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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Experimental inference of neutral and impurity transport in Alcator C-Mod using high-resolution x-ray and ultra-violet spectra

We present experimental inferences of cross-field impurity transport coefficients for AlcatorC-Mod plasmas without edge-localized modes, leveraging a novel forward model for the entire Ca Kα spectrum, including satellite lines within the spectral range, to compare to high-resolution x-ray imaging crystal spectroscopy (XICS). These measurements are complemented by extreme ultra-violet (EUV) spectroscopy that constrains transport closer to the edge. Using new atomic data sets for both XICS and EUV analysis has enabled consideration of line ratios across both spectral ranges and has increased the accuracy of inferred transport coefficients. Inclusion of charge exchange between edge thermal neutrals and impurities is shown to be extremely important in C-Mod pedestals. Atomic D neutral densities from experimental D Lyα measurements at the midplane are compared to SOLPS-ITER simulations, finding good agreement. Bayesian inferences of impurity transport coefficients are presented for L-, EDA H-, and I-mode discharges, making use of the Aurora package for forward modeling and combining our spectroscopic constraints. Experimentally inferred diffusion profiles are found to match turbulent transport models at midradius within uncertainties, using both quasilinear gyro-fluid TGLF SAT-1 and nonlinear ion-scale gyrokinetic CGYRO simulations. Furthermore, significant discrepancies in convection are observed in some cases, suggesting difficulties in predictions of flat or hollow impurity profiles.

neutrals↗
DOE OSTI2021

Two-photon above-threshold ionization of helium

Multiphoton ionization provides a clear window into the nature of electron correlations in the helium atom. In the present study, the final-state energy range extends up to the region near the N = 2 and N = 3 ionization thresholds, where two-photon ionization proceeds via continuum intermediate states above the lowest threshold. Our calculations are performed using multichannel quantum defect theory (MQDT) and the streamlined R-matrix method. The sum and integration over all intermediate states in the two-photon ionization amplitude is evaluated using the inhomogeneous R-matrix method developed by Robicheaux and Gao. The seamless connection of that method with MQDT allows us to present high-resolution spectra of the final-state Rydberg resonances. Our analysis classifies the resonances above the N = 2 threshold in terms of their group theory quantum numbers. Furthermore, their dominant decay channels are found to obey the previously conjectured propensity rule far more weakly for these even-parity states than was observed for the odd-parity states relevant to single-photon ionization.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Solvation dynamics of choline fluoride in ethylene glycol – Water mixtures

Due to their favorable physical properties, deep eutectic solvents (DESs) have been of interest in many applications in the energy and industry sectors. In this study, we investigate the new deep eutectic solvent ethalineF (a 1:2 M ratio of choline fluoride:ethylene glycol) and the effect of water addition on the solvation dynamics compared to the better known ethaline (a 1:2 M ratio of choline chloride:ethylene glycol). Femtosecond transient absorption spectroscopy and NMR diffusometry were used to study the solvation dynamics. 19 F NMR spectra were obtained for ethalineF solutions with different water additions. The 19 F NMR spectra show two fluorine signals; one can be assigned to fluoride anions solvated by EG and the other signal to fluoride anions associated with the much heavier and less mobile choline cation. Density, viscosity, conductivity, and E T (30) polarity measurements are also provided for ethalineF. The nuclear spin nature of fluorine atom permits to measure the self-diffusion coefficient of the negative ion in addition of the cation diffusion in a choline halide based deep eutectic solvent system. This allows to calculate the NMR-predicted conductivities σ NMR . The obtained ion dissociation coefficients α D at different water concentrations suggest a partial ion dissociation which was commensurate with the relatively low ion conductivity. Furthermore, the observed modest conductivity of ethalineF is explained in terms of ion association as well as the gradual increase in conductivity caused by the addition of water.

19F NMR↗
DOE OSTI2025

Benchmarking DFT Accuracy in Predicting O 1s Binding Energies on Metals

X-ray photoelectron spectroscopy (XPS) is a powerful tool for probing the electronic structure and composition of materials, particularly metals and metal oxides of relevance to solar cells and catalysis. Density functional theory (DFT) is often used to support XPS peak assignments, but its reliability for predicting oxygen species is not well established. Here, we compile a large data set of experimental oxygen binding energies and evaluate corresponding DFT predictions. We find that as the binding energies of metal-bound atomic oxygen species increase, especially above ≈530 eV, there is a general decrease in the accuracy of DFTpredicted values. Thus, high-binding-energy atomic oxygen species, such as those proposed as active for selective Ag-catalyzed epoxidation, are less well represented. The chemical nature of the oxygen species also influences accuracy, with molecularly bound species more reliably captured across the entire range of energies. These findings illustrate the limitations of DFT for interpreting XPS spectra and provide a benchmark for improving computational methods.

