bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer analysis of a-Si:H p-i-n solar cells with a hydrogenated microcrystalline silicon p layer
Authors: Palit N. , Chatterjee P.
Published: July 2002

An integrated electrical–optical model has been used to analyze the performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells having a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) window layer. Our attempts to simulate various experimentally measured solar cell characteristics of such cells available in the literature indicate that for the same thickness, μc-Si:H p layers may have different mobility band gaps (Eμ), which may be linked to different crystalline volume fractions (Fc). Modeling reveals that there is both an optimum value for Eμ (therefore Fc), as well as an optimum thickness of the μc-Si:H p layer for obtaining the best solar cell performance. A thin (8–10 nm) film having a rather low crystalline volume fraction (high Eμ) was found from our computer analysis to give the best results. This is because as the band gap of μc-Si:H p layer decreases (i.e., its crystallinity increases), the valence band offset at the p/i interface increases, leading to a collapse of the bulk electric field. Also investigated was the effect of sandwiching an intrinsic or boron-doped carbide buffer layer between the μc-Si:H p layer and the intrinsic absorber. In both cases the open-circuit voltage (Voc) and the fill factor (FF) improve markedly on account of the reduction of the large potential drop at the p/i interface, which ensures more field over the intrinsic absorber layer. However, whereas for an i-a-SiC:H buffer, the cell performance still depended on the mobility band gap of the μc-Si:H p layer; for a p-a-SiC:H buffer, the conversion efficiency (η) could be made to attain nearly the same value, regardless of Eμ(p-μc-Si:H), simply by adjusting the thickness of the p-a-SiC:H layer. Compared to a “standard” p-i-n cell employing a single p-a-SiC:H window, both combinations show improvements in Voc and FF, but a fall in the short-circuit current density (Jsc), leading to a small net increase in η.
Valence band photoemission, band bending, and ionization energy of GaAs(100) treated in alcoholic sulfide solution
Authors: Lebedev Mikhail V. , Aono Masakazu
Published: July 2002

Valence band and Fermi level position on a p-GaAs(100) surface treated in the solution of ammonium sulfide in isopropanol have been studied by ultraviolet photoemission spectroscopy. Sulfur treatment and subsequent annealing affect features of 0.8 and 3.7 eV binding energy in normal emission valence band spectrum of a bare semiconductor and the effect is related to the variation of surface chemical bonds. It has been found that after sulfur treatment surface band bending and ionization energy increase by 0.4 and 0.3 eV, respectively. Annealing at 310 °C or higher results in the decrease of the ionization energy with the temperature. The position of the surface Fermi level sharply changes from 1.5–1.20 to 0.85 eV above the valence band maximum when the surface is annealed at 360 °C.
Correlation between luminescence and structural properties of Si nanocrystals
Authors: Iacona Fabio , Franzò Giorgia , Spinella Corrado
Published: July 2002

Strong room-temperature photoluminescence (PL) in the wavelength range 650–950 nm has been observed in high temperature annealed (1000–1300 °C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 °C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation between structural (nanocrystal radius and width of the size distributions) and optical (wavelength and width of the PL peaks) data indicates that light emission from the annealed SiOx films is due to carrier recombination in the Si nc, and it can be interpreted in terms of carrier quantum confinement. The possible reasons for the quantitative discrepancy between the experimentally measured luminescence energy values and the theoretical calculations for the enlargement of the band gap with decreasing the crystal size are also discussed.
Kinetics and role of C, O, and OH in low-pressure nanocrystalline diamond growth
Authors: Teii Kungen , Ito Haruhiko , Hori Masaru , Takeo Takashi , Goto Toshio
Published: July 2002

A simple low-pressure condition at 80 mTorr has been employed to study the kinetics and role of C, O, and OH in diamond growth by using inductively coupled CO/CH4/H2 and O2/CH4/H2 plasmas. Vacuum ultraviolet absorption spectroscopy (VUVAS) and actinometric optical emission spectroscopy (OES) were used to examine the densities of ground-state C atoms and emissive species such as OH, C2, and O, respectively. Diamond films consisting of nanocrystallites with sizes as small as 20 nm were obtained on positively biased Si substrates only when CH4 was fed. Both diamond and nondiamond growth were enhanced with increasing CO for a fixed CH4 concentration of 5%, while diamond growth was suppressed with increasing O2. Comprehensive discussion along with the VUVAS and OES results suggested that the C atoms resulting mainly from CO by electron impact dissociation had a close relation with the formation of C2 or still larger species as the precursors to nondiamond phase, while the OH radicals resulting predominantly by loss reactions of the byproduct O atoms with H2 and CH4 were highly responsible for the enhanced diamond growth. A large amount of O atoms from O2 was shown to affect the initial nucleation stage seriously. The results support the growth chemistry of diamond from H-hybridized carbon radicals fragmented from CH4 rather than from H-stripped carbon radicals.
Electro-optic characteristics of 4-domain vertical alignment nematic liquid crystal display with interdigital electrode
Authors: Hong S. H. , Jeong Y. H. , Kim H. Y. , Cho H. M. , Lee W. G. , Lee S. H.
Published: July 2002

