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At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Localized surface plasmon and exciton interaction in silver-coated cadmium sulphide quantum dots

Localized surface plasmon and exciton coupling has been investigated on colloidal solutions of silver-coated CdS nanoparticles (NPs), synthesized by gamma irradiation. Two broad photoluminescence (PL) bands (blue/red) corresponding to band to band and defect state transitions have been observed for the bare and coated samples. In case of bare CdS NPs, the intensity of the red PL peak is about ten times higher than the blue PL peak intensity. However, on coating the CdS NPs with silver, the peak intensity of the blue PL band gets enhanced and becomes equal to that of the red PL band. High-resolution transmission electron microscopic (HRTEM) images adequately demonstrate size distribution of these metal/semiconductor nanocomposites. UV-Vis absorption studies show quantum confinement effect in these semiconductor quantum dot (SQD) systems. Absorption spectrum of silver-coated SQDs shows signature of surface plasmon-exciton coupling which has been theoretically verified.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

FIRST ULTRAVIOLET REFLECTANCE SPECTRA OF PLUTO AND CHARON BY THE HUBBLE SPACE TELESCOPE COSMIC ORIGINS SPECTROGRAPH: DETECTION OF ABSORPTION FEATURES AND EVIDENCE FOR TEMPORAL CHANGE

We have observed the mid-UV spectra of both Pluto and its large satellite, Charon, at two rotational epochs using the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) in 2010. These are the first HST/COS measurements of Pluto and Charon. Here we describe the observations and our reduction of them, and present the albedo spectra, average mid-UV albedos, and albedo slopes we derive from these data. These data reveal evidence for a strong absorption feature in the mid-UV spectrum of Pluto; evidence for temporal change in Pluto's spectrum since the 1990s is reported, and indirect evidence for a near-UV spectral absorption on Charon is also reported.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

WIDE-FIELD SURVEY OF EMISSION-LINE STARS IN IC 1396

We have made an extensive survey of emission-line stars in the IC 1396 H II region to investigate the low-mass population of pre-main-sequence (PMS) stars. A total of 639 H{alpha} emission-line stars were detected in an area of 4.2 deg{sup 2} and their i' photometry was measured. Their spatial distribution exhibits several aggregates near the elephant trunk globule (Rim A) and bright-rimmed clouds at the edge of the H II region (Rim B and SFO 37, 38, 39, 41), and near HD 206267, which is the main exciting star of the H II region. Based on the extinction estimated from the near-infrared color-color diagram, we have selected PMS star candidates associated with IC 1396. The age and mass were derived from the extinction-corrected color-magnitude diagram and theoretical PMS tracks. Most of our PMS candidates have ages of <3 Myr and masses of 0.2-0.6 M{sub Sun }. Although it appears that only a few stars were formed in the last 1 Myr in the east region of the exciting star, the age difference among subregions in our surveyed area is not clear from the statistical test. Our results may suggest that massive stars were born after the continuous formation of low-mass stars for 10 Myr. The birth of the exciting star could be the late stage of slow but contiguous star formation in the natal molecular cloud. It may have triggered the formation of many low-mass stars at the dense inhomogeneity in and around the H II region by a radiation-driven implosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MODELING PHOTODISINTEGRATION-INDUCED TeV PHOTON EMISSION FROM LOW-LUMINOSITY GAMMA-RAY BURSTS

Ultra-high-energy cosmic-ray heavy nuclei have recently been considered as originating from nearby low-luminosity gamma-ray bursts that are associated with Type Ibc supernovae. Unlike the power-law decay in long duration gamma-ray bursts, the light curve of these bursts exhibits complex UV/optical behavior: shock breakout dominated thermal radiation peaks at about 1 day, and, after that, nearly constant emission sustained by radioactive materials for tens of days. We show that the highly boosted heavy nuclei at PeV energy interacting with the UV/optical photon field will produce considerable TeV photons via the photodisintegration/photo-de-excitation process. It was later predicted that a thermal-like {gamma}-ray spectrum peaks at about a few TeV, which may serve as evidence of nucleus acceleration. The future observations by the space telescope Fermi and by the ground atmospheric Cherenkov telescopes such as H.E.S.S., VERITAS, and MAGIC will shed light on this prediction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A MODEL FOR THE ORIGIN OF HIGH DENSITY IN LOOPTOP X-RAY SOURCES