Adsorption↗
DOE OSTI2020

Multifrequency stimulated Raman scattering of light in a calcite single crystal

Using pulses of multifrequency stimulated Raman scattering (SRS) in calcite, nonlinear photoluminescence in a stilbene molecular crystal is excited. When multifrequency SRS is excited by the radiation of a Nd{sup 3+} : YAG laser with a wavelength of 1064 nm, eleven anti-Stokes components are observed in the visible spectrum with an average frequency shift of 1086 cm{sup −1} between them. Using the second harmonic of the Nd {sup 3+} : YAG laser radiation allowed three anti-Stokes and four Stokes components to be recorded. The large spectral range of the frequency comb facilitated effective reduction in the duration and increase in the intensity of the radiation pulse of multifrequency SRS. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Resolving the dynamic correlated disorder in KTa 1- x Nb x O 3

Understanding the complex temporal and spatial correlations of ions in disordered perovskite oxides is critical to rationalize their functional properties. Here, in this study, we provide new insights into the longstanding controversy regarding the off-centering of transition metal ions in the archetypal ferroelectric alloy KTa 1-x Nb x O 3 (KTN). By mapping the full energy (E) and wavevector (Q) dependence of the dynamical structure factor S(Q, E) using neutron scattering, and rationalizing our observations with atomistic simulations leveraging machine learning, we fully resolve the static vs dynamic nature of diffuse scattering sheets, as well as their composition (x) and temperature dependence. Our first-principles simulations, extended with machine-learning molecular dynamics, reproduce both inelastic neutron spectra and diffuse features, and establish how dynamically-correlated transition metal off-centerings couple to phonons, unifying local and collective viewpoints. This study sheds new light into an exemplary ferroelectric systems and shows the importance of mapping the full S(Q, E) to reveal critical spatio-temporal correlations of atomic disorder from which functional properties emerge.

42 ENGINEERING↗
DOE OSTI2021

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions

Charge transport processes at interfaces which are governed by complex interfacial electronic structure play a crucial role in catalytic reactions, energy storage, photovoltaics, and many biological processes. Here, the first soft X-ray second harmonic generation (SXR-SHG) interfacial spectrum of a buried interface (boron/Parylene-N) is reported. SXR-SHG shows distinct spectral features that are not observed in X-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 {\AA}, wherein changes as small as 0.1 {\AA} result in easily detectable SXR-SHG spectral shifts (ca. 100s of meV). As SXR-SHG is inherently ultrafast and sensitive to individual atomic layers, it creates the possibility to study a variety of interfacial processes, e.g. catalysis, with ultrafast time resolution and bond specificity.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1982

Theoretical studies of photoexcitation and ionization in H2O

Theoretical studies using Franck-Condon and static-exchange approximations are reported for the complete dipole excitation and ionization spectrum in H2O, where (1) large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground state equilibrium geometry, and (2) previously devised moment-theory techniques are employed in constructing the continuum oscillator-strength densities from the calculated spectra. Comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact-excitation cross sections, and dipole and synchrotron-radiation studies of partial-channel photoionization cross sections. The calculated partial-channel cross sections are found to be atomic-like, and dominated by 2p-kd components. It is suggested that the latter transition couples with the underlying 1b(1)-kb(1) channel, accounting for a prominent feature in recent synchrotron-radiation measurements.

Diercksen, G. H. F.↗
NASA NTRS1987

Reactions of Atomic Oxygen (O(3P)) with Polybutadienes and Related Polymers

Thin films of the following polymers were exposed at ambient temperature to ground-state oxygen atoms (O(3P)), generated by a radio-frequency glow discharge in O2: cis- and trans-1,4-polybutadienes (CB and TB), amorphous 1,2-polybutadiene (VB), polybutadienes with different 1,4/1,2 contents, trans polypentenamer (TP), cis and trans polyoctenamers (CO and TO), and ethylene-propylene rubber (EPM). Transmission infrared spectra of CB and TB films revealed extensive surface recession, or etching, unaccompanied by any microstructural changes within the films, demonstrating that the reactions were confined to the surface layers. Contrary to the report by Rabek, Lucki, and Ranby (1979), there was no O(3P)-induced cis-trans isomerization in CB or TB. From weight-loss measurements, etch rates for polybutadienes were found to be markedly dependent on vinyl content, decreasing by two orders of magnitude from CB (2% 1,2) to structures with 30 to 40% 1,2 double bonds, thereafter increasing by half an order of magnitude to VB (97% 1,2). Relative etch rates for EMP and the polyalkenamers were in the order: EMP is greater than CO (or TO) is greater than TP is greater than CB. The sole non-elastomer examined, TB, had an etch rate about six times that of CB, ascribable to a morphology difference. Cis/trans content had a negligible effect on the etch rate of the polyalkenamers. Mechanisms involving crosslinking through units are proposed for the unexpected protection imparted to polybutadienes by the 1,2 double bonds.