We have fabricated a vertically aligned 4-domain nematic liquid crystal display cell with thin film transistor. Unlike the conventional method constructing 4-domain, i.e., protrusion and surrounding electrode which needs additional processes, in this study the pixel design forming 4-domain with interdigital electrodes is suggested. In the device, one pixel is divided into two parts. One part has a horizontal electric field in the vertical direction and the other part has a horizontal one in the horizontal direction. Such fields in the horizontal and vertical direction drive the liquid crystal director to tilt down in four directions. In this article, the electro-optic characteristics of cells with 2 and 4 domain have been studied. The device with 4 domain shows faster response time than normal twisted-nematic and in-plane switching cells, wide viewing angle with optical compensation film, and more stable color characteristics than 2-domain vertical alignment cell with similar structure.
Electrically controlled light scattering from thermoreversible liquid-crystal gels
Authors: Janssen Rob H. C. , Stümpflen Volker , Broer Dirk J. , Bastiaansen Cees W. M. , Tervoort Theo A. , Smith Paul
Published: July 2002

Thermoreversible gels of the liquid-crystal LC-E7 with 1,3:2,4-Di-O-benzylidene-D-sorbitol (DBS) form white light-scattering films that are reversibly switchable to a clear state by ac electric fields. The light scattering by the gelled films is an intrinsic material property that originates in the phase diagram of the system displaying a monotectic-type equilibrium (“mesotectic”) among a liquid, a solid, and a mesophase at extremely low concentrations of DBS. Electro-optical characteristics and demonstrated viscoelastic behavior of the films produced indicate the applicability of DBS/LC-E7 in large area scattering-based flat panel displays and projection systems.
Theory of longitudinal magnetoimpedance in wires
Authors: Ménard D. , Yelon A.
Published: July 2002

A theory of giant magnetoimpedance (GMI) in anisotropic magnetic wires is presented. The theory is valid over a broad field and frequency range. The emphasis is put on the GMI response in the low field region, where the wire is not saturated. The behavior of the wire may be described using three magnetic modes of mixed electromagnetic and spin wave character and one nonmagnetic mode, which is uncoupled from the magnetic response of the material. The properties of these four modes are discussed, with particular attention to the exchange-conductivity effects, which play a decisive role at moderate frequency. Application of the theory to real material is examined, with an outline of its applicability and its limitations. Predictions of the model compare well with experimental data on CoFeSiB wire.
Direct theoretical ab initio calculations in exchange coupled copper(II) dimers: Influence of the choice of the atomic basis set on the singlet–triplet splitting in modeled and real copper dimers
Authors: de Loth Philippe , Daudey Jean-Pierre , Astheimer Harald , Walz Leonhard , Haase Wolfgang
Published: July 2002

The singlet–triplet splitting (2J=EST) is calculated in bis-[bromo-(N,N-diethylaminoethanolato)copper(II)], belonging to the group of alkoxo-bridged copper(II) dimers, by a recently proposed ab initio method. The singlet–triplet splitting energy is obtained by a perturbation expansion closely related to Anderson’s formalism. The role of all valence electrons is taken into account and it is shown to be very important in order to obtain a satisfying agreement with the experimental result. Two important facts, from a computational point of view, are tested: influence of the atomic basis set size, and influence of modifying of the ligands. Changes in the basis set of the bridging atoms can induce variations in the results from 10% to 20%. As a final result, the theoretical value of the real complex is calculated to 2J(calc)=−1095 cm−1 and agrees with the experimental one 2J(exp)=−817 cm−1.
Unimolecular reactions and phase space bottlenecks
Authors: Davis Michael J. , Gray Stephen K.
Published: July 2002

