmethane with varying proportions of the N,N'bispropargyl diamine incorporated as a crosslinking agent. The propargylcontaining diamines were crosslinked thermally and characterized by infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Attempts were also made to crosslink polyamide films by exposure to ultraviolet light, electron beam, and gamma radiation.

St.clair, A. K.↗
NASA NTRS1995

Effects of Surface Treatments on Secondary Electron Emission from CVD Diamond Films

Secondary electron emission (SEE) properties of polycrystalline diamond films grown by chemical vapor deposition (CVD) were measured. The total secondary yield (sigma) from as-grown samples was observed to be as high as 20 at room temperature and 48 while heating at 700 K in vacuum. Electron-beam-activated, alkali-terminated diamond films have shown stable values of sigma as high as 60 when coated with CsI and similarly high values when coated with other alkali halides. Diamond coated with BaF2 had a stable sigma of 6, but no enhancement of the SEE properties was observed with coatings of Ti or Au. Hydrogen was identified to give rise to this effect in as-grown films. However, electron beam exposure led to a reduction in sigma values as low as 2. Exposure to a molecular hydrogen environment restored sigma to its original value after degradation, and enabled stable secondary emission during electron beam exposure. Atomic hydrogen and hydrogen plasma treatments were performed on diamond/Mo samples in an attempt to increase the near-surface hydrogen concentration which might lead to increased stability in the secondary emission. Raman scattering analysis, scanning electron microscopy, and Auger electron spectroscopy (AES) confirmed that hydrogen plasma and atomic hydrogen treatments improved the quality of the CVD diamond significantly. Elastic recoil detection (ERD) showed that heating as-grown diamond targets to 7OO K, which was correlated with an increase in sigma, removed contaminants from the surface but did not drive hydrogen from the diamond bulk. ERD showed that the hydrogen plasma treatment produced an increase in the hydrogen concentration in the near-surface region which did not decrease while heating in vacuum at 700 K, but no improvement in the SEE properties was observed.

Mearini, G. T.↗
NASA NTRS1988

Characterization of GaAlAs optical waveguide heterostructures grown by molecular beam epitaxy

Multiple-layer GaAlAs optical waveguide heterostructures have been grown by MBE. These samples were designed to operate at 840 nm with negligible coupling of guided light to the absorbing GaAs substrate. The Al concentration was 13 percent for the guiding layer and was 16 percent for the cladding layers. The process for growing waveguide layers was calibrated primarily by high-energy electron diffraction, with the optical quality confirmed by photoluminescence measurements. Channel waveguide structures having widths of 5 microns were etched in a low-pressure magnetically confined multipolar plasma reactor. The resulting waveguide structures were characterized by Raman spectroscopy, ellipsometry, AES, and optical-waveguide loss measurements.

Radens, C. J.↗
NASA NTRS1999

Ionized Outflows in 3-D Insights from Herbig-Haro Objects and Applications to Nearby AGN

HST shows that the gas distributions of these objects are complex and clump at the limit of resolution. HST spectra have lumpy emission-line profiles, indicating unresolved sub-structure. The advantages of 3D over slits on gas so distributed are: robust flux estimates of various dynamical systems projected along lines of sight, sensitivity to fainter spectral lines that are physical diagnostics (reddening-gas density, T, excitation mechanisms, abundances), and improved prospects for recovery of unobserved dimensions of phase-space. These advantages al- low more confident modeling for more profound inquiry into underlying dynamics. The main complication is the effort required to link multi- frequency datasets that optimally track the energy flow through various phases of the ISM. This tedium has limited the number of objects that have been thoroughly analyzed to the a priori most spectacular systems. For HHO'S, proper-motions constrain the ambient B-field, shock velocity, gas abundances, mass-loss rates, source duty-cycle, and tie-ins with molecular flows. If the shock speed, hence ionization fraction, is indeed small then the ionized gas is a significant part of the flow energetics. For AGN'S, nuclear beaming is a source of ionization ambiguity. Establishing the energetics of the outflow is critical to determining how the accretion disk loses its energy. CXO will provide new constraints (especially spectral) on AGN outflows, and STIS UV-spectroscopy is also constraining cloud properties (although limited by extinction). HHO's show some of the things that we will find around AGN'S. I illustrate these points with results from ground-based and HST programs being pursued with collaborators.

Cecil, Gerald↗
NASA NTRS1986

Deep levels in as-grown and Si-implanted In(0.2)Ga(0.8)As-GaAs strained-layer superlattice optical guiding structures

Trap levels in about 2-micron In(0.2)Ga(0.8)As(94 A)/GaAs(25 A) strained-layer superlattices, suitable for optical waveguides, have been identified and characterized by deep-level transient spectroscopy and optical deep-level transient spectroscopy measurements. Several dominant electron and hole traps with concentrations of approximately 10 to the 14th/cu cm, and thermal ionization energies Delta-E(T) varying from 0.20 to 0.75 eV have been detected. Except for a 0.20-eV electron trap, which might be present in the In(0.2)Ga(0.8)As well regions, all the other traps have characteristics similar to those identified in molecular-beam epitaxial GaAs. Of these, a 0.42-eV hole trap is believed to originate from Cu impurities, and the others are probably related to native defects. Upon Si implantation and halogen lamp annealing, new deep centers are created. These are electron traps with Delta-E(T) = 0.81 eV and hole traps with Delta-E(T) = 0.46 eV. Traps occurring at room temperature may present limitations for optical devices.

Dhar, S.↗