bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin valves exchange biased by IrMn/PtMn laminated antiferromagnets
Authors: Anderson G. W. , Huai Yiming
Published: July 2002

This investigation has examined the use of the laminated antiferromagnet PtMn/IrMn to exchange bias spin valves in an attempt to combine the high exchange field and blocking temperature of PtMn with the good pinning profile of IrMn. The effect of varying the IrMn thickness was examined for top and bottom spin values exchange biased by CoFe20Å/IrMn/PtMn. For top spin valves, Hex rapidly decreases as IrMn thickness is increased, with the exchange pinning virtually disappearing by 20 Å. As the thickness continues to increase the pinning reappears, and by 40 Å of IrMn an exchange field of 500 Oe and coercivity of 100 Oe are observed. This exchange field is slightly higher than that observed for CoFe/IrMn40 spin valves (400 Oe as deposited, 470 Oe after annealing). For bottom spin valves the same trends are observed, although the pinning never completely disappears. Also, the exchange field of 600 Oe is lower than that obtained for IrMn (670 Oe), presumably due to the rougher interface of the PtMn underlayer. For both top and bottom spin valves the laminated antiferromagnet showed improvement in thermal stability compared to as deposited and annealed IrMn spin valves.
Phase separation in inhomogeneous mixed-valence systems: Application to manganites
Authors: Schlottmann P.
Published: July 2002

Manganites are inhomogeneous mixed-valence systems in which the Mn ions coexist in two magnetic configurations, Mn4+ and Mn3+. We consider a simple alloy-analogy model with a charge interaction between nearest neighbor Mn ions. In addition there is an antiferromagnetic (AF) (superexchange) interaction between nearest neighbor ions of equal valence, while ions of different valence are coupled ferromagnetically (FM) (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of AF and charge ordered FM domains. In a magnetic field the FM domains increase at the expense of the AF ones.
Training effects and the microscopic magnetic structure of exchange biased Co/CoO bilayers
Authors: te Velthuis S. G. E. , Berger A. , Felcher G. P. , Hill B. K. , Dahlberg E. Dan
Published: July 2002

Exchange bias of a partially oxidized thin film of ferromagnetic Co was studied by magnetization measurements and polarized neutron reflectivity (PNR). The magnetization curve shows strong effects of training with cycling of the magnetic field. Reflectivity measurements with the field parallel to the cooling field showed the onset of spin-dependent diffuse scattering—off the specular reflection—after a training cycle. Such scattering, of the Yoneda type, is due to misaligned Co domains possibly close to the Co/CoO interface. Subjecting the field cooled Co/CoO pair to a field perpendicular to the cooling field causes a rotation of the magnetization. The PNR measurements confirmed earlier susceptibility studies by indicating that the rotation of the magnetization is reversible in fields up to 400 Oe. The rotation of the magnetization of Co is uniform across the film thickness.
New memory effect in ferro/antiferromagnetic multilayers
Authors: Kodama R. H. , Edelstein A. S. , Lubitz P. , Sieber H.
Published: July 2002

Previous magnetization and ferromagnetic resonance measurements showed that when coupling was observed between ferromagnetic layers separated by an antiferromagnetic NiO layer, it was always ferromagnetic. Here we report magnetization measurements that explain this result. We find that the coupling is only strongly ferromagnetic when the sample is cooled with the two ferromagnetic layer magnetizations parallel. Cooling with the magnetizations antiparallel causes the coupling to become nearly zero. Thus, the coupling retains a memory of the relative orientation of the ferromagnetic layer magnetizations during cooling.
Investigation of advanced position error signal patterns in patterned media
Authors: Lin Xiangdong , Zhu Jian-Gang , Messner William
Published: July 2002

A potential approach to extending current thin film media to 100 Gbits/in2 recording density is physical patterning of the media, perhaps via an etching technique. An advantage of patterning is the capability of creating advanced position error signal (PES) patterns that are difficult or impossible to generate with writing by a recording head. This study uses spin stand tester to investigate various PES patterns generated by a focused ion beam etching technique on conventional thin film disks. For all practical purposes, the patterned PES eliminates transition noise, track edge noise, and erase band, therefore, demonstrates a much better PES signal than conventionally written servo patterns.
Magnetic field study of Ce0.8La0.2Al3
Authors: Pietri R. , Kumar P. , Andraka B.
Published: July 2002

A partial substitution of La for Ce in CeAl3 leads to large anomalies in the low temperature specific heat, presumably of magnetic origin, and results in a strong reduction of γ. Magnetic fields up to 14 T have little effect on the temperature position of the anomaly in Ce0.8La0.2Al3 but attenuate its magnitude. Specific heats of Ce0.8La0.2Al3 and CeAl3 approach each other in large fields of order 14 T. Magnetic fields increase γ of Ce0.8La0.2Al3.
Anomalous magnetic response of Ce1−xLaxAl3
Authors: Osborn Raymond , Goremychkin Eugene A. , Rainford Brian D. , Sashin Igor L. , Murani Amir P.
Published: July 2002

We have been conducting a systematic investigation of the effect of chemical pressure on the ground state properties of the heavy fermion system Ce1−xLaxAl3. We observe the development of an inelastic magnetic response below a characteristic temperature T* (2.2 K at x=0.2,2.1 K at x=0.5) from the high-temperature quasielastic response. This inelastic response is also extremely sensitive to applied pressure. Our observations are consistent with the predictions of the anisotropic Kondo model.
Nuclear magnetic resonance and nuclear quadrupole resonance studies of Ni61 in heavy fermion compounds Ce7Ni3
Authors: Mishina S. , Iwamoto Y. , Kohori Y. , Ueda K. , Kohara T. , Jung M-H. , Takabatake T.
Published: July 2002