We discuss the phase space bottlenecks for the unimolecular decomposition of HeI2. An intermolecular bottleneck can be defined in phase space which is a ‘‘point of no return’’ or critical configuration for the unimolecular reaction, even though there is no barrier to dissociation for the simple case studied here, HeI2 restricted to T-shaped geometry (two degrees of freedom). In addition, one can find intramolecular bottlenecks and the combination of the two types of bottlenecks yields theoretical reaction rates which are in excellent agreement with classical numerical simulations. Rates, derived from an approach using only intermolecular bottlenecks or approaches using both intermolecular and intramolecular bottlenecks, are two orders of magnitude better than simple RRKM rates. The paper further demonstrates how the phase space bottlenecks can be employed to discern probabilities of complex formation and discusses the extension of the present work to other types of reactive systems.
On the bonding in Fe2(CO)9
Authors: Bauschlicher Charles W.
Published: July 2002

The bonding in Fe2(CO)9 is analyzed using an SCF wave function for a large basis set. There is no direct Fe–Fe metal–metal bond. The bridging CO’s hold the two Fe(CO)3 fragments together by σ donation into the empty Fe–Fe dπ orbital and metal donation from the dπ* orbital into the CO 2π* orbital. The bonding of the terminal CO’s is similar to that in Ni(CO)4 and the equatorial CO groups in Fe(CO)5.
Difference in energy between cubic and hexagonal ice
Authors: Handa Y. Paul , Klug D. D. , Whalley Edward
Published: July 2002

The difference in energy between ice Ic and Ih has been determined calorimetrically in order to help test water–water potential energies. It is −50.5±2.2 J mol−1 at 200 K, and −47±∼3 J mol−1 when adjusted to zero temperature using measured vibrational frequencies.
Temperature-dependent Raman study of ammonium perchlorate single crystals: The orientational dynamics of the NH+4 ions and phase transitions
Authors: Chakraborty T. , Khatri S. S. , Verma A. L.
Published: July 2002

A detailed temperature-dependent study of the Raman spectra of oriented single crystals of NH4ClO4 is reported in the spectral regions of lattice modes and internal vibrations of the ClO−4 and NH+4 ions between 10 and 300 K. The internal modes of the ClO−4 ions show splitting into several components due to site and correlation field effects. The linewidth, frequency shift, and intensities of some of the internal modes of the ClO−4 and NH+4 ions and the frequency shift of a few lattice modes show anomalous temperature dependence around 180 and at 40 K. These anomalies have been explained in terms of phase transformations associated with the changes in hydrogen bonding strength and reorientational freedom of the NH+4 ions in the lattice. The low temperature transition at 40 K exhibits a sharp and discontinuous anomaly in some of the spectral parameters measured in this study which is associated with order–disorder-type transition. The measured linewidth of the ν′1 mode in the diagonal scattering configuration can be understood in terms of vibrational dephasing of the ν1 excited state due to vibrational–librational coupling. The estimated activation energies in the 50–160 K and 10–40 K temperature ranges are found to be 141 and 51 cm−1, respectively, which correspond to the observed NH+4 librational frequencies.
Coherent pulse sequence induced control of selectivity of reactions: Exact quantum mechanical calculations
Authors: Tannor David J. , Kosloff Ronnie , Rice Stuart A.
Published: July 2002

We present a novel approach to the control of selectivity of reaction products. The central idea is that in a two-photon or multiphoton process that is resonant with an excited electronic state, the resonant excited state potential energy surface can be used to assist chemistry on the ground state potential energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited state potential energy surface. Different propagation times, in turn, can be used to generate different chemical products. There are many cases for which selectivity of product formation should be possible using this scheme. We illustrate the methodology with numerical application to a variety of model two degree of freedom systems with two inequivalent exit channels. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum mechanical calculations.
Kinetic oscillations in the catalytic CO oxidation on Pt(100): Computer simulations
Authors: Möller P. , Wetzl K. , Eiswirth M. , Ertl G.
Published: July 2002

The previously observed phenomena of temporal and spatial self-organization during the catalytic oxidation of CO on a Pt(100) surface were computer simulated by use of the cellular automaton technique. The underlying model is footed on the knowledge about the individual reaction steps (adsorption, desorption, surface structural transformation, etc.), which in turn formed the basis of a previous theoretical treatment in terms of the formulation and solution of a set of coupled differential equations. The present result nicely show the formation and propagation of two-dimensional patterns, and they reproduce qualitatively well all the experimental observations. The development of macroscopic patterns even with an a priori perfectly homogeneous surface is a particularly interesting effect.
Population lifetimes of OH(v=1) and OD(v=1) stretching vibrations of alcohols and silanols in dilute solution
Authors: Heilweil E. J. , Casassa Michael P. , Cavanagh R. R. , Stephenson J. C.
Published: July 2002

