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Polycyclic aromatic hydrocarbons and fullerenes as decomposition products of hydrogenated amorphous carbon

ASTROPHYSICAL JOURNAL 1997年第2期489卷 L193-L195页

作者： Scott, A Duley, WW Pinho, GP Guelph Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo ON Canada N2L 3G1

The gaseous products evolved from solid hydrogenated amorphous carbon (HAC) under UV irradiation have been sampled using time of flight (TOF) mass spectrometry. A notable feature is the appearance of an extended sequence of aromatic carbon clusters containing in excess of 30 carbon atoms and including fullerenes such as C-50, C-60, and C-70. Under certain conditions, clusters containing up to several hundred C atoms are detected. The radiation-induced decomposition of HAC is found to occur in a series of stages in which small (<20 atom) molecules including alkanes and unsaturated carbon chain (C=C)(n)H-m radicals with m = 1-4 are the first to be released. There is also evidence in these mass spectra for the ejection of small dehydrogenated polycyclic aromatic hydrocarbon (PAH) molecules. After these lighter molecules have been released, we observe a strong mass peak near 500 amu corresponding to clusters with similar to 40 C atoms. These clusters, which are protographitic aromatic structures, have been detected in IR spectra of HAC and show absorption and emission features at 3.3, 3.4, and 6.2 mu m and other wavelengths characteristic of PAH molecules. These data show that the decomposition of HAC in interstellar shocks may be a source of large PAH and fullerene molecules.

关键词： infrared, ISM, lines and bands ISM, molecules molecular processes

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Infrared emission spectra from hydrogenated amorphous carbon

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ASTROPHYSICAL JOURNAL 1997年第2期490卷 L175-L177页

作者： Scott, AD Duley, WW Jahani, HR Guelph Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

Thermal emission spectra from hydrogenated amorphous carbon (HAC) over the 2.5-15 mu m range have been obtained at temperatures between 573 and 773 K. These spectra are similar, but not identical, to absorption spectra of HAC samples subjected to the same thermal cycle. A distinct 3.29 mu m aromatic CH emission is found in samples heated to temperatures in excess of 723 K. This emission is also observed at lower temperatures in samples that have been thermally cycled to higher temperature. Laboratory emission spectra from HAC are shown to provide a good simulation of 3.2-3.6 mu m emission from dust in the extended atmospheres of post-asymptotic giant branch objects. The ratio of 3.4 and 3.3 mu m emission in such objects may be a useful indicator of evolutionary status.

关键词： infrared, ISM, lines and bands ISM, molecules stars, AGB and post-AGB

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Self-consistent-field theory of a brush of randomly branched polymers

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Physical Review E 1997年第2期55卷 1660-1660页

作者： Shi-Min Cui Zheng Yu Chen Guelph-Waterloo Program for Graduate Work in Physics and Department of Physics University of Waterloo

The conformational properties of randomly branched polymers grafted at one end on a planar surface in the good solvent regime are investigated by using a Flory-type scaling theory and by solving a self-consistent-field model numerically. The average monomer height obtained from the self-consistent-field model is shown to be in agreement with the scaling behavior predicted from the scaling theory. The density profile is found to have a near-parabolic form with some discrepancies near the surface and the brush end.

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Nonexponential dynamic relaxation of randomly branched polymers in good solvents

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Physical Review E 1997年第6期56卷 7017-7017页

作者： Josh Kemp Zheng Yu Chen Guelph-Waterloo Program for Graduate Work in Physics and Department of Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

Monte Carlo simulations were carried out to study the dynamics of randomly branched polymers in good solvents. Two types of time scales were observed: fast relaxation times corresponding to the internal contraction motions and slower relaxation times corresponding to the overall rotational motions of the polymers. The former is associated with autocorrelation functions that exhibit nonexponential decay behavior, a signature of the dynamics of random systems. The latter is associated with the usual exponential decay behavior, typical of linear or regularly branched polymers.

关键词： Molecular dynamics

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Charge-density waves and density of states in the intercalated tunnel structure TlxNb3Te4

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Physical Review B 1997年第3期56卷 1175-1175页

作者： F. W. Boswell G. A. Scholz J. C. Bennett Guelph-Waterloo Program for Graduate Work in Physics Waterloo Campus University of Waterloo Ontario Canada N2L 3G1

Tunnel structures, into which a variety of atoms can be intercalated, offer a promising approach for investigation of the interaction between charge-density waves (CDW’s) and foreign atoms. For this purpose single crystals of TlxNb3Te4 with 0<~x<~1 were prepared. CDW’s were observed below room temperature for all crystals, despite the wide range of Tl content. The wave vectors were determined by electron diffraction and the transition temperatures by resistivity measurements. For x<0.25 the transition temperatures were less than those of the pure material, while for x>0.25 both the transition temperatures and wave vectors increased strongly and passed through a maximum near x=0.75. The results were interpreted in terms of the crystal structure and the variation of the electron density of states at the Fermi level, as additional electrons were contributed to the host crystal with increasing Tl content. This is an example of a quasi-one-dimensional, inorganic system in which the transition temperature and CDW vector can be conveniently tuned by adjustments of the intercalate content.

