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At least 397 records · Page 22

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NASA NTRS1989

The optical counterpart of the X-ray source H0253+193 - A distant, high-luminosity RS Canum Venaticorum system

Optical and near-infrared photometry in the VRIJHK and CO (2.35 microns) bands and optical spectroscopy and polarimetry reveal that the optical counterpart of the X-ray source H0253+193 is most likely a RS CVn binary system. The results show a visual extinction of 11.3 mag to the star, with an IR excess possibly due a cooler binary companion or circumstellar material. It is suggested that the I-band polarization of 8.1 + or - 1.8 may be induced in the starlight by passage through the molecular cloud. The lack of strong H-alpha emission in the optical spectrum argues against the possibility of a classical T Tauri star, and the IR excess argues against a naked T Tauri star. The wide energy spectrum found in the dereddened optical and near-infrared photometry supports the binary star model.

Clemens, Dan P.↗

NASA NTRS1991

The carbon-rich proto-planetary nebula IRAS 22272 + 5435

Ground-based photometry and spectroscopy is presented for IRAS 22272 + 5435, a 9th mag star with a large infrared excess. The flux distribution is 'double-peaked', with a visible and near-infrared component due to the reddened photosphere and a far-infrared component presumably due to a detached dust shell. About equal amounts of energy are detected from each. The spectrum is that of a peculiar G supergiant, Gp Ia. In addition, strong molecular bands of C3 and C2 are observed. The flux distribution is modeled and, together with published molecular-line radio observations, allows the determination of basic parameters of the central star and the dust shell. The source has the characteristics one would expect of a proto-planetary nebula, an object in transition from the asymptotic giant branch to the planetary nebula phase.

Hrivnak, Bruce J.↗

NASA NTRS1998

Near-infrared spectroscopy of the proto-planetary nebula CRL 618 and the origin of the hydrocarbon dust component in the interstellar medium

A new 2.8-3.8 micrometers spectrum of the carbon-rich protoplanetary nebula CRL 618 confirms the previous detection of a circumstellar 3.4 micrometers absorption feature in this object (Lequeux & Jourdain de Muizon). The high resolution and high signal-to-noise ratio of our spectrum allow us to derive the detailed profile of this absorption feature, which is very similar to that observed in the spectrum of the Galactic center and also resembles the strong 3.4 micrometers emission feature in some post-asymptotic giant branch stars. A weak 3.3 micrometers unidentified infrared band, marginally detected in the CRL 618 spectrum of Lequeux & Jourdain de Muizon, is present in our spectrum. The existence of the 3.4 micrometers feature implies the presence of relatively short-chained, aliphatic hydrocarbon materials (-CH2-/-CH3 approximately = 2-2.5) in the circumstellar environment around CRL 618. It also implies that the carriers of the interstellar 3.4 micrometers feature are produced at least in part in circumstellar material, and it calls into question whether any are produced by the processing of interstellar ices in dense interstellar clouds, as has been previously proposed. Other features in the spectrum are recombination lines of hydrogen, rotational and vibration-rotation lines of molecular hydrogen, and a broad absorption probably due to a blend of HCN and C2H2 bands.

Spectroscopy, Near-Infrared↗

NASA NTRS2004

Time-Average Measurement of Velocity, Density, Temperature, and Turbulence Using Molecular Rayleigh Scattering

