bluuebunny/crossref_metadata_2025 huggingface dataset:
Angular dispersion of the easy axis in a magnetically soft film as determined by vibrating sample magnetometry
Authors: Jen S. U. , Chen W. L.
Published: July 2002

In this paper, we briefly described how to use the vector or biaxial vibrating sample magnetometer (VSM) to determine the angular dispersion of easy axis in a magnetically soft film. For each angle of rotation φ of the external field H, a pair of the Mx-hysteresis and My-hysteresis loops could be obtained. Then, we changed the φ setting from negative to positive, with φ=0° indicating the exact direction of the hard axis of the film. Note, the easy axis has been induced by a deposition field h during fabrication of the film. From the characteristics of the Mx-hysteresis and My-hysteresis loops, the condition |φ|<φc≅1° defines the “in-center zone,” and the condition |φ|>φc defines the “off-center zone.” The types of switching modes for the in-center and off-center zones, respectively, are different. Only the hysteresis behaviors in the latter zone are understood well for detailed explanations. Then, the normalized change ΔMy/MS due to the wall motions at the switching field |H|=HSW can be used to indicate the angular dispersion Δθ in the film. Co70Fe10Ni20 films were made with various deposition conditions in order to cause different degrees of dispersion in them. The result shows that it is possible to study the angular dispersion in a magnetically soft film via the vectorial VSM measurements.
Prediction of the periods of multilayers prepared by multitarget sputtering
Authors: Martin Nicolas , Rousselot Christophe
Published: July 2002

A simple theoretical approach is proposed to predict the periodicity and thickness of multilayers synthesized by a hypothetical multitarget sputtering process. A sputtering technique involving two separated targets is investigated first. Assuming some physical and chemical properties of the bulk materials and taking into account the geometry and operating conditions imposed by the process, some mathematical relationships can be deduced to determine the alternations and dimensions of each sublayer as well as the final thickness of the coating. An extension of the proposed model is therefore developed for a process involving any target number which enables us to establish the experimental setup required to synthesize any period. The advantages and weaknesses of the theoretical analysis due to some simplifying assumptions are discussed and some improvements are also suggested. The influence of some process parameters on some characteristics of the materials, especially density, are studied so as to show that their effect can also be added to the presented theory.
Formation of extended states in disordered two-dimensional In0.4Ga0.6As/GaAs(311)B quantum dot superlattices
Authors: Lan Sheng , Akahane Kouichi , Song Hai-Zhi , Okada Yoshitaka , Kawabe Mitsuo , Nishimura Tetsuya , Nishikawa Satoshi , Wada Osamu
Published: July 2002

Formation of extended states or minibands in two-dimensional (2D) In0.4Ga0.6As/GaAs(311)B quantum dot superlattices (QDSLs) is directly demonstrated in time-resolved photoluminescence measurements. At a low excitation density of 1 W/cm2, photoluminescence transients with ∼15 ps rise time and ∼25 ps decay time are observed. Both rise and decay times are found to increase with increasing excitation density. The excitons in 2D QDSLs exhibit different relaxation and recombination behaviors as compared to those in quantum wells and quantum dots. A physical model treating 2D QDSLs as disordered systems containing localized and extended states can successfully interpret all of the experimental observations.
Reduction of dissipation in a thermal engine by means of periodic changes of external constraintsa)
Authors: Escher Claus , Ross John
Published: July 2002

We consider a thermal engine driven by chemical reactions, which take place in a continuous flow, stirred tank reactor fitted with a movable piston. Work can be produced by means of a heat engine coupled to the products and to an external heat bath, and by the piston. Two modes of operation are compared, each with fixed input rate of chemicals: one with periodic variation of an external constraint [mode (b)], in which we vary the external pressure, and one without such variation [mode (a)]. We derive equations for the total power output in each of the two modes. The power output in mode (b) can be larger than that of mode (a) for the same chemical throughput and for the same average value of the external pressure. For a particularly simple case it is shown that the total power output in mode (b) is larger than that in (a) if work is done by the piston. At the same time the entropy production is decreased and the efficiency is increased. The possibility of an increased power output is due to the proper control of the relative phase of the externally varied constraint and its conjugate variable, the external pressure and the volume. This control is achieved by the coupling of nonlinear kinetics to the externally varied constraint. Details of specific mechanisms and the occurrence of resonance phenomena are presented in the following article.
Proton nuclear magnetic resonance and line shape analysis in the alkali metal hydroxides: LiOH, NaOH, KOH, and RbOH
Authors: Amm D. T. , Segel S. L. , Heyding R. D. , Hunter B. K.
Published: July 2002

The proton NMR line shapes of the anhydrous alkali metal hydroxides at 27 and 200 MHz are presented. The structure in the proton resonances of NaOH, KOH, and RbOH is explained using a four-spin planar zig–zag chain model calculation. Second moment calculations show that the observed line shapes are consistent with known crystal structures. The crystal structures of KOH and RbOH, as determined by x-ray powder diffraction, are confirmed.
Quantum Monte Carlo calculation of the singlet–triplet splitting in methylene
Authors: Reynolds Peter J. , Dupuis Michel , Lester William A.
Published: July 2002

