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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Density Functional Theory Study of the Adsorption and Dissociation of Copper(I) Acetamidinates on Ni(110): The Effect of the Substrate

In order to assess the role of a metal substrates on the thermal chemistry of adsorbed acetamidinate metalorganic compounds, we have studied the surface chemistry of copper(I)–N,N'-dimethylacetamidinate on Ni(110) using density functional theory and contrasted it with similar calculations we previously carried out on Cu(110). At low coverages, it was found that, in its most stable configuration, the molecular adsorption of copper(I)–N,N'-dimethylacetamidinate dimers occurs with the Cu atoms occupying surface hollow sites. In this work, the ligands reorient away from those metal centers, and the N atoms develop new direct bonds with surface Ni atoms. However, this configuration is not stable and decomposes by losing both ligands to the surface. In the final state, the two ligands bind via their N atoms to Ni sites one lattice space away from the sites where the Cu atoms remain, with their molecular planes perpendicular to that of the surface. This is in contrast with what happens in the case of adsorption on Cu(110), where the Cu atoms from the metalorganic complex still occupy hollow sites but where only one of the ligands breaks away and binds directly to the surface; the other remains on top of the two Cu ions. In terms of the energetics of adsorption and decomposition, the reactions on Ni(110) are much more exothermic than on Cu(110). Further analysis of the distribution of charge within the adsorbates shows a minor reduction of the Cu atoms of the dimer upon interaction with the surface; full reduction to metallic copper is complete only when both ligands have fully migrated to their new Ni surface sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Adsorption Properties of Au−Ni Surface Alloys with a Nonstoichiometric Moiré Structure: A Density Functional Theory Study

Due to the large lattice mismatch between gold and nickel, gold–nickel surface alloys can form unique nonstoichiometric overlayer structures characterized by a moiré pattern and subsurface defects. For this work, we performed density functional theory (DFT) calculations to study the adsorption of molecular oxygen, atomic hydrogen, and atomic carbon on a gold–nickel(111) surface alloy with 0.46 monolayer gold randomly distributed in the surface layer. We observed six distinct adsorption structures for molecular oxygen characterized by intramolecular stretching frequencies of <700, 729, 795, 857, 929, and 1004 cm –1 , which describe well the experimentally observed high-resolution electron energy-loss spectra. Surface atomic hydrogen adsorption is associated with adsorbate–surface modes in the ∼1000 cm –1 range, while subsurface hydrogen can have features as low as ∼400 cm –1 . We observed a unique adsorption structure for atomic carbon inside the surface dislocation loop defect, which explains the experimentally observed low carbon-surface mode at ∼340 cm –1 . Our study sheds light on the unique adsorption properties of the gold–nickel surface alloys and helps with rationalizing vibrational frequency experimental studies for this system.

adsorption↗
NASA NTRS1969

A theory of theories

Theory comparisons with facts obtained by reduction of observed data

Sturrock, P. A.↗
NASA NTRS1972

On the use of the Hill variables in artificial satellite theory - Brouwer's theory.

By reformulating Brouwer's first-order satellite theory in terms of the Hill variables, it is demonstrated that these variables possess several advantages over the Delaunay variables. The final algorithm is simpler and more compact, there is no singularity at zero eccentricity, and the process of deriving the perturbations is considerably simplified. An algorithm for the computation of position and velocity is presented which is significantly simpler and faster in use than Brouwer's algorithm. Moreover, whereas the latter algorithm breaks down for circular and equatorial orbits, the present one holds for all eccentricities less than one and for all inclinations, with the exception of the critical inclination.

Aksnes, K.↗
NASA NTRS1974

Correlation effects in the theory of combined Doppler and pressure broadening. I - Classical theory

An investigation is conducted of the combined effects of radiator-perturber collisions and radiator translational motion in the context of foreign gas broadening of optical transitions in neutral radiators. Questions concerning the speed-dependent collision frequency are considered and aspects of general theory are explored, taking into account the correlation function, the ensemble average, and the kinetic equation formalism. An elementary solution is discussed along with a one-perturber approximation, inverse power law model calculations, and a comparison with the Voigt profile.

Ward, J.↗
NASA NTRS1975

Observations and theory of Mg II lines in early type stars. II - Theory and predicted profiles. III - The observations and a comparison with the predictions

Profiles of the UV Mg II lines in the spectra of early type stars are computed in a grid of model atmospheres with effective temperatures between 8000 and 35,000 K and log g values of 2.5 to 4.0 using the LTE and non-LTE theories of line formation. The theoretical results indicate that the line cores are strengthened by non-LTE effects over the entire temperature range, resonance-line wings are weaker in the cooler models than in the LTE case, and very large deviations from LTE occur in the hot low-gravity models. These predictions are compared with the equivalent widths of the UV Mg II lines in 106 stars and of the visual lines in 48 stars (spectral types O4 to A3). The observed equivalent widths of normal stars in luminosity classes II through V are found to agree with the predictions over the entire range from O8 to A2 if a certain Mg/H abundance is adopted. The line intensities observed in supergiants, Be, Bp, and Ap stars are discussed.

Snijders, M. A. J.↗
NASA NTRS1976

Kinetic theory of turbulent compressible flows and comparison with classical theory

A complete set of turbulent correlations is given in terms of expansion coefficients in a double series of Hermite polynomials of a two-particle correlation in phase space. Only two of these coefficients, corresponding to Reynolds stress and turbulent heat flux, are shown to appear in gasdynamic equations of turbulence. This is accomplished by incorporating the phase-space correlation in the collision integral of Boltzmann's equation and by deriving generalized Navier-Stokes and Fourier transport relations. In contrast to the classical formalism, the simple expression remains invariant in form whether the flow is compressible or not. Nevertheless, a bilinear transformation of fluctuating quantities shows that the two formalisms are identical with regard to terms of double correlations. The kinetic theory justifies nonexistence of the higher-order correlations characteristic of compressible-turbulence equations in the classical regime.

Tsuge, S.↗
NASA NTRS1979

A theory of local and global processes which affect solar wind electrons. I - The origin of typical 1 AU velocity distribution functions - Steady state theory

A kinetic theory for the velocity distribution of solar wind electrons which illustrates the global and local properties of the solar wind expansion is proposed. By means of the Boltzmann equation with the Krook collision operator accounting for Coulomb collisions, it is found that Coulomb collisions determine the population and shape of the electron distribution function in both the thermal and suprathermal energy regimes. For suprathermal electrons, the cumulative effects of Coulomb interactions are shown to take place on the scale of the heliosphere itself, whereas the Coulomb interactions of thermal electrons occur on a local scale near the point of observation (1 AU). The bifurcation of the electron distribution between thermal and suprathermal electrons is localized to the deep solar corona (1 to 10 solar radii).

Scudder, J. D.↗