bluuebunny/crossref_metadata_2025 huggingface dataset:
On the pressure dependence of the polarization of diamagnetic muons and muonium in pure noble gases: Is there a missing fraction?
Authors: Turner Ralph Eric , Senba Masayoshi
Published: July 2002

The amplitudes of the signals in μSR exhibit pressure dependencies which are associated with the stopping dynamics of muonium atoms and diamagnetic muon species observed when muons are thermalized in pure noble gases. To explain this effect, a set of coupled rate equations, with time dependent rates and based upon quantal Boltzmann equations, have been developed to describe the spin dynamics for the thermalization of the two species. These, by definition positive, rates depend upon time through the translational single particle density operators associated with each species. Thus, to exactly solve the spin dynamics, the coupled kinetic Boltzmann equations for the stopping process must also be solved. Furthermore, the rate equations also contain spin dynamics generated by the muonium hyperfine interaction. It is the presence of this hyperfine interaction which leads to the loss of polarization for low pressure gases. The coupled quantal rate equations have been solved for a model of the stopping dynamics in which the rates, taken as square box functions of time, describe the charge exchange regime wherein muonium is both formed and ionized by subsequent collisions. Two post charge exchange extensions of this model are now considered. Following the charge exchange region, in the loss model, it is assumed that the rate of muonium formation is zero while the ionization rate is not. On the other hand, the capture model assumes that the ionization rate is zero while the muonium formation rate is not. Fits are presented to the available data for argon, krypton, xenon and neon. Since argon has both diamagnetic and paramagnetic signals then independent fits to each may be compared. A single set of fitting parameters has been found which describes both signals. This single fit requires a further extension of the models. In particular, a missing fraction must be assumed! The missing fraction model is also required when fits are made to the krypton and xenon data. On the other hand, the fit to the neon data is inconclusive. This is the first suggestion that a missing fraction may exist in the pure noble gases. Such missing fractions have been well established in condensed phases.
Coherent anti-Stokes Raman spectra study of CF3 and NO photofragments of CF3NO
Authors: Bozlee B. J. , Nibler J. W.
Published: July 2002

Coherent anti-Stokes Raman (CARS) spectra are reported for the nascent products of visible laser photolysis of CF3NO. The results indicate a high degree of rotational excitation in both CF3 and NO fragments following C–N bond cleavage. The rotational band shapes observed for both photoproducts are consistent with those predicted from simple prior statistical calculations and correspond to Boltzmann temperatures in excess of 1100 K. This first detection of the CF3 radical by a Raman technique confirms earlier low resolution infrared assignments of the ν1 symmetric C–F stretching vibration and yields an improved value of 1088.6±0.5 cm−1 for this fundamental mode. With several simplifying assumptions, band contour calculations are used to estimate changes in B and C rotational constants for this transition.
Vibrational-state distribution of IF from the reaction F+3-iodopropene: An example of radical resonance energy participation
Authors: Collins S. T. , Trautmann M. , Wanner J.
Published: July 2002

The role of the double bond and allyl radical resonance energy has been investigated in the reaction F+C3H5I→IF+C3H5. The vibrational state distributions of the IF product from this reaction and from the corresponding saturated iodo-hydrocarbon reaction F+C3H7I→IF+C3H7 have been determined using laser-induced fluorescence. A bimodal distribution for the reaction F+C3H5I has been observed in contrast to the monotonically decreasing distribution for the reaction F+C3H7I. The bimodal distribution consists of a monotonically decreasing branch, identical to the results for the case of F+C3H7I, and an inverted branch which accounts for the majority of vibrational excitation. The statistical part of the distribution is ascribed to an abstraction mechanism with 〈 f′v〉I =0.07, whereas the inverted part can be explained in terms of an addition–elimination mechanism with 〈 fv〉II =0.59. The branching ratio for the two channels is 5.7:1 in favor of the addition–elimination mechanism. The spectra show that the allyl radical product resonance energy is transformed into vibrational excitation of IF for mechanism II. This can be understood in mechanistic terms.
High power laser photochemistry. Production of neutral atomic and small molecular fragments by UV multiphoton dissociation
Authors: Chen Peter , Pallix Joan B. , Chupka William A. , Colson Steven D.
Published: July 2002

Atomic carbon, Ch, CH2, and CH3, produced by UV multiphoton dissociation, have been detected by resonant MPI on picosecond and nanosecond time scales. Large yields of neutral fragments produced within the pulse width of the dissociation laser suggest that exceedingly rapid radiationless transitions must occur, suppressing ionization of the parent molecule.
Finite size corrections to Madelung’s number
Authors: Wilson John W. , Heinbockel John H. , Outlaw Ronald A.
Published: July 2002

A correction to the usual Madelung form for the electrostatic energy of cubic ionic crystals due to the finite size of real atomic ions is derived.
Hyperpolarizability dispersion measured for Kr and Xe
Authors: Shelton D. P.
Published: July 2002

The frequency dependence of the second hyperpolarizabilities (γ ) of Kr and Xe have been measured in the visible by means of gas phase electric-field-induced second-harmonic generation (ESHG). A simple model relating the dispersion of the nonresonant electronic contribution to γ for different third-order nonlinear optical processes is derived. This model is tested by comparing the results obtained here with those obtained by other methods.
Disentangling of two intertwined chains
Authors: Baumgärtner A. , Muthukumar M.
Published: July 2002