The fixed-node quantum Monte Carlo (QMC) method is used to calculate the total energy of CH2 in the 3B1 and 1A1 states. For both states, the best QMC variationally bounded energies lie more than 15 kcal/mol (0.024 h) below the best previous variational calculations. Subtracting these energies to obtain the singlet–triplet splitting yields Te=9.4±2.2 kcal/mol. Adjusting for zero-point energies and relativistic effects, we obtain T0=8.9±2.2 kcal/mol. This result is in excellent agreement with the recent direct measurements of McKellar et al. of T0=9.05±0.06 kcal/mol, and of Leopold et al. of ∼9 kcal/mol, as well as with recent threoretical investigations which indicate an energy gap of 9–11 kcal/mol. We summarize the QMC method, discuss a possible scheme for iteratively correcting the procedure, and note that the present results were obtained using only single determinant functions for both states, in contrast to conventional ab initio approaches which must use at least two configurations to properly describe the singlet state.
Local modes and complex formation in H+OH collisions: Collinear reactions
Authors: Howard Robert E. , Thompson Donald L.
Published: July 2002

We compute quasiclassical trajectories on the Schinke–Lester potential energy surface to study complex formation in collinear H+OH collisions at relative translational energies between 0.5 and 1.0 eV. At all energies except those just above the entrance channel barrier, the great majority of collisions are direct. Vibrational phase relationships and energy resonances determine the probability of forming a complex and its lifetime. At 0.5 eV we found two trajectories that followed regular, periodic paths for 10–12 vibrations. At higher energies long-lived complexes are characterized by chaotic motion, a pattern of beats between local modes, and a tendency to share energy.
The linearly driven parametric oscillator: Application to collisional energy transfer
Authors: Gazdy Bela , Micha David A.
Published: July 2002

The time-evolution operator is explicitly constructed for a general linearly driven parametric quantum oscillator, equivalent to a harmonic oscillator driven by linear plus quadratic potentials. The method is based on an algebra of operators which are bilinear in the position and momentum operators, and form a closed set with respect to commutation. The obtained result requires only integrals over time and the solution of two coupled first order linear differential equations related to the classical equations of motion. The model is used to obtain vibration-translation probabilities in a collinear collision of an atom with a diatomic molecule. Numerical calculations have been performed for systems with several mass combinations and potential parameters. Approximation methods are compared, and criteria are established to determine when it is necessary to go beyond the popular linearly driven harmonic oscillator.
The lifetime of the fluorescent excited state in solid, liquid, and vapor phase cyclohexane
Authors: Wickramaaratchi M. A. , Preses J. M. , Holroyd R. A. , Weston Ralph E.
Published: July 2002

The fluorescence lifetimes of cyclohexane in the solid, liquid, and vapor phase were measured using VUV synchrotron radiation from the National Synchrotron Light Source at BNL. The lifetime of liquid and solid cyclohexane changes with temperature, and the decay rate is a combination of a temperature-dependent Arrhenius-type process and a temperature-independent process. The results for liquid and solid cyclohexane from 59 to −49 °C can be represented by the expression 1/τ=[5.3×108+4.2×1012 exp(−5100/1.987 T)] s−1. The latest lifetime measurements available in the literature for cyclohexane are in good agreement with our values measured between 20 and 25 °C. The lifetime of liquid cyclohexane at a given temperature is independent of the excitation energy. However, in the vapor phase the lifetime strongly depends on the excitation energy.
ESR of chromium metal atoms and molecules in matrices
Authors: Van Zee R. J. , Baumann C. A. , Weltner W.
Published: July 2002

Chromium metal, natural and isotopically enriched in 53Cr (I=3/2), was vaporized and trapped in solid neon, argon, and krypton at 4 K. An extensive ESR spectrum of 16 lines between 200 and 7000 G in neon and argon matrices was found to be due to an axial molecule with S=3, ‖D‖=0.110 cm−1, g⊥=1.9970, g∥=1.9996. 53Cr spectra showed essentially isotropic hyperfine interaction of the six unpaired spins with apparently only one nucleus and an accompanying electric field gradient at that nucleus. It is concluded that the molecule is a trigonally distorted tetrahedral Cr4 or possibly a square-based pyramidal Cr5. The unique apical atom is bonded relatively weakly to the ‘‘base’’ and is the locus of almost all of the s character among the unpaired spins. In argon and krypton matrices, another species with S=3 is observed near g=2.0 but with only a small zero-field splitting (‖D‖=0.0049 cm−1, g∥=2.005, g⊥=2.0015 in argon). The possibilities of it being a weakly perturbed Cr atom in an asymmetrical matrix site or a Crn molecule, where n(≥3) is the number of equivalent atoms, are discussed.
Photodissociation of chlorine isocyanate
Authors: Bell D. D. , Coombe R. D.
Published: July 2002

Gaseous ClNCO exhibits a continuous ultraviolet absorption feature peaked near 250 nm. Photolysis of this compound at 249 nm produces bright blue emission identified as the A 2Σ+→X 2Π transition in NCO. The excited NCO arises from a collisional process in which N(2D) atoms, produced directly by the photodissociation, react with the parent ClNCO. The rate constant for this process is k=(2.1±0.2)×10−11 cm3 s−1. From production of N(2D) by the 249 nm photolysis, the heat of formation of gaseous ClNCO is inferred to be ΔHf(ClNCO)≥52.9 kcal mol−1. ClNCO is metastable with respect to the ground electronic states of NCl and CO by 8.2 kcal mol−1.
Thermal energy electron detachment rate constants. The electron detachment from azulene− and the electron affinity of azulene
Authors: Grimsrud E. P. , Chowdhury S. , Kebarle P.
Published: July 2002

