bluuebunny/crossref_metadata_2025 huggingface dataset:
Relative coherency strain and phase transformation history in epitaxial ferroelectric thin films
Authors: Foster C. M. , Pompe W. , Daykin A. C. , Speck J. S.
Published: July 2002

Experimental evidence is presented to verify the quantitative predictions of interfacial defect theory as applied to strain relief in epitaxial PbTiO3 thin films through the formation of 90° domains. Epitaxial PbTiO3 thin films grown by metal-organic chemical vapor deposition on MgO(001), SrTiO3(001), LaAlO3(001), and SrRuO3(001)/SrTiO3(001) substrates are examined using four-circle x-ray diffraction and transmission electron microscopy. The data represents a detailed examination of the ...c/a/c/a... 90° domain patterns that develop during the paraelectric to ferroelectric (PE→FE) phase transition as the film is cooled from the growth temperature. Three independent measurements of the relative coherency strain (er) are reported. The data quantitatively and self-consistently verify the crystallographic rotations predicted by the concept of the relative coherency strain and demonstrate the validity of domain stability maps in understanding the phase transformation history in epitaxial ferroelectric thin films.
Real-time, in situ infrared study of etching of Si(100) and (111) surfaces in dilute hydrofluoric acid solution
Authors: Niwano Michio , Miura Taka-aki , Kimura Yasuo , Tajima Ryo , Miyamoto Nobuo
Published: July 2002

The chemical nature of Si(100) and (111) surfaces during immersion in dilute hydrofluoric acid (HF) solution was investigated ‘‘in situ’’ and in real time using infrared absorption spectroscopy in the multiple internal reflection geometry. In dilute HF solution, the Si surface is not completely terminated with hydrogen, but may be covered in part with hydrogen-associated Si fluorides, such as SiH2(SiF) and SiH2F2. It is found that the hydrogen coverage of the surface depends on the HF concentration of the solution. At HF concentrations above 1%, the surface concentration of Si hydrides is reduced while that of Si fluorides is enhanced. We confirm that rinsing in water following HF immersion leads to complete hydrogen termination of the surface. Based on the present experimental results, we suggest that in dilute HF the Si surface is in chemical equilibrium with the solution to allow the coexistence of Si hydrides and Si fluorides on the surface.
Structure and magnetic moment of α″-Fe16N2 compound films: Effect of Co and H on phase formation (invited)
Authors: Takahashi Migaku , Takahashi H. , Nashi H. , Shoji H. , Wakiyama T. , Kuwabara M.
Published: July 2002

In order to determine the intrinsic value of magnetic moment, σs, of α″-Fe16N2 compound, the magnetic moment of (α″+α′)-Fe16N2 films is discussed in connection with the degree of N site ordering in nitrogen–martensite. To establish the more general relation between Fe–N martensite structure and σs, the Fe–N system is expanded to Fe–Co–N and Fe–H–N systems. As a result, it is found that (1) The existence of completely the same structure as bulk α″ phase was reconfirmed even in the film form by the precise structural analysis using x-ray diffraction, transmission electron microscopy and Mössbauer analysis, (2) Fe–H martensite was synthesized by using sputtering under Ar+H2 atmosphere, (3) Stable formation of α″-(Fe100−XCoX)16N2 (X=10–30) phase through N site ordering by postannealing is proposed to be fairly difficult due to the sudden decrement of the phase decomposition temperature, Tp.d., of α′-(Fe100−XCoX)–N (X=0–30). Tp.d. of α′-(Fe100−XCoX)–N (X=0–30) phase decreases from 200 °C to RT with increasing Co and N contents, and (4) The intrinsic value of saturation magnetization of the α″-Fe16N2 phase is convinced to be no more than 240 emu/g (≊2.4 μB per Fe atom on average) at 300 K.
Iron gettering mechanisms in silicon
Authors: Benton J. L. , Stolk P. A. , Eaglesham D. J. , Jacobson D. C. , Cheng J.-Y. , Poate J. M. , Ha N. T. , Haynes T. E. , Myers S. M.
Published: July 2002

Boron implantation into silicon offers a unique system for studying the gettering mechanisms of Fe. Using deep level transient spectroscopy to monitor the remaining Fe in the gettered region and secondary-ion-mass spectroscopy to measure the concentration of Fe redistributed to the B region, we show that the gettering mechanisms can be quantitatively described. A combination of Fermi-level-induced Fe+ charge-state stabilization and Fe+–B− pairing acts to lower the free energy of Fe in p+ regions. This can lead to Fe partition coefficients as high as 106 at a p+/p interface at temperatures below ≊400 °C. The dynamic response of the system is diffusion limited during the cooling cycle. B gettering is more effective than gettering produced by Si implantation damage and more effective than trapping by a neutral impurity such as C. These mechanisms also make a large contribution to the effective gettering of Fe by p/p+ epitaxial silicon wafers. The Fermi-level/pairing gettering mechanism is also expected to operate for Cr and Mn.
Stacking faults in BaTiO3 particles synthesized from organic precursor
Authors: Cho Woo-Seok , Hamada Etsuo , Takayanagi Kunio
Published: July 2002