The nonequilibrium process of disentangling of two self-avoiding polymer chains is investigated using Monte Carlo methods. The initial configuration of the two chains corresponds to a double helix of M turns. Chains consisting of N=8M+1 segments with M=2, 4, and 8 have been simulated. The disentangling process is found to take place in two distinct stages. The first step is the softening of the original double helix configuration to form interpenetrating chains with their centers of mass not far away from each other. This process takes a typical time of the order of N3.0 ± 0.2 . During the first stage, the center of mass of the either strand obeys the diffusion law, with the diffusion coefficient D∼N−(1.6±0.2) . The second stage involves the actual unraveling of the interpenetrating chains to form the isolated coils. The time taken for this step is found to scale as N3.3±0.2 . After the disentangling is complete, we recover the Rouse behavior, D∼N−1 for the center of mass diffusion of each coil.
The collapse transition in three-dimensional linear and ring polymers
Authors: Bishop Marvin , Michels J. P. J.
Published: July 2002

The collapse transition for three-dimensional linear and ring polymers is investigated by Brownian dynamics. It is found that the mean-square radius of gyration, 〈S2〉, displays a different power law for strong and weak attractive forces between chain units, N: for rings 〈S2〉∼N1.2 (weak forces) and N2/3 (strong forces), whereas for linear drains 〈S2〉2(N−1)1.2 (weak forces) and (N−1)2/3 (strong forces).
The orientation of NH3D+ in ammonium nickel–sulfate hydrate: A new type of crystalline phase
Authors: Trapani Andrew P. , Strauss Herbert L.
Published: July 2002

We report the discovery of a new type of phase. The N–D bonds in dilute NH3D+ in the title compound are oriented to a maximum extent at 29 K, but this orientation decreases at both higher and lower temperatures. We attribute the decrease as the temperature is lowered to an increase in the tunneling of the NH3D+ ions.
Spontaneous vibrational decay rate in the KF electronic ground state
Authors: Bedding David R. , Moran Thomas I.
Published: July 2002

We have measured the spontaneous vibrational decay rates in the KF electronic ground state for v=2 → 1 and v=1 → 0 transitions in both the J=1 and the J=2 rotational states using a velocity selected molecular-beam technique. The four decay rates are self-consistent and reduce to a 1 → 0 decay rate of 2.6±0.4 s−1, which agrees with rates inferred from spectroscopic data.
The effects of excited-state diffusion and saturation on the strong field reflection and transmission properties of materials
Authors: Band Yehuda B. , Sewell G.
Published: July 2002

We propose a method for calculating the effects of saturation of optical transitions (and other nonlinear polarization mechanisms), and the effects of particle diffusion, on the reflection and transmission of light through a medium when surface reflections are important. We use a multiple shooting technique to solve the two-point boundary value problem specified by the nonlinear wave equation for the electric field and the rate equation for the excited state population density of the medium, and their boundary conditions. The method can be used to describe both saturable absorption of organic dyes in a Fabry–Perot cavity and strong field reflection and transmission of semiconductors. As an illustrative example, the intensity dependence of the reflection and transmission from a slab of InSb is calculated.
On the Clausius virial of inhomogeneous systems
Authors: Henderson J. R.
Published: July 2002

This paper sets out to clarify the nature of the Clausius virial evaluated inside a specified volume of arbitrary geometry and in the presence of a general one-body external field. The standard form of the virial theorem is not obtained when intermolecular forces act across the surface of the volume. A modification of the virial is given in order to reinstate the relationship with the normal pressure acting at the surface of the volume.
Stability and thermodynamics of ligand-free tin–gold clusters
Authors: Kingcade J. E. , Gingerich K. A.
Published: July 2002

The gaseous molecules AuSn, AuSn2, Au2Sn, Au2Sn2, and AuSn3 have been indentified in a mass spectrometric study of the Knudsen effusate from the condensed Au–Sn system. Third-law enthalpies of the various reactions involving these species, and for the smaller clusters also second-law enthalpies, were determined from the experimental equilibrium measurements and estimated thermodynamic functions. For the polyatomic molecules several different structures were considered. The dissociation energy D○0 of AuSn(g) was obtained as 252.6±7.2 kJ mol−1 and the heat of formation ΔH○f,298 was evaluated as 414.6±7.2 kJ mol−1. The atomization energies ΔH○a,0 and the standard heats of formation ΔH○f,298 for the polyatomic molecules, in kJ mol−1, are: AuSn2, 483.1±11.7 and 489.4±11.7; Au2Sn, 540.5±20.4 and 493.0±20.4; AuSn3, 788±28 and 477±28; Au2Sn2, 726±35 and 610±35.
Overtone spectra of helical polymers: Infrared absorption and vibrational circular dichroism
Authors: Lalov I. J. , Svetogorski D. A. , Turkedjiev N. P.
Published: July 2002

A theory is developed for the IR absorption and vibrational circular dichroism (VCD) spectra of helical polymers in the frequency range of the frequency sum for two vibrational excitons. Bound and unbound two-exciton states are shown to exist, using the model for first nearest neighbors interactions (applicable to the amide II vibration). The dielectric permittivity tensor and the gyration tensor are found in the following cases: (a) oriented helices; (b) isotropic solutions of helical polymers. The frequency dependence of the various VCD terms is studied in the vicinity of the bound two-exciton states and in the band of unbound two-exciton states.
A semiclassical model for orientation effects in electron transfer reactions
Authors: Cave Robert J. , Klippenstein Stephen J. , Marcus R. A.
Published: July 2002

