bluuebunny/crossref_metadata_2025 huggingface dataset:
Profiling nitrogen in ultrathin silicon oxynitrides with angle-resolved x-ray photoelectron spectroscopy
Authors: Chang J. P. , Green M. L. , Donnelly V. M. , Opila R. L. , Eng J. , Sapjeta J. , Silverman P. J. , Weir B. , Lu H. C. , Gustafsson T. , Garfunkel E.
Published: July 2002

Angle-resolved x-ray photoelectron spectroscopy (AR–XPS) is utilized in this work to accurately and nondestructively determine the nitrogen concentration and profile in ultrathin SiOxNy films. With furnace growth at 800–850 °C using nitric oxide (NO) and oxygen, 1013–1015 cm−2 of nitrogen is incorporated in the ultrathin (⩽4 nm) oxide films. Additional nitrogen can be incorporated by low energy ion (15N2) implantation. The nitrogen profile and nitrogen chemical bonding states are analyzed as a function of the depth to understand the distribution of nitrogen incorporation during the SiOxNy thermal growth process. AR–XPS is shown to yield accurate nitrogen profiles that agree well with both medium energy ion scattering and secondary ion mass spectrometry analysis. Preferential nitrogen accumulation near the SiOxNy/Si interface is observed with a NO annealing, and nitrogen is shown to bond to both silicon and oxygen in multiple distinct chemical states, whose thermal stability bears implications on the reliability of nitrogen containing SiO2.
An extended model of the Barkhausen effect based on the ABBM model
Authors: Clatterbuck D. M. , Garcia V. J. , Johnson M. J. , Jiles D. C.
Published: July 2002

The Barkhausen model of Alessandro et al. [J. Appl. Phys. 68, 2901 (1990)] has been extended to nonstationary domain wall dynamics. The assumptions of the original model limit, its use to situations where the differential permeability, and time derivative of applied field are constant. The later model of Jiles et al. assumes that the Barkhausen activity in a given time interval is proportional to the rate of change of irreversible magnetization which can be calculated from hysteresis models. The extended model presented here incorporates ideas from both of these. It assumes that the pinning field and domain wall velocity behave according to the Alessandro model, but allows the rate of change of the magnetic flux to vary around a moving average which is determined by the shape of the hysteresis curve and the applied magnetic field wave form. As a result, the new model allows for changes in permeability with applied field and can also reproduce the frequency response of experimental Barkhausen signals.
Micromagnetic simulations of thermally activated magnetization reversal of nanoscale magnets
Authors: Brown Gregory , Novotny M. A. , Rikvold Per Arne
Published: July 2002

Numerical integration of a stochastic Landau–Lifshitz–Gilbert equation is used to study dynamic processes in single-domain nanoscale magnets at nonzero temperatures. Special attention is given to including thermal fluctuations as a Langevin term, and the fast multipole method is used to calculate dipole–dipole interactions. It is feasible to simulate these dynamics on the nanosecond time scale for spatial discretizations that involve on the order of 104 nodes using a desktop workstation. The nanoscale magnets considered here are single pillars with large aspect ratio. Hysteresis-loop simulations are employed to study the stable and metastable configurations of the magnetization. Each pillar has magnetic end caps. In a time-dependent field the magnetization of the pillars is observed to reverse via nucleation, propagation, and coalescence of the end caps. In particular, the end caps propagate into the magnet and meet near the middle. A relatively long-lived defect is formed when end caps with opposite vorticity meet. Fluctuations are more important in the reversal of the magnetization for fields weaker than the zero-temperature coercive field, where the reversal is thermally activated. In this case, the process must be described by its statistical properties, such as the distribution of switching times, averaged over a large number of independent thermal histories.
Micromagnetic simulations of submicron cobalt dots
Authors: Parker G. J. , Cerjan C.
Published: July 2002

Numerical simulations of submicron Co extruded elliptical dots were performed to illustrate the relative importance of different physical parameters on the switching behavior in the easy direction. Shape, size, magnetic moment magnitude, and the magnitude and distribution of the crystalline anisotropicity were varied. The simulation represents magnetostatic, exchange, and crystalline anisotropicity fields on a structured mesh using finite difference techniques. The smooth boundary of the dots is accurately represented by use of the embedded curve boundary method. Agreement with experimental hysteresis measurements of submicron dot arrays is obtained when an appropriate angular distribution of the grain anisotropicity axes is invoked.
Thickness dependence of domain formation in epitaxial PbTiO3 thin films grown on MgO (001) substrates
Authors: Lee K. S. , Baik S.
Published: July 2002

Strain relaxation and its effect on domain formation of epitaxial PbTiO3 thin films grown on MgO (001) substrates were investigated as a function of film thickness by two-dimensional reciprocal space mapping using synchrotron x-ray diffraction. Within a few hundreds of angstrom region, it was observed that c-domain abundance, α, was critically dependent on film thickness. As the film thickness increased further, α was saturated at a value of ∼0.75. The HK mesh scan on PbTiO3 (100) reflections revealed that directional tilting of a domains with four-fold symmetry began to develop as the film thickness exceeded 650 Å. Thermodynamic equilibrium relief of the coherency strain was evaluated based on Mattews–Blakslee criteria that determine thickness dependent misfit accommodation. This theoretical consideration with experimental results led us to conclude that the unrelaxed residual misfit strain has a significant effect on the domain formation, particularly in the region below the thickness of 1000 Å.
Magnetization dynamics as derived from magneto impedance measurements
Authors: Carara Marcos , Baibich M. N. , Sommer R. L.
Published: July 2002