BaTiO 3 powder has been synthesized by heat treating (BaTi) citrate polyester resins at 500 °C for 8 h. X-ray powder diffractometry (XRD) and Raman spectroscopy data suggest the presence of a high-temperature hexagonal phase at room temperature. However, high-resolution transmission electron microscopy indicates that the peaks that were ascribed to the above phase in the Raman and XRD spectra may originate from stacking faults in the particles. Our results disprove the existence of the high-temperature hexagonal BaTiO3 at room temperature.
Influence of barrier impurities on the magnetoresistance in ferromagnetic tunnel junctions
Authors: Jansen R. , Moodera J. S.
Published: July 2002

The effect of barrier impurity atoms on the magnetoresistance of ferromagnetic tunnel junctions has been investigated. For that purpose, Co/Al2O3/Ni80Fe20 junctions were prepared with submonolayer amounts of Co, Pd, Cu, or Ni incorporated into the middle of the insulating oxide. The junction magnetoresistance (JMR) was measured at 77 and 300 K and referenced to that of simultaneously prepared control junctions without impurities. The JMR decays approximately linearly with increasing impurity content, the slope depending sensitively on the type of element. The decrease is most pronounced for Ni, with the normalized JMR going down at a rate of 0.39 Å−1 of material (at 77 K), whereas Co shows a relatively weak decay of only 0.08 Å−1. Pd and Cu represent intermediate cases. At 300 K, the suppression of JMR is slightly higher. Results are interpreted in terms of spin-flip scattering of tunneling electrons by the impurities.
Finite element analysis of the influence of a fatigue crack on magnetic properties of steel
Authors: Shi Y. , Jiles D. C.
Published: July 2002

Fatigue can affect the magnetic properties of materials due to microstructural changes. Previous investigations have shown that several structure sensitive magnetic properties, such as coercivity Hc and remanence Br, changed systematically as a result of fatigue. When approaching failure the accumulated changes in microstructure resulted in the occurrence of fatigue cracks and the magnetic properties showed dramatic changes which mainly resulted from the geometrical changes in samples due to the cracks. It was found that the remanence Br followed the changes in stress, while the coercivity Hc sometimes showed different trends. In this article the influence of the size and the position of a fatigue crack on magnetic field and magnetic induction were studied using finite element modeling. Models were constructed to simulate the geometry of the test sample and sensor. It was found that, for a given coil current in the exciting coil, the magnetic induction was mainly determined by the geometry of the crack, while the magnetic field was influenced by both the size and the position of the crack.
High conductance magnetoresistive tunnel junctions with multiply oxidized barrier
Authors: Wong P. K. , Evetts J. E. , Blamire M. G.
Published: July 2002

We have fabricated 8×8 μm2 mesa junctions from ultrahigh vacuum magnetron sputter-deposited Nb/Fe/Al2O3/Al/CoFe/Nb heterostructures. The Al2O3 barrier was prepared by the technique of multiple oxidation. The effect of multiple oxidation on the junction magnetoresistance (JMR) was investigated. JMR up to 6.2% at room temperature and 9.2% at 77 K was obtained. Junctions with the highest JMR have resistance-area products in the range required for device applications.
Upconversion mechanism in Er3+-doped fluorozirconate glasses under 800 nm excitation
Authors: Tsuda M. , Soga K. , Inoue H. , Inoue S. , Makishima A.
Published: July 2002

The upconversion emission intensities of Er3+ ion around 550 and 660 nm in fluorozirconate glasses were measured under 800 nm excitation. Though energy transfer processes played an important role in upconversion mechanism at high concentration of ErF3, those have not been treated quantitatively. The energy transfer rates were calculated from the optical parameters assuming some distribution of Er3+ ions. We calculated the upconversion intensities around 550 and 660 nm by using rate equations. It was found that the dependence of upconversion emission intensities on the ErF3 concentration could be reproducible and the principal upconversion mechanism could be evaluated.
Molecular dynamics studies of silica wafer bonding
Authors: Timpel Dirk , Schaible Max , Scheerschmidt Kurt
Published: July 2002

Molecular dynamics simulations are performed to investigate the atomic processes initiated by the adhesion of two silica surfaces, which are covered with adsorbates of oxygen, hydrogen or water molecules. The calculations describe the mechanism of hydrophilic silicon wafer bonding in terms of empirical potentials assumed. The challenge of the macroscopically relevant computations is to understand and to predict the formation of covalent bonds as a function of initial silica structures, external forces, adsorbates, and annealing temperatures applied.
Magnetic properties of RCo3B2 (R=rare earth) compounds
Authors: Ido H. , Nashima O. , Ito T. , Kaneda T. , Saito Y. , Konno K. , Yoshida H. , Motokawa M.
Published: July 2002

In order to investigate the crystalline electric field at R (R=rare earth) site and the induced magnetic moment at Co site in RCo3B2 compounds, high field magnetization process up to 15 T and paramagnetic susceptibility have been measured at various temperatures. It is found by analysis of the experimental data that (1) the crystal field parameter A20 is very large (∼−2000Ka0−2) and (2) significant magnetic moment (0.15–0.93μB) is induced at the Co site, in RCo3B2 compounds.
Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures
Authors: Wei J. Y. T. , Yeh N.-C. , Fu C. C. , Vasquez R. P.
Published: July 2002