The rate constant kd and its temperature dependence for thermal electron detachment from the azulene negative ion Az−⇄ad Az+e was determined by observing, at suitably high temperature, the decay of Az− with time. A pulsed electron beam, high ion source pressure mass spectrometer was used. The reaction mixtures typically contained 4 Torr CH4 as bath gas and azulene and perfluoromethylcyclohexane (C7F14) at pressures of a few mTorr. The C7F14 which has a large electron attachment coefficient was used as electron scavenger in order to suppress the (re)attachment of electrons to Az. The experimental data points for kd could be fitted equally well with the expressions kd =2.5×106 T3/2 (s−1)exp[−14.5 (kcal/mol)/RT] and kd =1.14×1011 (s−1)exp[−15.7 (kcal/mol)/RT]. The activation energies are close to the electron affinity E.A.(Az)=16 kcal/mol obtained in earlier determination from this laboratory. Consistent values for the equilibrium constant K of the attachment–detachment equilibrium can be obtained when K is evaluated from E.A.(Az) and third law entropies of the reactants and from K=ka/kd, where ka and kd are experimentally determined quantities. The ka values were taken from Christophorou.
Vibrational energy flow into a reactive coordinate: A theoretical prototype for a chemical system
Authors: Holme Thomas A. , Hutchinson John S.
Published: July 2002

A realistic model is developed to illustrate vibrational energy flow into a reactive coordinate. The isomerization of C≡N–H to H–C≡N is enhanced nonstatistically through overtone excitations of the H–N bond. The mechanism determined for this theoretical model is dominated by resonant energy transfer from the anharmonically suppressed H–N mode into a combination of the C≡N plus the bending reaction coordinate. Implications of this classical prototype to mode specific chemistry, in particular the prediction of possible quantum tunnelling enhancement, are discussed in terms of experimentally observable systems.
Effect of an electric field on a methane molecule. I. Infrared analysis of methane (CH4–CD4) adsorbed in NaA zeolite in the temperature range 150–20 K
Authors: De Lara E. Cohen , Kahn R. , Seloudoux R.
Published: July 2002

The spectra of methane adsorbed at low temperature in NaA zeolite show two strong effects of the field existing in the cavities: (i) the appearance of the ν1 forbidden band which increases when decreasing the temperature and (ii) the splitting of the ν3 degenerate band. The variation with T of its components lead to the conclusion that the molecule progressively is oriented in a C3V configuration with respect to the field.
Laboratory measurement of the P(2,1) submillimeter transition frequency of H3O+
Authors: Plummer Grant M. , Herbst Eric , De Lucia Frank C.
Published: July 2002

The P(2,1) transition frequency of the rotation-inversion spectrum of the molecular ion H3O+ has been measured in the laboratory to be 307 192.41 (5) MHz. Measurement to this frequency should enable radioastronomers to search for H3O+ in interstellar clouds that are warm enough to excite this transition in emission.
Zero field NMR and NQR with selective pulses and indirect detection
Authors: Millar J. M. , Thayer A. M. , Bielecki A. , Zax D. B. , Pines A.
Published: July 2002

Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo- and heteronuclear systems are presented as an illustration of these techniques.
Clustering and percolation in multicomponent systems of randomly centered and permeable spheres
Authors: Chiew Y. C. , Stell G. , Glandt E. D.
Published: July 2002

For a multicomponent system of particles in equilibrium, an exact integral equation is derived for the pair connectedness function (which measures the probability that two particles, with centers a distance r apart, are connected). The pair connectedness function, mean cluster size, and percolation thresholds for mixtures of randomly centered (noninteracting) spheres and permeable spheres are then obtained analytically in the Percus–Yevick (PY) approximation. (The permeable-sphere model provides a one-parameter bridge from randomly centered sphere mixtures to hard-sphere mixtures.) For this family of models, connectedness is defined by particle overlap. It is found that, within the PY approximation, a multicomponent mixture of randomly centered spheres percolates at ξ3=π/6∑iρiR3i =1/2, independent of the concentration and size distributions of the particles. For the permeable-sphere case the percolation threshold depends on the relative densities, size, and interparticle permeability among the species. The loci of the percolation thresholds of a binary mixture of permeable spheres are explicitly determined.
Memory and entropy in c
i
s–t
r
a
n
s isomerization
Authors: Statman D. , Robinson G. W.
Published: July 2002

We present here some molecular dynamics results concerning the role of frequency-dependent friction in the barrier crossing problem. Friction along the reaction coordinate can be about a factor of 2 less than that for the bulk solvent. Another effect that has been found to cause disagreement between the Grote–Hynes theoretical isomerization rate and the molecular dynamics rate is an entropy contribution resulting from a change of hydrodynamic volume upon isomerization.
Photoselection study of the Br2 A→X emission system in an argon matrix
Authors: Nicolai Jean-Philippe , Heaven Michael C.
Published: July 2002