An approximate solution to the single-particle Schrödinger equation with an oblate spheroidal potential well of finite depth is presented. The electronic matrix element HBA for thermal electron transfer is calculated using these wave functions, and is compared with values of HBA obtained using the exact solution of the same Schrödinger equation. The present method yields accurate results for HBA, within the oblate spheroidal potential well model, and is useful for examining the orientational effects of the two centers on the rate of electron transfer.
Hemiquantal mechanics. I. Vibrational predissociation of van der Waals molecules
Authors: Halcomb Lawrence L. , Diestler D. J.
Published: July 2002

The ‘‘hemiquantal’’ equations (HQE), which pertain to a system consisting of a quantally-behaving (‘‘light’’) subsystem coupled to a classically-behaving (‘‘heavy’’) one, result from a partial classical limit of Heisenberg’s equations of motion. In effect, all heavy particles are required to follow precisely their classical trajectories. The HQE are applied to vibrational predissociation in a collinear model of the van der Waals molecule He ⋅ ⋅ ⋅I2(B). Here, the vibration of I2 is the classical subsystem and the motion of He relative to the center of mass of I2 is the quantal subsystem. In this case, the HQE comprise a partial differential equation (Schrödinger’s equation for the He motion) coupled to two ordinary differential equations (Hamilton’s equations for the I2 vibration). These were solved numerically on the CYBER 205 supercomputer by means of an algorithm that uses a second-order predictor–corrector for Hamilton’s equations and second-order time differencing for Schrödinger’s equation. A scheme based on the fast Fourier transform was used to evaluate the spatial derivative of the wave function. The computed rates of vibrational predissociation are compared with the results of previous quasiclassical and fully quantal calculations and with experimental results.
Solvent isotope effects on the thermodynamics of hydrophobic hydration
Authors: Zichi D. A. , Rossky P. J.
Published: July 2002

A procedure is outlined to evaluate the intermolecular vibrational contribution to solvent isotope effects for hydrophobic hydration. The method utilizes librational power spectra obtained from a classical molecular dynamics simulation and assumes harmonic hindered rotational motion for solvent molecules. Application is made to the transfer process of Ar from light to heavy water. The results of the calculation indicate that, while librational effects are quite significant, there exist additional structural contributions to the thermodynamics of transfer.
Conductivity and ordering on the planar honeycomb lattice: A Monte Carlo study of transport and order parameters in β″ aluminas
Authors: Pechenik Alexander , Whitmore D. H. , Ratner Mark A.
Published: July 2002

The effects of ordering on the conductivity of carriers on a planar honeycomb lattice is studied using a modified Monte Carlo technique. The hopping model includes repulsion between carriers on nearest-neighbor sites. In agreement with previous work, we find an ordered region on the phase diagram for ion density ρ in the range 0.41≤ρ≤0.59 for sufficiently low temperatures. Within this composition range of the phase diagram, ordering effects on the conductivity and on its temperature dependence are very substantial; previous results which did not correctly include the ordering effects yield quite different conductivities. We also calculate correlation factors, diffusion coefficients, and several different kinds of order parameters. Experiments on systems such as Ba++ β″-alumina and AgCrSe2 should be able to probe these ordered regions, and our results predict the influence of structure on the transport of the mobile ionic species in such compounds.
Hemoglobin recombination spin selectivity
Authors: Doetschman D. C. , Szumowski J.
Published: July 2002

Group theory and the spin Hamiltonian are employed in a theoretical treatment of the spin dependence of the tunneling recombination of CO with hemoglobin following photolysis. Second order spin-orbit coupling between the 1A1 and 5T2 states of the octahedral five-coordinate Hb heme iron is considered in the presence of lower D2 symmetry fields. It is assumed that the totally symmetric 1A1 character that is mixed into the lowest spin multiplet of 5T2 is responsible for recombination to the 1A1 state of the recombined hemoglobin. The problem of identifying which spin multiplet, 5B1, 5B2, 5B3, of 5T2 in lower D2 symmetry lies lowest has been solved. When the field and temperature dependence of the HbCO tunneling recombination rate is calculated, 5B2 is in best agreement with experiments on HbCO. With the formulation developed for diamagnetic HbCO, it is relatively simple to include a small exchange interaction between the NO and Hb iron heme while assuming that the recombination of HbNO and HbCO is otherwise similar. The effect of an exchange interaction as small as a few cm−1 is formally similar to the application of a very large magnetic field and predicts an increase of the recombination rate in agreement with experiment.
Quantum mechanical microcanonical rate constants from direct calculations of the Green’s function for reactive scattering
Authors: McCurdy C. William , Garrett Bruce C.
Published: July 2002