The heavy fermion compound Ce7Ni3 is an antiferromagnet with TN∼1.7 K. According to the high pressure experiment, this compound has undergone a transition to a non-Fermi liquid state at ∼0.32 GPa and has recovered a Fermi liquid state at ∼0.62 GPa again. Before the high pressure nuclear magnetic resonance (NMR) study through two transitions, we have performed NMR experiments of Ni61, Co59 and Cu63 in order to clarify the electronic states at ambient pressure. Two kinds of partially overlapping Ni NMR spectra were observed in Ce7Ni3 containing an enriched Ni61. This result may be ascribed to two intrinsic local fields presumably due to the lattice distortion, etc. All the relaxation rates studied here are proportional to temperature above 1.4 K in the paramagnetic state, which indicates that the systems are in the Fermi liquid state at ambient pressure. The increase of (1/T1T) with increasing Co concentration, CCo, tells us that the partial density of states on Ni-3d states at Fermi level, N(EF)Ni, increase with CCo. On the contrary, CCu-independent (1/T1T) values indicate N(EF)Ni keep almost constant with Cu doping.
a-Si buffer layer for improvement of crystallographic and magnetoresistance characteristics of Co/Cu multilayers
Authors: Shimizu Yutaka , Nishimura Koichi , Saito Hiroyuki , Nakagawa Shigeki , Naoe Masahiko
Published: July 2002

Co/Cu multilayers deposited on an amorphous Si (a-Si) buffer layer revealed good crystallinity and lower resistivity than ones deposited on other buffer layers. The periodicity and the sharpness at the interface between each layer were not as good as those in the multilayer with a Ni–Fe buffer. Therefore, the multilayer with a Si buffer exhibited smaller resistivity ρ but slightly smaller change of resistivity Δρ. The annealing process improved the periodicity and the sharpness at the interface in the Co/Cu multilayer with a Si buffer, and the value of the magnetoresistance ratio in the multilayer with a Si buffer (17.1%) became larger than that with Ni–Fe (15.5%).
Control of the permeability loss-peak frequency of Ni81Fe19 thin films through laser ablation of triangular and square cluster geometries
Authors: Grimes Craig A. , Lumpp Janet K.
Published: July 2002

Laser ablation arrays of triangular and square shaped clusters, comprised of 23 μm diam circular holes, are defined upon 100 nm thick Ni81Fe19 films used to control the rf permeability spectra. Cluster-to-cluster spacing is varied from 200 to 600 μm. For each geometry it is found that the loss peak frequency and permeability magnitude shift lower, in a step-wise fashion, at a cluster-to-cluster spacing between 275 and 300 μm. The nonlinear shift in the behavior of the permeability spectra correlates with a dramatic increase in domain wall density.
Magnetoimpedance effect in the nanocrystalline Fe–Zr–Cu–B–Al alloy system
Authors: Lee Heebok , Lee Kyeong-Jae , Kim Yong-Kook , Kim Taik-Kee , Kim Chong-Oh , Yu Seong-Cho
Published: July 2002

The magnetoimpedance (MI) of Fe92−x−yZr7BxCu1Aly (x=2,4,6,8,y=0,0.5,1,1.5) alloys has been measured to investigate the influence of the structural changes in the crystallization process as well as the changes of the soft magnetic properties such as permeability, magnetic anisotropy, etc. after thermal treatment. The MI measurements were carried out along the ribbon axis with longitudinal magnetic field. The nanocrystalline alloy system was prepared by the rapid quenching technique in an Ar atmosphere. Ultrasoft magnetic behavior has been observed in all samples annealed at 550 °C. The x-ray diffraction and transmission electron microscopy results show the α-Fe phase appeared in the annealed alloy system at 550 °C. The maximum magnetoimpedance ratio value in Fe84Zr7B3Cu1 alloy annealed at 550 °C was reached as much as 1100%. The field annealing does not contribute to improve the magnetoimpedance effect in these nanocrystalline alloy system.
Environmental effects on big molecule with spin 1/2
Authors: Chiorescu I. , Wernsdorfer W. , Barbara B. , Müller A. , Bögge H.
Published: July 2002

The V15 polyoxovanadate molecule is made of 15 spins 1/2 with antiferromagnetic couplings. It is a low spin/big molecule with spin S=1/2. Contrary to large spins/big molecules of the Mn12−ac type, V15 has no energy barrier against spin rotation. Nevertheless, it shows magnetic hysteresis. This new phenomenon characterized by a “butterfly” hysteresis loop is due to the effect of the environment on the quantum rotation of the entangled 15 spins of the molecule, in which the phonon density of states is not at its equilibrium (phonon bottleneck). This behavior is very likely specific to big molecules with low spin.
Response to “Comment on ‘ZnGeP2 birefringence and its temperature dispersion using polarized interference’ ” [J. Appl. Phys. 87, 4638 (2000)]
Authors: Ohmer Melvin C.
Published: July 2002