In this work, the magnetization dynamics of soft magnetic materials is studied with the aid of transverse differential permeability μ(Iac,f,Hdc) spectra. Contributions to the magnetization processes from domain wall motion and rotation of the magnetization can be extracted from the transverse differential permeability data which are in turn obtained from impedance Z(Iac,f,Hdc) spectra. In particular, an iteration method is used to extract μ(Iac,f,Hdc) from Z(Iac,f,Hdc) data. The approach is tested in samples with a very well known domain structure, namely (110)[001]FeSi3%. Permeability spectra μ(Iac,f,Hdc) were obtained in the frequency range (100 Hz⩽f⩽100 kHz), probe current range (0.1⩽Iac⩽50 mA) and dc magnetizing field range (0⩽Hdc⩽500 Oe). It is shown that the method developed in this article can be efficiently used to identify and study different dynamic processes driven by the probe current and controlled by the external dc field. In particular, it is shown that the method provides the tools to separate the reversible and irreversible parts of these processes.
A search for strain gradients in gold thin films on substrates using x-ray diffraction
Authors: Leung O. S. , Munkholm A. , Brennan S. , Nix W. D.
Published: July 2002

The high strengths of gold thin films on silicon substrates have been studied with particular reference to the possible effect of strain gradients. Wafer curvature/thermal cycling measurements have been used to study the strengths of unpassivated, oxide-free gold films ranging in thickness from 0.1 to 2.5 μm. Films thinner than about 1 μm in thickness appear to be weakened by diffusional relaxation effects near the free surface and are not good candidates for the study of strain gradient plasticity. Our search for plastically induced strain gradients was thus limited to thicker films with correspondingly larger grain sizes. Three related x-ray diffraction techniques have been used to investigate the elastic strains in these films. The standard dhkl vs sin2 Ψ technique has been used to find the average strain through the thickness of the films. The results are consistent with wafer curvature measurements. We have also measured a number of dhkl’s as a function of penetration depth to construct depth-dependent dhkl vs sin2 Ψ plots. These data show that the residual elastic strain is essentially independent of depth in the film. Finally, a new technique for sample rotation has been used to measure the dhkl’s for a fixed set of grains in the film as a function of penetration depth. Again, no detectable gradient in strain has been observed. These results show that the high strengths of unpassivated gold films relative to the strength of bulk gold cannot be rationalized on the basis of strain gradients through the film thickness. However, a sharp gradient in strain close to the film substrate interface cannot be ruled out.
Microstructure and magnetic properties of mechanically milled nanograined PrxCo100−x (x=15.4−20.5) powders
Authors: Chen Zhongmin , Zhang Yong , Hadjipanayis G. C.
Published: July 2002

The structure, microstructure, and magnetic properties of high-coercivity PrCo5-based PrxCo100−x (x=15.4–20.5) powders synthesized by mechanical milling and subsequent annealing were systematically studied as a function of Pr content. While the magnetization decreases monotonously with the Pr content, the coercivity increases, reaching a maximum of 24.1 kOe in Pr19Co81, and then decreases for higher Pr content. As a result, the maximum energy product goes through a broad peak of about 11.3–11.8 MGOe at the 16.7–18.0 at. % Pr content. Microstructural studies reveal that a uniform nanoscale PrCo5/Pr2Co17 microstructure with an average grain size of about 15–30 nm is developed in powders with Pr content up to 19 at. %. The volume fraction of the Pr2Co17 decreases with Pr content and a nearly single PrCo5 structure is obtained in Pr19Co81 powders. Further increase in the Pr content leads to the presence of the less-hard Pr2Co7 phase in the form of large grains, resulting in lower coercivities. Evidence of intergranular exchange coupling is found in all the samples. The results suggest that the observed magnetic hardening originates from the high anisotropy field of the PrCo5 phase and the uniform nanoscale microstructure developed by the processing used.
[1,3] Sigmatropic thermal rearrangements as vector fields on the 2 sphere
Authors: Fernández Ariel
Published: July 2002

The nature of the symmetry-forbidden pathway for the rearrangement of bicyclo [3.2.0]hept-2-enes(1) to norborn-2-enes(2) is analyzed making use of a combinatorial-topological method. In agreement with experiment evidence, it is assumed that the triplet sheet does not cross the singlet manifold within the locus in the nuclear configurations space where the thermal process occurs. The intrinsic reaction coordinate trajectory, as defined by Fukui, corresponding to the retention of configuration at the migrating group is shown to pass through a concerted critical configuration and not through a diradical species. Equations relating the numbers of paths for steepest descent joining different kinds of critical configurations are derived and tested.
Self-consistent linear response study of the polarizability of molecular nitrogen
Authors: Soven Paul
Published: July 2002

The static polarizability of molecular nitrogen is computed in a local-density-functional self-consistent linear response framework. The work reported here investigates that applicability of a theory of this type to a study of observable response functions and evaluates the utility of a simple approximation to the density–density response function of the molecular system. Computed quantities are in good agreement with experiment.
Comparison of proton transfers in (S2H5)+ and (O2H5)+
Authors: Scheiner Steve , Bigham Larry D.
Published: July 2002

