493 research outputs found
A System of Subroutines For Iteratively Reweighted Least Squares Computations
A description of a system of subroutines to compute solutions to the iteratively reweighted least squares problem is presented. The weights are determined from the data and linear fit and are computed as functions of the scaled residuals. Iteratively reweighted least squares is a part of robust statistics where "robustness" means relative insensitivity to moderate departures from assumptions. The software for iteratively reweighted least squares is cast as semi-portable Fortran code whose performance is unaffected (in the sense that performance will not be degraded) by the computer or operating-system environment in which it is used. An [ell sub1] start and an [ell sub2] start are provided. Eight weight functions, a numerical rank determination, convergence criterion, and a stem-and-leaf display are included.
Exploring Pd adsorption, diffusion, permeation, and nucleation on bilayer SiO<sub>2</sub>/Ru as a function of hydroxylation and precursor environment: From UHV to catalyst preparation
The hydroxylation-dependent permeability of bilayer SiO2 supported on Ru(0001) was investigated by XPS and TDS studies in a temperature range of 100K to 600K. For this, the thermal behavior of Pd evaporated at 100K, which results in surface and sub-surface (Ru-supported) binding arrangements, was examined relative to the extent of pre-hydroxylation. Samples containing only defect-mediated hydroxyls showed no effect on Pd diffusion through the film at low temperature. If, instead, the concentration of strongly bound hydroxyl groups and associated weakly bound water molecules was enriched by an electron-assisted hydroxylation procedure, the probability for Pd diffusion through the film is decreased via a pore-blocking mechanism. Above room temperature, all samples showed similar behavior, reflective of particle nucleation above the film and eventual agglomeration with any metal atoms initially binding beneath the film. When depositing Pd onto the same SiO2/Ru model support via adsorption of [Pd(NH3)4]C2 from alkaline (pH12) precursor solution, we observe notably different adsorption and nucleation mechanisms. The resultant Pd adsorption complexes follow established decomposition pathways to produce model catalyst systems compatible with those created exclusively within UHV despite lacking the ability to penetrate the film due to the increased size of the initial Pd precursor groups
High-intensity two-frequency photoassociation spectroscopy of a weakly bound molecular state: theory and experiment
We investigate two-frequency photoassociation of a weakly bound molecular
state, focusing on a regime where the ac Stark shift is comparable to the
halo-state energy. In this "high-intensity" regime, we observe features absent
in low-intensity two-frequency photoassociation. We experimentally measure the
spectra of Sr atoms coupled to the least bound state of the
Sr ground electronic channel through an intermediate electronically
excited molecular state. We compare the spectra to a simple three-level model
that includes a two-frequency drive on each leg of the transition. With
numerical solution of the time-dependent Schrodinger equation, we show that
this model accurately captures (1) the existence of experimentally observed
satellite peaks that arise from nonlinear processes, (2) the locations of the
two-photon peak in the spectrum, including ac Stark shifts, and (3) in some
cases, spectral lineshapes. To better understand these numerical results, we
develop an approximate treatment of this model, based on Floquet and
perturbation theory, that gives simple formulas that accurately capture the
halo-state energies. We expect these expressions to be valuable tools to
analyze and guide future two-frequency photoassociation experiments.Comment: 12 pages, 7 figure
Enantiomerically Pure Tetravalent Neptunium Amidinates: Synthesis and Characterization
The synthesis of a tetravalent neptunium amidinate [NpCl((S )âPEBA)] (1 ) ((S )âPEBA=(S ,S )âN ,NâČâbisâ(1âphenylethyl)âbenzamidinate) is reported. This complex represents the first structurally characterized enantiopure transuranic compound. Reactivity studies with halide/pseudohalides yielding [NpX((S )âPEBA)] (X=F (2 ), Br (3 ), N3 (4 )) have shown that the chiralityâatâmetal is preserved for all compounds in the solid state. Furthermore, they represent an unprecedented example of a structurally characterized metalâorganic Np complex featuring a NpâBr (3 ) bond. In addition, 4 is the only reported tetravalent transuranic azide. All compounds were additionally characterized in solution using paraâmagnetic NMR spectroscopy showing an expected Câsymmetry at low temperatures
Electron stimulated hydroxylation of a metal supported silicate film
Water adsorption on a double-layer silicate film was studied by using infrared reflectionâabsorption spectroscopy, thermal desorption spectroscopy and scanning tunneling microscopy. Under vacuum conditions, small amounts of silanols (SiâOH) could only be formed upon deposition of an ice-like (amorphous solid water, ASW) film and subsequent heating to room temperature. Silanol coverage is considerably enhanced by low-energy electron irradiation of an ASW pre-covered silicate film. The degree of hydroxylation can be tuned by the irradiation parameters (beam energy, exposure) and the ASW film thickness. The results are consistent with a generally accepted picture that hydroxylation occurs through hydrolysis of siloxane (SiâOâSi) bonds in the silica network. Calculations using density functional theory show that this may happen on SiâOâSi bonds, which are either parallel (i.e., in the topmost silicate layer) or vertical to the film surface (i.e., connecting two silicate layers). In the latter case, the mechanism may additionally involve the reaction with a metal support underneath. The observed vibrational spectra are dominated by terminal silanol groups (Îœ(OD) band at 2763 cmâ1) formed by hydrolysis of vertical SiâOâSi linkages. Film dehydroxylation fully occurs only upon heating to very high temperatures (âŒ1200 K) and is accompanied by substantial film restructuring, and even film dewetting upon cycling hydroxylation/dehydroxylation treatment
Frustrated H-Induced Instability of Mo(110)
Using helium atom scattering Hulpke and L"udecke recently observed a giant
phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An
explanation which is able to account for this and other experiments is still
lacking. Below we present density-functional theory calculations of the atomic
and electronic structure of the clean and hydrogen-covered Mo(110) surfaces.