In 1984 Andreev et al. [Sov. J. Quantum Electron. 14, 1021 (1984)] initiated a controversy by reporting that theory fails to predict the experimental phase matching angles for second harmonic generation of CO2 pump lasers or even that phase matching is possible at the longer wavelengths. As the phase matching angle under discussion is determined by the refractive indices, Andreev speculated that the index data of Boyd et al. [Appl. Phys. Lett. 18, 301 (1971)] were incorrect and that by assuming a larger birefringence, the discrepancy could be resolved. However, it was found that these index values were remarkably accurate. Bhar and Kumbhakar [J. Appl. Phys. 87, 4638 (2000)], speculate that the required birefringence is obtained as a result of a 180 °C rise in the crystal temperature due to absorption. In this reply, it is argued that their speculation is inconsistent with the results of numerous experiments and most importantly, that this large temperature rise occurs cannot be accepted as a fact without experimental verification.
Origin of asymmetric growth during solid-state amorphization studied with molecular-dynamics simulation
Authors: Zhang Q. , Lai W. S. , Liu B. X.
Published: July 2002

Solid-state interfacial reaction in the Ni/Nb multilayers upon thermal annealing is investigated with molecular-dynamics simulation. The result shows that amorphization takes place at medium temperatures and the growth of the amorphous interlayer presents an asymmetric behavior due to faster consuming of the Ni layer than the Nb layer. Consequently, a Ni-enriched amorphous phase is formed together with some unreacted Nb before complete amorphization, which agrees well with the experimental observations. Moreover, it is revealed that the Nb lattice can accommodate a large number of Ni atoms and still retain crystalline structure, while a small amount of Nb atoms induce a spontaneous decay of the Ni lattice, which is essentially the physical origin of the asymmetric growth observed in not only the Ni–Nb system but also in the other systems studied so far.
Electron field emission from tetrahedral amorphous carbon films with multilayer structure
Authors: Zhao J. P. , Chen Z. Y. , Wang X. , Shi T. S.
Published: July 2002

A multilayer structure with alternating metal and semiconductor layers is proposed to occur in tetrahedral amorphous carbon (ta-C) films prepared by using an intermittent layer-by-layer deposition method. In this model, the multilayers can be represented as A/B/A/B/…/A/B/A stacks, in which A is considered to be a semimetallic sp2-rich graphite-like layer with B being a semiconducting sp3-rich diamond-like layer. According to the proposed structural model, the electron field emission properties of the ta-C multilayers that could be modulated by adjusting the total number of layers, layer thickness and sp3 content of each layer have been predicted. Correspondingly, three kinds of ta-C multilayers were designed and deposited to confirm this model by enabling us to measure the electron field emission properties. Agreement between the prediction and the experimental results has been observed. It was found that field emission from ta-C multilayers can be optimized by changing the number of layers, layer thickness and sp3 content of each layer. In our experiments, a threshold electric field (Eth) as low as ∼5 V/μm has been obtained for field emission from ta-C multilayers with a total of 20 layers and with a 10 nm layer thickness.
Systematic study of molecular anions within the self-consistent-field approximation: OH−, CN−, C2H−, NH−2, and CH−3
Authors: Lee Timothy J. , Schaefer Henry F.
Published: July 2002

The title molecular anions have been studied at the self-consistent-field (SCF) level of theory with several different basis sets. The smallest of these bases is triple zeta (TZ) in quality while the largest can be labeled near Hartree–Fock limit. The dependence of proton affinities, dipole moments, harmonic frequencies, and infrared intensities on the inclusion of diffuse functions in the basis set is investigated. It is concluded that at the SCF level of theory the addition of diffuse s and p functions (for first row elements) is necessary in order to obtain reliable results. This is true especially for NH−2 and CH−3. A method to extend any standard Gaussian basis set is suggested. Finally, predictions are made for some of the as yet unobserved fundamentals of the anions.
Dynamics of a two-level system coupled to a dissipative bath: Comparisons of analytical theories with Monte Carlo simulation
Authors: Behrman E. C. , Wolynes P. G.
Published: July 2002

We apply the quantum dynamical Monte Carlo technique to the calculation of the complex time correlation function of the flux through a SQUID, and compare the results with those of three representative theories: a random phase approximation (RPA), an effective adiabatic approximation (EA), and an optimized random phase approximation (ORPA). The RPA quickly becomes unreliable as the damping is increased beyond a negligible amount. For small coupling, the variational information contained in the EA theory results in superior predictions of initial decay rates, but for moderate coupling the inherent nonergodicity of the theory leads to incorrect results for the long time behavior of the correlation function. The ORPA fails to renormalize the initial frequency properly, but succeeds in predicting the correct qualitative behavior even at moderately large values of the damping. This illustrates the power of the QDMC as a benchmark against which to test approximations, and as a bridge between their regions of applicability.
Theoretical study of the dipole moments of selected alkaline-earth halides
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry , Ahlrichs Reinhart
Published: July 2002

A b initio calculations at the self-consistent-field (SCF), singles plus doubles configuration-interaction (SDCI), and coupled pair functional (CPF) level are reported for the dipole moments, μe, and dipole derivatives, (∂μ/∂r)re, of the X 2Σ+ ground states of BeF, BeCl, MgF, MgCl, CaF, CaCl, and SrF. For comparison, analogous calculations are performed for the X 1Σ+ state of KCl. The CPF results are found to be in remarkably better agreement with experiment than are the SCF and SDCI results. Apparently higher excitations are required to properly describe the radial extent along the bond axis of the remaining valence electron on the alkaline-earth metal.
Atom–atom potentials via electron gas theory
Authors: Spackman Mark A.
Published: July 2002