Measurement of time-averaged velocity, density, temperature, and turbulence in gas flows using a nonintrusive, point-wise measurement technique based on molecular Rayleigh scattering is discussed. Subsonic and supersonic flows in a 25.4-mm diameter free jet facility were studied. The developed instrumentation utilizes a Fabry-Perot interferometer to spectrally resolve molecularly scattered light from a laser beam passed through a gas flow. The spectrum of the scattered light contains information about velocity, density, and temperature of the gas. The technique uses a slow scan, low noise 16-bit depth CCD camera to record images of the fringes formed by Rayleigh scattered light passing through the interferometer. A kinetic theory model of the Rayleigh scattered light is used in a nonlinear least squares fitting routine to estimate the unknown parameters from the fringe images. The ability to extract turbulence information from the fringe image data proved to be a challenge since the fringe is broadened by not only turbulence, but also thermal fluctuations and aperture effects from collecting light over a range of scattering angles. Figure 1 illustrates broadening of a Rayleigh spectrum typical of flow conditions observed in this work due to aperture effects and turbulence for a scattering angle, chi(sub s), of 90 degrees, f/3.67 collection optics, mean flow velocity, u(sub k), of 300 m/s, and turbulent velocity fluctuations, sigma (sub uk), of 55 m/s. The greatest difficulty in processing the image data was decoupling the thermal and turbulence broadening in the spectrum. To aid in this endeavor, it was necessary to seed the ambient air with smoke and dust particulates; taking advantage of the turbulence broadening in the Mie scattering component of the spectrum of the collected light (not shown in the figure). The primary jet flow was not seeded due to the difficulty of the task. For measurement points lacking particles, velocity, density, and temperature information could reliably be recovered, however the turbulence estimates contained significant uncertainty. Resulting flow parameter estimates are presented for surveys of Mach 0.6, 0.95, and 1.4 jet flows. Velocity, density, and temperature were determined with accuracies of 5 m/s, 1.5%, and 1%, respectively, in flows with no particles present, and with accuracies of 5 m/s, 1-4%, and 2% in flows with particles. Comparison with hotwire data for the Mach 0.6 condition demonstrated turbulence estimates with accuracies of about 5 m/s outside the jet core where Mie scattering from dust/smoke particulates aided in the estimation of turbulence. Turbulence estimates could not be recovered with any significant accuracy for measurement points where no particles were present.

Mielke, Amy F.↗

NASA NTRS1985

Far-infrared observations of Sagittarius B2: Reconsideration of source structure

New moderate-angular-resolution far-infrared observations of the Sagittarius B2 star-forming region are presented, discussed, and compared with recent radio molecular and continuum observations of this source. In contrast to previous analyses, its far-infrared spectrum is interpreted as the result of a massive frigid cloud overlying a more-or-less normal infrared source, a natural explanation for the object's previously-noted pecularities. The characteristics derived for the obscuring cloud are similar to those found for the W51 MAIN object. Both sources have high sub-millimeter surface brightness, a high ratio of sub-millimeter to far-infrared flux, and numerous regions of molecular maser emission.

Thronson, H. A., Jr.↗

NASA NTRS1986

Far-infrared observations of Sagittarius B2 - Reconsideration of source structure

New moderate-angular-resolution far-infrared observations of the Sagittarius B2 star-forming region are presented, discussed, and compared with recent radio molecular and continuum observations of this source. In contrast to previous analyses, its far-infrared spectrum is interpreted as the result of a massive frigid cloud overlying a more-or-less normal infrared source, a natural explanation for the object's previously-noted peculiarities. The characteristics derived for the obscuring cloud are similar to those found for the W51 MAIN object. Both sources have high sub-millimeter surface brightness, a high ratio of sub-millimeter to far-infrared flux, and numerous regions of molecular maser emission.

Thronson, H. A., Jr.↗

NASA NTRS1997

Molecularly Oriented Polymeric Thin Films for Space Applications

The increased commitment from NASA and private industry to the exploration of outer space and the use of orbital instrumentation to monitor the earth has focused attention on organic polymeric materials for a variety of applications in space. Some polymeric materials have exhibited short-term (3-5 yr) space environmental durability; however, future spacecraft are being designed with lifetimes projected to be 10-30 years. This gives rise to concern that material property change brought about during operation may result in unpredicted spacecraft performance. Because of their inherent toughness and flexibility, low density, thermal stability, radiation resistance and mechanical strength, aromatic polyimides have excellent potential use as advanced materials on large space structures. Also, there exists a need for high temperature (200-300 C) stable, flexible polymeric films that have high optical transparency in the 300-600nm range of the electromagnetic spectrum. Polymers suitable for these space applications were fabricated and characterized. Additionally, these polymers were molecularly oriented to further enhance their dimensional stability, stiffness, elongation and strength. Both unoriented and oriented polymeric thin films were also cryogenically treated to temperatures below -184 C to show their stability in cold environments and determine any changes in material properties.