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-polarized quasiparticles.
Spectra and energy levels of trivalent samarium in strontium fluorapatite
Authors: Gruber John B. , Zandi Bahram , Ferry Michael , Merkle Larry D.
Published: July 2002

Absorption and fluorescence spectra are reported for Sm3+(4f5) as a dopant in crystals of strontium fluorapatite, Sr5(PO4)3F. Spectra were obtained at sample temperatures near 10, 100 K, and room temperature. The absorption spectra covered the wavelength range between 2840 and 520 nm and were analyzed for the detailed crystal-field splitting of the 6HJ, 6FJ, 4G(4)5/2, and 4F(3)3/2 multiplet manifolds of Sm3+(4f5). Fluorescence spectra were observed from the 4G(4)5/2 to the 6H5/2 (ground state), 6H7/2, 6H9/2, and part of the 6H11/2 manifolds. The strongest fluorescence is observed to the 6H9/2 manifold. The distribution of the fluorescence intensities is similar to that observed for the Sm3+-doped glass laser. A detailed crystal-field splitting calculation was carried out for the 14 multiplet manifolds of Sm3+ that are lowest in energy. A rms deviation of 8 cm−1 was obtained between 49 experimental and calculated energy Stark levels. The calculation also predicts fluorescence branching ratios and the 4G(4)5/2 radiative lifetime, in reasonable agreement with our measured values.
Measurements of the electron energy distribution function in molecular gases in an inductively coupled plasma
Authors: Singh Harmeet , Graves David B.
Published: July 2002

A tuned, cylindrical Langmuir probe has been used to measure the electron energy distribution function (EEDF) in atomic and molecular gases in an inductively coupled plasma. We have discussed the precautions necessary for making Langmuir probe measurements in fluorocarbon plasmas. The ionic and neutral composition of the plasma is measured using mass spectrometry. While the EEDFs in argon are non-Maxwellian, the EEDFs in molecular gases are found to be approximately Maxwellian at low pressures (<20 mTorr) in the gases studied (N2, O2, CF4). The EEDFs in argon–molecular gas mixtures change from Maxwellian to two-temperature distributions, as the fraction of argon is increased in the plasma. At higher pressures, the molecular gases exhibit EEDFs reflecting the electron collision cross sections of these gases. In particular, N2 plasmas show a “hole” in the EEDF near 3 eV due to the resonant vibrational collisions. O2 plasmas show a three-temperature structure, with a low-energy high-temperature electron group, a low-temperature intermediate-energy electron group, and a high-temperature high-energy tail. The fractional degree of dissociation in the N2 and O2 plasmas is below 0.1, with the parent molecules and molecular ions being the dominant species. The spatial variation of the EEDF in an oxygen plasma at low pressures (10–20 mTorr) is found to be consistent with the nonlocal theory.
Interface chemistry and structure of multiply oxidized barrier spin-tunnel junctions
Authors: Ozkaya D. , Dunin-Borkowski R. E. , Petford-Long A. K. , Wong P. K. , Blamire M. G.
Published: July 2002

Nb/Fe/Al 2 O 3 /CoFe/Nb spin-tunnel junctions fabricated using a multiple oxidation technique have been characterized using high spatial resolution electron microscopy techniques. Junction magnetoresistance values up to 6.2% at room temperature and 9.2% at 77 K were obtained for junctions fabricated using this technique. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy were used to study the chemistry and interface structure of the barrier layer; elemental mapping showed the degree of chemical homogeneity across the layers and high spatial resolution electron energy loss spectroscopy revealed changes in the oxidation state and d-shell occupancies of Fe and Co across the layers, which need to be considered when modelling the spin-tunneling effect. Pinholes across the barrier were also observed by high resolution electron microscopy.
Diode laser absorption spectroscopy of D3O+: Determination of the equilibrium structure and potential function of the oxonium ion
Authors: Sears Trevor J. , Bunker P. R. , Davies P. B. , Johnson S. A. , S̆pirko V.
Published: July 2002

High resolution measurements of the two components of the ν2 (umbrella) fundamental mode of the fully deuterated oxonium ion (D3O+) are reported. The spectra were obtained by diode laser absorption in a plasma containing D2 and D2O. Analysis of the observed spectra yields precise estimates of the two band origins and molecular parameters describing the states involved. By combining these data with the available high resolution data for H3O+, using the nonrigid invertor Hamiltonian, we are able to determine the equilibrium structure and the vibrational potential function; also we predict the inversion spectrum of D3O+ and the 2v2–v2 hot band spectra of H3O+ and D3O+.
Simulation of the dynamics of electron transfer reactions in polar solvents: Semiclassical trajectories and dispersed polaron approaches
Authors: Warshel Arieh , Hwang Jenn-Kang
Published: July 2002