It has been shown previously [Miller, Schwartz, and Tromp, J. Chem. Phys. 79, 4889 (1983)] that the cumulative reaction probability, N(E), is given by a quantal trace, N(E)=2π2ℏ2 tr[Fδ(E−H) Fδ E−H)], where F is a symmetrized flux operator for flux through a surface dividing reactants from products, and δ(E−H) is the density operator related to the full Green’s operator, G+(E), by δ(E−H)=−Im G+(E)/π. Discretizing the coordinate space representation of the Schrödinger equation for the Green’s function leads to a set of linear matrix equations, and it is shown that these can be solved by an efficient recursive technique requiring little computer storage. Several simplifications are possible in this application because the Green’s function is only required near the dividing surface. A complete procedure is outlined which includes the exact solution of the recursion equations outside the interaction region. Two other approaches are explored: a discretized complex coordinate technique and the coordinate space solution of the Lippmann–Schwinger equation for the Green’s function. Results and an analysis of the numerical behavior of these procedures are given for model one-dimensional systems. The essential features of the extension of the recursive method to multidimensional systems are given.
High temperature exchange and linewidth anisotropy in an ion-radical salt of TCNQ
Authors: Bondeson S. R. , Holman R. W.
Published: July 2002

The EPR linewidth of triphenylmethylarsonium (TCNQ)2 was measured as a function of crystal orientation at room temperature. The anisotropy is reproduced by calculating the static dipole–dipole interaction in the dense triplet exciton system and including the modulation by the exchange coupling between excitons. The exchange frequency is found to be ωe=6.7×109 s−1. A discussion of the effects of adsorbed molecular oxygen is also included.
On the atomic roughness of solid–vapor interfaces: Significance of many-body interactions
Authors: Chen Jenn-Shing , Ming Nai-Ben , Rosenberger Franz
Published: July 2002

It is traditionally assumed that the pair energies of atoms on the surface are site independent and equal to the pair energy in the bulk of the crystal (constant bond energy approach). We have assumed that the internal energy of an atom, ψ, depends nonlinearly on the number of nearest neighbors n in the form ψ(n)=−(an+bn2), where a and b are material constants that can be evaluated from experimentally determined heats of sublimation and energies of bulk vacancy formation. This simple approximation of the actual many-body interactions accommodates site-dependent pair energies. Based on this phenomenological ‘‘variable bond model’’ and the Bragg–Williams approximation, occupation fractions of an equilibrium surface are obtained in two-level and three-level models. These provide a criterion of surface roughness for specific orientations. Application of the criterion to copper, lead, and zinc yields good agreement with experimental observations of anisotropic melting, while the traditional constant bond approach fails. It is shown that in addition to these systems many other metals possess negative b values, i.e., exhibit ‘‘bond strengthening’’ on surfaces. Similar conditions are shown to exist in numerous ionic, covalent, and van der Waals solids. Hence, one must expect that constant bond models underestimate the surface roughness for a large group of materials.
Surface-suppressed electron resonance spectroscopies
Authors: Barkley P. Glenn , Hornak Joseph P. , Freed Jack H.
Published: July 2002

Surface-suppressed electron resonance spectroscopies (SSERS) refers to the phenomenon in which stable paramagnetic radicals adsorbed on clean (noble) metal surfaces have their ESR signal suppressed. This phenomenon is studied in some detail by ultra-high-vacuum ESR (UHV-ESR) and cyclotron resonance from microwave-induced secondary electron emissions (CREMSEE) in combination with conventional thermal desorption. The UHV-ESR is performed in situ on the inner surface of the microwave cavity after leaking in stable nitroxides as previously described by Nilges and Freed. In the SSERS phenomenon, the first layers of nitroxide deposited on the metal surface do not give rise to an observable ESR signal, even though a significant decrease in the CREMSEE microwave power threshold, Pt, is observed. It is consistently found (for several nitroxides and noble metals) that an ESR signal is observed only when Pt has dropped to of the order of 20 (±10)% of its initial value. The ESR signal then increases monotonically with increased dosage. Also, a correlation between these measured reductions in Pt, and the estimated surface coverages required for initial observation of the ESR signal, is suggested by our results. It may be that SSERS is (partly) due to the first layer of nitroxide interacting strongly by exchange forces to the surface conduction band of the metal, so that the ESR signal is too broadened to be observed. Subsequent nitroxide layers may also be affected through their interaction with previous layers by weaker exchange forces. This is consistent with experiments in which a related but diamagnetic species is

utilized to form the initial adsorbed layers, and it is found to act as an ‘‘insulator’’ for the subsequent nitroxide layers. (On the other hand, surfaces pretreated with O2 or O2/H2O mixtures had very little effect on SSERS observed with Ag surfaces, although it had some effect, in the sense of a weak insulator, with Cu surfaces.) The change in ESR signal upon warming was correlated with the observed pressure changes. In some cases there are unusual nonmonotonic variations of the ESR signal strength, inconsistent with observed desorption of nitroxide, that are also believed to be due to SSERS. The possible role of temperature-dependent surface wetting effects is briefly considered.
Infinite heat capacity anomaly for two-level systems with a mean-field coupling
Authors: Stillinger Frank H.
Published: July 2002

This paper examines the statistical mechanics of collections of two-level systems that experience a nonlinear mean-field coupling. The coupling reduces the two-level excitation energy by an amount that depends on the number of excitations already present; in other words the energy spectrum manifests a cooperative ‘‘softening’’ phenomenon. An exact solution in the infinite system limit is presented for key aspects of a specific case, the ‘‘logarithmic’’ model. This model exhibits a symmetric infinite heat capacity anomaly with divergence exponent equal to 2/3. Furthermore, the divergence evolves from maxima with heights asymptotically proportional to N1/2 as the number N of coupled two-level systems increases to infinity.
Collisional deactivation of two-photon laser excited xenon 5p5 6p. I. State-to-state reaction rates
Authors: Böwering N. , Bruce M. R. , Keto J. W.
Published: July 2002