The energetics and electronic rearrangements associated with proton transfer between S atoms in (H2S–H–SH2)+ are calculated using ab initio molecular orbital methods and compared with similar data in the first-row analog (H2O–H–OH2)+. The full potential energy surface of (S2H5)+, calculated as a function of the H-bond length as well as the position of the proton, contains two equivalent minima separated by a small energy barrier, whereas the surface of (O2H5)+ contains a single minimum corresponding to a symmetric position for the central proton. In both cases the energy barrier to transfer increases as the H bond is lengthened. This rise is noticeably less steep in the case of (S2H5)+, a fact attributed to the greater ease with which a proton may be pulled a given distance from each SH2 subunit in the absence of the other. Enlargements of the proton transfer barriers also result from angular distortions of each H bond; these increases are qualitatively quite similar in the two systems. There is a great deal of resemblance also in the electronic redistribution patterns accompanying proton transfer in the two systems. However, the greater polarizability of SH2 as compared to OH2 leads to greater overall charge transfer between the subunits in (H2S–H–SH2)+ and to larger extent of spatial regions of density change.
Investigation of polarization diffusion and relaxation dynamics of low temperature glasses
Authors: Köhler W. , Breinl W. , Friedrich J.
Published: July 2002

We investigated the optical anisotropy and the normalized areas of persistent spectral holes in organic glasses over almost 1 week (4 decades). The systems studied were 1,4-dihydroxyanthraquinone and tetracene in an EtOH/MeOH glass. The areas of the holes decay on a logarithmic time scale, but the anisotropy remains constant. We draw the following conclusions: (i) There is no reorientation of dye molecules in the glass. (ii) Molecules relaxing from the product state reassume the position in the frequency domain and in space which they had before hole burning. (iii) A dye molecule seems to be strongly coupled just to one two level system (TLS).
CO product energy distribution in the photodissociation of methylketene and acrolein at 193 nm
Authors: Fujimoto G. T. , Umstead M. E. , Lin M. C.
Published: July 2002

CO product vibrational energy distributions in the photodissociation of the two C3H4O isomers, methylketene (CH3CHCO) and acrolein (CH2CHCHO), at 193 nm have been measured by CO laser resonance absorption. The CO from methylketene was found to be vibrationally excited up to v=7, and from acrolein v=6, with average vibrational energies of 3.4±0.3 and 2.7±0.7 kcal/mol, respectively. The similarities observed in the appearance times and in the vibrational energy content of the CO formed in the two systems support our previous conclusion that in the case of acrolein isomerization to methylketene takes place prior to the dissociation process: CH2CHCHO+hν→CH3CHCO*†→CH3CH+CO†. The CO vibrational energy distributions observed in both systems agree closely with the statistical distribution predicted assuming that ethylidene rather than ethylene is formed in the photodissociation reaction.
Susceptibility measurements of liquid crystal–quasispherical solute mixtures in the isotropic phase near the nematic–isotropic transition
Authors: Rosenblatt Charles
Published: July 2002

Magnetically induced optical birefringence in the isotropic liquid crystal phase was used to study binary mixtures of pentylcyanobiphenyl (5CB) and two quasispherical impurities: carbon tetrachloride and tetraethyltin. For solute mole fraction concentrations 0≤X≤0.7, both the nematic orientational susceptibility dS/dH2 vs temperature and the supercooling limit T* were obtained vs X. It was found that both quantitites differ from existing theories for the 5CB/tetraethyltine mixtures, probably requiring the inclusion of short range correlations in the various models.
Rotation of a large molecule in liquids: A resonance-enhanced light scattering study
Authors: Uphoff R. , Dorfmüller Th.
Published: July 2002

Depolarized resonance-enhanced light scattering spectra of the cationic dye auramine-O have been obtained in methanol, ethanol, and 2-propanol solutions at high dilution and with different counterions. The spectra could all be fitted with a single Lorentzian. The rotational correlation times thus obtained conform to the Arrhenius equation and the activation energies are equal to the activation energies of the macroscopic shear viscosity. The Debye–Stokes–Einstein equation applies to the data, but the hydrodynamic volumes of gyration obtained by this procedure are dependent upon the solvent and the nature of the counterion. This effect is discussed in terms of solute–solvent interactions and of ion pairing.
Thermodynamic properties of water in the subcooled region. II
Authors: Leyendekkers J. V. , Hunter R. J.
Published: July 2002

The surface effect in fine capillaries is considered and an analysis made of the volumetric properties of water in the subcooled region, using emulsion, capillary, and bulk specific volume (v) data. The analysis indicates that the emulsion values of v are probably too large below 253 K and consequently the expansivity (α) values are too large numerically. Compressibility, heat capacity, and ultrasonic and refractive index data are used to estimate the expansivity in the subcooled region. All these data support conclusions from the analysis of v. Below 250 K the Cp/Cv ratios from emulsion data are much larger than those from ultrasonic data and those estimated from a variety of other data. The overall conclusion (from parts I and II) is that the question concerning the accuracy of emulsion data below 253 K must remain open at present.
Theoretical study and experimental observations of lattice vibrations in squaric acid
Authors: Bougeard D. , Samuelsen E. J. , Semmingsen D.
Published: July 2002

The dispersion relations of lattice vibrations in several symmetry directions have been calculated for the room temperature structure using Buckingham type atom–atom potentials. Satisfactory agreement is obtained with inelastic neutron scattering observations, also presented in the present paper.
Statistical mechanics of ion–dipole mixtures: An exactly solvable model in one dimension
Authors: Vericat Fernando , Blum Lesser
Published: July 2002