Golub, Morton A.↗
NASA NTRS1988

Multichannel processes of H2O in the 18 eV region

Measurements were made of: (1) the fluorescence cross sections of OH(A 2Sigma+) fragments; (2) the absolute cross sections producing H atoms in the n = 2, 3, and 4 states; (3) the cross section for producing excited O atoms which has an upper limit of 5 x 10 to the -21 sq cm; and (4) the fluorescence cross section for producing H2(a 3Sigma g +) fragments. It is shown that, in the 16-20 eV region, there are excellent correspondences in the peak positions and spacings among the photoabsorption, photoionization spectra, and fluorescence functions of OH(A) and H(n).

Wu, C. Y. Robert↗
DOE OSTI2023

Manipulation of Scattering Spectra with Topology of Light and Matter

Structured lights, including beams carrying spin and orbital angular momenta, radially and azimuthally polarized vector beams, as well as spatiotemporal optical vortices, have attracted significant interest due to their unique amplitude, phase front, polarization, and temporal structures, enabling a variety of applications in optical and quantum communications, micromanipulation, and super-resolution imaging. In parallel, structured optical materials, metamaterials, and metasurfaces consisting of engineered unit cells—meta-atoms, opened new avenues for manipulating the flow of light and optical sensing. While several studies explored structured light effects on the individual meta-atoms, their shapes are largely limited to simple spherical geometries. However, the synergy of the structured light and complex-shaped meta-atoms has not been fully explored. Here, in this paper, the role of the helical wavefront of Laguerre–Gaussian beams in the excitation and suppression of higher-order resonant modes inside all-dielectric meta-atoms of various shapes, aspect ratios, and orientations, is demonstrated and the excitation of various multipolar moments that are not accessible via unstructured light illumination is predicted. The presented study elucidates the role of the complex phase distribution of the incident light in shape-dependent resonant scattering, which is of utmost importance in a wide spectrum of applications ranging from remote sensing to spectroscopy.

42 ENGINEERING↗
DOE OSTI2025

PAH101: A GW+BSE Dataset of 101 Polycyclic Aromatic Hydrocarbon (PAH) Molecular Crystals

Abstract The excited-state properties of molecular crystals are important for applications in organic electronic devices. TheGWapproximation and Bethe-Salpeter equation (GW+BSE) is the state-of-the-art method for calculating the excited-state properties of crystalline solids with periodic boundary conditions. We present the PAH101 dataset ofGW+BSE calculations for 101 molecular crystals of polycyclic aromatic hydrocarbons (PAHs) with up to ~500 atoms in the unit cell. To the best of our knowledge, this is the firstGW+BSE dataset for molecular crystals. The data records include theGWquasiparticle band structure, the fundamental band gap, the static dielectric constant, the first singlet exciton energy (optical gap), the first triplet exciton energy, the dielectric function, and optical absorption spectra for light polarized along the three lattice vectors. The dataset can be used to (i) discover materials with desired electronic/optical properties, (ii) identify correlations between DFT andGW+BSE quantities, and (iii) train machine learned models to help in materials discovery efforts.

Science & Technology - Other Topics↗
DOE OSTI2023

Size distribution of polycyclic aromatic hydrocarbons in space: an old new light on the 11.2/3.3 μm intensity ratio

The intensity ratio of the 11.2/3.3 μm emission bands is considered to be a reliable tracer of the size distribution of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). This paper describes the validation of the calculated intrinsic infrared (IR) spectra of PAHs that underlie the interpretation of the observed ratio. The comparison of harmonic calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra reveals a consistent underestimation of the 11.2/3.3 μm intensity ratio by 34%. IR spectra based on higher level anharmonic calculations, on the other hand, are in very good agreement with the experiments. While there are indications that the 11.2/3.3 μm ratio increases systematically for PAHs in the relevant size range when using a larger basis set, it is unfortunately not yet possible to reliably calculate anharmonic spectra for large PAHs. Based on these considerations, we have adjusted the intrinsic ratio of these modes and incorporated this in an interstellar PAH emission model. This corrected model implies that typical PAH sizes in reflection nebulae such as NGC 7023 – previously inferred to be in the range of 50 to 70 carbon atoms per PAH are actually in the range of 40 to 55 carbon atoms. The higher limit of this range is close to the size of the C 60 fullerene (also detected in reflection nebulae), which would be in line with the hypothesis that, under appropriate conditions, large PAHs are converted into the more stable fullerenes in the ISM.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1977

Computer programs for the interpretation of low resolution mass spectra: Program for calculation of molecular isotopic distribution and program for assignment of molecular formulas

Two FORTRAN computer programs for the interpretation of low resolution mass spectra were prepared and tested. One is for the calculation of the molecular isotopic distribution of any species from stored elemental distributions. The program requires only the input of the molecular formula and was designed for compatability with any computer system. The other program is for the determination of all possible combinations of atoms (and radicals) which may form an ion having a particular integer mass. It also uses a simplified input scheme and was designed for compatability with any system.