Laser-induced fluorescence of two-photon excited Xe 5p5 6p is analyzed to obtain collisional branching fractions and state-to-state reaction rates. The intramultiplet quenching is found to be described surprisingly well by a simple statistical model. Based on the molecular potentials involved, curve crossing mechanisms are discussed.
Lattice dynamics and specific heat of α-helical poly(L-alanine)
Authors: Datye Vandana K. , Krimm S.
Published: July 2002

We have calculated the phonon dispersion relation in α-poly(L-alanine) with all atoms in the chemical repeat explicitly included and using a recently refined force field for the polypeptide chain. The phonon density of states shows very good agreement with the inelastic neutron scattering data for this polypeptide. The specific heat for T<150 K yields better agreement with experiment than did earlier calculations. At higher temperatures the discrepancy with experiment remains. We present a simple model calculation which suggests that the source of this discrepancy may lie in the anharmonicity of the methyl torsion mode.
Resonant photoemission in MnF2
Authors: Hayes Robert G. , Sham T. K.
Published: July 2002

Photoelectron spectra are reported for MnF2 using the National Synchrotron Light source radiation (NSLS) as excitation radiation. Resonance structures, defined as the enhancement of intensity are discussed. (AIP)
Interaction potential and chiral discrimination between an alanine molecule and a quartz surface
Authors: Vega L. , Breton J. , Girardet C. , Galatry L.
Published: July 2002

The calculation of the interaction energy between a quartz crystal surface and a chiral molecule is considered. The various contributions (electrostatic, dispersion, hydrogen bond, and repulsion) to this energy are separately treated and computed using atom–atom summation or Fourier expansion techniques. The discussion of the resulting potential surface allows us to exhibit an influence of the chiral character of the adsorbed molecule; this may amount to a few percent of the total interaction energy for closest approaches. A possible connection with the experimental results is suggested.
Two-photon ionization of atomic systems treated by means of the complex coordinate method
Authors: Froelich P. , Flores-Riveros A.
Published: July 2002

A L2 discretization of the continuum within the scope of the complex coordinate technique is employed to obtain the differential cross section for two-photon ionization of atomic systems. The method avoids explicit construction of the continuum final state and instead dispersion relations are utilized to evaluate the appropriate transition matrix element controlling the process, to second order of perturbation theory in the nonrelativistic dipole approximation. The method is applied to the two-photon ionization of atomic hydrogen.
Carbon spin-lattice relaxation rate measurements during single proton excitation. A new method of internuclear distances determination
Authors: Rossi Claudio
Published: July 2002

In this paper by combining carbon spin-lattice relaxation rate measurements obtained under condition of broadband proton decoupling (RBB1) and single proton excitation (RSPE1) , 1H–13C internuclear distances have been determined in solution. The experimental results obtained by excitation of H6 proton of thymidine can be used to calculate proton–carbon distances in thymidine. Moreover in this paper the use of RSPE1 experimental parameter for obtaining structural and dynamical information is proposed.
Raman spectrum of ferroelectric PbDPO4: Comparison with PbHPO4
Authors: Ohno N. , Lockwood D. J. , Kuok M. H.
Published: July 2002

The Raman spectrum of ferroelectric PbDPO4 (TC≂450 K) has been measured at temperatures between 10 and 484 K in the frequency region 8–3000 cm−1. Mode assignments have been made by comparison with similar Raman measurements on PbHPO4 (TC≂310 K). It is found that the lattice modes and PO4 internal modes are scarcely affected by the isotope change, although the high-frequency hydrogen modes do show the expected shift to lower frequency in the deuterium compound. The overall similarities in the spectra of the two compounds both above and below TC indicate that PbDPO4 has an acentric crystal structure above TC, as was recently discovered for PbHPO4. The Raman data indicate the phase transitions in PbDPO4 and PbHPO4 are of the order–disorder type.
Statistical mechanics in homopolypeptides. II. Theoretical approach for denaturation process induced by pH change
Authors: Vila J.
Published: July 2002

In this paper we present a statistical mechanics analysis of the helix–coil transition induced by pH change in homopolypeptide. We propose a one-dimensional model where the contribution from each site to the grand partition function includes contribution of ionic origin (due to the repulsive interaction between similarly charged groups) and contribution of nonionic origin (due to the break of intramolecular hydrogen bond, characteristic of the α helix). Through an accurate solution of the model, we derive analytical expressions for a fraction of internal hydrogen bond θ, dissociation degree of the homopolypeptide α, the melting temperature Tm, change of free energy, ΔG, etc. Then we compare the theoretical curve of titration processes with experimental data and we obtain a very good agreement. We note that the model does not have fitting parameters.
O− detected by ESR as a primary electron-excess defect in x-irradiated K2SO4
Authors: Byberg J. R.
Published: July 2002