The statistical mechanics of a mixture of hard core ions and dipoles, with or without an applied external field, is solved exactly in one dimension using the generating function technique of Baxter. This function is shown to be the solution of a nonlinear Hill-type equation. In the plasma limit we obtain explicit expressions for the thermodynamic properties of the bulk phase and the potential drop and differential capacitance of the electrical double layer as well. The expressions obtained resemble the mean spherical approximation results.
The flow of dilute polymer solution in a narrow channel. I. The slip effect in simple shear flow
Authors: Goh C. J. , Atkinson J. D. , Phan-Thien N.
Published: July 2002

Existing experimental data have suggested that the flow of a dilute polymer solution near a wall exhibits a slip effect. In the case of a plane shear flow, the suspension of macromolecules contributes a nonuniform stress field profile across the channel which causes the velocity field to deviate from its otherwise linear profile. The apparent slip velocity can be calculated from such deviation. In this paper, we consider the flow problem of a suspension of bead-spring molecules in the presence of solid reflective boundary. Analytical and simulation solutions to the problem are obtained which do not require any small shear rate assumption. The added stresses contributed by the molecules are used to derive the slip velocity and the effective viscosity. Extension to nonlinear Warner spring model is also attempted by the simulation and the results give rise to some qualitative features which cannot be accounted for by the linear model.
Optical excitation and emission spectra of Cs2NaErCl6
Authors: DePiante Anne , Richardson F. S. , Hasan Zameer-ul
Published: July 2002

Optical excitation and emission spectra are reported for the Cs2NaErCl6 system under variable-temperature (8.5–298 K) conditions. Detailed line assignments are made for the 4I15/2←4S3/2 and 4I15/2←4F9/2 high-resolution emission spectra, and all five crystal-field levels split out of the 4I15/2 ground multiplet are located. These results combined with our previously reported absorption results provide the locations of 35 crystal-field levels spanning 11 multiplets of the Er3+ ion in Cs2NaErCl6. Energy level calculations based on a nine-parameter 4f-electron Hamiltonian operating within the 4f11 configuration of Er3+ give results in good agreement with experiment. Excitation spectra for 4I15/2←4S3/2 emission are reported at several temperatures over the 340–540 nm spectral region. Emission spectra excited by near-ultraviolet radiation (centered at 383 nm) are reported over the 405–725 nm spectral region for sample temperatures ranging from 8.5 to 298 K. All but one of the seven multiplet-to-multiplet emission bands observed in this region decrease in intensity as the sample temperature is increased. The 4I13/2←4G11/2 emission exhibits an increase in intensity with increasing temperature.
A semiclassical quantization using arbitrary trajectories
Authors: Ramaswamy Ramakrishna
Published: July 2002

A simple method of obtaining the (first-order) semiclassical eigenvalue spectrum from information pertaining to arbitrary nonquantizing tori is discussed. This method relies on the existence of a unique energy functional for tori, and the observation that for most typical potentials, the energy can be expressed as a polynomial function of the actions.
On 2H NMR and molecular motion of t-butyl groups in solidsa)
Authors: Ripmeester J. A. , Ratcliffe C. I.
Published: July 2002

Although 1H NMR generally cannot be used to distinguish between methyl group (C3) rotation, and rotation of the whole t-butyl group about its threefold axis (C′3 motion), 2H NMR line shapes are shown to be quite diagnostic as to which motion is present. In agreement with neutron scattering results C′3 motion was shown to be faster than C3 motion for t-butyl chloride and t-butyl bromide at ∼100 K. Also, C′3 motion is faster than C3 motion for trimethylamine in its 10.25 hydrate.
Optical spectra and crystal field analysis of Sm3+ in the cubic Cs2NaYCl6 host
Authors: Foster David R. , Richardson F. S. , Schwartz R. W.
Published: July 2002

Optical absorption and emission measurements are reported for the Cs2NaSmCl6 and Cs2NaYCl6:Sm3+ (1 or 30 mol %) systems under variable-temperature (10–298 K) conditions. Twenty-six crystal-field levels spanning 13 multiplets of the Sm3+ 4f 5 electronic configuration are located and assigned on the basis of the experimental data, and these energy level results are rationalized in terms of a nine-parameter 4f-electron Hamiltonian. The most intense electric-dipole vibronic lines observed in the spectra are assigned to one-phonon vibronic transitions associated with the three odd-parity ‘‘moiety’’ modes of the SmCl3−6 octahedral clusters ν3(t1u), ν4(t1u), and ν6(t2u). Intensity calculations are reported for both the no-phonon magnetic-dipole (origin) transitions and the one-phonon (ν3, ν4, and ν6) electric-dipole vibronic transitions.
Intersoliton hopping in lightly doped polyacetylene: Assistance by vibrational motion of the dopant
Authors: Yamabe Tokio , Tanaka Kazuyoshi , Yamanaka Shozo , Koike Tsuneaki , Fukui Kenichi
Published: July 2002

The rate of intersoliton hopping associated with the interchain conduction mechanism in lightly doped polyacetylene has been studied explicitly including a certain vibrational motion of the dopant molecule. The model system which was used consists of a neutral soliton and a charged soliton alternating between these chains. Conventional time-dependent perturbation theory was applied to the effective Hamiltonian developed here to study the system. We find that, under the assistance of the vibrational motion of the dopant along the axis between the two soliton sites, the rate of this intersoliton hopping is largely accelerated up to the order of that of the vibrational motion.
7Li-NMR relaxation of hydrated zeolite Li–X
Authors: Basler Wolf D.
Published: July 2002