Picosecond infrared pump–probe experiments determined the vibrational population lifetimes (T1) of the hydroxyl fundamental stretching mode OH(v=1) in 12 alcohols (R3COH) and 8 silanols (R3SiOH) in dilute room temperature CCl4 solutions. T1 for the silanols is in the range 185<T1<292 ps, while T1 for the alcohols is much less (T1<80 ps). The deuterium-exchanged analogs (COD and SiOD) exhibit population relaxation times similar to protonated hydroxyls. An analysis of the vibrational energy levels corresponding to modes involving the four bonds nearest the hydroxyl groups of these molecules is used to qualitatively explain the trends of the observed T1 lifetimes for these systems. Solution T1 lifetimes are also compared to those previously measured for OH(v=1) on the surface of silica and in other condensed-phase, room temperature systems.
Statistical mechanics of helical worm-like chains. XV. Excluded-volume effects
Authors: Shimada Jiro , Yamakawa Hiromi
Published: July 2002

The expansion factors for the mean-square end-to-end distance and radius of gyration are studied for the helical worm-like (HW) chain with the excluded-volume effects incorporated in the Yamakawa–Stockmayer scheme. In this scheme, approximate closed expressions for them are expressed in terms of the excluded-volume parameter z and the first-order perturbation coefficient K(L) as a function of the total contour length L of the chain. The ring-closure probabilities necessary for the evaluation of K(L) are evaluated by a slight modification of the method previously developed for the Kratky–Porod (KP) worm-like chain. The actual evaluation is carried out for the values of the HW model parameters in their limited ranges, and also for the KP chain. It is then numerically shown that K(L) for the HW chain is approximately equal to that for the KP chain. From a simple analysis, this conclusion may be expected to be generally correct, so that the expansion factors for the HW chain may be expressed in terms of the simple K(L) for the KP chain. The derived closed expressions for them may be regarded as valid for any ordinary flexible chains of arbitrary length.
Dissociative recombination of H+2, HD+, and D+2 by collisions with slow electrons
Authors: Nakashima Keiji , Takagi Hidekazu , Nakamura Hiroki
Published: July 2002

Using the multichannel quantum defect theory (MQDT), dissociative recombination of H+2, HD+, and D+2 via the two-electron excited superexcited state 1Σg(2pσu)2 is studied for each specified initial vibrational state vi (≲4) of molecular ions at low electron energies 0.02≲E≲1.0 eV. The necessary basic physical quantities, electronic coupling V(R) and quantum defect as a function of internuclear distance R, are obtained from our previous scattering calculations. As a by-product we have also obtained the singlet scattering d-wave partial cross sections for the various vibrational transitions of molecular ions by electron impact. It is found that the Franck–Condon like factor between vibrational state and dissociative continuum is most decisive in characterizing the overall magnitude and energy dependence of the cross sections. Even the vibrational transitions occur predominantly via the dissociative superexcited state. This suggests that the preionization mechanism of the states of the 1Σg symmetry is very different from that of 1Σu and 1Πu in the vicinity of ionization threshold.
Exact analytic formula for calculating Franck–Condon factors using the Kratzer potential
Authors: Waldenstro/m S. , Razi Naqvi K.
Published: July 2002

To calculate Franck–Condon factors (i.e., squares of the vibrational overlap integrals for two electronic states of a diatomic molecule), the potential energy curve in each state is approximated by the function proposed originally by Kratzer. The resulting factors q(v′,v″) are compared with experimental data for the following band systems: CN(A 2Π–X 2Σ+), N2(B 3Πg–A 3Σ+u; a 1Πg–X 1Σ+g; C 3Πu–X 1Σ+g), and CO(A 1Π–X 1Σ+). Very satisfactory agreement between theory and experiment is observed for low values of the vibrational quantum numbers v′ and v″ (v′,v″≤3).
Polymer shapes in two, four, and five dimensions
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

The average asymmetry of two-, four-, and five-dimensional linear and ring polymers, with and without excluded volume, is investigated numerically via Brownian dynamics. It is found that the average asymmetry of nonexcluded volume chains is in good agreement with theoretical predictions in all dimensions studied. The distribution of the average asymmetry is also determined. This distribution indicates that two-dimensional ring chains are likely to be nearly symmetrical. Moreover, the distribution of the maximum eigenvalues of the tensor of the moment of inertia is also in agreement with the theoretical predictions.
Molecular conformation of polyaniline oligomers: Optical absorption and photoemission of three-phenyl molecules
Authors: Conwell E. M. , Duke C. B. , Paton A. , Jeyadev S.
Published: July 2002