Fay, Catharine C.↗

NASA NTRS1975

Measurement of the spectrum of the submillimeter cosmic background

The spectrum of the night sky has been measured in the wave number range from 3 to 40 per cm using a fully calibrated liquid-helium-cooled balloon-borne spectrophotometer at an elevation of 39 km. A model based on the known molecular parameters was used to subtract the atmospheric emission. In the range from 4 to 17 per cm, the spectrum of the background radiation is that of a blackbody with a temperature of about 2.99 K.

Woody, D. P.↗

DOE OSTI2021

Natural Charge-Transfer Analysis: Eliminating Spurious Charge-Transfer States in Time-Dependent Density Functional Theory via Diabatization, with Application to Projection-Based Embedding

For many types of vertical excitation energies, linear-response time-dependent density functional theory (LR-TDDFT) offers a useful degree of accuracy combined with unrivaled computational efficiency, although charge-transfer excitation energies are often systematically and dramatically underestimated, especially for large systems and those that contain explicit solvent. As a result, low energy electronic spectra of solution-phase chromophores often contain tens to hundreds of spurious charge-transfer states, making LR-TDDFT needlessly expensive in bulk solution. More nefariously, intensity borrowing by the low-energy charge-transfer states can affect intensities of the valence excitations even if those excitation energies are accurate. At higher excitation energies, it is difficult to distinguish spurious CT states from genuine charge-transfer-to-solvent (CTTS) excitations. In this work, we introduce an automated diabatization scheme that enables fast and effective screening of the CTTS acceptor space in bulk solution. Our procedure introduces the concept of “natural charge-transfer orbitals”, which provide a means to isolate characteristic pairs of orbitals that are most likely to participate in a CTTS excitation. The projection of these orbitals onto solvent-centered virtual orbitals provides a criterion for defining the most important solvent molecules in a given excitation. We apply this method to analyze an ab initio molecular dynamics (MD) trajectory of I-(aq) and report the lowest-energy CTTS band in the absorption spectrum. Our results are in excellent agreement with experimental measurements for bulk I-(aq), and only one-third of the water molecules in the I-(H2O)96 simulation cell need to be described with LR-TDDFT in order to obtain excitation energies that are converged to < 0.1 eV. The tools introduced herein will improve the accuracy, efficiency, and usability of LR-TDDFT in solution-phase environments

Carter-Fenk, Kevin D.↗

NASA NTRS1985

Characterization of a soluble polyimide

The solid phase thermal imidization of a soluble 4,4-prime-bis (3,4-dicarboxyphynoxy) diphenyl sulfide dianhydride plus 2,2-bis 4-(4-aminophenoxyl)phenyl hexafluoropropane (BDSDA-BDAF) polyimide precursor staged between 40 and 325 C was characterized by gel permeation chromatography, inherent viscosity, and number average molecular weight measurements. The information obtained indicated that this polyamic acid experienced an initial reduction in molecular weight during cure before achieving its ultimate molecular weight as a polyimide. The event was most likely related to the development of an anhydride band that appeared in the infrared spectrum around 1850/cm as the material cured. Correlation through infrared spectroscopy with a previous study dealing with several common, but insoluble, polyimide precursors, suggested that polyimides, in general, exhibit this trend in molecular weight behavior.

Young, Philip R.↗

NASA NTRS1993

Extreme Ultraviolet Emission Spectrum of CO_2 Induced by Electron Impact at 200 eV

We present the extreme ultraviolet (EUV) emission spectrum of CO_2 induced by electronimpact at 200 eV. There are 36 spectral features which are identified with a resolution of 0.5 nmover the wavelength range of 40 to 125 nm. Absolute emission cross sections were obtained for eachof these features. The EUV emission spectrum induced by electron impact consist of atomicmultiplets of CI,II and OI,II,III as well as CO and CO^+ molecular band systems produced bydissociative excitation. The CI (119.4 nm) multiplet is the strongest feature of CI with a peak crosssection of 3.61 x 10^(-19) cm^2 at 200 eV. The strongest feature of OI in the EUV spectrum is theOI (99.0 nm) multiplet with a peak cross section of 3.59 x 10^(-19) cm^2 at 200 eV.