Excitation of the Br2 A 3Π(1u) state by 532 nm radiation was examined in a low temperature (15 K) argon matrix. Fluorescence polarization anisotropies were measured in order to assign the absorption system by which the A state was excited. Previous investigators had suggested that the A state was populated by B→A transfer following absorption by the B–X system. The present study demonstrates unambiguously that the A state is populated by direct A–X absorption.
A systematic procedure for estimating the orientation distribution for nonspherical Rayleigh particles
Authors: Caruthers James M. , Franses Elias I.
Published: July 2002

The inverse problem of estimating the orientation distribution of monodisperse nonspherical Rayleigh particles via electromagnetic scattering is systematically analyzed. Using all the possible angular and polarization information available in Rayleigh scattering, at most 15 averages or moments of the orientation distribution function can be obtained. For monodisperse axisymmetric Rayleigh particles with a plane of symmetry normal to the axis (spheroids, finite cylinders, etc.), the orientation distribution function is expanded in terms of the complete set of surface harmonics. The first 15 coefficients of the expansion can be determined unambiguously without any assumption about the form of the distribution. No additional coefficients can be determined with all conceivable experiments. For distributions which are azimuthally symmetric around the direction of incident light, only three coefficients are nonzero. Average orientation and structure factors are calculated in terms of these coefficients. Computer simulation of experimental errors reveals certain instrument precision requirements for successful measurements. The procedure is applied to a simulated example of spheroids whose orientation distribution is determined by rotational Brownian diffusion.
Reverse saturable absorption of short polarized light pulses
Authors: Band Yehuda B.
Published: July 2002

We describe the absorption characteristics of polarized light pulses of short temporal duration in media with a ground state absorption cross section and large excited state absorption cross section. The effects of rotational diffusion, pulse depletion, finite pulse propagation time, and transition dipole moment orientation are properly treated. Significant enhancement of the reverse saturation due to excited state absorption results when the ground and excited state absorption dipole moments are parallel.
Near-infrared spectra of the NaK molecule
Authors: Huennekens J. , Walker T. G. , McClain S. C.
Published: July 2002

In this paper we present near-infrared absorption, thermal emission, and laser-induced fluorescence spectra obtained in high density sodium–potassium mixtures. These spectra are compared to spectra calculated from recently published NaK potentials and dipole moments and the agreement is seen to be quite good. We have also observed significant laser-induced fluorescence to the red of the 2 1∑+ → 1 1∑+ NaK band (A band). From laser power and density dependences of this long-wavelength fluorescence, as well as from comparison to calculated spectra, we have tentatively identified this emission as the lowest NaK triplet band 1 3∏ → 1 3∑+ which has not previously been reported. We have also observed a shoulder or satellite feature in potassium vapor under similar conditions. It is possible that this is a potassium trimer emission band.
Excited state absorption of Cr3+ in K2NaScF6 and Gd3Ga2(MO4)3, M=Ga, Al
Authors: Andrews Leonard J. , Hitelman Stewart M. , Kokta Milan , Gabbe David
Published: July 2002

The excited state absorption (ESA) spectra of Cr3+ in the elpasolite K2NaScF6 and the garnets Gd3Sc2(GaO4)3 (GSGG) and Gd3Sc2(AlO4)3 (GSAG) were measured from 0.3 to 1.8 μ, exclusive of the fluorescence region, using pulsed laser excitation. The two ligand field transitions 4T1g(a)←4T2g and 4T1g(b)←4T2g were located in the garnets at 9000 and 19 400 cm−1 (19 200 cm−1 for the elpasolite). In addition, intense UV charge transfer bands were found in the garnets, but not the elpasolite. The elpasolite spectrum confirms that major differences exist between the ESA of 2Eg and 4T2gCr3+ excited states. A single configuration coordinate model is developed which is adequate for predicting energies but not the bandwidths of 2Eg and 4T2g ESA transitions between ligand field levels.
Vibrational spectra of chemisorbed NO2 and condensed N2O4 on the Ru(001) surface
Authors: Schwalke U. , Parmeter J. E. , Weinberg W. H.
Published: July 2002

Vibrational electron energy loss spectroscopy combined with thermal desorption mass spectrometry have been used to investigate the adsorption and decomposition of NO2 on the Ru(001) surface. The results indicate that the initial NO2 adsorption is dissociative at 80 K. The reaction products, molecularly adsorbed NO and atomically adsorbed oxygen, passivate the surface, and subsequent (submonolayer) adsorption is molecular. The molecularly adsorbed NO2 is bound weakly (∼9 kcal/mol) through the nitrogen atom with C2v symmetry. With increasing exposure, the formation of N2O4 dimers in the condensed multilayer is observed.
Semiclassical vibration–rotation spectra of gaseous and physisorbed molecules
Authors: Adams John E.
Published: July 2002

A semiclassical spectral intensity method is applied to the calculation of vibration–rotation spectra both of isolated molecules and of molecules physically adsorbed on a solid surface. For the case of an isolated HCl molecule, we are able to generate discrete vibration–rotation spectral lines, the frequencies and integrated intensities of which agree well with the available literature values. Line shapes obtained for this case exhibit no evidence of broadening beyond the theoretical resolution of the calculation. Physisorption of the HCl on an Ar(111) surface leads, however, to a collapse of the R, P band structure at low rotational energies, with the free rotor dynamical limit being reached only at higher energies. The transition between dynamical regimes is associated with substantial line broadening and shifts in the line centers as well as with an increase in the desorption rate. A loss of rotational phase coherence appears to represent the principle line broadening mechanism in the present calculations.
Electron paramagnetic resonance study of the phase transition in Cu2+-doped CaCd(CH3COO)4⋅6H2O
Authors: Misra Sushil K. , Kumar Krishan
Published: July 2002