Simulation methods for exploring the microscopic aspects of electron transfer (ET) reactions are developed. For the high temperature limit, where anharmonicity effects are crucial, we develop a semiclassical trajectory (ST) method. This method treats the reaction by considering the time dependence of the electronic energy gap along the classical trajectories of the solvent molecules. The ST approach, which appears as an ad hoc approach, might be at present the most rigorous practical models for simulating ET reactions. That is, this method reproduces the results of the quantum mechanical harmonic test case in the high temperature statistical limit. More importantly, for anharmonic systems it provides a rate constant which depends on the proper activation free energy; this dependence cannot be evaluated by any of the current quantum mechanical methods. For the low temperature range we develop a ‘‘dispersed polaron’’ model that uses the Fourier transform of the microscopic energy gap to evaluate the harmonic Franck–Condon factors of the system. These Franck–Condon factors are then used to evaluate the quantum mechanical harmonic rate constant. The potential of our approach is outlined by considering its implementation in studies of several key problems. This includes: (i) Studies of the effect of solvent dynamics and the role of dielectric relaxation times. (ii) Studies of the actual microscopic activation free energy. (iii) Exploration of the role of vibronic channels in the inverted region. (iv) Transition between the adiabatic and diabatic limits.
Ultraslow molecular motions in crystalline polyoxymethylene. A complete elucidation using two-dimensional solid state NMR
Authors: Kentgens A. P. M. , de Boer E. , Veeman W. S.
Published: July 2002

The slow magic angle spinning 2D exchange NMR experiment applied to semicrystalline polyoxymethylene shows the existence of ultraslow molecular rotations in crystalline regions of the material. By simulating the two-dimensional spinning sideband spectra, which are characteristic for the molecular motion involved, it is found that a rotation of the polyoxymethylene helical chains over 200° is responsible for the observed spectra. The activation energy is 20 kcal/mol in good agreement with values obtained for the α relaxation in dynamic mechanical and dielectric studies.
Effect of metal ion perturbers on the triplet state of naphthalene in naphthalene crown ether–metal ion complexes. II. Stark effect
Authors: Ghosh Sanjib , Petrin Michael , Maki August H. , Sousa Lynn R.
Published: July 2002

The lowest triplet state energy and the zero-field splitting parameters ‖D‖ and ‖E‖ have been determined for 1,5-naphtho-22-crown-6, 1,8-naphtho-21-crown-6, 2,3-naphtho-20-crown-6, and 2,3-naphtho-17-crown-5, and their complexes with various metal ions (Na+, K+, Cs+, Ba+2, Hg+2, Ag+, Tl+) by low temperature phosphorescence and optical detection of magnetic resonance (ODMR) spectroscopy in zero applied magnetic field. The results have been interpreted in terms of the Stark effect arising from the metal ions bound to the crown cavity. The maximum Stark effect on the triplet state energy and on the ZFS parameters is observed when the perturber ions are located along the in-plane long axis. A Stark effect is not observed when the ions are placed along the out-of-plane axis.
Core–core and core–valence correlation
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Taylor Peter R.
Published: July 2002

The effect of 1s core correlation on properties and energy separations has been analyzed using full configuration-interaction (FCI) calculations. The Be 1S–1P, the C 3P–5S, and CH+ 1Σ+–1Π separations, and CH+ spectroscopic constants, dipole moment and 1Σ+–1Π transition dipole moment have been studied. The results of the FCI calculations are compared to those obtained using approximate methods. In addition, the generation of atomic natural orbital (ANO) basis sets, as a method for contracting a primitive basis set for both valence and core correlation, is discussed. When both core–core and core–valence correlation are included in the calculation, no suitable truncated CI approach consistently reproduces the FCI, and contraction of the basis set is very difficult. If the (nearly constant) core–core correlation is eliminated, and only the core–valence correlation is included, CASSCF/MRCI approaches reproduce the FCI results and basis set contraction is significantly easier.
Hybrid mechanics: A combination of classical and quantum mechanics
Authors: Huber Daniel , Heller Eric J.
Published: July 2002

Because classical mechanics is so much easier to handle than quantum mechanics, the time evolution of wave functions for molecular dynamics is often calculated using semiclassical methods. The errors of such methods grow, in general, faster than linearly with time, although they may be quite small for small, but finite times. We therefore propose to use a semiclassical method to calculate the quantum mechanical time propagator for a finite time step (say 1/10 of a vibrational period) and to use this propagator and quantum mechanics for longer times. To describe the quantum time propagator we use a basis set that can describe regions in phase space that are not necessarily rectangular, but can have any shape, that will become important in applications to higher dimensions. We give numerical examples to demonstrate the accuracy of the method.
Microwave spectrum and molecular structure of the N2–H2O complex
Authors: Leung Helen O. , Marshall Mark D. , Suenram R. D. , Lovas F. J.
Published: July 2002

The a-type, K=0 microwave spectrum of the N2–H2O complex has been observed using a pulsed molecular beam Fabry–Perot cavity microwave spectrometer. Seven isotopic species have been studied in the range of 5–23 GHz.The N2–H2O complex exhibits tunneling motions similar to the 1→2 tunneling motion of the H2O–DOD complex which gives rise to four components for each rotational transition. The molecular constants obtained for the ground tunneling (A1) state of 14N2–HOH are: B̄=2906.9252(2) MHz, DJ =0.043 486(15) MHz, and eQq(14N)=−4.253(2) MHz. The structure has a nearly linear N–N–HO geometry with a N–H distance of 2.42(4) Å and an OHN angle of 169° [RO–N=3.37(4) Å]. The electric dipole moment along the a principal axis of inertia was determined for the 15N2–HOH species with μa =0.833(3) D.
The crystal–liquid interface of a body-centered-cubic-forming substance: Computer simulations of the r−6 potential
Authors: Laird Brian B. , Haymet A. D. J.
Published: July 2002

