bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas dynamics and electromagnetic processes in high-current arc plasmas. Part II. Effects of external magnetic fields and gassing materials
Authors: Schlitz Lei Z. , Garimella Suresh V. , Chan S. H.
Published: July 2002

Arc plasmas in switching devices are investigated in the presence of external and self-induced magnetic fields, as well as of gassing materials, using a three-dimensional computational model. The model development and validation for two- and three-dimensional arc columns with a self-induced magnetic field and natural-convection effects was presented in L. Z. Schlitz et al. [J. Appl. Phys. 85, 2540 (1999)]. The present paper discusses further development of the model to enable an investigation of the effects of external magnetic fields and the presence of gassing materials. Steady-state solutions with an external transverse magnetic field (0–20 mT) imposed on a wall-stabilized air arc column are presented, followed by a study of the transient response of the arc in an open-ended arc chamber in a 0.68 ms time span. Ablation from the side walls of the arc chamber made of gassing materials (and the resulting mixing of hydrogen and air) is shown to increase the arc voltage dramatically due to the increase in heat transfer around the arc body.
Nucleation in ferromagnetic nanowires—magnetostatics and topology
Authors: Braun Hans-Benjamin
Published: July 2002

Magnetization reversal in nanowires can be accomplished locally via thermal nucleation of soliton-antisoliton pairs. It is demonstrated that the total nonlocal magnetostatic energy can be reduced to a local anisotropy if the particle diameter is smaller than the magnetic length scale. Topological restrictions of the curling mode are discussed. Barrier energies for nucleation at sample ends are computed and close agreement with recent experiments on Ni-nanowires is found.
Analysis of structural changes in plasma-deposited fluorinated silicon dioxide films caused by fluorine incorporation using ring-statistics based mechanism
Authors: Pankov V. , Alonso J. C. , Ortiz A.
Published: July 2002

Fluorinated silicon dioxide (SiOF) films were prepared by remote plasma enhanced chemical vapor deposition using SiF4, O2, H2, and He reaction gases. Fourier transform infrared spectroscopy studies accompanied by molecular orbital (MO) modeling were used to explain structural changes in SiOF films caused by F incorporation. On the basis of the results of MO modeling, it was shown that F atoms incorporated into the SiOF network only slightly affect the geometry of ring units, the main building blocks of SiOF film network, and cannot cause strong changes in the value of the average Si–O–Si angle, 〈θ〉. Ring-statistics-based mechanism is proposed to explain the increase in 〈θ〉 in SiOF films with F content. It is supposed that interaction of highly reactive F species from the incoming flux with the growing SiOF network during deposition process induces the preferential conversion of the most strained small-order ring units into those of higher order characterized by larger 〈θ〉. As a result, the 〈θ〉 in SiOF film increases and film structural homogeneity improves with increasing the F content in the incoming flux. It is assumed that structural changes in SiOF films caused by F incorporation are not the abrupt transition from the ring-built SiOF network to the ring-free chain-like network, but a continuous shift of the ring distribution function towards the large-size high-order rings which causes the increase in 〈θ〉 within SiOF network, a reduction in the film density, and an enhancement of the film moisture absorptivity.
Structural, optical, and electronic properties of cubic TiNx compounds
Authors: Kang Ji Hoon , Kim Kwang Joo
Published: July 2002

Structural, optical, and electronic properties of cubic TiNx compounds were investigated by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and dc resistivity measurements. The samples were thin films prepared by dc reactive magnetron sputtering of a Ti target at various nitrogen partial pressures in an argon–nitrogen gas mixture. XRD and XPS analyses on the samples indicated a linear increase of the lattice parameter of the compounds with the nitride composition in the 0.7<x<1.2 range. The measured dielectric functions of the compounds by SE show significant low-energy shift of the screened plasma frequency as the nitride composition increases while the interband-transition structures above 3 eV remain at about the same energy. The dielectric functions of the compounds in the intraband-transition region are well described by the Drude model with constant plasma frequency and relaxation time. The estimated dc resistivity using the Drude parameters is minimized near x=1 and its variation with the nitride composition is in good agreement with the result of dc resistivity measurements on the compounds.
Conduction mechanisms in Ta2O5/SiO2 and Ta2O5/Si3N4 stacked structures on Si
Authors: Chaneliere C. , Autran J. L. , Devine R. A. B.
Published: July 2002