A detailed X-band EPR study of a single crystal of CaCd (CH3COO)4⋅6H2O doped with Cu2+ has been made from room temperature to liquid helium temperature. The crystal undergoes only one reversible structural phase transition over this temperature range; this occurs at 130±1 K. As the temperature is lowered, in the general directions of the magnetic field, the single set of four hyperfine lines at room temperature splits into four sets of four hyperfine lines each at just below 130±1 K, indicating the presence of four magnetically inequivalent ions below this temperature. Careful EPR measurements below the phase-transition temperature for the magnetic field directions in several selected planes have been made to study the orientations of the magnetic axes of the four inequivalent Cu2+ ions. It is found that at lower temperatures the magnetic axes of the four inequivalent ions are symmetrically located, 5° away from the c axis, retaining the fourfold symmetry relating the chains. The principal values and direction cosines of the g2 and A2 tensors at different temperatures, and for different Cu2+ sites, are evaluated rigorously on a digital computer, using the method of least-square fitting, employing simultaneously all resonant line positions obtained for several orientations of the external magnetic field in three mutually perpendicular planes.
The FFT method for determining semiclassical eigenvalues: Application to asymmetric top rigid rotors
Authors: Duchovic Ronald J. , Schatz George C.
Published: July 2002

A Fourier transform method for calculating action variables and semiclassical eigenvalues in molecules starting from Cartesian coordinates and momenta is developed and applied to the determination of rotational semiclassical eigenvalues for rigid asymmetric top molecules. The method involves calculating rotational actions by using Fourier representations of the symmetric top action-angle variables to express the integrals. The question of what is the optimum quantization axis is considered, and it is found that the same primitive semiclassical eigenvalues are obtained independent of which principal axis is used for quantization provided that the replacement J → J+ 1/2 is made. The replacement J → [J(J+1)]1/2 leads to eigenvalues that are slightly dependent on quantization axis, but more accurate. For either choice of J replacement, the primitive eigenvalues are found to be double valued when trajectory motion is librational relative to both the prolate and oblate top quantization axes, and also when motion is rotational relative to both axes. This problem can be resolved by implementing uniform semiclassical theory, but this is hard to generalize to nonrigid molecule eigenvalue calculations. Here we propose several simple modifications to the primitive semiclassical method which determine uniquely defined eigenvalues for both the J(J+1) or (J+ 1/2 )2 replacements. These modified primitive semiclassical methods are also applicable to nonrigid molecule calculations, and one of these (the linear interpolation method) is found to give eigenvalues that are quite accurate without requiring information beyond that which is needed in a primitive semiclassical calculation.
Non-Gaussian influence functional for quantum systems
Authors: Allinger Kurt , Carmeli Benny , Chandler David
Published: July 2002

We consider the quantum paths (in Euclidean time) through a space of N multiparticle states, and we derive the influence functional for a subset of m primary states in this space. The influence functional results from integrating out the occupation of the N-m nonprimary or bath states holding fixed the time-dependent occupation path through the primary states. The division into bath states and primary states is completely general in principle, though in practice, physical considerations will guide the choice of partitioning. The exact influence functional is nonlocal in time and very complicated in form. We argue, however, by a variational calculation that a simplified approximate functional can be constructed which is very accurate. Unlike the standard Gaussian influence functionals which are inapplicable for processes involving the large amplitude fluctuations of charge transfer and fermionic exchange, the influence functionals described herein are appropriate for models of these processes.
Hydrodynamics of magnetic and dielectric colloidal dispersions
Authors: Hubbard J. B. , Stiles P. J.
Published: July 2002

We discuss the hydrodynamic behavior of colloidal suspensions, the particles of which possess electric or magnetic moments. Particular attention is given to the case where an external electric or magnetic field acts on a system in which the polarization does not relax instantaneously, so that reorientational Brownian motion is coupled to both the field and to hydrodynamic degrees of freedom. Magnetosonic and magnetoviscous effects are derived, with emphasis on anisotropy with respect to the external field.
MCSCF-CI calculations of infrared transition probabilities in the CH− and NH− ions
Authors: Mänz Ullrich , Zilch André , Rosmus Pavel , Werner Hans-Joachim
Published: July 2002

Potential energy, electric dipole, and electronic transition moment functions have been calculated for several bound electronic states of CH− and NH− from accurate electronic wave functions. Spectroscopic constants for the X 2Π state of NH− have been compared with the data recently obtained by laser induced autodetachment spectroscopy. Spectroscopic constants of the A 2Σ+ state of NH− and radiative X 2Π–A 2Σ+ transition probabilities are presented. Spectroscopic constants and infrared transition probabilities for the X 3Σ− and the a 1Δ states of CH− have been calculated. The radiative lifetime of v=1 in CH−(X 3Σ−) is found to be in a very good agreement with a recent experimental value. The transition probabilities are for both ions much larger than those of their neutral counterparts, and increase in the series OH−, NH−, CH−.
Comment on ‘‘Remarks on the mutual diffusion of Brownian particles’’
Authors: Altenberger Andrzej R. , Tirrell Matthew
Published: July 2002