The Gordon–Kim electron gas model is used to derive a set of short range repulsive potentials for homoatomic pairs containing atoms up to Kr. These potentials correlate extremely well with experimental van der Waals and nonbonded radii. Single exponential fits to the repulsive potentials are given, and are combined with an approximate dispersion energy term derived from experimental and theoretical atomic dipole polarizabilities and C6 constants to form a set of internally consistent atom–atom potentials of the exp-6 form.
Water-like particles at surfaces. II. In a double layer and at a metallic surface
Authors: Gardner Anne A. , Valleau John P.
Published: July 2002

This paper continues the Monte Carlo study of the TIPS2 fluid next to surfaces by examining the structure (i) under a superimposed field imitating that of an ionic double layer next to a charged electrode, and (ii) near an uncharged ‘‘metallic’’ surface. The results are compared to those next to an uncharged, inert wall. The double layer field has remarkably little effect on the structure; by contrast, the highly polarizable surface appears to freeze the surface region.
Monte Carlo simulation of hydrophobic interaction
Authors: Luzar A. , Bratko D. , Blum L.
Published: July 2002

The solvation force between neutral surfaces in water is studied by running the grand canonical ensemble Monte Carlo simulations on the system containing up to 102 water-like particles between parallel smooth plates immersed in the fluid. The water molecules are modeled like hard spheres with embedded point dipoles and orientation dependent adhesion potential. The pressure between the walls as a function of the distance displays oscillations resembling those observed experimentally, The overall attraction between the plates is observed despite the absence of direct wall–wall or wall–fluid attractive forces.
Dynamics of vibrationally excited ozone formed by three-body recombination. I. Spectroscopy
Authors: Rawlins W. T. , Armstrong R. A.
Published: July 2002

Spectrally resolved infrared fluorescence near 10 μm from vibrationally excited O3(ν3) has been observed in a cryogenic reactor facility at low pressure. The excited O3(v) is formed principally by three-body recombination of O and O2 in flowing, microwave-discharged O2/Ar mixtures at 1 Torr and 80 K. The spectral resolution is sufficient to permit identification and assignment of band centers for up to five quanta of stretching excitation. The observed transition frequencies are consistent with those predicted from a Darling–Dennison perturbation treatment. The spectra, which sample the recombination/deactivation sequence in its early to middle stages, indicate surprisingly little intermode coupling, being predominantly ν3 in character. There is also evidence of O3(v) excitation by near-resonant V–V coupling between O3 and O2(v=2). The spectroscopic analysis of the data is described, and considerations for scaling the spontaneous transition probabilities with vibrational level are discussed.
‘‘Equilibrium’’ and dynamic behavior of thin films of a perfluorinated polyether
Authors: Montfort J. P. , Hadziioannou G.
Published: July 2002

We have measured the equilibrium forces between two mica surfaces separated by a thin liquid film of a perfluorinated polyether. The observed force law was monotonically repulsive and had a range which was on the order of ten times the radius of gyration of the polymer molecules. The results can be explained by hypothesizing that some of the polymer chains are effectively ‘‘pinned’’ to the surfaces and thus are not free to exchange with chains in the bulk reservoir on the time scale of the experiments. We have also investigated the response of the same polymer melt to both sinusoidal and step perturbations in the distance between the two surfaces as a function of the film thickness. We have developed a simple rheological model for a linear, viscoelastic fluid which relates the experimental data to the effective viscosity of the confined fluid. For surface separations greater than approximately 1500 Å, the effective viscosity of the fluid film was found to be the same as the macroscopically measured value reported for the bulk polymer. For a surface separation of 840 Å, the effective viscosity was found to be greater than the bulk value. The results obtained from the dynamic experiments agreed with those obtained from the transient experiments.
Rydberg-like properties of rotational–vibrational levels and dissociation continuum associated with alkali–halide charge–transfer states
Authors: Pan Shao-hua , Mies Frederick H.
Published: July 2002

Fully quantal calculations for the vibrational bound levels and adjoining continuum of alkali–halide charge–transfer states have been made using new algorithms derived from the Milne quantum number function for the radial motion of the diatom. Because of the asymptotic Coulombic interaction, the molecular potential actually supports an infinite number of bound vibrational states which conform to a Rydberg-like spectrum and can be characterized by a vibrational quantum defect. As predicted by conventional quantum defect theory for perturbed Rydberg series, we also observe that the vibrational quantum defect extrapolates across the dissociation limit to yield the threshold elastic scattering phase shift for the dissociative continuum state. The implicit dependence of the defect on the rotational state of the molecule yields a complete description of the rotational–vibrational spectrum. Because the system is heavy, the mass and rotational dependence of the computed quantum defects generally conform to appropriate WKB-like predictions. The usefulness of our numerical results in forthcoming investigations of two body collisional charge–transfer processes is also indicated.
Benzene clustered with N2, CO2, and CO: Energy levels, vibrational structure, and nucleation
Authors: Nowak R. , Menapace J. A. , Bernstein E. R.
Published: July 2002