In this paper, the conduction mechanisms in Ta2O5/SiO2 and Ta2O5/Si3N4 stacked structures on Si are investigated both experimentally and theoretically. Amorphous Ta2O5 films (20–60 nm thick) were deposited by low pressure chemical vapor deposition or electron cyclotron resonance plasma enhanced chemical vapor deposition and some layers were annealed for crystallization at 800 °C in O2. The Si3N4 layers were formed by plasma nitruration or low pressure chemical vapor deposition. The SiO2 films studied were intentionally obtained by dry oxidation of the Si substrates, or as a result of the Ta2O5 deposition process (due to the oxidizing atmosphere), or of the Ta2O5 postdeposition annealing treatment under O2. The conduction mechanisms were identified by comparing the experimental current–voltage traces to the theoretical curves calculated in steady-state regime by using the Kirchhoff voltage law and the current continuity equation. In amorphous Ta2O5, the conduction mechanisms identified are the electronic hopping process and the Poole–Frenkel effect. For the corresponding interfacial SiO2 or Si3N4 films, the current transport is governed by tunneling processes or trap-modulated mechanisms, depending on the nature and deposition method of these interfacial layers. In crystalline Ta2O5 on SiO2 capacitors, no combination of basic conduction mechanisms can correctly fit the experimental curves, certainly due to the complex structure of crystalline Ta2O5 (high inhomogeneity and cracks).
Plasma chemical vapor deposition of hydrocarbon films: The influence of hydrocarbon source gas on the film properties
Authors: Schwarz-Selinger T. , von Keudell A. , Jacob W.
Published: July 2002

Hydrocarbon films were prepared by electron cyclotron resonance plasma deposition from different hydrocarbon source gases at varying ion energies. The source gases used were the saturated hydrocarbons CH4, C2H6, C3H8, C4H10 (n- and iso-) and the unsaturated hydrocarbons C2H4 and C2H2 as well as mixtures of these gases with hydrogen. Film deposition was analyzed in situ by real-time ellipsometry, and the resulting films ex situ by ion-beam analysis. On the basis of the large range of deposition parameters investigated, the correlation between hydrocarbon source gas, deposition parameters, and film properties was determined. The film properties are found to be influenced over a wide range not only by the energy of the impinging ions, but also by the choice of source gas. This is in contrast to a widely accepted study where no dependence of the film properties on the source gas was observed, this being ascribed to a “lost-memory effect.” A strong correlation was found between the hydrogen content of the films and the film properties. This strong correlation is explained on the basis of the random-covalent-network model.
Effect of sol rheology on the uniformity of spin-on silica xerogel films
Authors: Nitta S. V. , Jain A. , Wayner P. C. , Gill W. N. , Plawsky J. L.
Published: July 2002

Spin-on xerogels, which are promising candidates for use as interlayer dielectric materials in future microelectronic devices, change from a Newtonian liquid to a solid gel during processing. Since the rheology of the sol may affect the uniformity of the xerogel films produced, here we relate the rheology of a two-step, acid-base catalyzed, sol-gel system to the thickness and porosity profiles across xerogel films of importance to the microelectronics industry. We also analyze the effect of spin speed on the thickness and porosity of the films. Our rheological studies of the xerogel sol demonstrated that the sol changes from Newtonian far from the gel point, to shear thinning close to the gel point. On films spin coated with shear-thinning sols there is a region of uniformity extending for a distance of about 5 mm from the center. The film thickness and porosity are highest in this region and both quantities decrease towards the edge. If the sol is spun in its Newtonian regime, the resulting films are uniform (<3% thickness and porosity). The predictions of film thickness and uniformity based on simple models for spin coating a Newtonian and truncated power law fluid were found to be in good agreement with the experimental observations. The film thickness was varied from 0.485 to 1.9 μm by adjusting the spin coating speed from 5900 to 1000 rpm. Over this sixfold range of speed, the porosity was almost constant and varied by less than 10% while the thickness varied by about a factor of 4.
Dependence of indium–tin–oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies
Authors: Sugiyama Kiyoshi , Ishii Hisao , Ouchi Yukio , Seki Kazuhiko
Published: July 2002

The effect of the method used to clean indium–tin–oxide (ITO) on its work function was investigated by ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy. With only ultrasonic cleaning in the organic solvent, considerable carbon contamination remained on the ITO surface and the work function was low (4.5 eV). In contrast, ultraviolet (UV)–ozone treatment removed significant carbon contamination, with an increase in the work function to 4.75 eV, which improves the hole-injection efficiency into the organic hole-transport layer in organic electroluminescent devices. Although carbon contamination on the ITO surface was also removed by Ar+ sputtering, it was accompanied by the removal of oxygen from ITO, and the work function was reduced (4.3 eV). Three factors, i.e.,: (i) C-containing contaminants, (ii) the O/In ratio, and (iii) the In/Sn ratio on the ITO surface affect the work function. The present results and those of other workers suggest that these three factors affect the work function in the order: (ii)>(i)>(iii), and (i) is the main cause of the increase in the work function in the UV–ozone or O2 plasma treatments.
Si–H bonding in low hydrogen content amorphous silicon films as probed by infrared spectroscopy and x-ray diffraction
Authors: Mahan A. H. , Gedvilas L. M. , Webb J. D.
Published: July 2002

