Photoconductivity in high purity,semiconducting CdF2

The photoconductivity spectra of high purity, semiconducting CdF₂, containing low concentrations of rare-earth donors, are interpreted in terms of a shallow trap 1260 ± 100 cm⁻¹ below the bottom of the conduction band and an excited bound state 1030 cm⁻¹ above the ground state.     

Heterogeneous in vivo MR images of soft tissue tumors: guide to gross specimen sampling.

Twelve soft tissue tumors preoperatively studied with magnetic resonance (MR) imaging contained areas of inhomogeneous intralesional signal intensity. The pathologist's selection of biopsy sites from gross specimens was based on specific MR images selected by a radiologist and depended on close cooperation between the radiologist and pathologist. The histopathology so sampled, further depended on the lesion's size, careful gross specimen orientation, discrete sampling, and a specially developed Grid Analysis technique. Our initial experience indicates that (1) histology may vary within different and often neighboring sites of a single soft tissue lesion; (2) that such variability is common in soft tissue tumors; (3) that histological variability within different sites of a single lesion influences signal heterogeneity; and (4) that although differing MR signal intensities per se are not tissue specific, MR may enhance the histopathological accuracy of pathologists who can be directed to particular gross specimen sites by magnetic resonance images. To our knowledge such a specific, detailed effort has not been made.     

Clustering on surfaces

In this review we summarize the current theoretical and experimental understanding of clustering phenomena on surfaces, with an emphasis on dynamical properties. The basic theoretical concepts to predict evolving cluster size distributions are presented, with extensions to less restrictive assumptions, such as including the influence of non-zero deposition rates. The discussion of experimental results to test these concepts is preceded by a brief introduction of the experimental techniques used in morphological cluster studies. Finally, two important physical surface quantities, surface diffusion coefficients and adatom binding energies, are used to demonstrate the impact of clustering in their understanding.     

Trapping effects in the luminescence of Zn2GeO4:Mn2+

Free carriers are created in this material under sub-band-gap excitation and the effects of these on the luminescence are studied. The slow rise of emission, long afterglow and infrared stimulability are explained with a model utilizing an empirical set of configuration coordinate curves. The rise and afterglow curves are fitted to solutions to a rate equation which is based on the postulation of only one kind of trapping center.     

Extending Pummerer reaction chemistry. Synthesis of (+/-)-dibromophakellstatin by oxidative cyclization of an imidazole derivative

The diastereoselective oxidative cyclization of a dihydrooroidin derivative is reported. The thioimidate product formed by application of a new variant of the Pummerer reaction serves as a precursor to dibromophakellstatin. [reaction: see text]     

Resolution of GPC columns

A resolution index for gel-permeation chromatograph (GPC) columns is defined by the relation log (RI) = αW, where RI is the resolution index, α is the slope of the calibration curve of logarithm of molecular weight against peak position for narrow polymer fractions, and W is the width of the GPC curves for narrow fractions. This resolution index is calculated approximately for several GPC columns individually and in series combinations by using data obtained on narrow fractions of polystyrene. The index increases with number of columns and with flow rate over a limited range of flow rate, does not vary much with solvent, and does not correlate well with plate count obtained with low molecular weight molecules. It is believed that the resolution index provides an improved indication of the performance of GPC columns.     

A Forcing Mechanism for Spiral Density Waves in Galaxies

In this paper, we consider gas dynamical models of spiral galaxies in which there is a rigidly rotating, weakly barlike structure in the central regions. It is found that, in the neighborhood of the corotation circle, this barlike structure forces a trailing spiral wave. Such a driven wave could then propagate inwards to the bar and complete a feedback loop to maintain the spiral structure. For basic distributions typical of our Galaxy, if a ten-per-cent oval distortion is assumed, the strength of the induced spiral field is found to be of the order of a few per cent of the axisymmetric field, in agreement with observational data.     

The Debye-Waller factors for NbC

X-ray measurements and model calculations of the Debye-Waller factors, or mean-square displacements, for NbC are reported. The models used are the extended-shell model of Weber, Bilz and Schröder and the double-shell model of Weber. Excellent agreement between theory and experiment is achieved.