7Li-NMR spin-lattice relaxation T1 of hydrated zeolite Li–X has been determined at 23.5 MHz between 140 and 295 K for samples with different Fe content. The results show that relaxation by paramagnetic Fe impurities is dominating near the T1 minimum in samples which contain 100 ppm Fe. The whole spectrum of all Li ions is observed by NMR, the free induction decay is nonexponential at 295 K.
High-pressure Brillouin scattering studies in the cubic phase of CsCN
Authors: Strössner K. , Hochheimer H. D.
Published: July 2002

The pressure and temperature dependence of the elastic constants of CsCN have been studied in the orientationally disordered cubic phase by Brillouin spectroscopy. A softening of the symmetry adapted elastic constants (c11−c12)/2 and c44 is observed in the temperature range 193 to 300 K as the order–disorder phase transition is approached from above. This behavior is seen in the whole pressure range up to 0.6 GPa. The pressure dependence the volume corrected elastic constants has been determined and a comparison with the corresponding values of the other alkali cyanides has been made.
Charge transfer, vibrational excitation, and dissociative adsorption in molecule–surface collisions: Classical trajectory theory
Authors: Holloway S. , Gadzuk J. W.
Published: July 2002

The consequences of charge transfer processes occurring when a molecular beam of diatomic molecules is directed upon a solid surface are here considered. In analogy with resonance electron scattering from molecules or harpooning processes in atom–diatom collisions, the incident beam could either be scattered into a highly vibrationally excited molecular state, dissociatively scattered, or dissociatively adsorbed due to formation of temporary negative molecular ions which enable redistribution of the incident translation energy of the beam into intramolecular degrees of freedom. In this work, the exact classical trajectories for the diatomic molecule, including internal vibrational motion, are calculated for motion over model diabatic potential surfaces in which surface hopping due to charge transfer/harpooning is accounted for. Connections between classes of trajectories and topological features of the potential energy surfaces (PES) are illustrated. The model is used to study the average translational to vibrational energy transfer as a function of incident kinetic energy and of PES parameters. Branching ratios between scattered and dissociatively adsorbed molecules are obtained as a function of both incident translational and total energy and the role of the intermediate negative ion resonance in influencing the dynamics of molecular processes at surfaces is illustrated. Comparison with quantum mechanical theories is given in a subsequent paper.
Comment on the restricted ensemble averaging processes for axially symmetric ESR centers under orientational diffusion
Authors: Tang Sheng-Zhang , Lee Sook
Published: July 2002

It is shown that one of the restricted ensemble averaging processes recently formulated by Kivelson and Lee for an axially symmetric ESR system is valid only for the ESR symmetry axis parallel to the ESR external magnetic field, but not for other general orientations. The other averaging process is valid for all orientations.
Loss of phase coherence in the inversion of ammonia as a model for the racemization of chiral molecules
Authors: Macomber James D.
Published: July 2002

It is shown that if an ensemble of ammonia molecules in the gas phase could be prepared with the nitrogen atoms displaced (localized) to one side of the plane of the hydrogen atoms, the ensemble-averaged expectation value of this displacement would oscillate with time due to inversion. It is also shown that the amplitude of this oscillation gradually would decrease due to various relaxation processes and approach zero at equilibrium. It is suggested that similar damped oscillations should constitute the dominant gas-phase racemization mechanism for disymmetric amines and other chiral molecules, whenever intramolecular pathways dominate the process. In such cases, the racemization curve (optical rotation vs time) should be the Fourier transform of the inversion spectrum.
Harmonic tunneling corrections to sudden and adiabatic transition state theory
Authors: Pollak Eli
Published: July 2002

Harmonic tunneling corrections are incorporated within semiclassical adiabatic and sudden transition state theory. Good agreement is obtained with CS and sudden quantal computations. Analysis of the transition state theories at the level of rotationally averaged cross sections leads to a new interpretation of the quantal sudden computation and to convergence of the adiabatic and sudden approximations in reactive scattering.
Collision-induced energy transfer in laser-excited dichloromethane
Authors: Gao W. B. , Shen Y. Q. , Häger J. , Krieger W.
Published: July 2002

Laser-induced infrared fluorescence was used to measure the collision-induced vibrational energy relaxation in dichloromethane (CH2Cl2). The activation and deactivation of the υ8 and υ3/υ9 vibrational modes was measured after excitation of CH-stretching vibrations near 3000 cm−1. Rate constants for the energy transfer are deduced leading to a simplified model for the vibrational relaxation in CH2Cl2.
Thermodynamics of reversible adsorption of amphiphiles from solution onto interface near saturation
Authors: Zhou N. F. , Chen L. W.
Published: July 2002

Based on the fundamental thermodynamical relations for surface phases given by Guggenheim and the rigorous discussions by Vochten, Petre, and Defay, the reversible entropy and enthalpy differences for the amphiphile molecules in between the adsorbed surface and the bulk phases of solution were derived and calculated respectively. The entropy of the amphiphile solutes at the interface is lower than that in the bulk when the adsorption is below saturation. However, when the adsorption approaches saturation the entropy difference becomes either zero or positive. These features can be reasonably explained by the adsorption-orientation effect of the amphiphile solute molecules in the interfacial adsorbed layer.
Exact solution to the linearized Poisson–Boltzmann equation in cylindrical coordinates
Authors: Rice Richard E.
Published: July 2002