A systematic series of hydrogenated amorphous silicon (a-Si:H films) has been deposited by the hot wire chemical vapor deposition (HWCVD) technique onto crystalline silicon substrates, and the H bonding has been examined by infrared spectroscopy. All deposition parameters were kept the same, except that the substrate temperature (TS) was varied to affect changes in the film H content. Although the peak position of the Si–H stretch mode changes minimally with increasing substrate temperature, the stretch mode shape changes, becoming more intense (compared to the height of the wag mode) and considerably narrower. We show, through annealing experiments, that this narrow stretch mode may be a universal feature of low H content films, and suggest interpretations for this finite (narrow) linewidth. By correlations with x-ray diffraction data, we also show that the narrowing of the stretch mode peak for low H content HWCVD films is an indication of improved lattice ordering, and suggest that this improved ordering might also exist for other types of low H content a-Si:H films as well. However, for the as-grown HWCVD films the narrowing of the stretch mode peak width at lower H contents does not completely compensate for the increase in peak height, and as a result the integrated intensity of the peak mode (relative to that of the wag mode) increases. We comment on the differences between as-grown, low H content a-Si:H HWCVD films and high H content films annealed to reduce the film H content to comparable levels, and discuss possible reasons for these intensity changes versus sample H content.
Backward mode detection of laser-induced wide-band ultrasonic transients with optoacoustic transducer
Authors: Karabutov Alexander A. , Savateeva Elena V. , Podymova Natalia B. , Oraevsky Alexander A.
Published: July 2002

Time-resolved piezoelectric detection of wide-band ultrasonic transients induced by laser pulses in absorbing medium was studied. An optoacoustic transducer was developed for measuring the profiles of ultrasonic transients propagating in backward direction out of the laser-irradiated medium. For this purpose, an optical fiber for delivery of laser pulses to the surface of absorbing medium and a wide-band lithium niobate acoustic transducer were incorporated in one compact system, optoacoustic front surface transducer (OAFST). The transducer possesses temporal resolution (rise time) of 3.5 ns, low effective thermal noise pressure (10 Pa), and high sensitivity of piezoelectric detection (10 μV/Pa) over the ultrasonic frequency range from 1 to 100 MHz. Nd:YAG laser pulses at 355 nm were employed to generate distribution of acoustic sources in water solutions of potassium chromate with various concentrations. A temporal course of ultrasonic transients launched into an optically and acoustically transparent medium, coupled to the absorbing medium, was studied. Ultrasonic profiles experimentally measured at the site of laser irradiation were compared with profiles calculated using theoretical model. Experimental curves were in a good agreement with theoretical profiles. The backward detection mode permitted accurate reconstruction of the axial distribution of heat deposition in the laser-irradiated homogeneous or layered medium from the measured profiles. OAFST may become a useful modality for optoacoustic imaging in biological tissues and nondestructive evaluation of industrial materials.
The effect of humidity on probe-sample interactions in near-field scanning optical microscopy
Authors: Wei P. K. , Fann W. S.
Published: July 2002

The shear force damping between a tapered near-field probe and a well-cleaned glass sample depends on the humidity. A slow decay region, up to few hundred Å, appeared in the onset of the damping curves as the humidity increased. This humidity-dependent interaction force is of viscous type and ∼0.1 nN. The frictional coefficient increases faster than the estimation from Newton’s law of viscosity as the probe and sample distance decreases. This is attributed to the enhancement of viscosity in thin water layer.
Magnetic structure and spin reorientation of the Mn ions in NdMnO3
Authors: Wu S. Y. , Kuo C. M. , Wang H. Y. , Li W.-H. , Lee K. C. , Lynn J. W. , Liu R. S.
Published: July 2002

The crystal structure and magnetic ordering of the Mn spins in polycrystalline NdMnO3 have been investigated by means of neutron diffraction and ac magnetic susceptibility measurements. The compound crystallizes into an orthorhombic symmetry of space group Pbnm. Three peaks were observed in the temperature dependence of the in-phase component of the ac susceptibility, χ′(T). Neutron diffraction measurements show that all three peaks observed in χ′(T) are of magnetic origin, and are associated with the ordering and the reorientation of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were observed, resulting in a noncollinear magnetic structure. The Mn spins order at Tm≈75 K, and the moment saturates at ∼20 K at 〈μz〉=2.21 μB.
Exchange-stiffness constant in cold-worked and nanocrystalline Ni measured by elastic small-angle neutron scattering
Authors: Michels A. , Weissmüller J. , Wiedenmann A. , Barker J. G.
Published: July 2002

We present a new method for determining the exchange-stiffness constant A of a ferromagnetic bulk material by field-dependent elastic small-angle neutron scattering (SANS). In the limit of high applied magnetic field H, for which the scattering volume is a single magnetic domain and the magnetization is nearly aligned with the direction of the applied field, a combination of micromagnetics theory with neutron scattering formalism suggests closed-form expressions for the differential scattering cross section as a function of the scattering vector and of H. Based on these results it is suggested that the exchange-stiffness constant can be extracted from experimental SANS data recorded as a function of H. At ambient temperature we have applied this method to polycrystalline cold-worked Ni and nanocrystalline electrodeposited Ni, finding exchange-stiffness constants of (8.2±0.2)×10−12 and (7.6±0.3)×10−12 J/m, respectively. Measurement at 5 K yields a value of (9.2±0.2)×10−12 J/m for the nanocrystalline sample, a temperature dependence that agrees qualitatively with data in the literature. In addition to the value of A, the technique supplies information on the spatial structure of the magnetic anisotropy field.
Magnetotransport and magnetic properties of molecular-beam epitaxy L1 FePt thin films
Authors: Yu J. , Ruediger U. , Kent A. D. , Farrow R. F. C. , Marks R. F. , Weller D. , Folks L. , Parkin S. S. P.
Published: July 2002