Miller, R. A.↗
NASA NTRS1984

On the acceleration of energetic ions in Jupiter's magnetosphere

Several aspects of the problem of high-energy ions in the Jovian magnetosphere are addressed. Voyager observations pertaining to the problem of high-energy ions in the magnetosphere are summarized, and the charge exchange emission of fast neutral sulfur and oxygen atoms and their subsequent recapture by electron impact, charge exchange, and photoionization is considered. Solutions are given to the diffusion equation assuming a source of ions injected with a gyroenergy corresponding to pickup in the middle and outer magnetosphere. It is concluded that no reasonable model parameters exist to produce the required steep spectra of the particle observations with only pickup and adiabatic radial diffusion included. A local acceleration mechanism based on nonadiabatic wave-particle interactions is needed. The assumptions and model predictions of stochastic acceleration by MHD turbulence for the Jovian magnetosphere are described. The model makes a specific correspondence between MHD wave spectrum properties and particle spectrum properties at energies above the Alfven energy.

Barbosa, D. D.↗
NASA NTRS1988

Imaging graphite in air by scanning tunneling microscopy - Role of the tip

Atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact have been obtained. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current-voltage (I-V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. The observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.

Colton, R. J.↗
NASA NTRS2006

Accurate VUV Laboratory Measurements of Fe III Transitions for Astrophysical Applications

We report preliminary measurements of Fe III spectra in the 1150 to 2500 A wavelength interval. Spectra have been recorded with an iron-neon Penning discharge lamp (PDL) between 1600 and 2500 A at Imperial College (IC) using high resolution Fourier (FT) transform spectroscopy. These FT spectrometer measurements were extended beyond 1600 A to 1150 A using high-resolution grating spectroscopy at the National Institute of Standards and Technology (NIST). These recorded spectra represent the first radiometrically calibrated measurements of a doubly-ionized iron-group element spectrum combining the techniques of vacuum ultraviolet FT and grating spectroscopy. The spectral range of the new laboratory measurements corresponds to recent HST/STIS observations of sharp-lined B stars and of Eta Carinae. The new improved atomic data can be applied to abundance studies and diagnostics of astrophysical plasmas.

Blackwell-Whitehead, R. J.↗
NASA NTRS2006

White-Light Whispering-Gallery-Mode Optical Resonators

Whispering-gallery-mode (WGM) optical resonators can be designed to exhibit continuous spectra over wide wavelength bands (in effect, white-light spectra), with ultrahigh values of the resonance quality factor (Q) that are nearly independent of frequency. White-light WGM resonators have potential as superior alternatives to (1) larger, conventional optical resonators in ring-down spectroscopy, and (2) optical-resonator/electro-optical-modulator structures used in coupling of microwave and optical signals in atomic clocks. In these and other potential applications, the use of white-light WGM resonators makes it possible to relax the requirement of high-frequency stability of lasers, thereby enabling the use of cheaper lasers. In designing a white-light WGM resonator, one exploits the fact that the density of the mode spectrum increases predictably with the thickness of the resonator disk. By making the resonator disk sufficiently thick, one can make the frequency differences between adjacent modes significantly less than the spectral width of a single mode, so that the spectral peaks of adjacent modes overlap, making the resonator spectrum essentially continuous. Moreover, inasmuch as the Q values of the various modes are determined primarily by surface Rayleigh scattering that does not depend on mode numbers, all the modes have nearly equal Q. By use of a proper coupling technique, one can ensure excitation of a majority of the modes. For an experimental demonstration of a white-light WGM resonator, a resonator disk 0.5-mm thick and 5 mm in diameter was made from CaF2. The shape of the resonator and the fiberoptic coupling arrangement were as shown in Figure 1. The resonator was excited with laser light having a wavelength of 1,320 nm and a spectral width of 4 kHz. The coupling efficiency exceeded 80 percent at any frequency to which the laser could be set in its tuning range, which was >100-GHz wide. The resonator response was characterized by means of ring-down tests in which the excitation was interrupted by a shutter having a rise and a fall time of 5 ns. The ring-down time of photodiodes and associated circuitry used to measure the interrupted excitation and the resonator output was <1 ns. Figure 2 shows the shapes of representative input and output light pulses. The average ring-down time was found to be 120 ns, corresponding to Q=2x10(exp 8). The variations of Q with the laser carrier frequency were found to be <5 percent. Hence, the resonator was shown to have the desired white light properties.

Matsko, Andrey↗