Super-hot (SH) looptop sources, detected in some large solar flares, are compact sources of HXR emission with spectra matching thermal electron populations exceeding 30 MK. High observed emission measure (EM) and inference of electron thermalization within the small source region both provide evidence of high densities at the looptop, typically more than an order of magnitude above ambient. Where some investigators have suggested such density enhancement results from a rapid enhancement in the magnetic field strength, we propose an alternative model, based on Petschek reconnection, whereby looptop plasma is heated and compressed by slow magnetosonic shocks generated self-consistently through flux retraction following reconnection. Under steady conditions such shocks can enhance density by no more than a factor of four. These steady shock relations (Rankine-Hugoniot relations) turn out to be inapplicable to Petschek's model owing to transient effects of thermal conduction. The actual density enhancement can in fact exceed a factor of 10 over the entire reconnection outflow. An ensemble of flux tubes retracting following reconnection at an ensemble of distinct sites will have a collective EM proportional to the rate of flux tube production. This rate, distinct from the local reconnection rate within a single tube, can be measured separately through flare ribbon motion. Typical flux transfer rates and loop parameters yield EMs comparable to those observed in SH sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

AGE SPREAD IN W3 MAIN: LARGE BINOCULAR TELESCOPE/LUCI NEAR-INFRARED SPECTROSCOPY OF THE MASSIVE STELLAR CONTENT

We present near-infrared multi-object spectroscopy and JHK{sub s} imaging of the massive stellar content of the Galactic star-forming region W3 Main, obtained with LUCI at the Large Binocular Telescope. We confirm 15 OB stars in W3 Main and derive spectral types between O5V and B4V from their absorption line spectra. Three massive young stellar objects are identified by their emission line spectra and near-infrared excess. The color-color diagram of the detected sources allows a detailed investigation of the slope of the near-infrared extinction law toward W3 Main. Analysis of the Hertzsprung-Russell diagram suggests that the Nishiyama extinction law fits the stellar population of W3 Main best (E(J - H)/E(H - K{sub s}) = 1.76 and R{sub K{sub s}} = 1.44). From our spectrophotometric analysis of the massive stars and the nature of their surrounding H II regions, we derive the evolutionary sequence of W3 Main and we find evidence of an age spread of at least 2-3 Myr. While the most massive star (IRS2) is already evolved, indications for high-mass pre-main-sequence evolution are found for another star (IRS N1), deeply embedded in an ultracompact H II (UCH II) region, in line with the different evolutionary phases observed in the corresponding H II regions. We derive a stellar mass of W3 Main of (4 {+-} 1) Multiplication-Sign 10{sup 3} M{sub Sun} by extrapolating from the number of OB stars using a Kroupa initial mass function and correcting for our spectroscopic incompleteness. We have detected the photospheres of OB stars from the more evolved diffuse H II region to the much younger UCH II regions, suggesting that these stars have finished their formation and cleared away their circumstellar disks very fast. Only in the hyper-compact H II region (IRS5) do the early-type stars seem to be still surrounded by circumstellar material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MORPHO-KINEMATIC ANALYSIS OF THE POINT-SYMMETRIC, BIPOLAR PLANETARY NEBULAE Hb 5 AND K 3-17, A PATHWAY TO POLY-POLARITY

The kinematics of the bipolar planetary nebulae Hb 5 and K 3-17 are investigated in detail by means of a comprehensive set of spatially resolved high spectral resolution, long-slit spectra. Both objects share particularly interesting characteristics, such as a complex filamentary, rosette-type nucleus, axial point-symmetry, and very fast bipolar outflows. The kinematic information of Hb 5 is combined with Hubble Space Telescope imagery to construct a detailed three-dimensional model of the nebula using the code SHAPE. The model shows that the large-scale lobes are growing in a non-homologous way. The filamentary loops in the core are proven to actually be secondary lobes emerging from what appears to be a randomly punctured, dense, gaseous core and the material that forms the point-symmetric structure flows within the lobes with a distinct kinematic pattern and its interaction with the lobes has had a shaping effect on them. Hb 5 and K 3-17 may represent a class of fast evolving planetary nebulae that will develop poly-polar characteristics once the nebular core evolves and expands.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

