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Deviation from the single-particle model in the angular distribution of thorium L3 x rays in proton-impact ionization

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Physical Review A 1993年第4期48卷 3062-3062页

作者： T. Papp J. L. Campbell J. A. Maxwell Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

Angular distributions of L3 x-ray transitions, including electric-dipole-forbidden ones, were measured for 1-, 2-, and 3-MeV proton impact on a thorium target, where the excitation and detection systems were cylindrically symmetric. A sophisticated spectrum-analysis technique was applied, where both the Lorentzian broadening of the transitions and the Si(Li) detector response function with various tailing features were taken into account. The anisotropy-parameter ratios are expected to be independent of the ionization process, and to be characteristic of the x-ray transitions in the independent-particle model. The ratios of the anisotropy parameters of the electric-dipole-allowed transitions were in disagreement with this expectation, even when the higher-order multipole contributions were taken into account. These results follow the same trend as earlier angular distribution and angular correlation measurements. Additionally, the electric-dipole-forbidden LtL3M2) and Ls(L3M3) transitions had a P4(cos(θ)) term, which is not expected within the framework of the single-particle-model predictions. Possible causes for this discrepancy are explored.

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Reaction dynamics of metastable helium molecules and atoms near 4.2 K

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Physical Review A 1993年第1期48卷 364-364页

作者： D. W. Tokaryk R. L. Brooks J. L. Hunt Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

Infrared absorption spectra from metastable helium atoms (2s 3S, 2s 1S) and molecules (a Σu+3) were previously acquired by irradiating dense helium gas near 4.2 K with a pulsed proton beam [R. L. Brooks, J. L. Hunt, and D. W. Tokaryk, J. Chem. Phys. 91, 7408 (1989)]. The molecular spectrum was unusual because the observed rovibrational distribution within the a Σu+3 state was far from thermal equilibrium. Three different rovibrational groups were observed: (i) v=0, N=1 (‘‘thermal’’ molecules); (ii) v=0, 9≤N≤21 (rotationally excited molecules); and (iii) 10≤v≤12, N=1 (vibrationally excited molecules). In this work, the time evolutions of members of these three molecular populations were studied both during irradiation and in the subsequent afterglow. In addition, the evolution of the 2s 3S–2p 3P atomic line was investigated. This study quantitatively explores the reaction dynamics of the metastable molecule in the gas phase near 4.2 K. Gas pressures between 100 and 750 Torr were used. Time-resolved data were taken with a transient-digitizer system and summed for several thousand cycles of the pulsed proton beam. The absorption measurements were converted to time-resolved number densities with the aid of theoretical transition moments. The analysis required that the data be fit to the solutions of sets of coupled differential equations with a nonlinear least-squares-fit routine. The results provide insight into the complicated reactions involved in generating the unusual molecular distribution and into the reactions between the metastable molecules, metastable atoms, and the background helium gas.

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Stochastic Treatment of Adsorbate Local-Potential Effects on Surface-Ion Neutralization

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Physical Review Letters 1993年第24期71卷 4075-4075页

作者： Zoran L. Mišković Sydney G. Davison Frank O. Goodman Department of Applied Mathematics University of Waterloo Waterloo Ont. N2L 3G1 Canada Institute of Nuclear Sciences-Vinča Belgrade Department of Physics University of Waterloo Guelph-Waterloo Program for Graduate Work in Physics

The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newns Hamiltonian. The adsorbate-induced random modulation of the projectile orbital energy level is treated as a stochastic process, and its role in the charge transfer is evaluated analytically, in the Gaussian-process approximation, for low projectile speeds and low surface coverage by alkali atoms.

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Photoinduced photocurrent decay in non-hydrogenated amorphous silicon nitride prepared by ion-beam-assisted reactive deposition

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Philosophical Magazine B 1993年第6期67卷

作者： S. Zhang D. E. Brodie Guelph-Waterloo Program for Graduate Work in Physics Waterloo Campus Waterloo Ontario Canada N2L3G1

Non-hydrogenated amorphous silicon nitride (a-SiNx) films have been prepared using ion-beam-assisted reactive deposition and N2for the ion source gas. For a fixed ion beam voltage, a fixed Si deposition rate and a fixed substrate temperature, the N concentration in the film is determined by the ion beam current. At low N concentrations, the dark conductivities are dominated by hopping transport via the gap states. When the N content is increased, the gap state density is reduced and activated conductivities are observed. Room-temperature dark conductivities varying by ten orders of magnitude and optical bandgaps from 1.3 to 3.5 eV were obtained by changing the N concentration. Films with a ratioNN/NSiof N to Si content greater than 0.7 were photoconducting and this degraded slowly over time with exposure to a mercury light source. Since care was taken to eliminate H from these films and no H was detected in the films by infrared absorption, this would suggest that H may not be needed for the photoinduced degradation mechanism in a-SiNx. In addition, evidence is given that suggests sample polarization occurs and this could explain the photoconductivity fatigue observed.