ESR spectra of K2SO4 crystals irradiated with x rays at 26 K contain signals from a novel type of defect in three inequivalent configurations. The g tensors, linewidths, and hyperfine splittings from one potassium nucleus all indicate that the defects are structurally analogous to the primary electron-excess defects [O−,ClO−3 ] in KClO4. Hence the spectra are assigned to the complex [O−,SO2−3 ]. The directions in the K2SO4 lattice of the unique axes of the three g tensors support this assignment. The occurrence of dissociative electron attachment to SO2−4 and ClO−4 contrasts the well-established existence of the radical ions SeO3−4 and BrO2−4.
Calculation of the deuterium electric field gradient in DF
Authors: Kim Hae-Won , Hameka Hendrik F. , Zeroka Daniel
Published: July 2002

We report theoretical results of the electric field gradient at the position of the deuterium nucleus in the DF molecule. The purpose of the calculation is the evaluation of a correction term to the expectation value of the electric field gradient with respect to a linear combination of atomic orbitals-molecular orbital self-consistent field (LCAO-MO SCF) function proposed by Ransil. Our method is based on a variation-perturbation method that was proposed previously. The expectation value of q0D (el) is qD =−2.8964, the corrected value is qD (el)=−2.9056, and the experimental value is qD (exp)=−2.9241 (all values are expressed in terms of atomic units). The difference between our theoretical value and the experimental result is 0.6%.
Test of Hubbard–Onsager’s dielectric friction theory for ion migration in mixed solvents. I. Ethanol–water systems
Authors: Ibuki K. , Nakahara M.
Published: July 2002

In order to test the Hubbard–Onsager (HO) dielectric friction theory for ion mobility in mixtures of polar solvents, the theory has been generalized within the same framework to a system which has two dielectric relaxation times by using the superimposed Debye polarization fields related to the two kinds of dipoles. The same form of linear differential equation for the determination of the velocity field is reached only with the modified definition of the HO radius. The numerical solutions to the differential equation are fitted to a polynomial to facilitate a systematic application of the HO theory. The HO theory is reasonably well successful in predicting the dependence of the residual friction coefficient Δζ (=ζ−4πηR) on ion size and solvent composition for the alkali metal and halide ions even if no effects of the ions on solvent properties are taken into account. It turns out that the HO theory is by far better than the Zwanzig theory. Limitations of the HO theory combined with bulk properties of solvent are exhibited for the relatively large ions, in particular, below the solvent composition where the solvent has a maximum viscosity.
Reaction of CF3 radicals on fused silica between 320 and 530 K
Authors: Selamoglu N. , Rossi M. J. , Golden D. M.
Published: July 2002

The reaction between CF3 radicals and silicon oxide (fused silica) surface was studied in a VLPΦ flow reactor (∼0.1–3 mTorr) as functions of surface temperature (320–530 K) and CF3 concentration. The CF3 radicals were generated from CF3I by CO2 laser photolysis, and the subsequent gas-phase reaction products were followed by mass spectroscopy. The surface reaction was found to yield CO, HF, CO2, COF2, and SiF4. It was found that H2O residing on the silicon oxide surface was largely responsible for the oxygen- and hydrogen-containing products, and that little etching of the SiO2 itself occurred under these conditions. The rates for the irreversible surface loss of CF3, and for the formation of CO were both first order with respect to [CF3]. These were found to be temperature dependent with Ea≂4.7 and 7.5 kcal/mol, respectively. The CF3 surface loss rate indicates that the sticking coefficient for this radical on quartz is between 0.0014–0.017 for the temperature range of this study.
Molecular and crystal structure of the nonlinear optical material: 2-(N-prolinol)-5-nitropyridine (PNP)
Authors: Twieg R. J. , Dirk C. W.
Published: July 2002

The structure [C10H13N3O3,Mr=223.23, monoclinic, P21, a=5.182(1) Å, b=14.964(4) Å, c=7.045(1) Å, β=106.76(1)°, V=523.1 Å3, Z=2, Dx=1.42 Mg mm−3, μ=9.0 cm−1, F(000)=236, 23±1 °C, R=(Σ‖‖F0‖−‖Fc‖‖/Σ‖F0‖) =0.047 based on 930 reflections with F20>3.0σ(F20)] of this compound reveals it to optimally satisfy many of the structural and symmetry restrictions required for optical second harmonic generation (SHG). The molecules are extraordinarily well aligned to optimize noncritically phase matched SHG with the amino nitrogen to nitro nitrogen charge transfer axis making an angle of 59.6° with the unique axis (b). The mean aromatic plane sits nearly perfectly in the (101) crystallographic plane which may be a natural cleavage surface through which normal propagation may produce noncritically phase matched SHG. Powder optical second harmonic generation measurements (λ=1.06μ; Nd-YAG laser) yield a value of 140.0x relative to powdered urea. Molecules related by a screw operation and the translation [0,0,1] are intermolecularily hydrogen bonded [amino–H/nitro–O; 2.20(10) Å]. The structure is essentially isostructural to the earlier reported structure of N-(4-nitrophenyl)-(L)-prolinol (NPP) which appears to possess slightly enhanced second harmonic efficiency.
Inelastic processes in ion/surface collisions: Direct recoil ion fractions as a function of kinetic energy
Authors: Rabalais J. Wayne , Chen Jie-Nan
Published: July 2002