Two-color time-of-flight mass spectroscopy is employed to study the van der Waals (vdW) clusters of benzene(N2)n (n≤8), benzene(CO2)n (n≤7), and benzene(CO)n (n=1,2) created in a supersonic molecular jet. Potential energy calculations of cluster geometries, normal coordinate analysis of vdW vibrational modes, and calculations of the internal rotational transitions are employed for the assignment of the benzene(solvent)1 cluster spectra in the 000 and 610 regions of the benzene 1B2u←1A1g transition. The respective vibronic and rotational selection rules for these clusters are determined based on the appropriate point groups and molecular symmetry groups of the clusters. Good agreement between the calculated and experimental spectra is obtained with regard to the vdW vibrational and internal rotational modes. The solvent molecules rotate nearly freely with respect to benzene about the benzene–solvent bond axis in the benzene(solvent)1 clusters. In the excited state a small ∼20 cm−1 barrier to rotation is encountered. Studies of larger clusters (n>2) reveal a broad red shifted single origin in the 610 spectra. A linearly increasing cluster energy shift is observed as a function of cluster size. The cluster energy shifts are not saturated by one solvent molecule on each side of the aromatic ring; several solvent molecules effectively interact with the solute π electronic cloud. Both homogeneous and inhomogeneous nucleation take place for the clusters studied depending on the ratio of the solvent–solvent binding energy to the cluster binding energy.
Atomic force microscopy of polymeric liquid films
Authors: Mate C. Mathew , Lorenz Max R. , Novotny V. J.
Published: July 2002

We demonstrate the use of the atomic force microscope (AFM) for studying perfluoropolyether polymer liquid films as thin as ∼20 Å. With the AFM we are able to measure three distinct properties of the liquid film: (1) its thickness when the thickness of liquid on the AFM tip is taken into account, (2) the meniscus force acting on the AFM tip as a function of depth into the liquid film, and (3) the topography of the liquid/air interface. All three of these measurements can be done with a very high lateral resolution, ∼1000 Å, demonstrating the unique capability of AFM for studying liquid films. With AFM we have observed several interesting properties of these polymeric liquid films. First films thinner than ∼300 Å are fairly uniformly distributed, while films thicker than ∼300 Å slowly dewet the surface. Second, by measuring the meniscus radius of liquid in a micron sized hole on the surface, we can determine the disjoining pressure in a thin liquid film.
Two-dimensional zero-field nutation nuclear quadrupole resonance spectroscopy
Authors: Harbison Gerard S. , Slokenbergs Andris , Barbara Thomas M.
Published: July 2002

We introduce a new two-dimensional nuclear quadrupole resonance experiment, in which the first time period (t1) is the duration of the radiofrequency exciting pulse; and the second (t2) is the normal free precession of a quadrupolar nucleus at zero field. After double Fourier transformation, the result is a 2D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero field. For single crystals, this spectrum contains narrow lines, whose frequency, for axially symmetric tensors, is proportional to sin θ, where θ is the angle between the unique axis of the quadrupolar tensor and that of the transmitter/receiver coil. For polycrystalline samples we obtain powder line shapes which are reminiscent of high-field nuclear magnetic resonance (NMR) powder patterns, and which allow determination of the asymmetry parameter η, which has previously only been obtainable using Zeeman perturbed nuclear quadrupole resonance (NQR) methods. Both theoretical spectra and several experimental examples are presented.
Molecular dynamics study of the aqueous core of a reversed ionic micelle
Authors: Linse Per
Published: July 2002

The molecular structure and dynamics of a molecular electric double layer have been examined with the molecular dynamics simulation technique. A 50 ps trajectory from a simulation of a model system comprising 50 sodium ions and 1000 water molecules, spherically enclosed by a hydrophobic interface carrying 50 carboxylate groups, has been analyzed. The results show that the water structure is strongly distorted by the strong local fields and that the hydrophobic interface plays only a secondary role. The orientational polarization of the interfacial water molecules has been examined and compared with dielectric continuum theory. The dynamical results of this study show a reduced rate of translational as well as rotational motions of ionic hydrating water by a factor of 2–4 as compared to bulk water.
Nonlinear effect of an oscillating electric field on membrane proteins
Authors: Astumian R. Dean , Robertson Baldwin
Published: July 2002

The nonlinear response of a two-state chemical transition to an oscillating electric field is examined. A reaction for which this analysis is particularly relevant is a conformational transition of a membrane protein exposed to an ac electric field. Even a modest externally applied field leads to a very large local field within the membrane. This gives rise to nonlinear behavior. The applied ac field causes harmonics in the polarization and can cause a dc shift in the state occupancy, both of which can be observed and used to determine kinetic parameters. Fourier coefficients are calculated for the enzyme state probability in the ac field, exactly for infinite frequency, and in powers of the field for finite frequency. Kramers–Kronig relations are proved and response functions are given for the leading terms of the harmonics. The results are extended to the spherical symmetry relevant to suspensions of spherical cells, vesicles, or colloidal particles. If the protein catalyzes a reaction, free energy is transduced from the electric field to the output reaction, even if that reaction is electrically silent. Many transport enzymes are ideal examples. The ac field can cause the enzyme to pump ions or molecules through the membrane against an (electro)chemical potential. The efficiency of this energy transduction can be as high as 25%.
Equilibrium surface composition of diblock copolymers
Authors: Green Peter F. , Christensen Thomas M. , Russell Thomas P. , Jerome Robert
Published: July 2002