The interfaces between a bcc crystal and its melt are studied by molecular dynamics simulation. Three distinct crystal/melt interfaces, (100), (111), and (110) are studied. For all interfaces the variation with z, the coordinate perpendicular to the interfacial plane, of the single particle density (averaged over the directions perpendicular to z) and the diffusion constant are measured. Although the 10–90 widths of the density peak-height profiles differ significantly among the three interfaces (6, 9, and 7 molecular diameters, respectively), the corresponding 10–90 widths of the diffusion constant profiles are nearly identical with a common value of about four molecular diameters. This leads to the conclusion that the differences in apparent structural width are due primarily to geometric considerations and not to differences in average local molecular environments.
The torsional–rotational spectrum and structure of the formaldehyde dimer
Authors: Lovas F. J. , Suenram R. D. , Coudert L. H. , Blake Thomas A. , Grant Kimberley J. , Novick Stewart E.
Published: July 2002

The microwave spectra of (H2CO)2 and (D2CO)2 have been observed with a pulsed beam, Fabry–Perot cavity, Fourier transform microwave spectrometer. Both species exhibit a-type spectra which are split by internal rotation of each monomer unit and an interchange of donor–acceptor bonding roles analogous to the water dimer. Rotational analysis of each spectrum provides the constants A=18583.(54) MHz, 1/2 (B+C)=3272.105(34) MHz, and B−C=503.92(17) MHz for (H2 CO)2 and A=14 862.1(35) MHz, 1/2 (B+C)=3030.2366(37) MHz, and B−C=490.977(18) MHz for (D2CO)2. Stark effect measurements result in derived electric dipole components μa =0.858(4) D and μb=0.027(10) D for (H2 CO)2 and μa =0.908(4) D and μb=0.095(4) D for (D2CO)2. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units perpendicular in a nearly antiparallel orientation of the CO groups with a center-of-mass distance of 3.046(17) Å. The shortest carbon to oxygen distance (2.98 Å) and hydrogen to oxygen distance (2.18 Å) between the monomer units are indicative of a dual bond interaction to form a ring structure.
The effect of steady flow fields on the isotropic–nematic phase transition of rigid rod-like polymers
Authors: See Howard , Doi Masao , Larson Ronald
Published: July 2002

In this paper we study the stability of steady state solutions to Doi’s kinetic equation for rigid rod-like polymers in an arbitrary flow field. It is found that for a certain range of concentration, the addition of flow to an isotropic phase can induce a transition to the nematic phase, if the flow rate exceeds a certain critical value. This critical flow rate decreases as concentration is increased. For phase transitions occurring in weak flow, exact solutions to this problem are calculated. Unlike the ‘‘free energy’’ approach, these calculations are not restricted to potential flows.
Asymptotics of the correlation functions for fluids
Authors: Martynov G. A. , Sarkisov G. N.
Published: July 2002

An equation based on the Ornstein–Zernike theory is suggested to describe the asymptotic behavior of the two-particle distribution function. The equation is shown to be the general one. It is possible to study the long-range part of the correlation function for any interparticle interaction potential as well as for any closure of the Ornstein–Zernike equation. Calculations are performed for a hard-sphere particle system for hypernetted chain, Percus–Yevick, and Martynov–Sarkisov approximate integral equations.
X-ray, ultraviolet, and synchrotron radiation excited inner-valence photoelectron spectra of CH4
Authors: Göthe M. Carlsson , Wannberg B. , Karlsson L. , Svensson S. , Baltzer P. , Chau F. T , Adam M-Y.
Published: July 2002

The inner-valence electron states of the methane molecule have been studied by means of x-ray, synchrotron radiation, and UV-photoelectron spectroscopy. Five correlation satellites have been identified and a detailed study has been carried out of the 2a−11 single hole state. For this state a Franck–Condon analysis has been performed, suggesting an equilibrium bond distance of 1.279 Å. The vibrational lines have a Lorentzian shape and the linewidth increases gradually with the vibrational quantum number. This probably indicates a reduction of the lifetime of the vibrational states due to predissociation. A discussion of the potential curves related to the correlation satellites is included.
Photon echoes of polyatomic molecules in condensed phases
Authors: Yan Yi Jing , Mukamel Shaul
Published: July 2002

A theory of optical echo spectroscopies of large polyatomic molecules in condensed phases is developed. Using phase space correlation functions, we examine the interrelationships among the following optical measurements: ordinary photon echo, stimulated photon echo, accumulated photon echo, incoherent accumulated photon echo, and pump–probe absorption. Conditions for the elimination of inhomogeneous broadening in these experiments are specified. A multimode Brownian oscillator model is used to account for high frequency molecular vibrations, as well as solvent modes, and electronic dephasing processes. The effects of quantum beats, spectral diffusion, and homogeneous dephasing on the echo signals are studied and compared in detail with pump–probe and hole burning spectroscopy.
The structure of water and methanol in p-dioxane as determined by microwave dielectric spectroscopy
Authors: Mashimo Satoru , Miura Nobuhiro , Umehara Toshihiro , Yagihara Shin , Higasi Keniti
Published: July 2002