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TRANSPORT-PROPERTIES OF ANTIFERROMAGNETIC SUPERCONDUCTORS

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JOURNAL OF LOW TEMPERATURE physics 1992年第1-2期86卷 139-160页

作者： CHI, HG NAGI, ADS 1. Guelph-Waterloo Program for Graduate Work in Physics Department of Physics Univesity of Waterloo Waterloo Ontario

Transport properties of antiferromagnetic superconductors with T(N) < T(c) have been investigated. Detailed numerical results are given for SmRh4B4 by using the following model. The paramagnetic phase (T(N) < T < T(c)) is described by using Abrikosov-Gorkov theory of magnetic ions in an ordinary superconductor. In the AF phase (T less-than-or-equal-to T(N)), the effects of the molecular field H(Q)(T) and the elastic scattering of conduction electrons from spin fluctuations are included. Expressions for H(Q)(T) and the scattering rate from spin fluctuations have been derived. The aim has been to see if properties are enhanced or depressed by the AF ordering occurring below T(N). It is found that whereas the electronic thermal conductivity K(s), nuclear spin relaxation rate R(s), and the longitudinal ultrasonic attenuation alpha(s), are depressed by the AF ordering, the inverse of the magnetic penetration depth, [lambda(T)]-1, is enhanced below T(N). The effect of all types of impurities is included in the study. Theoretical results for K(s) and [lambda(T)]-1 agree with the experimental values for SmRh4B4 (experimental data for other properties are not available in literature). In the above, T(N) is the Neel temperature and T(c) is superconducting transition temperature.

关键词： air feed equipment Not Available transport properties conduction electrons Feeling of Sadness Spin fluctuations Superconductors

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A-SI-F AND A-SI-F-H PREPARED BY ION-BEAM-ASSISTED REACTIVE DEPOSITION

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JOURNAL OF APPLIED physics 1992年第4期72卷 1446-1453页

作者： ZHANG, S BRODIE, DE Guelph‐Waterloo Program for Graduate Work in Physics Waterloo Campus Waterloo Ontario N2L 3G1 Canada

a-Si:F and a-Si:F:H films have been prepared by ion-beam-assisted deposition using SiF4, SiH4+Ar, or SiF4+SiH4 as the gases for the ion source. Fluorine in a-Si eliminates some dangling bonds, increases the optical gap, and decreases the dark conductivity. The results are influenced mainly by the ion-beam energy used. The a-Si:F films do not exhibit an activated conductivity even up to 150-degrees-C, and no photoconductivity could be detected. However, film properties were significantly improved when a very small amount of H was added to the a-Si:F and much less than 1 at. % H produced films that were photoconducting and had activated conductivities. The properties of these a-Si:F:H are strongly dependent on both the fluorine concentration C(F) and the hydrogen concentration C(H). The deposition rate decreases with increasing SiF4 content in the source gas, and neither C(F) nor C(H) vary linearly with the change in the source gas ratio SiH4/(SiF4+SiH4). Hence, C(H) must be known and controlled in order to evaluate the effect of fluorine on the film's properties. By studying a series of a-Si:F:H samples containing the same C(H), it is noted that incorporating some F does improve the film's electrical properties. Annealing experiments suggest that an observed peak at 2100 cm-1 in the IR spectra of these a-Si:F:H films is not completely due to SiH2, as has been suggested by others.