INCIDENCE OF Mg II ABSORPTION SYSTEMS TOWARD FLAT-SPECTRUM RADIO QUASARS

The conventional wisdom that the rate of incidence of Mg II absorption systems, dN/dz (excluding 'associated systems' having a velocity {beta}c relative to the active galactic nucleus (AGN) of less than {approx}5000 km s{sup -1}), is totally independent of the background AGNs has been challenged by a recent finding that dN/dz for strong Mg II absorption systems toward distant blazars is 2.2 {+-} {sup 0.8}{sub 0.6} times the value known for normal optically selected quasars (QSOs). This has led to the suggestion that a significant fraction of even the absorption systems with {beta} as high as {approx}0.1 may have been ejected by the relativistic jets in the blazars, which are expected to be pointed close to our direction. Here, we investigate this scenario using a large sample of 115 flat-spectrum radio-loud quasars (FSRQs) that also possess powerful jets, but are only weakly polarized. We show, for the first time, that dN/dz toward FSRQs is, on the whole, quite similar to that known for QSOs and that the comparative excess of strong Mg II absorption systems seen toward blazars is mainly confined to {beta} < 0.15. The excess relative to FSRQs probably results from a likely closer alignment of blazar jets with our direction; hence, any gas clouds accelerated by them are more likely to be on the line of sight to the active quasar nucleus.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

INNOCENT BYSTANDERS: CARBON STARS FROM THE SLOAN DIGITAL SKY SURVEY

Among stars showing carbon molecular bands (C stars), the main-sequence dwarfs, likely in post-mass transfer binaries, are numerically dominant in the Galaxy. Via spectroscopic selection from the Sloan Digital Sky Survey, we retrieve 1220 high galactic latitude C stars, {approx}5 times more than previously known, including a wider variety than past techniques such as color or grism selection have netted, and additionally yielding 167 DQ white dwarfs. Of the C stars with proper motion measurements, we identify 69% clearly as dwarfs (dCs), while {approx}7% are giants. The dCs likely span absolute magnitudes M{sub i} from {approx}6.5 to 10.5. 'G-type' dC stars with weak CN and relatively blue colors are probably the most massive dCs still cool enough to show C{sub 2} bands. We report Balmer emission in 22 dCs, none of which are G-types. We find 8 new DA/dC stars in composite spectrum binaries, quadrupling the total sample of these 'smoking guns' for AGB binary mass transfer. Eleven very red C stars with strong red CN bands appear to be 'N'-type AGB stars at large Galactocentric distances, one likely a new discovery in the dIrr galaxy Leo A. Two such stars within 30' of each other may trace a previously unidentified dwarf galaxy or tidal stream at {approx}40 kpc. We explore the multiwavelength properties of the sample and report the first X-ray detection of a dC star, which shows strong Balmer emission. Our own spectroscopic survey additionally provides the dC surface density from a complete sample of dwarfs limited by magnitude, color, and proper motion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANALYSIS OF THE EARLY-TIME OPTICAL SPECTRA OF SN 2011fe IN M101

The nearby Type Ia supernova (SN Ia) SN 2011fe in M101 (cz = 241 km s{sup -1}) provides a unique opportunity to study the early evolution of a 'normal' SN Ia, its compositional structure, and its elusive progenitor system. We present 18 high signal-to-noise spectra of SN 2011fe during its first month beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This gives a clear picture of how various line-forming species are distributed within the outer layers of the ejecta, including that of unburned material (C+O). We follow the evolution of C II absorption features until they diminish near maximum light, showing overlapping regions of burned and unburned material between ejection velocities of 10,000 and 16,000 km s{sup -1}. This supports the notion that incomplete burning, in addition to progenitor scenarios, is a relevant source of spectroscopic diversity among SNe Ia. The observed evolution of the highly Doppler-shifted O I {lambda}7774 absorption features detected within 5 days post-explosion indicates the presence of O I with expansion velocities from 11,500 to 21,000 km s{sup -1}. The fact that some O I is present above C II suggests that SN 2011fe may have had an appreciable amount of unburned oxygen within the outer layers of the ejecta.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Chemical Shift of the K{sub α1} and K{sub α2} Lines of the X-ray Emission Spectrum of Yb(II)/Yb(III) Fluorides: a Quantum-Chemical Investigation