The surface composition of a series of symmetric diblock copolymers of polystyrene (PS) and polymethylmethacrylate (PMMA) was studied using x-ray photoelectron spectroscopy (XPS). Each copolymer was composed of N segments where N ranged from 270 to 5200. At large N, where the system is highly segregated, the surface is composed only of PS, the lower surface energy component. For smaller N, where the system is in the weak segregation limit, both the PS and PMMA segments coexist at the surface. The surface excess ψ1 of PS in the weak segregation limit is well described by ψ1=α−βN−1/2, in good agreement with mean field predictions. The constants α and β depend on the interactions between the unlike segments on the copolymer chain and on the surface energy differences between the components. We also show that the strength of the segmental interactions in the vicinity of the surface are weaker than from those in the bulk.
The rotational and tunneling spectrum of the H2S⋅CO2 van der Waals complex
Authors: Rice Jane K. , Coudert L. H. , Matsumura K. , Suenram R. D. , Lovas F. J. , Stahl W. , Pauley D. J. , Kukolich S. G.
Published: July 2002

The rotational spectra of H2S⋅CO2 and two deuterated forms have been observed using a pulsed-beam Fourier-transform microwave spectrometer. For each of the three complexes we assign a-type and c-type transitions which are split into a ‘‘weak’’ and a ‘‘strong’’ intensity component. The analysis based on that previously used for the (H2O)2 complex and modified for application to H2S⋅CO2, allowed us to assign internal rotation, inversion tunneling states of the H2S and CO2 units in the complex. The following rotational constants were determined for the ground tunneling state of each species: for H2S⋅CO2, A=11 048.0(26) MHz, B=2147.786(4) MHz, and C=1806.468(4) MHz; for HDS⋅CO2, A=10 769(35) MHz, B=2107.26(24) MHz, and C=1775.83(24) MHz; and for D2S⋅CO2, A=10 356.2(28) MHz, B=2065.376(8) MHz, and C=1746.122(8) MHz. The electric dipole moments were determined for the H2S⋅CO2 and D2S⋅CO2 species, resulting in the values μa=0.410(14) D and μc=0.822(10) D for the H2S⋅CO2 species. The structure of the complex has the CO2 and the S atom of H2S in a T-shaped configuration. The H2S plane is nearly orthogonal to the CO2–S plane with an angle of about 92° and the H2S⋅CO2 center-of-mass separation Rc.m. is 3.498(3) Å.
Photoelectron spectroscopy and electronic structure of clusters of the group V elements. III. Tetramers: The 2T2 and 2A1 excited states of P+4, As+4, and Sb+4
Authors: Wang Lai-Sheng , Niu B. , Lee Y. T. , Shirley D. A. , Ghelichkhani E. , Grant E. R.
Published: July 2002

Methods employing high resolution HeI (584 Å) photoelectron spectroscopy have been applied to the tetrameric clusters of the group V elements, to resolve details of vibronic and spin–orbit structure in the first three electronic states of P+4, As+4, and Sb+4. Measured spacings of distinct vibrational progressions in the ν1 mode for the 2A1 states of P+4 and As+4, yield vibrational frequencies of 577 (5) cm−1 for P+4 and 350 (6) cm−1 for As+4. Franck–Condon factor calculations suggest bond length changes for the ions in the 2A1 states of 0.054 (3) Å for P+4 and 0.060 (3) Å for As+4. Strong Jahn–Teller distortions in the ν2(e) vibrational mode dominate the structure of the 2E ground states of the tetrameric ions. Both Jahn–Teller and spin–orbit effects appear in the spectra of the 2T2 states of the tetrameric ions, with the spin–orbit effect being dominant in Sb+4 and the Jahn–Teller effect dominant in P+4. Vibrational structure is resolved in the P+4 spectrum, and the ν3(t2) mode is found to be the one principally active in the Jahn–Teller coupling. A classical metal-droplet model is found to fit well with trends in the IPs of the clusters as a function of size.
The visible photoabsorption spectrum and potential curves of ArN+
Authors: Broström L. , Larsson M. , Mannervik S. , Sonnek D.
Published: July 2002

The photoabsorption spectrum of ArN+ has been studied in the visible wavelength region by means of photofragment kinetic energy spectroscopy. The mass selected ions were accelerated to 25 keV and the ion beam was merged or crossed with a laser beam. The photoproducts Ar+/N and, weakly, Ar/N+ were observed as ArN+ was photodissociated with photons in the range 1.8–2.5 eV. This is the first observation of electronic transitions in ArN+. In order to interpret the results, potential energy curves of ArN+ were calculated with complete active space self-consistent field and multireference configuration interaction methods. The observed spectra are assigned to the A 3Π←X 3Σ−, C 3Σ−←X 3Σ− and C 3Σ−←A 3Π transitions. From these assignments a value of 2.16 eV for the dissociation energy of ArN+ is inferred.
The gas-phase thermochemistry of FeH
Authors: Schultz Richard H. , Armentrout P. B.
Published: July 2002

Endothermic hydride abstraction reactions of Fe+ with propane, cyclopropane, n-butane, cyclopentane, and acetaldehyde are studied in the gas phase as a function of ion translational energy in a guided ion beam mass spectrometer. The thresholds for these reactions are measured and used to derive 298 K values of D°(Fe–H)=1.63±0.08 eV (37.5±1.9 kcal/mol), D°(Fe+ –H−)=8.77±0.08 eV (202.3±1.9 kcal/mol), IE(FeH)=7.37±0.10 eV, PA(Fe−)=347.6±1.9 kcal/mol, and Δf H(FeH)=113.9±1.9 kcal/mol. This thermochemistry is compared with prior measurements and theoretical values.
An improved intermolecular potential for sulfur hexafluoride
Authors: Aziz Ronald A. , Slaman Martin J. , Taylor William L. , Hurly John J.
Published: July 2002