The magnetotransport and magnetic properties of chemically ordered (001) L10 FePt epitaxial thin films with small scale perpendicularly magnetized stripe domains have been investigated. Film growth conditions are used to systematically vary the degree of chemical order, the magnetic anisotropy, and magnetic domain sizes. The longitudinal and transverse (Hall) resistivities are correlated with both film chemical order and magnetic properties. The low-field magnetoresistance shows evidence of domain effects. In the highest anisotropy and most chemically ordered film studied, this low field magnetoresistance is consistent with an intrinsic domain wall scattering contribution to the resistivity.
The H3 potential surface revisited
Authors: Blomberg M. R. A. , Liu B.
Published: July 2002

New ab initio configuration interaction (CI) calculations have been performed to investigate the accuracy of the best analytical H3 potential surface, the so-called LSTH surface, for the description of the H+H2 (v=1) reaction. The suty is motivated by the discrepancies between experimental and theoretical total rate constants for the H+H2 (v=1) and D+H2 (v=1) reactions and also between the experimental and theoretical rotational distributions in the HD (v=1,2) product of the D+H2 (v=1) reaction. The errors in the LSTH surface are found to be too small rate constants that are obtained in most of the theoretical calculations. Also for the high lying linear parts of the potential surface the errors are very small, but it is harder to asses the effects of these small errors on the rotational distributions of the product HD (v=1,2). (AIP)
A perturbation theory of classical equilibrium fluids
Authors: Kang Hong Seok , Lee Choong Sik , Ree Taikyue , Ree Francis H.
Published: July 2002

A new perturbation theory which is reliable over a wide fluid region is presented. The new theory reduces to the theory of Weeks, Chandler, and Anderson at densities near or below the triple point density of a simple fluid but it can also accurately predict thermodynamic properties at higher densities near the freezing line of the fluid. This is done by employing an optimized reference potential whose repulsive range decreases with increase in density. Thermodynamic properties for Lennard-Jones, exponential-6, and inverse nth-power (n=12, 9, 6, and 4) potentials have been calculated from the new theory. Comparison of the calculated data with available Monte Carlo simulations and additional simulations carried out in this work shows that the theory gives excellent thermodynamic results for these systems. The present theory also gives a physically reasonable hard-sphere diameter over the entire fluid range.
Statistical thermodynamics of electrons and holes in semiconductors
Authors: Gokcen N. A.
Published: July 2002

A statistical thermodynamic treatment is presented for the formation of electrons and holes in nondegenerate semiconductors, and for the accompanying standard changes in the Gibbs energy ΔG°, entropy ΔS°, heat capacity ΔC°, and enthalpy ΔH°. The treatment leads to the derivation of a new form for the variation of ΔG° with temperature, i.e., ΔG°−ΔG° (at 0 K)≡E g−E g (at 0 K)=AT−BT ln T, where E g is the forbidden band gap energy, and A and B are theoretically derivable constants, requiring relatively small adjustments for each semiconductor. Reliable data for Si, Ge, GaAs, and GaP show that this equation and the derived properties ΔS°, ΔC°, and ΔH° are well represented as functions of temperature. The foregoing equation is also useful in predicting the temperature dependence of E g from two values of E g at two different temperatures. The resulting interpretation of ΔG° indicates a distinct possibility that E g is constant below a characteristic temperature close to 0 K.
Measurement and analysis of the infrared-active stretching fundamental (ν3) of UF6
Authors: Aldridge Jack P. , Brock Ernest G. , Filip Henry , Flicker Herbert , Fox Kenneth , Galbraith Harold W. , Holland Redus F. , Kim K. C. , Krohn Burton J. , Magnuson Dale W. , Maier William B. , McDowell Robin S. , Patterson Chris W. , Person Willis B. , Smith D. F. , Werner George K.
Published: July 2002

High-resolution spectra of the infrared-active stretching fundamental ν3 of 238UF6 have been obtained between 620.6 and 633.5 cm−1 using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF6 in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground- and excited-state values of the rotational constant B could be individually determined, and the U–F bond length in the ground vibrational state is r0=1.9962±0.0007 Å. The Q branch of 235UF6 has also been analyzed and the 235UF6–238UF6 ν3 isotope shift measured to be 0.603 79±0.000 17 cm−1. The isotope shift and the Coriolis constant ζ3 have been used to refine the general quadratic intramolecular force field of UF6, and the Cartesian displacement coordinates of both infrared-active fundamentals are illustrated and compared with those of SF6.
Geometry of polymer chains near the theta-point and dimensional regularization
Authors: Duplantier Bertrand
Published: July 2002