For the case of a concentration gradient across a charged membrane, the linearized Poisson–Boltzmann equation in cylindrical coordinates, is solved exactly by means of a finite Hankel transform. Although some of the required integrals cannot be analytically evaluated in general, they can be approximated with asymptotic expansions of the modified Bessel functions for large argument. Under these conditions the solution is identical to that obtained with the usual assumption that the electric potential can be separated into one term that depends on only the axial coordinate and a second that depends on both the axial and radial coordinates. Thus this separation, which is much simpler to apply than the transform method presented here, can be applied with confidence under the appropriate conditions.
Unimolecular decay paths of electronically excited species. VI. The Ã2E state of NH+3
Authors: Krier C. , Praet M. Th. , Lorquet J. C.
Published: July 2002

The Ã 2E state of the NH+3 ion is split by a Jahn–Teller interaction. The potential energy surface of the 2A″ component of state Ã undergoes a conical intersection with the surface of the ground state. This brings about an ultrafast relaxation channel.
The flow alignment of some nematic liquid crystals
Authors: Beens W. W. , de Jeu W. H.
Published: July 2002

A setup is described to determine the flow alignment angle θ0 in nematic liquid crystals. Results for compounds with and without a strongly polar terminal cyano group indicate that the antiparallel dipole correlation associated with cyano groups has no influence on θ0. The influence of the occurence of a smectic-A phase on θ0 is investigated on three homologous series. The results show that θ0 is a very sensitive probe for smectic-like short-range order. With increasing chain lengths a decrease of θ0 is found, until finally no flow alignment exists anymore. Interestingly, a similar behavior is found in a homologous series without smectic phases.
Experimental test of a recent theory of depolarized light scattering by molecular liquids
Authors: Savant-Ros E. , Bezot P. , Brot C.
Published: July 2002

Relative measurements of the intensities of the light scattered by three anisotropic molecules diluted in weakly depolarizing solvents are reported. Our aim was to test the expression recently derived by Brot for the model of the molecule in an ellipsoidal cavity embedded in a continuum. Our results are in good agreement with this expression, the fit of the ellipticity of the cavity yielding reasonable values, contrary to the results of an earlier treatment.
Energy partitioning in He2+/K collisions
Authors: Burkhardt C. E. , Earsom D. L. , Daulton T. L. , Garver W. P. , Leventhal J. J. , Gellene Gregory I. , Porter Richard F.
Published: July 2002

Final state distributions and state-specific reaction cross sections for 1.5–6 keV He2+/K collisions have been determined from data acquired using both neutralized ion beam spectroscopy and direct observation of luminescence from decay of excited collision products. While several minor reaction channels can be identified, it is found that near-resonant processes yielding He2(a 3Σu+) and He2(A 1Σu+) are predominant. These major channels, both of which have cross sections in excess of 100 Å2, lead to substantial production of energetic metastable molecules and VUV continuum radiation extending from about 750 to 850 Å.
Crystal field invariants and parameters for low-symmetry sites
Authors: Yeung Y. Y. , Newman D. J.
Published: July 2002

Relationships between crystal field invariants and superposition model parameters are established. Various strategies are examined for determining and assessing the reliability of crystal field parameters for lanthanide ions in low-symmetry sites. In particular, a new method of estimating crystal field parameters is proposed using crystal field invariants in conjunction with the superposition model. The importance of using a fixed reference frame throughout a series of fitting based on approximate symmetries is stressed and this leads us to identify a new simplified parametrization for lanthanide ions in LaF3 which is not related to an approximate symmetry.
The flow of dilute polymer solution in a narrow channel. II. Plane Poiseuille flow
Authors: Goh C. J. , Atkinson J. D. , Phan-Thien N.
Published: July 2002

As an extension of the previous paper, we derive a similar theory for the plane Poiseuille flow of a dilute polymer solution. The strict enforcement of reflective boundary conditions allows an approximate solution to the Fokker–Planck–Kolmogorov equation to be obtained which is found to be consistent with solutions obtained by Galerkin methods and Monte Carlo simulation. The approximate theory also allows us to derive analytical expressions for the slip velocity and the effective viscosity which are again consistent with the plane Couette flow results derived in the previous paper. The theory is also extended to the nonlinear Warner spring model via Monte Carlo simulation.
Stimulated two-photon induced transitions at high photon densities
Authors: Mavroyannis Constantine
Published: July 2002

We have considered the excitation spectra arising from stimulated two-photon transitions between two atomic levels of the same parity. The system consists of a three-level atom in the cascade configuration interacting with two strong electromagnetic fields whose initially populated modes ωa and ωb are in resonance with the two atomic transition frequencies, respectively. The Green function formalism is used to calculate the spectral function of the system near the frequencies ω=ωa+ωb, in the limit of high photon densities. The excitation spectra consist of Lorentzian lines peaked at the frequencies ω−ωa−ωb=0, ±Ωa/√2, ±Ωb/√2, ±Ω, and ±2Ω, where Ω2=(Ω2a+Ω2b)/2 and Ωa,Ωb are the Rabi frequencies of the two laser fields, respectively. Numerical calculations for selected values of the Rabi frequencies are graphically presented and discussed. The computed two-photon spectra are compared with those arising from the third-order nonlinear mixing of frequencies near ω=ωa+2ωb and ωb+2ωa, respectively, as well as with those of the stimulated Raman spectra near ω=±(ωa−ωb).
Bulk properties of two-phase disordered media. II. Effective conductivity of a dilute dispersion of penetrable spheres
Authors: Torquato S.
Published: July 2002