Dielectric measurements were performed on water–p-dioxane and methanol–p-dioxane mixtures using time domain reflectometry over the frequency range 0.1–10 GHz. In the case of water–p-dioxane mixtures, the relaxation strength normalized by the number of water molecules per unit volume is independent of the molar fraction of water xW if xW<0.83. There are no ordered micellelike clusters in the mixture. On the other hand, if xW is larger than 0.83, the normalized strength increases linearly with xW. The clusters of pure water which appear in this region are cyclic and consist of six molecules. In the case of methanol–p-dioxane, the normalized strength is independent of xM for xM<0.66 and increases linearly with xM for xM>0.66. However, the relaxation time of pure methanol is too large for clusters consisting of three molecules. It is suggested that the chainlike clusters form network structures.
Grafted polymer layers under shear: A Monte Carlo simulation
Authors: Lai Pik-Yin , Binder Kurt
Published: July 2002

Endgrafted polymers at surfaces exposed to a shear flow are modeled by a nonequilibrium Monte Carlo method where the jump rate of effective monomers to neighboring lattice sites against the flow direction is smaller than in the flow direction, assuming that this difference in jump rates is proportional to the local velocity of the flowing fluid. In the dilute case of isolated chains, the velocity profile is assumed linearly increasing with the distance from the surface, while for the case of polymer brushes the screening of the velocity field is calculated using a parabolic density profile for the brush whose height is determined self-consistently. Linear dimensions of isolated chains are obtained over a wide range of shear rates γ̇, and the deformation of the coil structure by the shear is studied in detail. For brushes it is found that the density profile and the distribution for the perpendicular coordinate zN of the free ends differ only little from the shear-free case, while the distribution of the free end coordinate xN parallel to the wall in the flow direction gets strongly modified. It is shown that the average scaled chain trajectory (〈zi〉/〈zN〉 as a function of 〈xi〉/〈xN〉, i labels the monomers along the chain) is a universal function independent of shear rate, while 〈xN〉 depends on γ̇, chain length N and grafting density σ in scale form, 〈xN〉/(σ1/3N3γ̇) is a function of N2σ5/3 only. Our results are compared with the recent theories of Rabin and Alexander or Barrat, respectively, and both similarities and differences are noted and discussed. The observed increase of the coil radius with the inclination of the chain produced by the flow is somewhat smaller in our model than it was in those theories.
Liquid–vapor interface of water–methanol mixture. II. A simple lattice-gas model
Authors: Matsumoto Mitsuhiro , Mizukuchi Hiroshi , Kataoka Yosuke
Published: July 2002

A simple lattice-gas model with a mean field approximation is presented to investigate qualitative features of liquid–vapor interface of water–methanol mixtures. The hydrophobicity of methanol molecules is incorporated by introducing anisotropic interactions. A rigorous framework to treat such anisotropy in a lattice-gas mixture model is described. The model is mathematically equivalent to an interfacial system of a diluted antiferro Ising spin system. Results of density profiles, orientational ordering near the surface, and surface excess thermodynamic quantities are compared with results of computer simulation based on a more realistic model.
Adsorption of hydrated halide ions on charged electrodes. Molecular dynamics simulation
Authors: Glosli James N. , Philpott Michael R.
Published: July 2002

Constant temperature molecular dynamics has been used to simulate the adsorption of hydrated halide ions X−=F−, Cl−, Br− and I−, and lithium ion Li+ on flat uniformly charged surfaces. The simulations were done with either 214 water molecules and two ions (Li+ and X−) in a box 2.362 nm deep or with 430 water molecules and the two ions in a box 4.320 nm deep. The boxes were periodically replicated in the xy directions. The magnitude of the surface charge on the box ends was ±0.11 e/(nm)2, corresponding to an electric field of 2×107 V/cm. The lateral dimensions of the simulation cell were 1.862 nm×1.862 nm (x×y) in each case. All of the water molecules and ions interacted with the end walls via a weak 9-3 potential. The Stillinger ST2 water model and parameters optimized for alkali halides interacting with the model ST2 water molecule were used in the calculations. Common particles of truncating the interactions at a finite distance (0.82 nm) and switching off Coulomb interactions at small distances were followed. The temperature was set at T=2.411 kJ/mol (290 K). Some of the properties calculated were distribution density profiles for ions and water across the gap important for comparisons with Gouy–Chapman theory, adsorbed ion–water pair correlation functions, and the number of water molecules in the first and second hydration shells of the ions as a function of time. The time spent by a water molecule in the hydration shell was calculated to be approximately ten times longer for lithium than any other ion.

