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The formation of H-2 by H-atom reaction with grain surfaces

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 1996年第2期279卷 591-594页

作者： Duley, WW Guelph Waterloo Centre for Graduate Work in physics University of Waterloo Waterloo Ontario Canada N2L 3G1

A model for H-2 formation by reaction of I-I-atoms with the surface of carbonaceous grains is developed. Such reactions, which involve interstitial I-I-atoms at low grain temperature and H-atoms bonded to -CH, -CH2 and -CH3 groups at higher temperature, occur promptly on the collision of an I-I-atom with a grain surface. The rate of hydrogen formation and the efficiency of reaction depend on both gas kinetic and dust temperature. The rate coefficient R (cm(3) s(-1)) for H-2 formation using this prompt-reaction model (PRM) can be calculated from laboratory data and is consistent with measured values of R, The dependence of R on ambient conditions in interstellar clouds can be easily estimated from this prompt-reaction model. R is found to be strongly dependent on gas kinetic temperature T-k and can exceed 10(-17) cm(3) s(-1), even for warm dust, when T-k greater than or equal to 300 K.

关键词： molecular processes ISM, abundances dust, extinction ISM, molecules

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Metastability of freely suspended liquid-crystal films

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Physical Review E 1996年第3期53卷 2466-2466页

作者： Y. Martinez A. M. Somoza L. Mederos D. E. Sullivan Department of Physics and Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

We investigate the conditions required for the existence of smectic-A liquid-crystal films freely suspended in vapor. This work is based on a molecular density-functional theory developed in earlier studies of wetting and layering transitions of smectic-A films at liquid-solid and liquid-vapor interfaces. It is emphasized that all freely suspended films are metastable with respect to either the absence of the film or the formation of the bulk smectic phase, depending on the relative stability of the bulk vapor and smectic phases. Films containing different numbers of layers correspond to local minima of the grand canonical potential, and are ‘‘stabilized’’ relative to each other by the presence of sufficiently large meta- stability barriers. The disappearance of these barriers corresponds physically to the ‘‘rupturing’’ of the films. It is demonstrated that surface enhancement of smectic ordering does not play an essential role in the existence of freely suspended films, but may influence the order of disappearance of the layer-layer metastability barriers on changing temperature. We relate these findings to the results of a recent experimental study of ‘‘layer thinning transitions’’ in freely suspended films. © 1996 The American Physical Society.

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Monte Carlo simulations of randomly branched polymers with annealed and quenched branching structures

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Physical Review E 1996年第6期53卷 6238-6238页

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

We present three-dimensional off-lattice Monte Carlo simulations on randomly branched polymers with annealed and quenched branching structures when monomers interact with each other through excluded volume interactions. The values of the critical exponent for the mean-square radius of gyration are estimated on the basis of scaling analysis as νa=0.49±0.01 (annealed) and νq=0.45±0.01 (quenched), which suggest that annealed and quenched randomly branched polymers belong to different universality classes. We further confirm the existence of a scaling parameter that was deduced earlier by the perturbation theory and the Flory-type argument. © 1996 The American Physical Society.

关键词： Polymers

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A statistical approach to acoustic monitoring of laser welding

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JOURNAL OF physics D-APPLIED physics 1996年第3期29卷 556-560页

作者： Gu, HP Duley, WW Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

Acoustic emission at frequencies between 20 Hz and 20 kHz during CO2 laser welding of steel contains important diagnostic information related to weld morphology, depth of penetration and the heat-affected zone. We have recorded acoustic spectra under a variety of welding conditions with mild and galvanized steel sheet and find that a well-defined spectrum is obtained under conditions that lead to an optimization of these weld characteristics. A statistical approach is developed which yields a predictive measure of weld quality, based on a comparison between real-time acoustic spectra, and that obtained under optimized conditions. This technique is applied to an analysis of bead on plate, lap and butt laser welds in mild and galvanized steel.

关键词： Laser beam welding

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Resonant acoustic emission during laser welding of metals

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JOURNAL OF physics D-APPLIED physics 1996年第3期29卷 550-555页

作者： Gu, HP Duley, WW Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

We observed sharp resonances at certain frequencies in the acoustic and optical emission from metals during laser welding with a modulated CO2 laser beam. Strong emission is observed at high-order harmonics of the laser modulation frequency. Harmonics whose frequencies overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced, relative to other harmonics whose frequencies lie outside these bands. The keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of laser radiation with the walls of the keyhole. When a CW beam is used, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overlapping but discrete frequency components. When the laser beam is modulated, a forced response is elicited resulting in a much simplified emission spectrum. These results offer some new insights into the physical mechanisms that convert laser radiation to heat in laser welding, drilling and cutting applications.

关键词： Acoustic emissions

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Holographic inferometry of isotherms during laser drilling of fused quartz

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JOURNAL OF physics D-APPLIED physics 1996年第5期29卷 1140-1145页

作者： Olfert, M Duley, WW Guelph - Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 2G1

The propagation of heat into fused quartz during drilling with CO2 laser radiation had been studied using differential holographic interferometry. This technique yields the temporal evolution of the thermally induced optical path difference in a cross section through the sample in a plane which contains the longitudinal axis of the drill hole. With the assumption of cylindrical symmetry, a portion of the temperature field in the sample has been obtained from these optical path length differences and is compared to the results of theoretical models for heat transfer from a high-aspect-ratio laser-heated drill hole. Isotherms derived in this way have been obtained during CO2 laser drilling of fused quartz in normal gravity and under the reduced gravity conditions of the NASA KC 135 aircraft under parabolic flight conditions. Heat transfer from a pre-drilled hole absorbing laser radiation has also been studied to evaluate the possible effect of material removal on heat transfer. Comparisons of these thermal profiles with those generated by numerical modelling of heat transfer from a laser-heated drill differences. Possible sources of these differences measured isotherms are discussed.