The calculation of the effective electrical conductivity σ* of a dilute dispersion of equisized spheres of radius R distributed with arbitrary degree of penetrability is considered. It is demonstrated that σ*, through second order in the inclusion volume fraction φ2, can be written in terms of the zero-density limits of the pair-connectedness and pair-blocking functions, and certain polarizability tensors which involve one and two inclusions. Rigorous upper and lower bounds on σ*, through order φ22, are shown to depend upon, among other quantities, the aforementioned pair distribution functions and are evaluated for two models: an interpenetrable-sphere model and a certain sphere distribution in which the minimum distance between sphere centers is greater than or equal to 2R. An approximate expression obtained for the low-density expansion of σ* for dispersions of penetrable spheres always lies between the derived bounds on σ*. The study demonstrates that the effect of connectivity of the inclusion phase on σ*, through second order in φ2, can be substantial relative to the conductivity of dispersions of spheres characterized by a pair-connectedness function that is zero for all sphere separations.
ESR spectrum of ClO trapped in KClO4
Authors: Byberg J. R.
Published: July 2002

The ESR spectrum of a paramagnetic defect produced in solid KClO4 by x irradiation at 26 K is attributed to the radical ClO since the magnetic hyperfine tensor and the nuclear quadrupole tensor of the defect are both satisfactorily accounted for in terms of the hyperfine and quadrupole constants of gaseous ClO combined with a crystal-field representation of the influence of the solid matrix.
Anomalous defect distributions in SrCl2
Authors: Wietfeldt J. R. , Wright J. C.
Published: July 2002

Site selective spectroscopy has been performed for SrCl2:Eu3+ samples. The Eu3+ lines have been classified into nine major sites. The anomalous behavior of these sites as a function of concentration and annealing temperature is similar to that of rare earth doped CaF2, except that clusters are more stable in SrCl2. These results for SrCl2, a material which has the fluorite structure but no fluoride, eliminate the possibility that the anomalous behavior in the fluorites is due to fluoride.
Study of mode specificity by the natural expansion analysis
Authors: Moiseyev Nimrod , Schatzberger Rachel , Froelich Piotr , Goscinski Osvaldo
Published: July 2002

The self-consistent field (SCF) method beyond its computational advantage lends meaning to the concept of local modes and its applicability implies mode specificity in the unimolecular reaction being studied. The natural expansion (NE) analysis of stable or metastable vibrational states is presented here first. The NE analysis enables us to determine the applicability of the SCF approximation, the optimal molecular modes for any given vibrational energy and therefore provides a positive test for mode specificity. Moreover, it is shown here that even when the internal coordinates of the Hamiltonian are not optimized and there are several dominant configurations in the NE, mode specificity can be indicated by analyzing these configurations. As examples, the mode specificity in the unimolecular decomposition of CO2 to CO+O and in the dissociation of formaldehyde to H2+CO are studied by the NE analysis.
Microwave investigation of lactonitrile. Potential functions to the hydroxyl and methyl group torsions
Authors: Caminati Walther , Meyer Rolf , Oldani Markus , Scappini Flavio
Published: July 2002

The rotational spectrum of lactonitrile, CH3CH(OH)CN, has been reinvestigated. The analysis has been extended to the CH3CH(OD)CN species and to several excited states of the hydroxyl and methyl internal rotations. The molecule exhibits two conformers with the hydroxyl hydrogen gauche with respect to the cyano group. The conformer A with the hydroxyl hydrogen adjacent to the methinic hydrogen is 118±30 cm−1 (75±30 cm−1 for the OD species) more stable than the conformer B with the hydroxyl hydrogen adjacent to the methyl group. The barrier height to the interconversion is about 400 cm−1. Rotational A–E splittings due to methyl internal rotation have been observed in the first excited state of both conformers, yielding effective V3 barriers of 3.7 kcal/mol for conformer A and 4.3 kcal/mol for conformer B. By using a one-dimensional flexible model treatment the potential function and the structural relaxation associated with the OH torsion have been determined from the level spacings and the variations of the rotational constants, upon excitation. The potential profile and the structural relaxation indicate a repulsion of the hydroxyl hydrogen atom by the methyl group and an attraction by the cyano group. A preliminary two-dimensional model including both torsions is applied to the parent species. A comparison of the respective results with observations suggests some potential energy coupling of the two torsions and/or structural relaxation associated also with methyl internal rotation.
Coalescent resonances in 4He/LiF(001) surface scattering
Authors: Bačić Z. , Bosanac S. D.
Published: July 2002

Coalescent resonances are investigated in the He/LiF(001) collisions. The giant resonance is found when the incident energy of the He atom is scanned. Similar resonance is not found when the incidence azimuthal angle is scanned. This is primarily due to the presence of other resonances which are degenerate with the coalescent ones.
Topological rubber-elasticity theory. III. Comparison of rigidities computed for model tetra-functional networks with experiments
Authors: Iwata Kazuyoshi
Published: July 2002