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Interface and chain confinement effects on the glass transition temperature of thin polymer films

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Physical Review E 1997年第5期56卷 5705-5705页

作者： J. A. Forrest K. Dalnoki-Veress J. R. Dutcher Department of Physics and Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature Tg of thin polystyrene (PS) films as a function of the film thickness h for two different molecular weights Mw. Three different film geometries were studied: freely standing films, films supported on a SiOx surface with the other film surface free (uncapped supported), and films supported on a SiOx surface and covered with a SiOx layer (capped supported). For freely standing films Tg is reduced dramatically from the bulk value by an amount that depends on both h and Mw. For h≲REE (the average end-to-end distance of the unperturbed polymer molecules), Tg decreases linearly with decreasing h with reductions as large as 60 K for both Mw values. We observe a large Mw dependence of the Tg reductions for freely standing films which provides the first strong evidence of the importance of chain confinement effects on the glass transition temperature of thin polymer films. For both the uncapped and capped supported films, Tg is reduced only slightly (<10K) from the bulk value, with only small differences in Tg (<4K) observed between uncapped and capped supported films of the same thickness. The results of our experiments demonstrate that the polymer-substrate interaction is the dominant effect in determining the glass transition temperature of PS films supported on SiOx.

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Post-irradiation flashes and continuous emission from solid deuterium

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Physical Review B 1997年第2期55卷 906-906页

作者： J. A. Forrest R. L. Brooks Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

Optical emission from proton-beam-irradiated solid deuterium near 800 nm has been studied after termination of the proton beam. The continuous red emission shows a residual intensity that persists over 30 min from termination of the beam. Optical flashes can also be thermally triggered over 10 min after termination of irradiation. Such triggered flashes are shown to quench the infrared absorption of Stark-shifted charge-induced features. Ultraviolet photons can stimulate this red emission after termination of irradiation with no measurable decrease in intensity for 40 min. The cause of this continuous emission and optical flashes is discussed in the light of these results.

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Surface-enhanced ordering and layer-thinning transitions in freely suspended smectic-A films

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Physical Review E 1997年第2期55卷 2030-2030页

作者： Y. Martínez-Ratón A. M. Somoza L. Mederos D. E. Sullivan Instituto de Ciencia de Materiales Consejo Superior de Investigaciones Científicas Cantoblanco Madrid E-28049 Spain Department of Physics and Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

We propose a physical mechanism which leads to surface-enhanced smectic-A ordering (SESO) at the free surface of a model liquid crystal. We also provide an explanation, based on a density-functional theory, for recent experimental results for the melting behavior of freely suspended smectic (FSS) films. It is shown that stepwise layer-thinning transitions do not usually occur during melting of FSS films, despite the presence of SESO. We find that thinning transitions similar to those observed experimentally occur under conditions such that the film interior melts to a nematic rather than isotropic liquid phase.

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X-ray natural widths, level widths and Coster-Kronig transition probabilities

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AIP Conference Proceedings 1997年第1期389卷 431-446页

作者： T. Papp J. L. Campbell D. Varga Institute of Nuclear Research of the Hungarian Academy of Sciences Debrecen Bem tér 18/c P.f. 51 H-4001 Hungary Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario N1G2W1 Canada

A critical review is given for the K-N7 atomic level widths. The experimental level widths were collected from x-ray photoelectron spectroscopy (XPS), x-ray emission spectroscopy (XES), x-ray spectra fluoresced by synchrotron radiation, and photoelectrons from x-ray absorption (PAX). There are only limited atomic number ranges for a few atomic levels where data are available from more than one source. Generally the experimental level widths have large scatter compared to the reported error bars. The experimental data are compared with the recent tabulation of Perkins et al. and of Ohno et al. Ohno et al. performed a many body approach calculation for limited atomic number ranges and have obtained reasonable agreement with the experimental data. Perkins et al. presented a tabulation covering the K-Q1 shells of all atoms, based on extensions of the Scofield calculations for radiative rates and extensions of the Chen calculations for non-radiative rates. The experimental data are in disagreement with this tabulation, in excess of a factor of two in some cases. A short introduction to the experimental Coster-Kronig transition probabilities is presented. It is our opinion that the different experimental approaches result in systematically different experimental data.

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Micro-PIXE analysis of silicate reference standards for trace Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo and Pb, with emphasis on Ni for application of the Ni-in-garnet geothermometer

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Canadian Mineralogist 1996年第1期34卷 37-37页

作者： Campbell, J.L. Teesdale, W.J. Kjarsgaard, B.A. Cabri, L.J. University of Guelph Guelph-Waterloo Program for Graduate Work in Physics Guelph ON Canada

Trace-element analysis is a powerful tool for studying numerous processes of research interest in the Earth Sciences; as well, it has important applications of interest to the mineral exploration community. Here, we present the results of a comprehensive study of five internationally recognized silicate reference standards (BHVO-1, GXR-5, GSR-5, MAG-1 and GSD-50) utilizing micro-PIXE analysis. Results of trace analyses for Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo and Pb are presented. Measured data for the five standards exhibit agreement within + or -10%, and commonly better, with recommended levels of concentration in the 50-400 ppm range. Detection limits (for a typical 3- to 4-minute analysis of an accumulated charge of 2.5 microCoulombs) are in the range of 2-10 ppm, with precisions of 1-10%. A detailed study of the X-ray spectra in the vicinity of the Fe, Ni and Co lines illustrates that the GUPIX software is able to correctly fit the complex spectra and thus provide accurate Ni concentrations. Further tests on Ni in garnet are reported, in the context of current application of the Ni-in-garnet geothermometer in diamond exploration; it appears important to resolve the difference between the two published calibrations of this important geothermometer.

关键词： Acccuracy Garnet Geostandards Glasses Micro-PIXE analysis Ni-in-garnet geothermometer Trace elements

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