Chemical shifts of the K{sub α1} and K{sub α2} lines (the 2p{sub 3/2} → 1s{sub 1/2} and 2p{sub 1/2} → 1s{sub 1/2} transitions, respectively) of the X-ray emission spectrum of the Yb atom in fluorides have been calculated by ab initio modeling the electronic structure. The valence transition Yb(II) → Yb(III) has been analyzed by examples of YbF{sub 2}, YbF{sub 3}, and Yb{sub 2}F{sub 4} molecules and YbF{sub 2}{sup +} cation. The relativistic pseudopotential and basis sets corresponding to it have been constructed for the ytterbium atom. They were used in calculations by the two-component noncollinear version of the density functional theory (DFT) with the PBE0 exchange-correlation functional. Results for the three-coordinated Yb(II) in the FYbF{sub 2}YbF dimer demonstrated a very weak dependence of the chemical shift on the coordination number of the Yb atom and on the molecular association of ytterbium difluoride. Chemical shifts of the X-ray emission spectrum for the ytterbium compound are related mainly to the change in the occupation of the 4 f shell.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Effect of the fabrication conditions of SiGe LEDs on their luminescence and electrical properties

SiGe-based n{sup +}–p–p{sup +} light-emitting diodes (LEDs) with heavily doped layers fabricated by the diffusion (of boron and phosphorus) and CVD (chemical-vapor deposition of polycrystalline silicon layers doped with boron and phosphorus) techniques are studied. The electroluminescence spectra of both kinds of LEDs are identical, but the emission intensity of CVD diodes is ∼20 times lower. The reverse and forward currents in the CVD diodes are substantially higher than those in diffusion-grown diodes. The poorer luminescence and electrical properties of the CVD diodes are due to the formation of defects at the interface between the emitter and base layers.

36 MATERIALS SCIENCE↗
DOE OSTI2012

GAS EXCITATION IN ULIRGs: MAPS OF DIAGNOSTIC EMISSION-LINE RATIOS IN SPACE AND VELOCITY

Emission-line spectra extracted at multiple locations across 39 ultraluminous infrared galaxies have been compiled into a spectrophotometric atlas. Line profiles of H{alpha}, [N II], [S II], [O I], H{beta}, and [O III] are resolved and fit jointly with common velocity components. Diagnostic ratios of these line fluxes are presented in a series of plots, showing how the Doppler shift, line width, gas excitation, and surface brightness change with velocity at fixed position and also with distance from the nucleus. One general characteristic of these spectra is the presence of shocked gas extending many kiloparsecs from the nucleus. In some systems, the rotation curves of the emitting gas indicate motions that suggest gas disks, which are most frequent at early merger stages. At these early merger stages, the emission line ratios indicate the presence of shocked gas, which may be triggered by the merger event. We also report the general characteristics of the integrated spectra.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Optical properties of nanocrystalline WO{sub 3} and WO{sub 3-x} thin films prepared by DC magnetron sputtering

The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, with different oxygen vacancy (V{sub o}) concentration, have been studied by spectrophotometry and photoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the films exhibit similar band gap energies, E{sub g} ≈ 2.9 eV. The absorption spectra of the films show two pronounced absorption bands in the near-infrared region. One peak (P1) is located at approximately 0.7 eV, independent of V{sub o} concentration. A second peak (P2) shifts from 0.96 eV to 1.16 eV with decreasing V{sub o} concentration. Peak P1 is assigned to polaron absorption due to transitions between tungsten sites (W{sup 5+} → W{sup 6+}), or an optical transition from a neutral vacancy state to the conduction band, V{sub o}{sup 0} → W{sup 6+}. The origin of peak P2 is more uncertain but may involve +1 and +2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to 3.10 eV in the more sub-stoichiometric and 2.40 to 3.02 eV in the less sub-stoichiometric films. The low energy emission bands agree well with calculated optical transition energies of oxygen vacancy sites, with dominant contribution from neutral and singly charged vacancies in the less sub-stoichiometric films, and additional contributions from doubly charged vacancy sites in the more sub-stoichiometric films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

High efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde by solvothermal-synthesized ZnIn{sub 2}S{sub 4} microspheres under visible light irradiation

Hexagonal ZnIn{sub 2}S{sub 4} samples have been synthesized by a solvothermal method. Their properties have been determined by X-ray diffraction, ultraviolet–visible-light diffuse reflectance spectra, field emission scanning electron microscopy, nitrogen adsorption–desorption and X-ray photoelectron spectra. These results demonstrate that ethanol solvent has significant influence on the morphology, optical and electronic nature for such marigold-like ZnIn{sub 2}S{sub 4} microspheres. The visible light photocatalytic activities of the ZnIn{sub 2}S{sub 4} have been evaluated by selective oxidation of benzyl alcohol to benzaldehyde using molecular oxygen as oxidant. The results show that 100% conversion along with >99% selectivity are reached over ZnIn{sub 2}S{sub 4} prepared in ethanol solvent under visible light irradiation (λ>420 nm) of 2 h, but only 58% conversion and 57% yield are reached over ZnIn{sub 2}S{sub 4} prepared in aqueous solvent. A possible mechanism of the high photocatalytic activity for selective oxidation of benzyl alcohol over ZnIn{sub 2}S{sub 4} is proposed and discussed. - Graphical abstract: Marigold-like ZnIn{sub 2}S{sub 4} microspheres were synthesized by a solvothermal method. The high visible photocatalytic activities of ZnIn{sub 2}S{sub 4} were evaluated by selective oxidation of benzyl alcohol to benzaldehyde under mild conditions. Display Omitted - Highlights: • Marigold-like ZnIn{sub 2}S{sub 4} microspheres were synthesized by a solvothermal method. • The solvents have a remarkably influence on the morphology and properties of samples. • It is the first time to apply ZnIn{sub 2}S{sub 4} for selective oxidation of benzyl alcohol. • ZnIn{sub 2}S{sub 4} shows high photocatalytic activity for selective oxidation of benzyl alcohol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

PHASE-AVERAGED SPECTRA AND LUMINOSITIES OF GAMMA-RAY EMISSIONS FROM YOUNG ISOLATED PULSARS

We study the phase-averaged spectra and luminosities of {gamma}-ray emissions from young, isolated pulsars within a revised outer gap model. In the revised version of the outer gap, there are two possible cases for the outer gaps: the fractional size of the outer gap is estimated through the photon-photon pair process in the first case (Case I), and is limited by the critical field lines in the second case (Case II). The fractional size is described by Case I if the fractional size at the null charge surface in Case I is smaller than that in Case II, and vice versa. Such an outer gap can extend from the inner boundary, whose radial distance to the neutron star is less than that of the null charge surface to the light cylinder for a {gamma}-ray pulsar with a given magnetic inclination. When the shape of the outer gap is determined, assuming that high-energy emission at an averaged radius of the field line in the center of the outer gap, with a Gaussian distribution of the parallel electric field along the gap height, represents typical emission, the phase-averaged {gamma}-ray spectrum for a given pulsar can be estimated in the revised model with three model parameters. We apply the model to explain the phase-averaged spectra of the Vela (Case I) and Geminga (Case II) pulsars. We also use the model to fit the phase-averaged spectra of 54 young, isolated {gamma}-ray pulsars, and then calculate the {gamma}-ray luminosities and compare them with the observed data from Fermi-LAT.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1981

Integrated fluxes for emission lines in the ultraviolet spectra of several planetaries

The IUE satellite observatory was used to obtain absolutely-calibrated emission line fluxes for diagnostic lines of multiply-ionized C, N, O, Si, Ne, and Ar which occur in the ultraviolet spectral region of planetary nebulae. These data, when combined with data from the blue, visual, and near infrared, provide improved estimates of ionic concentrations, plasma temperatures and densities, and elemental abundances.

Carpenter, K. G.↗