We consider the ab initio continuum theory of polymer chains in a theta solvent, interacting via two- and three-body forces, with a short range cutoff s0 and in continuous space dimension d(2<d≤3). We calculate the full expression in terms of d and s0 of the squared end-to-end distance R2 of a single chain, and of its squared radius of gyration R2G, to first order in the interactions. We show explicitly how the cutoff is removed in any d, up to d=3, to yield an effective three-parameter theory. We show how this procedure is exactly equivalent to dimensional regularization. This completely disproves recent claims that for polymers in a θ solvent, cut-off theories and dimensional regularization would not be equivalent near three dimensions, or in three dimensions. We locate numerical errors in calculations by others, which explain the discrepancies. We also give to all orders the general relation existing between cut-off and dimensional regularizations for the three-parameter model, for 2<d<3.
The electronic and vibrational energies of two double-welled 3Σ+u states of He2
Authors: Konowalow Daniel D. , Lengsfield Byron H.
Published: July 2002

The potential energy curves for the 3Σ+u states of He2 which correspond to the interactions of He 1s2 1S with He 1s2s 3S and He 1s2s 3S with He 1s2s 3S are obtained from second-order configuration interaction calculations which themselves are based on a multiconfiguration self-consistent field calculation. The basis set includes functions obtained by optimizing the dipole, quadrupole, and octapole polarizability of the 3S atom and the dipole and quadrupole polarizabilities of the ground 1S atom. For the lowest 3Σ+u state we estimate the dissociation energy to be De =15 740±60 cm−1; we agree to within about 4 cm−1 with the observed vibrational energy level differences, and agree nicely with the position and depth of the short-range minimum deduced from spectra. We also obtain a potential hump at intermediate separations whose main features agree very well with those obtained from scattering experiments. The potential curve exhibits a van der Waals minimum about 3.82 cm−1 deep at about 11.6 a0. The excited 3Σ+u state has a deep metastable well at small internuclear separations as well as a well in the intermediate region which is about 3430 cm−1 deep.
Observation of vibrationally resolved charge transfer in H++H2 at ECM=20 eV
Authors: Niedner G. , Noll M. , Toennies J. P. , Schlier Ch.
Published: July 2002

The doubly differential cross sections for both the scattered protons and H atoms have been measured at ELAB=30 eV (ECM=20 eV) from θLAB=0° to 12° (θCM=0° to 18°) for the reactions H++H2(v=0)→H++H2(vf) and →H+H+2(vf) . The energy resolution is sufficient to resolve final vibrational states in both channels. The comparison of both the angular and energy loss distributions for the two product channels provides the first clear experimental evidence of a two-step charge transfer mechanism: Vibrational excitation on the lower H++H2 surface is followed by charge transfer in the outgoing collision for only those H2 molecules which are excited vibrationally high enough (vf≥4) to overcome the endoergic barrier (ΔE=1.83 eV). The final vibrational distributions of H+2 appear to be very similar to those of H2 for vf≥4 indicating that for the angular range observed the charge transfer probability is the same for all vibrational states with vf≥4. The comparison with classical trajectory surface hopping (TSH) calculations points to some disagreement which probably can be attributed to the potential surface used.
Comparison of the structure of harmonic aqueous glasses and liquid water
Authors: Pohorille Andrew , Pratt Lawrence R. , LaViolette Randall A. , Wilson Michael A. , MacElroy Robert D.
Published: July 2002

Glassy structures of water were generated by rapidly quenching configurations of 64 and 343 molecules of liquid water. The potential energy was then expanded through quadratic order around local minima generated this way and properties of the resulting harmonic system were calculated. The results were used to test the extent to which the structure of liquid water is similar to that of a harmonic aqueous glass. The radial distribution functions for the glass are remarkably similar to those of the liquid. The vibrational density of states for the glassy water exhibits a gap between 300 and 400 cm−1. The normal modes below 300 cm−1 correspond to molecular translations while the modes above 400 cm−1 are ascribed to molecular librations. Translational modes are almost entirely responsible for the broadening of oxygen–oxygen radial distribution function of the quenched configuration. They are also primarily responsible for the broadening of other radial distribution functions. Vibrational density of states leads to classical and quantum free energies for the harmonic system equal −9.62±0.12 and −8.89±0.12 kcal/mol, respectively, at T=300 K. Both free energies were found to be insensitive to sample size and to the configurational differences between the quenched structures.
Projected unrestricted Mo/ller–Plesset second-order energies
Authors: Knowles Peter J. , Handy Nicholas C.
Published: July 2002

A practical scheme for obtaining unrestricted Mo/ller–Plesset energies at second order (UMP2), with spin contamination removed from the wave function, is described and demonstrated. The algebra increases with the number of spin projections, and it appears to be practical to include only two spin projections. Full formulas are given, for which the cost is proportional to M3, where M is the number of basis functions. The results of these calculations give remarkably enhanced accuracy at fractionally extra cost. Four examples are included to demonstrate this: (i) the potential curve for the symmetric dissociation of H2O; (ii) the potential curve for the abstraction of H from CH4; (iii) the barrier for the addition of H to C2H4; and (iv) the electron affinity of CN. It is recommended that this procedure should be added at the end of all UMP2 calculations.
A new method for analysis of reactive adsorbed intermediates: Bismuth postdosing in thermal desorption mass spectroscopy
Authors: Campbell Charles T. , Rodriguez J. A. , Henn F. C. , Campbell J. M. , Dalton P. J. , Seimanides S. G.
Published: July 2002