Kanik, I.↗

NASA NTRS1985

Spectral identification/elimination of molecular species in spacecraft glow

Computer models of molecular electronic and vibrational emission intensities were developed. Known radiative emission rates (Einstein coefficients) permit the determination of relative excited state densities from spectral intensities. These codes were applied to the published spectra of glow above shuttle surface and to the Spacelab 1 results of Torr and Torr. The theoretical high-resolution spectra were convolved with the appropriate instrumental slit functions to allow accurate comparison with data. The published spacelab spectrum is complex but N2+ Meinel emission can be clearly identified in the ram spectrum. M2 First Positive emission does not correlate well with observed features, nor does the CN Red System. Spectral overlay comparisons are presented. The spectrum of glow above shuttle surfaces, in contrast to the ISO data, is not highly structured. Diatomic molecular emission was matched to the observed spectral shape. Source excitation mechanisms such as (oxygen atom)-(surface species) reaction product chemiluminescence, surface recombination, or resonance fluorescent re-emission will be discussed for each tentative assignment. These assignments are the necessary first analytical step toward mechanism identification. Different glow mechanisms will occur above surfaces under different orbital conditions.

Green, B. D.↗

NASA NTRS1994

Diffuse continuum gamma rays from the Galaxy observed by COMPTEL

The diffuse Galactic continuum gamma-ray emission has been studied using the full Sky Survey from COMPTEL on the Compton Observatory CGRO. The diffuse emission appears to be visible in the whole 0.75-30 MeV range covered by the instrument, although a considerable contribution from unresolved point sources cannot be excluded. A correlation analysis using HI and CO surveys of the Galaxy is used to derive the Galactic emissivity spectrum, and this is consistent with a smooth continuation to the spectrum at higher energies derived by a similar analysis of COS-B data. The apparent conversion factor from integrated CO temperature to molecular hydrogen column density can also be determined from the correlation analysis. The value obtained is consistent with results from COS-B and other non-gamma-ray methods. Calculations of the emissivity spectrum from bremsstrahlung from a cosmic-ray electron spectrum based on propagation models are compared with the observations.

Strong, A. W.↗

NASA NTRS1981

On the spectrum of turbulent magnetic fields

Theoretical power spectra of magnetic fields subject to turbulent fluid motions in the kinematic regime are presented, and previous theories are reviewed, with reference to magnetic fields on the sun. Magnetic field diffusion in turbulence with persistent eddies is predicted to be described by an effective negative magnetic diffusivity. It is found that observations cannot be explained on the basis of turbulent kinematic theories unless the turbulent motions are three-dimensional, and the effective diffusivities are larger than the molecular diffusivities. Lower bounds on the turbulent viscosity are derived, suggesting that dynamical processes controlling the magnetic field spectrum occur at least 15,000 km below the surface. The results, which remain consistent with the assumption that effective diffusivity is uniform, suggest that surface magnetic field observations can be used as a diagnostic for subsurface flows.

Knobloch, E.↗

DOE OSTI2020

Molecular structure determination: Equilibrium structure of pyrimidine (m-C 4 H 4 N 2 ) from rotational spectroscopy (reSE) and high-level ab initio calculation (re) agree within the uncertainty of experimental measurement