For the full adsorbate monolayer the calculations provide evidence for a strong
Fermi surface nesting instability. This explains the observed anomalies and
resolves the apparent inconsistencies of different experiments.Comment: 4 pages, 2 figures, submitted to PR
Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO<sub>4</sub>] tetrahedra
The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO4] tetrahedra strongly bonded to the Ru(0001) surface through the SiâOâRu linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO4] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO4]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., [similar]1135 cm-1 for a monolayer film, [similar]1300 cmâ»-1 for the bilayer structures, and [similar]1250 cmâ»-1 for the bulk-like vitreous silica
Comparison of Neurite Orientation Dispersion and Density Imaging and Two-Compartment Spherical Mean Technique Parameter Maps in Multiple Sclerosis
BACKGROUND: Neurite orientation dispersion and density imaging (NODDI) and the spherical mean technique (SMT) are diffusion MRI methods providing metrics with sensitivity to similar characteristics of white matter microstructure. There has been limited comparison of changes in NODDI and SMT parameters due to multiple sclerosis (MS) pathology in clinical settings. PURPOSE: To compare group-wise differences between healthy controls and MS patients in NODDI and SMT metrics, investigating associations with disability and correlations with diffusion tensor imaging (DTI) metrics. METHODS: Sixty three relapsing-remitting MS patients were compared to 28 healthy controls. NODDI and SMT metrics corresponding to intracellular volume fraction (v_{in}), orientation dispersion (ODI and ODE), diffusivity (D) (SMT only) and isotropic volume fraction (v_{iso}) (NODDI only) were calculated from diffusion MRI data, alongside DTI metrics (fractional anisotropy, FA; axial/mean/radial diffusivity, AD/MD/RD). Correlations between all pairs of MRI metrics were calculated in normal-appearing white matter (NAWM). Associations with expanded disability status scale (EDSS), controlling for age and gender, were evaluated. Patient-control differences were assessed voxel-by-voxel in MNI space controlling for age and gender at the 5% significance level, correcting for multiple comparisons. Spatial overlap of areas showing significant differences were compared using Dice coefficients. RESULTS: NODDI and SMT show significant associations with EDSS (standardised beta coefficient â0.34 in NAWM and â0.37 in lesions for NODDI vin; 0.38 and â0.31 for SMT ODE and vin in lesions; p < 0.05). Significant correlations in NAWM are observed between DTI and NODDI/SMT metrics. NODDI vin and SMT vin strongly correlated (r = 0.72, p < 0.05), likewise NODDI ODI and SMT ODE (r = â0.80, p < 0.05). All DTI, NODDI and SMT metrics detect widespread differences between patients and controls in NAWM (12.57% and 11.90% of MNI brain mask for SMT and NODDI v_{in}, Dice overlap of 0.42). DATA CONCLUSION: SMT and NODDI detect significant differences in white matter microstructure between MS patients and controls, concurring on the direction of these changes, providing consistent descriptors of tissue microstructure that correlate with disability and show alterations beyond focal damage. Our study suggests that NODDI and SMT may play a role in monitoring MS in clinical trials and practice
Aromaticity in a Surface Deposited Cluster: Pd on TiO (110)
We report the presence of \sigma-aromaticity in a surface deposited cluster,
Pd on TiO (110). In the gas phase, Pd adopts a tetrahedral
structure. However, surface binding promotes a flat, \sigma-aromatic cluster.
This is the first time aromaticity is found in surface deposited clusters.
Systems of this type emerge as a promising class of catalyst, and so
realization of aromaticity in them may help to rationalize their reactivity and
catalytic properties, as a function of cluster size and composition.Comment: 4 pages, 3 figure
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