关键词： AMORPHOUS SEMICONDUCTORS SILICON FLUORINE ADDITIONS HYDROGEN ADDITIONS ENERGY BEAM DEPOSITION FILMS ION BEAMS PHOTOCONDUCTIVITY ELECTRIC CONDUCTIVITY THIN FILMS

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PHOTOCONDUCTING TIO2 PREPARED BY SPRAY PYROLYSIS USING TICL4

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THIN SOLID FILMS 1992年第2期213卷 265-270页

作者： ZHANG, S ZHU, YF BRODIE, DE Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario N2L 3G1 Canada

TiO2 was prepared by the oxidation of TiCl4 at relatively low deposition temperatures using spray pyrolysis. For a fixed carrier gas (N2) flow rate, the substrate temperature is the critical parameter determining the film properties. Films made near 595 K contain only the anatase phase, adhere well to the glass substrate, and have a relative optical transmission of about 75% (between 450 and 850 nm), the value one calculates for the non-absorbing anatase phase in this region. These films have an absorption edge characteristic of a direct gap semiconductor with allowed transitions and optical gaps of 3.5-3.7 eV, depending on the porosity. At higher substrate temperatures (above 635 K), the resulting films have a cloudy appearance and they contain both the rutile and the anatase modifications, confirmed by X-ray diffraction. Lower temperature substrates reduce the deposition rate and yield more porous films. For the pure anatase films, the dark conductivities are about one order of magnitude larger, and the photoconductivity is about one order of magnitude smaller when measured with the sample in air, than those observed when the sample is in a vacuum. When the samples are in a vacuum, the conductivities of the clear films increase by about two orders of magnitude (from about 10(-10) to about 10(-8) S cm-1) when illuminated with light from an electrically programmable read-only memory eraser (mercury light), which gives an intensity of about 30-mu-w cm-2 at the sample. The photoresponse has two parts;the fast portion rises in less than 1 s to 99% of its final value, followed by a small slow rise portion. The slow response, which is more pronounced in the decay process, is interpreted as being due to the presence of surface trapping states.

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TRANSPARENT CONDUCTING ZNO FILMS DEPOSITED BY ION-BEAM-ASSISTED REACTIVE DEPOSITION

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THIN SOLID FILMS 1992年第1期213卷 109-112页

作者： ZHANG, DH BRODIE, DE Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario N2L 3G1 Canada

Highly transparent conducting ZnO films have been deposited using ion-beam-assisted reactive vacuum deposition. The zinc deposition rate was controlled by adding gallium to the zinc in an open Al2O3 crucible source. Oxygen was introduced into the system via a separate controlled leak and reacted with the zinc on the substrate. Mechanically stable polycrystalline conducting ZnO films having a preferred orientation were deposited with resistivities in the range from 4.0 x 10(-6) to 9.0 x 10(-6) OMEGA-m, with carrier densities of more than 2 x 10(26) m-3 and Hall mobilities between 2.8 X 10(-3) and 4.0 x 10(-3) m2 V-1 s-1. The average transmission exceeded 90% for films 350 nm thick in the wavelength range of the visible spectrum.

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CRYSTALLIZATION PROPERTIES OF A SUPERCOOLED METALLIC LIQUID

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JOURNAL OF CHEMICAL physics 1992年第1期96卷 513-516页

作者： QI, DW LU, J WANG, S Guelph‐Waterloo Program for Graduate Work in Physics Waterloo Campus University of Waterloo Waterloo Ontario Canada N2L 3G1

With the aid of molecular-dynamics technique, a calculation of the thermodynamic and structural properties of the crystallization of a supercooled metallic liquid is presented. The calculated results demonstrate a number of features of this crystallization. It now appears that the transition associated with this crystallization is a first-order-like phase transition in the Ehrenfest scheme.

关键词： CRYSTALLIZATION SUPERCOOLED LIQUIDS LIQUID METALS MOLECULAR DYNAMICS CALCULATIONS THERMODYNAMICS STRUCTURE FACTORS PHASE TRANSFORMATIONS MELTING POINTS KINETIC ENERGY COULOMB FIELD DISTRIBUTION FUNCTIONS

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RAMAN AND FAR-INFRARED SPECTRA OF CYANAMIDE AT 20-K AND 90-K - ATOM ATOM POTENTIALS AND LATTICE FREQUENCIES

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JOURNAL OF RAMAN SPECTROSCOPY 1992年第8期23卷 465-469页

作者： TORRIE, BH Guelph-Waterloo Program for Graduate Work in Physics Waterloo Campus University of Waterloo Waterloo Ontario N2L 3G1 Canada

The Raman and infrared spectra of cyanamide have been remeasured at lower temperatures (20 K) and higher laser powers (for the Raman). The recently obtained structure of cyanamide was used as a basis for calculations of the lattice dynamics. Standard models using atom-atom potentials of the exp - 6 form, an exponentially decaying attractive potential for the hydrogen bond and distributed dipoles on the atoms, gave good fits to the structural parameters and, with scaling, gave moderately good values for the lattice frequencies provided that it was assumed that the modes associated with librations about the long axis of the molecule were not observed.

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