Second virial coefficient data and viscosity were used to evaluate effective isotropic intermolecular potential functions proposed in the literature for sulfur hexafluoride. It was found that none of the potentials could predict the properties simultaneously. We have constructed a Morse–Morse–Spline–van der Waals (MMSV) potential which satisfactorily correlates second virial coefficient and viscosity data at the same time.
Flowing Afterglow Langmuir Probe measurement of the N+2(v=0) dissociative recombination rate coefficient
Authors: Canosa A. , Gomet J. C. , Rowe B. R. , Queffelec J. L.
Published: July 2002

A measurement of the dissociative recombination coefficient α is presented for reactions of electrons with ground state N+2(X 2Σ+g,v=0) ions at 300 K. The measurement has been made under truly thermalized conditions using a Flowing Afterglow Langmuir Probe (FALP). The rate constant was determined to be α[N+2(v=0)]=2.6×10−7 cm3 s−1 which is in good agreement with previous data except for the latest merged beam experiment.
A theoretical synthesis of polymers by using uniform localization of molecular orbitals: Proposal of an elongation method
Authors: Imamura Akira , Aoki Yuriko , Maekawa Koji
Published: July 2002

The elongation method, a new approach to calculating the electronic structure of polymers efficiently, is proposed as a model for real polymerization reactions following the normal chain reaction processes of initiation, propagation, and termination. The calculations are carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the end group of the cluster and an attacking molecule. As a first step of this treatment, the validity of our method is examined via application to regular and irregular model polymers consisting of hydrogen molecules at the extended Hückel level and comparing the results with those obtained from the conventional variational method. Furthermore, this treatment is performed on the real polymers such as polyethylene and polypropylene stereoisomers, and is proved to be reliable with advantages in computational time and storage. The approach is promising for application to very large systems on which direct variational calculations of the whole system are impossible. Moreover, it should be emphasized that this approach can mimic real reaction coordinates, or can be considered a theoretical synthesis of polymers.
Cyclohexane revisited: High pressure nuclear magnetic resonance rotating frame relaxation study of the dynamical solvent effects on the conformational isomerization of cyclohexane
Authors: Campbell D. M. , Mackowiak M. , Jonas J.
Published: July 2002

The main goal of this study is to extend the dynamic range of isomerization rates for cyclohexane in order to determine with high accuracy whether the barrier height to isomerization is pressure dependent. Therefore, the effect of pressure and temperature on the conformational isomerization of cyclohexane in carbon disulfide solvent has been investigated using the NMR (nuclear magnetic resonance) rotating frame relaxation technique. This technique, used for the first time in pressure studies of chemical exchange, allows the measurement of isomerization dynamics over a wide range of pressures and temperatures. By combining the rotating frame and NMR line shape techniques and generating the isoviscosity plots, it is shown that the barrier height to isomerization is independent of pressure. Since the experimental isomerization rate is accelerated by pressure, the viscosity dependence of the reduced transmission coefficient shows that the isomerization falls into the energy controlled (inertial) regime of the Kramers model in agreement with our earlier experimental findings. These experimental results, as interpreted in terms of stochastic models of isomerization reactions, indicate a strong collisional coupling and the presence of dynamical solvent effects.
The modeling of mixed-mode and chaotic oscillations in electrochemical systems
Authors: Koper Marc T. M. , Gaspard Pierre
Published: July 2002

We develop a simple but general three-variable model skeleton to describe complex nonlinear behaviors in electrochemical processes taking place at either a hanging mercury drop electrode (HMDE) or a rotating-disk electrode (RDE). We apply our formalism to the reduction of indium(III) at a HMDE in the presence of thiocyanate, a reaction known to exhibit complex mixed-mode and chaotic oscillations. Besides the role of the negative Faradaic impedance in destabilizing the electrochemical system, mass transport appears to be crucial as the model explicitly takes into account, in a truncated fashion, the time-dependent relaxation of the concentration profile. We study in detail the nonlinear dynamic behavior of our model of the indium/thiocyanate system and a RDE model. The models support mixed-mode sequences that appear either as incomplete Farey sequences or as periodic-chaotic sequences, which we discuss in terms of an incomplete homoclinic scenario whose definition and properties are worked out here. Our results compare very well to the experimental observations in the indium/thiocyanate system and the electrodissolution of a rotating copper disk in phosphoric acid. This satisfactory agreement strongly suggests that diffusion relaxation is an important phenomenon in electrochemical oscillations and could be the essential third variable in many dynamical electrochemical processes.
Molecular dynamics simulations of the effect of adsorption on SiO2 surfaces
Authors: Athanasopoulos D. C. , Garofalini S. H.
Published: July 2002