关键词： Laser applications

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The decomposition of hydrogenated amorphous carbon: A connection with polycyclic aromatic hydrocarbon molecules

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ASTROPHYSICAL JOURNAL 1996年第2期472卷 L123-&页

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

We have produced thin solid films of hydrogenated amorphous carbon (HAC) by vapor deposition in vacuum and have investigated the structure and infrared spectra of these materials under conditions in which they are at the point of decomposition. We find that the IR absorption spectrum of HAC under these conditions contains all the major spectral components seen in emission in nebulae. In addition, high-resolution scanning tunneling microscopy of HAC prior to decomposition reveals protographitic islands with dimensions similar to 1-5 nm showing that polycyclic aromatic hydrocarbon (PAH)-like molecular groups evolve from HAC. These experimental results suggest for the first time how PAHs and HAC may be related and how PAH molecules could evolve from HAC in regions of high excitation. The material produced as HAC decomposes is a very low density carbonaceous ''aerogel'' consisting primarily of these aromatic protographitic clusters in a weakly connected friable network. These protographitic clusters may be the source of interstellar graphite grains.

关键词： dust, extinction infrared, ISM, lines and bands ISM, abundances ISM, molecules

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Brillouin light scattering studies of the mechanical properties of polystyrene/polyisoprene multilayered thin films

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Journal of Polymer Science 1996年第17期34卷

作者： J. A. Forrest A. C. Rowat K. Dalnoki-Veress J. R. Stevens 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 (BLS) to investigate the mechanical properties of thin polystyrene (PS) and polyisoprene (PI) films incorporated in a multilayer geometry consisting of alternating layers of the two polymers. All samples had a total thickness h ∼ 1600 Å, and consisted of between 2 and 10 individual polymer films; individual layer thicknesses were as small as 160 Å. Ellipsometry was used to determine the thickness of the PS and PI layers. The velocities of several high-frequency film-guided mechanical waves were measured using BLS and the results are compared with values calculated using an effective medium approach. The effective elastic constants of the multilayered films were obtained from those determined for thick films of PS and PI. Remarkable agreement was obtained between the measured and calculated velocities even for samples in which the individual layer thicknesses were much less than the unperturbed size of the polymer molecules. These results suggest that the mechanical properties of polymers change very little even when the molecules are forced into highly confined geometries. © 1996 John Wiley & Sons, Inc.

关键词： polymer multilayered films mechanical properties

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INTERFACE FORMATION AND A STRUCTURAL PHASE-TRANSITION FOR THE SPHERICAL MODEL OF FERROMAGNETISM

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JOURNAL OF STATISTICAL physics 1995年第1-2期79卷 165-181页

作者： ALLEN, S Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Canada

A detailed analysis is reported examining the local magnetic susceptibility chi(r), in relation to the correlation function GIR) and correlation length xi, of a spherical model ferromagnet confined to geometry Omega = L(d-d') x infinity(d') (d' less than or equal to 2, d > 2) under a continuous set of twisted boundary conditions. The ''twist'' parameter ($) under bar tau in this problem may be interpreted as a measure of the geometry-dependent doping level of interfacial impurities (or antiferromagnetic seams) in the extended system at various temperatures. For tau(j) --> 0, For All j is an element of d-d', no seams are present except at infinity, whereas if tau(j) = 1/2, impurity saturation occurs. For 0 < tau(j) < 1/2 the physical domain Omega(phys) = D-d-d' x infinity(d') (D > L), defining the region between seams containing the origin, depends on temperature above a certain threshold (T > T-0). Below that temperature (T < T-0), seams are frozen at the same position (D approximate to L/2 tau, d-d' = 1), revealing a smoothly varying large-scale structural phase transition.

关键词： TWISTED BOUNDARY CONDITIONS LOCAL SUSCEPTIBILITY SPHERICAL MODEL FINITE-SIZE SCALING

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THEORY OF ROTATIONAL RELAXATION IN COHERENT RAMAN-SPECTROSCOPY

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CHINESE JOURNAL OF physics 1995年第4期33卷 305-333页

作者： LIU, WK Department of Physics Guelph- Waterloo Program for Graduate Work in Physacs (GWP) University of Waterloo

The theory of the nonlinear optical processes in recent high-resolution coherent Raman spectroscopic studies of gases is reviewed, and a unified description of various spectroscopic techniques in terms of the correlation functions of molecular polarizability tenser is presented. A theoretical description of spectral line shapes in terms of a relaxation matrix, which provides the connection between the microscopic intermolecular potential and the experimental observables, is discussed. We pay particular attention to ab initio scattering calculations starting from accurate interaction potentials for the linewidths, line shifts, and line-mixing parameters for the system of deuterium molecules interacting with a bath of inert gas atoms, under a wide range of perturber densities. It is clear that for simple systems involving foreign gas broadening, the theory has reached the maturity that first-principles calculation of line spectra is highly feasible. Future directions of theoretical research from both a fundamental point of view and practical considerations are indicated.

关键词： .potentials nonlinear optical shifts polarizability broadening intermolecular coherent Raman correlation functions linewidths line shapes Theory of Rotational relaxation matrix

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