Relations between diffusion coefficients in binary mixtures are explained. Explicit molecular expressions for the transport coefficients in the laboratory frame of reference are given.
The neighborhood of the three-phase regime in microemulsions
Authors: Kleinert H.
Published: July 2002

We extend a previous calculation and comparison with experiment of the shape of the three-phase regime, of a ternary mixture of water, oil, and tenside, in the space of concentrations and temperature x, y, τ into the neighborhood of the three-phase regime. Apart from the two- and one-phase boundaries, we determine the behavior of the intensity of scattered light and the interfacial energies. The agreement with experimental data is reasonable.
The semiclassical self-consistent field method for polyatomic vibrations: Use of hyperspherical coordinates for H2O and CO2
Authors: Gibson Lester L. , Roth R. M. , Ratner Mark A. , Gerber R. B.
Published: July 2002

The use of hyperspherical coordinates within the semiclassical self-consistent-field model (SCSCF) is investigated for two-mode models, corresponding to nonbending CO2 and H2O. The hyperspherical coordinate set is highly collective; since the screening which is characteristic of collective coordinates should result in increased accuracy of SCF methods, it might be expected that these coordinates would lead to very good SCF results. Numerical tests show that indeed the hyperspherical SCF results are accurate and superior to SCF either in local- or in normal-mode coordinates, the improvement being larger for CO2 than for H2O. These calculations show as does recent work on HCN that proper choice of the coordinate system can lead to much improved SCF results.
Novel effects in the laser induced fluorescence spectrum of C6F+6
Authors: Kennedy Richard A. , Miller Terry A. , Scharf Benjamin
Published: July 2002

The B̃ 2A2u←X̃ 2E1g(0) laser induced fluorescence excitation spectrum of the hexafluorobenzene cation in a free jet expansion has been measured from 21 500 to 22 580 cm−1. The spectrum displays a number of strong transitions involving the totally symmetric and the Jahn–Teller active vibrations. In addition, many weak transitions involving other vibrational modes are observed using high laser powers. These weak, interspersed transitions are readily detected because the ion is cooled in a supersonic expansion which results in sharp, well resolved lines. These weak transitions derive their intensities by a variety of mechanisms, the most novel of which depends on the presence of mixed cross-quadratic nuclear potential terms in the ground (2E1g) state. It is shown that these terms permit the observation of transitions from the vibrationless level of the ground state to combination levels of the excited state involving two modes of different symmetries. The identification of transitions of this type provides the first experimental demonstration of the occurrence of mixed cross-quadratic nuclear potential terms in spatially degenerate electronic states. The analysis of the spectrum has allowed overall the determination of fundamental frequencies for 16 of the 20 normal modes of C6F+6 in its B̃ 2A2u electronic state.
The optical spectra of matrix-isolated palladium–nitrogen complexes: An investigation by absorption, emission, and photoelectron spectroscopy
Authors: Schrittenlacher W. , Schroeder W. , Rotermund H. H. , Wiggenhauser H. , Grinter R. , Kolb D. M.
Published: July 2002

The optical spectra of palladium in neon and argon matrices containing up to 100% dinitrogen have been studied. Beside the known bands of isolated Pd atoms new strong bands assigned to weakly bonded Pd(N2)m (m=1, 2) complexes appear. The bands are attributed to three different types of transition. The dominant lines are essentially due to transitions localized at the Pd atom but strongly perturbed by a ‘‘crystal field’’ due to the weakly bonded N2 molecules. Secondly, a vibrational progression at lower energies is assigned to a Pd to N2 charge transfer transition and thirdly, at high energies, a vibrational progression assigned to a locally excited state of an N2 molecule perturbed by weak bonding to Pd is observed. No evidence has been found for the presence of Pd(N2)3. Photoelectron spectra of the Pd(N2)m complexes in neon have been observed. The Pd 4d photoemission peak is shifted with respect to the Pd atom in Ne by ∼1.1 eV to higher binding energies.
Electron vibrational spectroscopy of thin polyimide films
Authors: DiNardo N. J. , Demuth J. E. , Clarke T. C.
Published: July 2002

We describe high resolution electron energy loss (HREELS) and UV-photoemission (UPS) measurements performed to investigate the vibrational and electronic surface and interfacial properties of thin insulating polymer films. The measurements were performed in ultrahigh vacuum on PMDA–ODA films of thicknesses ∼30–50 and ∼1000 Å which were spun onto silicon or onto metallic substrates and on thin PMMA films deposited on metallic substrates. The general characteristics of our HREELS spectra are weak, diffuse electron scattering and broad vibrational bands which we have assigned by comparison to infrared absorption data. Cooling the polymer films which were spun onto Si substrates to ∼100 K resulted in a uniform reduction of the overall spectral broadening. This additional inelastic scattering contributing to the broadening appears to originate at the PMDA–ODA/Si interface.
On the phase transition in N-isopropylcarbazole
Authors: Nowak R. , Bernstein E. R.
Published: July 2002