The correlation between distance from the electrode and hydration number was studied and generally found to be pronounced for the larger anions. Comparison of the dynamics of the common ion Li+ for different anions revealed the subtle influence of a transcell interaction in the 2.362 nm thick film. In the given field, the smallest ions Li+ and F− remained fully solvated at all times. Chloride behaved quite differently. Part of the time this ion was far enough away from the electrode to be fully hydrated and part of the time it was in physical contact (i.e., physisorbed) on the electrode with no water molecules interposed between it and the electrode. Bromide favored contact adsorption over full hydration most of the time. Iodide was observed to be contact adsorbed almost all of the time. These simulations provide new insights on the behavior of strongly hydrated ions at surfaces and how the transition from noncontact to ‘‘contact’’ adsorption occurs.
New fundamental equations of thermodynamics for systems in chemical equilibrium at a specified partial pressure of a reactant and the standard transformed formation properties of reactants
Authors: Alberty Robert A. , Oppenheim Irwin
Published: July 2002

When temperature, pressure, and the partial pressure of a reactant are fixed, the criterion of chemical equilibrium can be expressed in terms of the transformed Gibbs energy G′ that is obtained by using a Legendre transform involving the chemical potential of the reactant that is fixed. For reactions of ideal gases, the most natural variables to use in the fundamental equation are T, P′, and PB , where P′ is the partial pressure of the reactants other than the one that is fixed and PB is the partial pressure of the reactant that is fixed. The fundamental equation for G′ yields the expression for the transformed entropy S′, and a transformed enthalpy can be defined by the additional Legendre transform H′=G′+TS′. This leads to an additional form of the fundamental equation. The calculation of transformed thermodynamic properties and equilibrium compositions is discussed for a simple system and for a general multireaction system. The change, in a reaction, of the binding of the reactant that is at a specified pressure can be calculated using one of the six Maxwell equations of the fundamental equation in G′.
Universality in surface catalysis with diffusion
Authors: Kaya H. , Erzan A. , Kadirgan F.
Published: July 2002

An irreversible surface reaction with diffusion is studied by Monte Carlo simulation. The exponents at the poisoning transition are found to be the same as their values in the absence of diffusion on the surface. The phase diagram is not altered. The average poisoning time for either species, as a function of their relative concentration x, diverges as ‖0.5−x‖−γ, with γ=0.9±0.1. The surface coverage by either species grows linearly for small times; for much longer times the majority species saturates as 1−exp(−const⋅t). The power spectrum of the fluctuations is Gaussian at the transition point.
Theory of transient anisotropy in molecular photodissociation
Authors: Waldeck Janet R. , Shapiro Moshe , Bersohn Richard
Published: July 2002

In this paper, we examine how vector properties of photodissociation with a coherent laser pulse evolve in time during and after the excitation process. In particular, we define and explore the transient anisotropy parameter β(t). We apply our formulation to the case of the predissociation of the NaI molecule. We predict that under certain conditions, β(t) displays a beating phenomenon, in which the system oscillates periodically between a parallel-type and a perpendicular-type angular distribution. Depending on the excitation pulse, this ‘‘vectorial’’ beating may be accompanied by a faster beating similar to the ‘‘scalar’’ vibrational beating observed in the past in the NaI system. It is shown that the transient anisotropy beating may be used to obtain experimentally the time-dependent average interfragment separation, and that different J states are characterized by different interfragment separation curves.
Algebraic variational and propagation formalisms for quantal dynamics calculations of electronic-to-vibrational, rotational energy transfer and application to the quenching of the 3p state of sodium by hydrogen molecules
Authors: Tawa Gregory J. , Mielke Steven L. , Truhlar Donald G. , Schwenke David W.
Published: July 2002

Two approaches, the outgoing wave variational principle (OWVP) and R-matrix propagation (RMProp), are presented for quantum dynamics calculations of inelastic scattering in systems involving two coupled potential energy surfaces (PES). The two formalisms are applied to Na(3p 2P) collisions with H2 at a total energy of 2.426 eV with zero and unit total angular momentum. This provides a challenging test case because the accessible region of the excited-state potential energy surface intersects the ground-state surface conically and involves H–H internuclear distances that are far larger than their equilibrium values in the ground state. We present the details of the formalisms for treating coupled surfaces, and we present converged results for the quenching probability and final vibrational–rotational quantum state distributions of the quenching agent. Convergence of the transition probabilities is established by convergence checks within each formalism, by obtaining the same results with laboratory-frame and body-frame basis functions in the OWVP formalism, and by obtaining the same results with the OWVP as with RMProp.
Property evaluation using the Hartree–Fock-density-functional-theory method: An efficient formalism for first- and second-order properties
Authors: Sekino Hideo , Oliphant Nevin , Bartlett Rodney J.
Published: July 2002

We present an efficient formalism for property evaluation using the Hartree–Fock-density- functional-theory method. The formalism uses the relaxed density concept for first-order properties which allows us to compute many different components of a property as well as many different properties once we have solved a single set of linear equations spanning the particle–hole space of the system. The density matrix representation of the method indicates that the method accounts for the correlation correction to Hartree–Fock only through particle–hole space. We also show why conventional density functionals based upon a local density and its gradient fail to account for electromagnetic effects. For second-order properties, we show that no extra linear equations need to be solved, once the regular coupled perturbed Hartree–Fock equations are solved.
Laboratory investigations of gaseous sulfuric acid formation via SO3+H2O+M→H2SO4+M: Measurement of the rate constant and product identification
Authors: Reiner Thomas , Arnold Frank
Published: July 2002