Time-of-flight (TOF) spectra of the scattered and recoiled particles resulting from 1–10 keV Ar+ ions impingent on surfaces of MgO, Mg(OH)2, graphite, Si, and SiO2 have been obtained. Measurements of directly recoiled (DR) neutrals plus ions and neutrals only are used to calculate positive and negative ion fractions Y+,− from DR events. These positive and negative ion yields observed for DR of H, C, O, and Si have distinctly different behavior as a function of ion kinetic energy. The Y+ values exhibit a ‘‘threshold-type’’ behavior with a steep rise followed by a slowly rising or plateau region at higher energy. The Y− values exhibit a maximum in the low energy region followed by a decreasing yield as energy increases. The Y−/Y+ ratio for C and O is very sensitive to the amount of hydrogen present, with the Y+ yields dropping as hydrogen concentration increases. The recently developed model for electronic transitions in keV ion/surface collisions which considers Auger and resonant transitions along the ion trajectory and electron promotions in the quasidiatomic molecule of the close atomic encounter is extended to include DR events. Analytical expressions for Y+,− are derived for the case of surface atoms in positive, neutral, and negative bonding environments. These model expressions are fitted to the experimental data, allowing determination of the probabilities of ionization in the close atomic encounter and of electron capture along the outgoing trajectory.
A stabilized vapor liquid interface in deuterated cyclohexane–methanol mixtures
Authors: Schmidt J. W.
Published: July 2002

The ellipticity of the vapor–liquid interface above mixtures of methanol (CH3OH) and deuterated cyclohexane (C6D12) has been measured in the vicinity of the consolute point. The data above and below Tc are consistent with models for the structure of the interface adapted from the theory of Widom and Ramos-Gomez and Widom. As three-phase coexistence is approached, the quantity of methanol adsorbed at the interface increases whether ordinary or deuterated cyclohexane is used in the mixture. If ordinary cyclohexane were used for studies within the three-phase region below Tc, thick, unstable, lenticular films of the methanol-rich liquid phase would form at the interface and lead to nonreproducible data. In the present measurements lenticular films do not form when deuterated cyclohexane is used in the mixture. (The entire methanol-rich phase then forms on top.) The gravitationally stable interface above the deuterated mixture can be studied reliably in the three-phase region below Tc even though the liquid phases are nearly density matched.
Energy dissipation mechanism of the optically excited molecules in solvents: A trajectory study for a photoisomerization process of the π-conjugated molecule in Ar and water
Authors: Ohmine Iwao
Published: July 2002

The energy dissipation mechanism of an optically excited molecule in solution is studied by using a classical molecular dynamics (MD) calculation. We chose ethylene in Ar or water as a model system and perform the MD calculation to analyze the solvent response to the optically excited ethylene motions that are large in magnitude and high in frequency. It is found that the energy dissipation is very fast; it is in the order of a picosecond in water and of a few to a few tens of picoseconds in Ar. The energy decay rate strongly depends on each ethylene mode and on the nature of solvent–solvent interaction. Due to the characteristic form of water–water interaction, that is strong and sensitive to the mutual geometrical changes, a large water kinetic fluctuation occurs. The ethylene motions couple to this water fluctuation efficiently transferring the ethylene energy to the water libration energy, that is immediately distributed into the various inter- and intrawater modes. A multistep collision process leading the energy flow from the ethylene internal vibration→the ethylene rotation→the solvent molecule motion, not accounted for in the gas-like models such as isolated binary collision model, is a pathway for the fast energy dissipation in Ar and water. We also employ a simple model of an oscillator in Ar to make a detail analysis of the energy decay mechanism, especially of its dependence on the oscillator amplitude, the solvent–solvent interaction, and the solvent density.
Statistical moments for random two-phase suspensions: A computational framework
Authors: Gillette Gregory J. , McCoy John J.
Published: July 2002

A new method for decomposing multipoint correlation functions is presented for two-phase suspension materials. The decomposition isolates various aspects of the substructure geometry such as inclusion shape and relative sizes, orientations, and positions of the inclusions. The method can be systematically applied to correlation functions of any order, and it allows them to be calculated directly from a given suspension substructure geometry. First the material properties of the phases are separated from the substructure geometry and this latter is described in terms of the inclusion phase alone. Second the geometric probabilities associated with the inclusion phase are separated into the probabilities associated with the individual inclusions taken singly, and in groups of two or more. The single inclusion probability is given by a geometry problem which is to be solved analytically (or numerically) for a given inclusion shape and orientation. The double inclusion probability, triple inclusion probability, etc., separate into (1) a probability function which depends on inclusion shape, size, and orientation; its calculation is reduced to a geometry problem like that of the single inclusion probability, and (2) inclusion distribution functions which contain configuration statistics.
Nitrogen-broadened linewidths and strengths of nitric oxide utilizing tunable diode laser spectroscopy
Authors: Phillips W. J. , Walker H. C.
Published: July 2002

Nitrogen-broadened linewidths and line strengths of NO fundamental band lines were measured utilizing tunable diode laser spectroscopy. The spectral lines measured were the P(4.5), P(5.5), and P(6.5) line of the 1/2 subband. Spectra were taken on a 1.75% NO/N2 mixture in the temperature range of 300 to 900 K and for pressures of 200, 400, and 600 Torr. The line strengths obtained in this work were found to be lower than those determined by an earlier work, and linewidths were close to those of previously published nitrogen-broadened half-widths for NO.
On the tricritical point of the nematic–smectic A phase transition in liquid crystals
Authors: Longa Lech
Published: July 2002