The elastic properties of N-isopropylcarbazole (NIPC), a pyroelectric molecular crystal, are investigated by Brillouin scattering. The full elastic constant tensor is determined at 295 K and the temperature dependences of the elastic constants are given. The major experimental finding reported is the anomaly of the LA a-axis mode governed by the c11 elastic constant. This mode exhibits a pronounced downward bending on both sides of the nonferroic, first order phase transition at ∼137 K. Symmetry allowed linear-quadratic and biquadratic couplings between the Brillouin zone boundary one-dimensional order parameter and the zone center strain introduced into the Landau free energy do not account for the observed anomaly. The transition is characterized in terms of a strong dispersion of the c11 elastic constant and a large dynamical critical behavior. By combining the c11 elastic constant data and the Brillouin scattering LA a-axis mode half-width data through a Landau–Khalatnikov process, one can extract a relaxation time satisfying a mean-field dependence characteristic of critical slowing down of the order parameter.
Ring dynamics and percolation in an excitable medium
Authors: Fraser Simon , Kapral Raymond
Published: July 2002

Statistical properties of the pattern formation process in excitable media are studied with the aid of a simple automaton model for the dynamics. When a quiescent medium is excited at local, randomly placed regions in the fluid, ‘‘rings’’ of excitation propagate through the medium. The evolution is studied as a function of the initial seeding probability of the locally excited regions and a scaled picture of the dynamics is presented, which holds for arbitrary dimensions of the medium. High initial seeding probabilities or the increase in the fraction of the medium which is excited as a result of the growth of the rings can lead to large connected regions of excitation in the medium. This process can be studied as a dynamical percolation problem. The percolation thresholds, where a connected region spans the system with certainty, are given as a function of the seeding probability and the time. At percolation the spatial configuration of the excited region possesses a fractal dimension of D=1.9. Chemically reacting systems which exhibit excitability, like the Belousov–Zhabotinskii reaction, are good candidates for the experimental observation of the phenomena described here.
Homogeneous and inhomogeneous structure in the vibrational overtone spectrum of tetramethyldioxetane
Authors: McGinley E. S. , Crim F. F.
Published: July 2002

Vibrational overtone predissociation spectra of tetramethyldioxetane, obtained by monitoring the products of the vibrational overtone initiated unimolecular decomposition in both a room temperature sample and in a free jet, reveal the interactions that are likely to be responsible for the structure in the spectrum. The room temperature measurements provide spectra of the second, third, and fourth CH stretching overtone transitions (3νCH, 4νCH, and 5νCH ) and of combination bands in which a single quantum of methyl deformation is excited along with the stretching vibration. These spectra show the greatest complexity for the lowest energy transition (3νCH) and become simpler for higher levels of excitation. Two prominent maxima in the pure CH stretching overtone spectra come from hydrogen atoms in inequivalent environments in the molecule, but additional structure probably reflects a Fermi resonance between the stretching vibration and the methyl deformation.
Precession induced modulation of fluorescence
Authors: Treffers M. A. , Korving J.
Published: July 2002

A technique is presented with which the alignment of molecular angular momenta in molecular beam scattering can be measured. In principle the method can be applied to any molecule as long as the polarization of an absorbed or emitted photon is related directly to the orientation of the molecular angular momentum and as long as the orientation of the molecular angular momentum can be rotated by precession in a magnetic field. The main advantage of the method is that the relative change in fluorescence intensity, as modulated by the magnetic field, is directly proportional to the alignment of the angular momenta. Both magnitude of the alignment and direction of the principal axis of the orientational distribution of the angular momenta can be determined from the field dependence of the fluorescence intensity. The applicability of the method is demonstrated with some experimental results in which the alignment of Na2 is determined after scattering by He.
Emission dynamics from doped crystals of (CH3)4NMnBr3(TMMB)-exciton trapping in a one-dimensional lattice
Authors: Rodriguez Waldo J. , Auerbach Roy A. , McPherson Gary L.
Published: July 2002

The red emission from (CH3)4NMnBr3 (TMMB) crystals is quenched by small concentrations of Cu2+. The emission decay curves from TMMB crystals doped with Cu2+ deviate dramatically from simple exponential form. Consistent with the linear chain structure of TMMB, the decay curves are accurately described by expressions derived to treat exciton trapping in a one-dimensional lattice. The analysis of the data indicate that the intrachain exciton migration is rapid, with a hopping frequency at room temperature of 1011 to 1012 s−1. The temperature dependence of the emission decay curves is consistent with an energy barrier to migration of 600 to 700 cm−1. Interchain migration apparently makes a negligible contribution to exciton trapping in TMMB with the interchain hopping frequency estimated to be less than 102 s−1. Within the limits of the experimental data, energy transport in TMMB is perfectly one-dimensional.
Classical trajectory study of the unimolecular dissociation of ammonia
Authors: Rice Betsy M. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