A new method which should have relatively general applicability for the identification and quantitative analysis of reactive adsorbed molecular intermediates in surface reactions will be described, and the first examples of its application will be presented. When a reactive intermediate is generated on a surface, it often has a tendency to dissociate before desorbing. Since dissociation generally requires additional free sites on the surface, dissociation can be suppressed and desorption correspondingly enhanced if the free sites on the surface can be properly poisoned. We have found that bismuth adatoms are very good inert site blockers, which can be postdosed to the surface of a transition metal containing a reactive adsorbed hydrocarbon without destroying the hydrocarbon. Whereas in the absence of bismuth, the hydrocarbon would completely dehydrogenate during thermal desorption spectroscopy (TDS) and liberate only H2 into the gas phase, after bismuth postdosing the reactive hydrocarbon desorbs intact for mass spectral identification and quantitative analysis. This method has been used to prove that adsorbed benzene is the initial product of the dehydrogenation of cyclohexane on Pt(111) at ∼235 K. In the absence of bismuth, this benzene all dissociates during TDS to liberate only H2, leaving graphitic carbon residue on the surface. When one-third monolayer of Bi is postdosed at 110 K, the dehydrogenation pathway is sterically poisoned and the adsorbed benzene quantitatively desorbs during TDS, where it is unambiguously identified by mass spectroscopy. By briefly heating the reactive adsorbed intermediate to increasing temperatures prior to Bi deposition, the thermal stability limits of the intermediate and the kinetic parameters for its dissociation can be established. This is demonstrated for the dehydrogenation reaction of adsorbed cyclopentene on Pt(111). Bismuth postdosing in thermal desorption mass spectroscopy (BPTDS) should be a very useful but inexpensive addition to surface analytical capabilities.
The electrical double layer: A solvable model
Authors: Cornu Françoise , Jancovici Bernard
Published: July 2002

In classical equilibrium statistical mechanics, the two-dimensional two-component Coulomb gas is exactly solvable at the special value of the reduced inverse temperature Γ=2. This is used for building an exactly solvable model of the electrical double layer. A charged hard wall (primitive electrode), a polarizable interface, an ideal conductor electrode, a semipermeable membrane are studied: the density profiles and correlation functions are computed. The differential capacity and the surface tension are also obtained.
Electric birefringence of segmentally flexible macromolecules with two subunits at arbitrary field strengths
Authors: Iniesta Angel , García de la Torre José
Published: July 2002

We first present the general treatment of the steady-state orientational distribution of a segmentally flexible macromolecule with two subunits in an electric field E. The orientation-dependent property that we consider is the electric birefringence Δn, although other electro-optical properties can be derived in the same way. In the low-field region linear in E2 we obtain the Kerr constant of macromolecules with a partially flexible joint, and explicit results are presented for a broken rod. A theory recently proposed by Yoshioka is used for the region of intermediate field strength, where Δn is quadratic in E2. The quadratic constant is also evaluated for hinged rods. For electric field of arbitrary strenght we have devised a Monte Carlo simulation procedure which has been checked using analytical results for straight rods. We have determined the extension of the E2 and E4 regions as a function of flexibility, using Yoshioka’s formulas and the simulation results. Finally, we have used literature data for the Kerr constant of myosin rod and its subfragments to obtain a value of the flexibility parameter that is in agreement with those for other properties.
Mo/ller–Plesset perturbation theory calculation of alkaline earth–rare gas complexes: Ground states of Mg–He and Mg–Ar
Authors: Funk David J. , Breckenridge W. H. , Simons Jack , Chal/asiński Grzegorz
Published: July 2002

We have used Mo/ller–Plesset perturbation theory as carried out through the fourth order for a determination of the potential parameters for the ground states of the alkaline–earth metal magnesium with argon and helium. Mg–He was found to have a potential minimum of ≈0.021 mhartree at an internuclear distance of 9.75 a0 and was determined to be capable of supporting a single vibrational level. Mg–Ar was found to have a potential minimum of ≈0.254 at an internuclear distance of 9 a0 , which is slightly shallower and at a larger internuclear distance than a recent experimental determination. A comparison of the homonuclear and heteronuclear bonding is presented and differences in the bonding are attributed to exchange–repulsion effects.
Differential delay equations in chemical kinetics: Some simple linear model systems
Authors: Epstein Irving R.
Published: July 2002

The reasons for and the effects of incorporating time lags into the differential equations of chemical kinetics are investigated for some simple model systems. Complex networks of first-order reactions may, in the limit of long times, be described by relatively simple systems of differential delay (differential difference) equations, in which the effects of intermediates are replaced by time lags. Explicit inclusion of time lags affords a more physically realistic description of nucleation in certain solid state reactions. An electrochemical cell with lagged feedback gives rise to a particularly simple and instructive delay equation. A two-chamber system in which molecules diffuse with a time lag through a membrane shows unexpected oscillatory, and seemingly nonconservative behavior, which is explained in terms of the number of molecules in passage through the membrane.
Determination of excess Gibbs free energy from computer simulation by the single charging-integral approach. I. Theory
Authors: Chialvo Ariel Augusto
Published: July 2002