The pure rotational spectrum of pyrimidine (m-C 4 H 4 N 2 ), the meta-substituted dinitrogen analog of benzene, has been studied in the millimeter-wave region from 235 GHz to 360 GHz. The rotational spectrum of the ground vibrational state has been assigned and fit to yield accurate rotational and distortion constants. Over 1700 distinct transitions were identified for the normal isotopologue in its ground vibrational state and least-squares fit to a partial sextic S-reduced Hamiltonian. Transitions for all four singly substituted 13 C and 15 N isotopologues were observed at natural abundance and were likewise fit. Deuterium-enriched samples of pyrimidine were synthesized, giving access to all eleven possible deuterium-substituted isotopologues, ten of which were previously unreported. Experimental values of rotational constants and computed values of vibration–rotation interaction constants and electron-mass corrections were used to determine semi-experimental equilibrium structures (r e SE ) of pyrimidine. The r e SE structure obtained using coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] corrections shows exceptional agreement with the re structure computed at the CCSD(T)/cc-pCV5Z level (≤0.0002 Å in bond distance and ≤0.03° in bond angle). Of the various computational methods examined, CCSD(T)/cc-pCV5Z is the only method for which the computed value of each geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The exceptionally high accuracy and precision of the structure determination is a consequence of the large number of isotopologues measured, the precision and extent of the experimental frequency measurements, and the sophisticated theoretical treatment of the effects of vibration–rotation coupling and electron mass. Taken together, these demanding experimental and computational studies establish the capabilities of modern structural analysis for a prototypical monocyclic aromatic compound.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Tensor Network Path Integral Study of Dynamics in B850 LH2 Ring with Atomistically Derived Vibrations

The recently introduced multisite tensor network path integral (MS-TNPI) allows simulation of extended quantum systems coupled to dissipative media. We use MS-TNPI to simulate the exciton transport and the absorption spectrum of a B850 bacteriochlorophyll (BChl) ring. The MS-TNPI network is extended to account for the ring topology of the B850 system. Accurate molecular-dynamics-based description of the molecular vibrations and the protein scaffold is incorporated through the framework of Feynman–Vernon influence functional. To relate the present work with the excitonic picture, an exploration of the absorption spectrum is done by simulating it using approximate and topologically consistent transition dipole moment vectors. Comparison of these numerically exact MS-TNPI absorption spectra are shown with second-order cumulant approximations. Finally, the effect of temperature on both the exact and the approximate spectra is also explored.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1994

A comparison of dynamical and molecular gas masses in very luminous infrared galaxies

Observations of the 2.3 micron bands of CO have been used to measure the central stellar velocity dispersions in NGC 1614 and IC 694. K-band images were used to constrain models of the stellar mass distribution. With these data and a spectrum of Arp 220 from the literature, we have determined the total mass within the central regions of all three galaxies. The central molecular gas masses inferred from (12)CO J = 1-0 fluxes are larger than the total masses determined from stellar kinematics. Subtraction of the likely stellar mass from the total mass yields upper limits for the H2 mass that are 4-10 times less than the previously reported masses. This result supports other arguments that the I(sub CO)/M(sub H2) ratio is not the same in ultraluminous galaxies as it is in Milky Way giant molecular clouds.

Shier, L. M.↗

DOE OSTI2024

Formation of Mn-rich interfacial phases in Co 2 Fe x Mn 1-x Si thin films

Here, we report the formation of Mn-rich regions at the interface of Co 2 Fe x Mn 1-x Si thin films grown on GaAs substrates by molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) with electron energy loss (EEL) spectrum imaging reveals that each interfacial region: (1) is 1–2 nm wide, (2) occurs irrespective of the Fe/Mn composition ratio and in both Co-rich and Co-poor films, and (3) displaces both Co and Fe indiscriminately. We also observe a Mn-depleted region in each film directly above each Mn-rich interfacial layer, roughly 3 nm in width in the x = 0 and x = 0.3 films, and 1 nm in the x = 0.7 (less Mn) film. We posit that growth energetics favor Mn diffusion to the interface even when there is no significant Ga interdiffusion into the epitaxial film. Element-specific X-ray magnetic circular dichroism (XMCD) measurements show larger Co, Fe, and Mn orbital to spin magnetic moment ratios compared to bulk values across the Co 2 Fe x Mn 1-x Si compositional range. The values lie between reported values for pure bulk and nanostructured Co, Fe, and Mn materials, corroborating the non-uniform, layered nature of the material on the nanoscale. Finally, SQUID magnetometry demonstrates that the films deviate from the Slater-Pauling rule for uniform films of both the expected and the measured composition. The results inform a need for care and increased scrutiny when forming Mn-based magnetic thin films on III-V semiconductors like GaAs, particularly when films are on the order of 5 nm or when interface composition is critical to spin transport or other device applications.

36 MATERIALS SCIENCE↗