Molecular dynamics computer simulations were used to study the adsorption of model Pt on silica and the effect of this adsorption on the structure of the silica surface. Silica glass and amorphized crystalline surfaces were used as the silica substrates. Results showed a 5–6 Å thick adsorbate/substrate interface and an approximately 1 Å compression of the substrate surface caused by the presence of the adsorbate film. A significant decrease in the Si–O–Si bond-angle distribution was observed as a result of the adsorption. The shift to smaller bond angles was caused by a distortion of the siloxane bonds in five- and six-membered rings rather than the formation of small sized rings. The smaller bond angles indicate that more reactive sites may exist in the surface.
Effect of double bonds on the dynamics of hydrocarbon chains
Authors: Rey Antonio , Kolinski Andrzej , Skolnick Jeffrey , Levine Yehudi K.
Published: July 2002

Brownian dynamics simulations of isolated 18-carbon chains have been performed, both for saturated and unsaturated hydrocarbons. The effect of one or several (nonconjugated) double bonds on the properties of the chains is discussed in terms of both equilibrium and dynamic properties. The introduction of a cis double bond increases the relaxation rates of the unsaturated chain with respect to the saturated alkane. On the other hand, coupling effects in the torsional transitions around a trans double bond make the dynamics of this unsaturated chain very similar to the saturated one. Based on these results, the parameters and moves of a dynamic Monte Carlo algorithm are tuned to reproduce the observed behavior, providing an efficient method for the study of more complicated systems.
Interactions between primitive electrical double layers
Authors: Tang Zixiang , Scriven L. E. , Davis H. T.
Published: July 2002

A free energy density functional theory is applied to predict the electrostatic force between charged surfaces immersed in the restricted primitive electrolyte, in which ions are represented by charged hard spheres and solvent by a uniform dielectric continuum. The particle correlation due to hard-core exclusions is incorporated in the nonlocal density functional of inhomogeneous hard sphere fluid. The ion–ion electrostatic correlation is treated in the mean spherical approximation. The surface force is found to depend strongly on the electrolyte concentration, surface charge density, and valency of the counterions. Attractive forces are often observed, especially in electrolytes containing divalent counterions. The maximum of the attraction can be an order of magnitude stronger than the van der Waals force evaluated at the same surface separation. The prediction of the classical Poisson–Boltzmann theory that the surface force is always repulsive is qualitatively correct only for monovalent electrolytes at low concentrations and surface charges. The results for an asymmetrical electrolyte resemble those of a symmetrical electrolyte with counterions of the same valency. The theoretical predictions agree well with available computer simulation results.
Comment on the rate of isomerization in molecules with a symmetric triple well potential
Authors: Jang Soonmin , Zhao Meishan , Rice Stuart A.
Published: July 2002

The classical theory of the rate of unimolecular isomerization developed by Gray and Rice, and extended by Zhao and Rice, is applied to the calculation of the rate of isomerization in model systems which have linear symmetric triple well potentials. Two of these wells define equally stable isomers A and A′; the third well, which defines a less stable intermediate B, is positioned along the reaction coordinate between A and A′. The model system also has a state C which corresponds to conformations with energies greater than the barrier energy between A and B. Both ‘‘direct’’ conversion A→C→A′ and ‘‘indirect’’ conversion A→C→B→C→A′ are allowed by the dynamics of the system. Then the rate of isomerization A→A′ depends on the extent to which the intermediate state B modifies the flow of points in the system phase space, and it is the dynamics associated with that modification which is the major point of interest in our study. We find a straightforward relationship between the areas in the Poincaré surface of section for the model system associated with states A, A′, B, and C and the rate of isomerization, namely, the larger the ratio of areas of C to B the faster the transformation A→A′. We also find that the Zhao–Rice version of the classical theory of isomerization rate yields values in good agreement with those derived from trajectory calculations and from the reactive island theory of DeLeon and co-workers.
Theory of carbon monoxide adsorption on NiAl(110)
Authors: Grillo M. E. , Castro G. R. , Doyen G.
Published: July 2002

We present results of a model Hamiltonian approach to CO single molecule adsorption on a semi-infinite NiAl(110) surface. CO adsorbs on the Ni rows in agreement with experimental findings. The calculated electron–hole excitation spectrum compares reasonably with direct and inverse photoemission data. The nature of the chemisorption bond is analyzed by partitioning the density matrix and the energy into components which are accessible to physical intuition. The results demonstrate that the d electrons are as important on the alloy as they are on the pure Ni component, although they play a different role. The extended nature of the alloy d wave functions determines the adsorption geometry. The character of the chemisorption bond is discussed in comparison to CO adsorption on the components Al and Ni and to CO/Cu.
Flow of microemulsions through microscopic pores
Authors: Ripple Dean , Berg Robert F.
Published: July 2002

The flow of the microemulsion dioctylsulfosuccinate (AOT)/decane/water through 0.1 and 1.0 μm diameter pores was studied for water droplet volume fractions ranging from 0 to 0.5. The viscosity of these microemulsions, as measured in a capillary viscometer with a bore of ≊1 mm diameter, exceeds by as much as a factor of 4 theoretical predictions of the viscosity of suspensions of hard or liquid spheres. If droplet clustering causes this viscosity enhancement, then flow of the microemulsions through pores with diameter small compared to the characteristic cluster size should display a large finite-size effect. The apparent viscosity of the microemulsions in pores differed from the viscosity measured in the capillary viscometer by less than 40% for 0.1 μm diameter pores and by less than 8% for 1.0 μm diameter pores. These differences are of the same scale as estimates of two effects: adsorption of droplets on the pore wall and the enhanced flow of suspensions near a wall. The absence of larger finite-size effects implies that clustering of droplets on length scales of 0.1 μm or larger does not contribute substantially to the microemulsion viscosity.