A recently proposed procedure for the determination of excess Gibbs free energy by computer simulation, due to Chialvo and Haile, and based on Kirkwood’s coupling parameter interpretation of the chemical potential, is revised and improved. New theoretical expressions for excess Gibbs free energy, excess enthalpy and excess volume, are derived in this paper. The new equations allow the simultaneous determination of these quantities via isobaric–isothermal (NPT) simulations involving coupling-parameter charging from only one ideal solution (i.e., pure component), irrespective of either the number of components in the mixture or the number of potential parameters involved. Theoretical expressions are derived which allow the determination of infinite dilution activity coefficients, pure component fugacity ratios, and Henry’s constant ratios without resorting to infinite dilution simulations.
Study of Ag(111) and Au(111) electrodes by optical second-harmonic generation
Authors: Guyot-Sionnest P. , Tadjeddine A.
Published: July 2002

We have measured the potential dependence of optical second-harmonic generation from Ag(111) and Au(111) electrodes at wavelengths below and above the free electron regime. In the free electron regime, the cubic nonlinear susceptibility is dominant, leading to similar effects for positive and negative charging. Above the free electron regime, the quadratic nonlinear susceptibility of the d-electrons is dominant, leading to different effects for positive and negative charging. We also followed the oxidoreduction of Au(111) and found that second-harmonic generation is very sensitive to the different steps of the reaction.
The Jahn–Teller instability of fivefold degenerate states in icosahedral molecules
Authors: Ceulemans A. , Fowler P. W.
Published: July 2002

The linear H⊗(g⊕2h) Jahn–Teller problem, relevant to the instability of icosahedral molecules in fivefold degenerate states, is analyzed in detail for the first time. The method of the isostationary function is used to identify all the extrema of the corresponding potential energy surface. Depending on one single mode-splitting parameter, two different coupling regimes are possible, favoring either pentagonal or trigonal minima. The saddle points on interconversion paths between equivalent minima are identified and the topology of the low-energy regions of the surface is determined. The results are found to be in agreement with the epikernel principle. In addition the symmetry characteristics of the principal warping term under the SO(5) symmetry group of electronic space are assigned.
Potassium promoted decomposition of methyl bromide on Ag(111)
Authors: Zhou X.-L. , Coon S. R. , White J. M.
Published: July 2002

Using TPD, ΔΦ, UPS, and XPS, the decomposition promoted by potassium of methyl bromide (CH3Br) on Ag(111) was studied. While adsorption of CH3Br on K-free Ag(111) lowers the work function, on K/Ag(111) it increases the work function. In contrast to the K-free surface, on which CH3Br adsorbs and desorbs with no detectable decomposition, it adsorbs both molecularly and dissociatively on both submonolayer and multilayer K covered Ag(111) at 100 K. A new adsorption state of CH3Br on K/Ag(111) desorbs at 160–165 K, higher than for desorption of CH3Br from the K-free surface (142–145 K). Most of CH3 radicals desorb upon dissociation. The small fraction retained reacts to form CH4 and C2H4 above 250 K. All Br atoms are retained and react with K to form KBr, which desorbs at 650 K. The dissociation, attributed to electron harpooning from K to CH3Br to form temporary CH3Br− ions which then dissociate, is not thermally activated. Mechanistic comparison is made with photon induced dissociation of CH3Br on K-free Ag(111).
The Onsager theory of the isotropic–nematic liquid–crystal transition: Biaxial particles in uniaxial phases
Authors: Tjipto-Margo Broto , Evans Glenn T.
Published: July 2002

Dense fluids of biaxial hard convex bodies exhibit a phase transition from an isotropic to a nematic (I–N) liquid–crystalline state. In the Onsager theory, the I–N transition is attributed to the tendency of pairs of molecules to minimize their excluded volume (or second virial coefficient, B2 ). In the present work, the mutual orientation dependence of B2 is calculated for hard biaxial ellipsoids and is expressed in a Wigner function expansion. Orientational distribution functions are determined for the Onsager model and for the Lee model, which incorporates some density correlations beyond those in the Onsager theory. In the uniaxial phase of biaxial molecules, the two nonzero order parameters calculated on the basis of hard-body models are in approximate accord with the findings of Maier–Saupe based theories. Introduction of biaxiality into a body has a pronounced effect: both the 〈P2〉 order parameter and the first orderness of the I–N transition are greatly reduced from that of comparable uniaxial bodies.
The distribution function of the radius of gyration of linear polymers in two and three dimensions
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

Monte Carlo simulations employing the pivot algorithm are used to generate excluded-volume chains on two- and three-dimensional lattices. The radius of gyration distribution function is calculated from the resulting configurations. The distributions are in excellent agreement with the scaled form postulated by Lhuillier.
Photoelectron spectroscopic studies of polyatomic molecules: Degree of orientation and ionization of rotationally state selected, oriented molecules
Authors: Chandra N. , Chakraborty M.
Published: July 2002

In this paper we report theoretical studies of angle-resolved photoelectron spectroscopy (ARPES) and of circular dichroism in photoelectron angular distribution (CDAD) for ionization in molecules oriented in a single ‖JKJMJ〉 rotational eigenstate. These processes have been investigated also as two of the possible alternatives to photodissociation to determine orientational distribution function of rotationally state selected, oriented molecules. Expressions are derived which can be used to calculate ARPES and CDAD for such molecular species from ab initio methods or to analyze these experimentally observed spectra for extracting information about the degree of orientation of the molecular framework. These formulas are put in their simplest possible forms using the transformation properties of the molecular point group to their full advantage. The ionization amplitude is thus shown to decompose into a sum of transitions each involving the final state wave function belonging to an irreducible representation of the point group of the target molecule. It is found that, similar to the case of photodissociation, one can determine the rotational quantum number J purely from experimental photoionization data.