A b initio comparison is made of the modulus computed for model tetrafunctional networks with experiments on poly(dimethyl–siloxane) (PDMS), polybutadiene (PB) and ethylene–propylene copolymer (EP) found in literature. The theory and computer programs presented in parts, I and II [K. Iwata, J. Chem. Phys. 76, 6363, 6375 (1982)] are improved further in the following points: (1) parameter L, which represents macroscopic shape of networks, is chosen as a parameter of topological distribution functions, (2) ‘‘THL network,’’ which has chain-connection pattern of tetrahedral lattice, is newly introduced as a model of tetrafunctional networks and (3) computer programs of part II are improved so that computational error becomes small enough for quantitative discussion. Reduced moduli, Ḡ=G/c0kT, are expressed in terms of the overlap parameter among strands, φ=(4πR3g/3)c0, and the volume expansion ratio relative to the as-formed volume, α, where c0 is the number concentration of elastically active strands and Rg, the unperturbed radius of gyration of the strands. It is found that (1) the shear modulus computed can be expressed in a compact form, Ḡtheor (φ,α)=Ḡph+φg(α), where Ḡph is a phantom network term and is far smaller than the second (i.e., topological) term, (2) a plot of Ḡexpt of PDMS, PB, and EP against φ produces a single curve, (3) qualitatively, Ḡtheor agree well with Ḡexpt but, quantitatively, Ḡtheor is overestimated by 50%–100%, (4) Ḡtheor is roughly proportional to α2/3, and (5) the Mooney constants 2C̄2, are far smaller than Ḡ. From these results it is concluded that the main source of rubber-elasticity is topological interaction among polymers.
Infrared laser spectroscopy of the ν2 mode of HCS+
Authors: Davies P. B. , Rothwell W.J.
Published: July 2002

Vibration-rotation absorption lines of HCS+ have been detected between 718 and 816 cm−1 by diode laser spectroscopy, and assigned to P-, Q-, and R-branch components of the ν2 bending fundamental. Their accurate analysis yields the first measurements of the rotational parameters B and D and the l-doubling constant q for the 0110 level. The band center was determined to be 766.453 (1) cm−1.
Comment on reactive diatom–solid surface collision: A quantum mechanical approach
Authors: Elber Ron
Published: July 2002

The previously reported results1, on the temperature effects on electron attchment rate constant to perfluorobenzene are further discussed in the light of recently reported works2–3. (AIP)
A Raman spectroscopic study of naphthalene:octafluoronaphthalene at high pressure and low temperature
Authors: Desgreniers S. , Kourouklis G. A. , Jayaraman A. , Kaplan M. L. , Schmidt P. H.
Published: July 2002

The crystalline 1:1 complex naphthalene:octafluoronaphthalene has been investigated at high pressure under ambient temperature, and down to 10 K at ambient pressure, by Raman spectroscopy. The complex is stable in the pressure range studied, 1 to 80 kbar. The lattice mode frequencies show a strong positive pressure dependence, while the internal modes are only weakly affected. The temperature and pressure dependencies of the phonon frequencies suggest that the bonding interaction between the molecular partners is of the usual van der Waals type, and the results are in agreement with the ‘‘sublattice’’ model proposed by Chen and Prasad. The mode Grüneisen γi’s have been obtained from the pressure data. From the measured (∂ν/∂T)p and (∂ν/∂P)T, the ‘‘explicit,’’ ‘‘implicit’’ contributions have been extracted.
Zero potential energy criterion for approximate wave functions
Authors: Koga Toshikatsu
Published: July 2002

A zero potential energy expression, which is a possible partner to the zero momentum energy expression presented previously, is proposed and discussed as a criterion for assessing the accuracy of approximate wave functions. Applicability of these criteria is illustrated and compared for several approximate wave functions for the 1sσg and 2pσu states of the H+2 molecule.
Laser-induced emission in the 1.9 μm region from excited state–excited state transitions in PuF6
Authors: Rice W. W. , Barefield J. E.
Published: July 2002

Laser-induced fluorescence (LIF) in the 1.9 μm region has been observed from room temperature PuF6 excited at 795 nm. The LIF has peaks at 1864±2, 1918±8, and 1983±2 nm and the emission waveforms have a temporal behavior similar to the PuF6 1 μm LIF that we have previously reported [J. Chem. Phys. 79, 2621 (1983)]. These spectral and temporal data suggest that the 1.9 μm LIF arises from vibronic transitions between electronically excited states of PuF6, i.e., 3Γ4(2)→3Γ4(1).
On controlled diffusion-limited drug release from a leaky matrix
Authors: Bunde A. , Havlin S. , Nossal R. , Stanley H. E. , Weiss G. H.
Published: July 2002

How fast can drug molecules randomly escape from a polymer matrix? This important question is of both scientific and practical importance, as increasing emphasis is placed on design considerations that can be addressed only if the physics of drug release is better understood. We study this problem using high accuracy Monte Carlo computer simulations. We find that the nature of drug release depends drastically on the dimension of the matrix and is different depending on whether the matrix is a normal Euclidean space or a fractal material such as a polymer, corresponding to the fact that the basic laws of physics are quite different in a fractal environment. We also find the surprising result that drug release is the same for noninteracting particles as it is for particles with hard-core excluded volume interactions, suggesting that the nature of the matrix is more important than the nature of the interactions among the drug particles in determining drug release.