An analytical potential-energy surface based on the Varandas–Murrell potential for equilibrium NH3 has been formulated using the results of scaled CI/6-31G* calculations to adjust the potential parameters to give the correct energies and geometries for equilibrium NH3, the inversion, and the NH2+H and NH+H2 dissociation channels. Microcanonical unimolecular decay coefficients have been calculated for both channels over the energy range 5.25–7.0 eV from classical trajectories. The overall dissociation mechanism is found to consist of two parallel first-order decay processes. Although the reaction thresholds for both channels are nearly identical on our potential-energy surface, it is found that dissociation to NH2+H is the major decomposition pathway at all energies. The computed product translational energy distributions for NH2+H are peaked at energies near zero, as expected for dissociation processes which have no barrier to the back reaction. In contrast, the corresponding distributions for the NH+H2 product are found to be shifted toward higher energies due to the presence of a 15 kcal/mol back-reaction barrier. Examination of the mechanistic details of individual trajectories shows that dissociation to NH+H2 occurs via a concerted elimination. Deuterium isotope effects are reported for the dissociation of ND3, NH2D, and NHD2.
Infrared diode laser spectroscopy of the PF radical
Authors: Yamada Chikashi , Chang Man Chai , Hirota Eizi
Published: July 2002

The infrared absorption spectrum of the PF radical in the X 3Σ− ground electronic state has been observed using a Zeeman modulated diode laser spectrometer. The radical was produced in a multiple-reflection absorption cell by a dc glow discharge in a mixture of PH3 and CF4. The F1 and F3 spin components of five vibration–rotation transitions of the v=1–0 band have been recorded in the region of 823 to 830 cm−1. By constraining molecular constants of the v=0 state to the values previously determined by microwave spectroscopy, the band origin, the vibration–rotation constant αB=B0−B1, and the vibrational change of the spin–spin interaction constant αλ=λ1−λ0 have been determined from the observed spectrum to be 837.816 36(70), 0.004 639 4(70), and −0.005 04(91) in cm−1, respectively, with one standard deviation in parentheses. The sign of αλ is the same as those of NCl and PCl, but is opposite to those of O2, SO, S2, and SeO.
He atom scattering from the graphite (0001) surface: Diffraction peaks, resonance splittings, and isolated resonances
Authors: Jónsson Hannes , Weare John H.
Published: July 2002

Experimental diffraction peaks and resonance splittings are analyzed with a close coupling scattering calculation to give a consistent He–graphite interaction potential. The interaction is modeled as a sum of effective He–C pair interactions. Previous, more approximate, estimates of bound state matrix elements of the potential were 30% larger than present estimates. The corrugation of the equipotential surface is estimated to be a little smaller than previous ‘‘hard wall’’ estimates. The results are compared with recent ab initio potentials.
Simplified sampling in quantum Monte Carlo: Application to H+3
Authors: Anderson James B.
Published: July 2002

A new procedure for sampling molecular wave functions in quantum Monte Carlo calculations is used to determine energies for a number of configurations of the molecular ion H+3. With the Green’s function fixed by use of a trial energy and with no drift terms required for importance sampling the procedure is both simple and efficient. For the equilibrium configuration of H+3 the energy is found to be −1.343 76±0.000 03 a.u. (i.e., ±0.02 kcal/mol).
Analysis of the quasimetallic reflection band by dynamical coherent potential approximation (DCPA)
Authors: Wada Yoshiki , Tokura Yoshinori , Koda Takao
Published: July 2002

A theoretical algorithm is presented for numerical calculation of quasimetallic reflection bands of molecular excitons by a unified model which takes account of exciton–intramolecular phonon coupling by the dynamical coherent potential approximation and the polariton effect (the directional dispersion effect) by an anisotropic dielectric tensor. This method is applied to analyze experimental quasimetallic reflection spectra of DCI (1-1′-diethyl 2-2′-cyanine iodide), TCNQ (tetracyanoquinodimethane), and CTIP [γ-cyclopropyl-bis(1,2,3′-trimethyl-indolenine-2-yl) pentamethinium tetrafluoroborate] crystals. From a comparison of experimental and calculated reflectivity spectra, important exciton parameters, such as the bandwidths and shapes of low dimensional density-of-states functions of excitons, have been estimated.
The generalized van der Waals theory applied to the electrical double layer
Authors: Boyle E. J. , Scriven L. E. , Davis H. T.
Published: July 2002

The generalized van der Waals theory expression for ion density distribution is used with Poisson’s equation to solve the electrical double-layer problem for 1:1, 2:1, and 2:2 restricted primitive model electrolytes in contact with a diffusively charged, hard, planar surface. Electrical potential and ion density profiles are compared to Monte Carlo results and are shown to be more accurate at lower electrolyte concentration and surface charge density than at higher. In addition, values for the diffuse layer potential drop are shown to be less exact than those predicted by the hypernetted chain equation, modified Gouy–Chapman theory and modified Poisson–Boltzmann equation, but could be brought closer to Monte Carlo results by an appropriate choice of excluded volume parameter. Surface tension, ion adsorption, and differential capacitance are also reported.
On the origin of the phase transitions in a class of mixed valence compounds
Authors: Stratt Richard M. , Adachi Steven H.
Published: July 2002

Crystals of several mixed-valence compounds featuring (FeIII)2(FeII)O units undergo a variety of phase transitions with the three main phases differing in the extent of the intramolecular delocalization of the extra electron. We point out that a published theory for this phenomenon is basically equivalent to a classical spin model. Although some intriguing quantal aspects are ignored, solution of the spin model by mean-field theory reveals a much richer phenomenology than expected. In particular, the model predicts that the middle phase (of the three observed) should be one in which only a sublattice contains a large fraction of delocalized molecules.