Expressions developed herein are used to study ARPES and CDAD for ionization in a1 orbital of those rotationally state selected and oriented spherical top molecules which transform according to the Td point symmetry group. In this case, the detection-integrated cross section, singly differential in molecular orientation, is found to be independent of the photoionization dynamics and directly gives the molecular orientational function. The other ARPES and CDAD formulas are shown to depend upon the dynamics through the integrated partial cross section σ̄, the angularly asymmetry parameter β̄, the phase shift of the continuum waves representing the photoelectron, and the phase of the dipole transition amplitudes. The formulation presented in this paper sets a methodology for the analysis of measurements and calculation of the photoelectron spectra in rotationally state selected and oriented molecules in general, spherical top Td molecules in particular. It is applied, as an example, to photoionization in 6a21 orbital of oriented CCl4 in a pure ‖JKJMJ〉 rotational state. We find, among other things, that the photoelectron angular distributions change significantly when either or both of the directions of molecular orientation and of polarization of ionizing radiation vary from parallel to perpendicular to the quantization axis.
Renormalization-group scaling theory for flexible and wormlike polymer chains
Authors: Chen Zheng Yu , Noolandi Jaan
Published: July 2002

Using an argument based on renormalization-group scaling, we study the structure of the functions corresponding to the mean square end-to-end separation, and radius of gyration, and the second virial coefficient for a flexible polymer chain including the finite effect of excluded volume. With the aid of recent accurate calculations of asymptotic properties [M. Muthukumar and B. G. Nickel, J. Chem. Phys. 86, 460 (1987)], we establish the form of the functions for these physical quantities at an arbitrary scaled excluded-volume parameter. We also propose approximations for the above-mentioned quantities for a wormlike polymer chain, which successfully interpolate the limiting flexible and hard-rod behavior.
Solute–solute correlations in infinitely dilute supercritical mixtures
Authors: Debenedetti Pablo G. , Chialvo Ariel A.
Published: July 2002

Asymptotic expressions for the solute–solute pair correlation integral and for the limiting slope of the composition dependence of the solute’s activity coefficient are derived for the situation in which the solvent’s critical point is approached while the mixture is kept at infinite dilution. Both quantities diverge to plus infinity and scale as the solvent’s compressibility, with prefactors that depend on the temperature, solvent density, and the infinite dilution limit of the solute–solvent direct correlation function integral.
Folding kinetics of proteins: A model study
Authors: Guo Zhuyan , Thirumalai D. , Honeycutt J. D.
Published: July 2002

We study the kinetics of folding of a heteropolymer containing a specific sequence of hydrophobic, hydrophilic, and neutral residues. The heteropolymer, representing the alpha carbon sequence of a model protein, folds into a β-barrel structure below a characteristic folding transition temperature. Several measures are introduced to probe the dynamics of approach to a folded state with particular emphasis on the kinetics of formation of nonbonded contacts starting from a high temperature random configuration. The measures of compactness are used to argue that even below the folding temperature, nonbonded contacts are formed at a very slow rate. This result is further corroborated by analyzing the dynamics of relaxation of the nonbonded interactions which are responsible for the formation of folded structures at low temperatures. We also show that below the folding transition temperature and for short times, the heteropolymer undergoes large structural fluctuations. Examination of the time dependence of the radius of gyration Rg shows that the heteropolymer is trapped in one minimum (characterized by a roughly constant Rg ) for arbitrary times followed by sudden excursions to neighboring minima. In this regime, multistate kinetics is appropriate. However, at longer times when the heteropolymer becomes trapped in one of the minima corresponding to the folded conformation, the folding process appears to be an all or none process. The implication of our results for the kinetics of in vitro folding of proteins are discussed.
Phase transitions in Gibbs films: Star textural defects in tilted mesophases
Authors: Hénon S. , Meunier J.
Published: July 2002

The formation of adsorbed films at the free surface of aqueous solutions of sodium hexadecanoate was observed by microscopy at the Brewster angle. A phase transition between a gas and a tilted phase occurs during these slow formations. The tilt of the molecules creates an observable optical anisotropy. We observed circular domains separated into regions of uniform tilt direction by straight defect lines, i.e., ‘‘star’’ textural defects very similar to those observed in freely suspended liquid crystal thin films and in Langmuir films of esters. The coalescence of domains leads to large regions of uniform tilt direction separated by straight or kink lines. A second transition towards a nontilted phase is observed. Quantitative information about the coefficients of the continuum elastic theory can be derived from our experiments.