Lifetime measurements,infrared and photoacoustic spectroscopy of NdP5O14

We have made a series of spectroscopic measurements to better understand the optical properties and concentration quenching characteristics of the stoichiometric laser material NdP₅O₁₄. The results of fluorescence lifetime measurements in the presence of different surface environments indicates that the surface condition affects the concentration quenching whereas lifetime measurements under applied uniaxial stress show that the presence of internal strains in the crystal is not significantly effective in fluorescence quenching. A comparison of the internal reflection spectrum with the normal infrared spectrum did not reveal any significant differences in the Nd³⁺ energy levels. Photoacoustic spectroscopy results proved difficult to interpret but appear to be consistent with the presence of surface quenching of excitons and indicate that the intrinsic quantum efficiency of Nd³⁺ ions in the pentaphosphate host in the absence of concentration quenching is approximately 0.90.     

Error Estimation for Discontinuous Galerkin Solutions of Two-Dimensional Hyperbolic Problems

We analyze the discretization errors of discontinuous Galerkin solutions of steady two-dimensional hyperbolic conservation laws on unstructured meshes. We show that the leading term of the error on each element is a linear combination of orthogonal polynomials of degrees p and p+1. We further show that there is a strong superconvergence property at the outflow edge(s) of each element where the average discretization error converges as O(h ^2p+1) compared to a global rate of O(h ^p+1). Our analyses apply to both linear and nonlinear conservation laws with smooth solutions. We show how to use our theory to construct efficient and asymptotically exact a posteriori discretization error estimates and we apply these to some examples.     

An integrable structure for type D spacetimes

A structure analogous to an almost complex structure on a manifold is presented. It is integrable for spacetime manifolds admitting two geodesic and shear-free null congruences. This fact sheds light on Newman's “complex coordinate transformations”.     

Observation of a metastable periodic structure for the (001) surface of KTaO3 after cleaving in situ

Helium atom diffraction experiments carried out under ultrahigh vacuum conditions on a freshly cleaved (001) surface of KTaO3 reveal metastable features which decay over a period of several hours. The initial He diffraction pattern contains large scattering intensity satellite peaks very close to the specular reflection beam. As time from cleaving elapses, the satellite intensities diminish virtually to zero while the specular intensity increases, and the diffraction pattern evolves into one consistent with the (1x1) bulk termination surface. The data are compared with model calculations for scattering from a series of terraces at two heights with a distribution of terrace lengths [Surf. Sci. 384, 15 (1997)].     

A method for the low-level (ng g(-1)) determination of perfluorooctanoate in paper and textile by liquid chromatography with tandem mass spectrometry

The determination of perfluorooctanoate (PFO) in articles of commerce has become increasingly important to understand if treated products are a possible source of PFO. An LC-MS/MS method for the determination of PFO in paper and textile using a dual labeled 13C-PFOA internal standard was successfully developed and validated. Residues of PFO were determined using an isocratic, reversed-phase high-performance liquid chromatography (HPLC) method with an ammonium acetate/methanol buffer. Ions monitored were 413 (parent) and 369 (daughter) for PFO and 415 (parent) and 370 (daughter) for dual labeled 13C-PFOA internal standard. As a precaution against ubiquitous PFO that occasionally occurs in mobile phase or instrument components, two Hypercarb cartridges (4 mm) were placed before the HPLC injector. Any PFO that was captured by the cartridges was removed before each injection by flushing the system with 100% methanol prior to equilibration with the isocratic mobile phase. Overall recovery and standard deviation over a 3 day validation regimen for samples (n=54-55) fortified with PFOA at 5, 50, and 200 ng g(-1) were 114+/-4.9% for textile and 110+/-7.6% for paper. The results also established a limit of detection (LOD) of 1 ng g(-1) in textile and 2 ng g(-1) in paper based upon S/N of the 5.0 ng g(-1) fortification versus the untreated paper and textile.     

Parallel adaptive hp-refinement techniques for conservation laws

We describe an adaptive hp-refinement local finite element procedure for the parallel solution of hyperbolic systems of conservation laws on rectangular domains. The local finite element procedure utilizes spaces of piecewise-continuous polynomials of arbitrary degree and coordinated explicit Runge-Kutta temporal integration. A solution limiting procedure produces monotonic solutions near discontinuities while maintaining high-order accuracy near smooth extrema. A modified tiling procedure maintains processor load balance on parallel, distributed-memory MIMD computers by migrating finite elements between processors in overlapping neighborhoods to produce locally balanced computations. Grids are stored in tree data structures, with finer grids being offspring of coarser ones. Within each grid, AVL trees simplify the transfer of information between neighboring processors and the insertion and deletion of elements as they migrate between processors. Computations involving Burgers' and Euler's equations of inviscid flow demonstrate the effectiveness of the hp-refinement and balancing procedures relative to non-balanced adaptive and balanced non-adaptive procedures.     

Laser site-selection time-resolved spectroscopy of NdP5O14

Laser site-selection spectroscopy shows the presence of Nd ions in non- equivalent crystal field sites in NPP. Time-resolved spectroscopy indicates that energy transfer does not take place between ions in these different types of sites.