We have measured the reaction SO3+H2O+M→H2SO4+M in a flow tube reactor in the pressure range between 31 and 260 mbar of synthetic air carrier gas. SO3 has been produced in the flow tube using a radical source for production of OH, which was converted to SO3 by reaction with SO2. The concentration of SO3 as well as the concentration of the reaction product has been measured by chemical ionization mass spectrometry (CIMS). Evidence has been found for the product being H2SO4. With increasing H2O-concentration in the flow reactor, an exponential decrease of the concentration of SO3 accompanied by a corresponding rise of the H2SO4-concentration has been observed. A detailed flow analysis has been carried out and the rate constants have been corrected for diffusive effects. The effect of heterogeneous reactions on the flow reactor walls has been investigated and could partially be quantified. From the experiment, values for the reaction rate constant in the range between 0.72 and 1.39×10−15 cm3 s−1 have been inferred. These values are thought to be due to gas phase reactions only. Within the experimental accuracy the reaction rate constant did not undergo a significant pressure variation. The mean rate constant averaged over the total pressure range is (1.2±0.2)×10−15 cm3 s−1.
Is slow thermal isomerization in viscous solvents understandable with the idea of frequency dependent friction?
Authors: Sumi Hitoshi , Asano Tsutomu
Published: July 2002

Thermal Z/E isomerization of substituted azobenzenes and N-benzylideneanilines takes place slowly after fast photoinduced E/Z isomerization. Its rate constant kobs is smaller than about 103 s−1 because of a high reaction barrier of about 50 kJ/mol. The pressure dependence of kobs measured in solvents as glycerol triacetate can well be understood in the framework of the transition state theory (TST) at low pressures. At high pressures, however, kobs begins to steeply decrease as the pressure increases, to be more exact, as the solvent viscosity η increases with the pressure, and the reaction enters the non-TST regime. Since the η-induced decrease of kobs at high pressures is slower than η−1, it cannot be described by the Kramers theory which regards the reaction as the barrier surmounting by Brownian motions regulated by frequency independent friction. Next, it was adjusted to the Grote–Hynes theory incorporating the idea of frequency dependent friction. The situation of kobs mentioned earlier enabled us to derive, without adjustable parameters, the correlation time τsc among random forces for friction due to solvent microscopic motions in the generalized Langevin equation on which the theory is based. At η∼107 Pa s, we obtained τsc∼1 ms. It is too long to justify the theory, since such a long-time correlation cannot be realized among random forces exerting on the isomerizing moiety with an angstrom dimension. It will also be shown that τsc must be so long unphysically as to be at least much longer than 1 ps even if kobs at low pressures is adjusted to the theory.
Vibrational relaxation of CO (v=1) by inelastic collisions with 3He and 4He
Authors: Reid J. P. , Simpson C. J. S. M. , Quiney H. M. , Hutson J. M.
Published: July 2002

Calculations of the vibrational relaxation rate constants of the CO–3He and CO–4He systems are extended to lower temperatures than in any previous calculation and a comparison made with new experimental results in the temperature range 35–295 K for CO–3He and previously published results in the range 35–2300 K for CO–4He. Both the coupled states (CS) and infinite-order sudden (IOS) approximations are used, with the self-consistent-field configuration interaction CO–He interaction potential of Diercksen and co-workers. The CS approximation is found to give a similar level of agreement with experiment for the two isotopic species, while the performance of the IOS approximation is system dependent. The discrepancy between experimental and theoretical IOS rate constants is quite different for collisions involving 3He and 4He, so that it is not profitable to compare IOS results directly with experiment for these two systems at temperatures below 300 K. The differences between the measured and the CS calculated rate constants for both the CO–4He and CO–3He systems are thought to be due predominantly to inaccuracies in the interaction potential. Relaxation rate constants for CO target molecules in collision with HD, D2 and H2 are compared with results involving 3He, 4He, and ‘‘2He,’’ revealing some systematic trends depending only on mass. However, for all hydrogen species there are marked upturns in the rate constants at low temperatures relative to those for helium atoms, while the rate constants for HD are greater than those for 3He throughout the temperature range. Calculations at small initial kinetic energies for the CO–He systems reveal an unexpected increase in relaxation cross section with reduction in kinetic energy. This implies that at very low temperatures the CO–He rate constants will show an upturn with decreasing temperature. The fact that this effect is smaller than that for the CO-hydrogen systems and occurs at lower temperatures is consistent with the shallower CO–He attractive well compared with that for CO–H2.
Temperature quench echoes in proteins
Authors: Xu Dong , Schulten Klaus , Becker Oren M. , Karplus Martin
Published: July 2002

Temperature quench echoes are analyzed in terms of the temperature–temperature correlation function in the harmonic approximation, and the resulting expressions are compared with molecular dynamics simulations. The relationship between the time dependence of the echo depth and the density of states is demonstrated for harmonic systems. For a protein, which has significant anharmonicity, the time dependence is dominated by relaxation effects that originate from dephasing of the periodic motions. A simple relaxation model is shown to provide a good description of the results observed in the simulations.
Exact affinity distributions for linear polyampholytes and polyelectrolytes
Authors: Koper G. J. M. , Borkovec M.
Published: July 2002

Ising models have been used to describe acid-base behavior of linear polyampholytes and polyelectrolytes. Alternatively, such titration curves are often interpreted in terms of an affinity distribution (pK spectrum). Here we show that such affinity distributions exist if the zeros of the grand partition function of the corresponding Ising model are negative and real. Furthermore, we derive the exact affinity distributions for various Ising models of linear polyelectrolytes and polyampholytes. Applications of the Ising model to the titration behavior of poly(acrylic acid) and poly(ethylenimine) are discussed.