Using the Landau expansion simple formulas are found that allow for systematic calculations of the tricritical temperature for a given molecular model. The theory is applied to some molecular models of the nematic–smectic A (NA) phase transition of liquid crystals. It is shown exactly that for a large class of molecular theories the NA tricritical temperature depends only on the couplings between the two lowest order translational order parameters and the orientational degrees of freedom. The relative importance of these couplings on tricritical behavior is studied in detail in the mean-field approximation. Numerical analysis is carried out for various molecular models of the smectic A phase including the McMillan model and its generalizations. It is shown that for an experimentally relevant class of models, the presence of second and higher order harmonics does not influence the tricritical temperature in an essential way, which means that the coupling between orientational and translational degrees of freedom is likely to be dominant in determining the character of the NA phase transition. Another interesting aspect of orientational-translational coupling is the possibility of the existence of the nematic–smectic A–reentrant nematic phase transition.
Double differential cross sections for K–Hg collisions in the 100 eV range. II. Particle-photon-coincidence measurements
Authors: Düren R. , Hübner H. , Kita S. , Krause U.
Published: July 2002

The angular dependence of cross sections with state analysis has been measured for the K–Hg interaction with collision energies of 100 and 200 eV. Two experiments have been performed: one with particle-photon coincidence to yield fine structure resolution of the K(42P) inelastic channel and one to resolve the P3/2 component with respect to the absolute value of mj.
Mechanism of phase transitions affecting intramolecular electron transfer in trinuclear mixed-valence transition-metal compounds
Authors: Kambara Takeshi , Hendrickson David N. , Sorai Michio , Oh Seung M.
Published: July 2002

Intramolecular electron delocalization in discrete mixed-valence transition metal complexes in the condensed phase depends not only on the electronic structure of a single complex but also sensitively on the details of the packing arrangement [D. N. Hendrickson, S. M. Oh, T.-Y. Dong, T. Kambara, M. J. Cohn, and M. F. Moore, Comments Inorg. Chem. 4, 329 (1985)]. The problem of how the cooperative properties of mixed-valence complexes in the solid state depend on the electron localization and/or delocalization in a single complex is studied theoretically. A phenomenological intermolecular interaction which depends on the sense and the magnitude of molecular distortion arising from the electron localization is introduced. A theoretical model is developed based on molecular field theory in order to show what types of phase transitions relating to the electron delocalization are possible in the trinuclear mixed-valence compounds and how the electronic structure of constituent molecules determines the type of phase transition. There are three types of phase transitions: (1) Order–disorder transition with respect to the alignment of the sense of molecular distortion associated with the electron localization; (2) static localization–delocalization transition, where the molecular distortion disappears above the transition temperature and electrons are coherently delocalized on three transition metal ions; (3) dynamical localization–delocalization transition in which the delocalization comes from fast electron transfer between three transition-metal ions and the molecular structure is changed from a static distortion to a dynamical distortion. The theoretical model is used to explain the observed temperature dependencies of heat capacity and Mössbauer spectra for the trinuclear mixed-valence complex

[Fe3O(O2CCH3)6(py)3](py), where (py) is pyridine [S. M. Oh. T. Kambara, D. N. Hendrickson, M. Sorai, K. Kaji, S. E. Woehler, and R. J. Wittebort, J. Am. Chem. Soc. 107, 5540 (1985); M. Sorai, K. Kaji, D. N. Hendrickson, and S. M. Oh, ibid. 108, 702 (1986)]. The first-order phase transition at ∼112 K is assigned as an order–disorder transition and the higher-order transition at ∼190 K is assigned as a dynamical localization–delocalization transition.
Perturbation approach to the computer simulation of dipolar fluids
Authors: Lado F.
Published: July 2002

The long range nature of the dipolar potential φ has made computer simulation of molecules with electric dipoles highly troublesome. We propose for such calculations a generalization of the Ceperley–Chester procedure for Coulomb fluids [Phys. Rev. A 15, 755 (1977)], separating φ into short range and long range parts, φSR and φLR. The reference system with φSR interaction can be studied using standard simulation methods for truly short range potentials. The corrections for φLR are then incorporated through solution of the reference-hypernetted chain equation. An illustrative calculation shows the correction procedure to be computationally reliable.
Molecular dynamics study of a two-dimensional system with screened Coulomb interactions
Authors: Cheng H. , Dutta P. , Ellis D. E. , Kalia R.
Published: July 2002

We have used molecular dynamics to study a two-dimensional system of point particles with screened Coulomb interactions, in a ‘‘box’’ whose size is allowed to vary. In addition to determining the familiar energy–temperature relations at both constant area and constant pressure, we have obtained pressure–area isotherms, diffusion constants, and pair-correlation functions. The results of our simulation are expected to be relevant to such systems as polar monolayers on the surface of water (Langmuir films) and thin colloidal suspensions. By monitoring the energy–temperature relation and the diffusion constant, we have observed first order solid–liquid transitions with hysteresis under both constant area and constant pressure conditions. However, the discontinuities in the pressure–area isotherms at the transition are almost imperceptible. Other thermodynamic relations such as area vs temperature at constant pressure, and the derivative of pressure vs area, are presented to support the analysis.