To examine the conformation of polyanilines with different degrees of oxidation two three-ring oligomers, N-N′-diphenyl-1,4-phenylenediamine (the ‘‘amine trimer’’) and N-N′-diphenyl-1,4-benzoquinodiimine (the ‘‘imine trimer’’) have been studied. The amine and imine trimers are model molecules for leucoemeraldine and pernigraniline, respectively. MNDO calculations for the cis and trans forms of the imine trimer predict the ground states to have almost identical energies. Both forms have the quinoid ring in the plane of the backbone and the torsion angle between the quinoid and benzenoid rings close to 90°. The density of valence states (DOVS) and optical absorption were calculated for different conformations of the amine and imine molecules using the CNDO/S3 model. Comparison of the calculated DOVS with photoemission data for the amine molecules indicates a conformation about the same as that predicted by MNDO for the ground state. The optically excited state for the amine, according to the comparison between calculated and measured optical absorption, could have the same geometry or possibly exhibit a larger torsion angle. In the case of the imine molecule, however, the existence of a low energy absorption at 2.8 eV requires the optically excited state to have a different geometry from that predicted by MNDO for the ground state. Analysis of the optical absorption using the CNDO/S3 model suggests that the quinoid ring is twisted out of the plane of the nitrogen backbone by ∼55° while the benzenoid ring is twisted by about the same angle in the opposite direction. The DOVS resulting from this conformation describes the photoemission data reasonably well, although these data are not well described by DOVS obtained using the ‘‘ground state’’ conformation predicted by the MNDO model.
Molecular dynamics simulation of water from 10 to 1273 K
Authors: Evans M. W. , Lie G. C. , Clementi E.
Published: July 2002

The molecular dynamics of liquid water have been simulated over a wide range of thermodynamic conditions using a five by five site–site model for the intermolecular potential recently developed by Evans. The model reproduces the pressure satisfactorily for temperatures up to 1273 K at constant density of 1 g/cm3. Within the uncertainty, the experimental pressure is also reproduced satisfactorily at the critical point. Two simulations at 773 and 1043 K have also been carried out at constant molar volume of 8.5 cm3/mol. The molecular dynamics of the sample were investigated at each state point with a range of auto- and cross-correlation functions. Some of these have been computed at constant molar volume over a 15 kbar range of pressure and 1000 K range of temperature. They suggest that diffusional dynamics in liquid water are largely determined by density. Some results at 10 (1 bar) and 77 K (1 bar) were obtained by ‘‘splat quenching’’ at constant molar volume. The oxygen–oxygen pair distribution functions from these simulations have been compared with the results available from amorphous solid water at these temperatures.
Rotationally induced vibrational mixing in formaldehyde
Authors: Sibert Edwin L.
Published: July 2002

Almost-degenerate perturbation theory is used to derive an effective Hamiltonian describing the vibrational states of H2CO. Eigenvalues have been determined for energies up to 8600 cm−1 above the zero-point energy. Both curvilinear and rectilinear representations of the vibrational dynamics are presented and explored. Although differences are observed between the two effective Hamiltonian matrix elements, their eigenvalues generally agree to better than a wave number for the energies studied. Using the Watson Hamiltonian, the mechanism of rotationally induced vibrationally mixing is investigated as a function of K, the projection of the total angular momentum onto the body-fixed a axis. The combination of a-axis Coriolis coupling and Fermi couplings leads to extensive vibrational mixing between the rotational–vibrational states in this energy regime.
The mechanism for the O2 dissociation on Ni(100)
Authors: Panas Itai , Siegbahn Per , Wahlgren Ulf
Published: July 2002

The dissociation of O2 on Ni(100) has been studied using a cluster model approach. The three principally different reaction pathways, over an on-top position, over a bridge position, and over a fourfold hollow position, were considered. The dissociation mechanisms were found to be very similar for these pathways. In the entrance channel a chemisorbed, peroxo-form, of molecular O2 is first formed, which is strongly bound to the Ni(100) surface by two polar covalent bonds. The binding energy at the fourfold hollow site is found to be 78 kcal/mol, which is about 20 kcal/mol larger than for the other two sites, and much larger than the chemisorption energies for the experimentally observed O2 on Pt(111) and Ag(110). The reason for this difference is discussed. In a simplified valence-bond picture the wave function of this molecularly bound O2 has a large component of a πu to πg excited state of O2. The dissociation of O2 then proceeds by two stepwise electron transfers from the surface over to the O2 3σu orbital, which completes the breaking of the O–O bond. In this latter process the energy passes over a local barrier, which is still far below the long distance asymptote, however. The local barrier height is much higher for the fourfold hollow dissociation, 35 kcal/mol over the local molecular minimum, than for the other two pathways, where the barrier height is only 6–8 kcal/mol. The 3d orbitals on nickel remain passive for all the three dissociation pathways, which is in line with the fact that also nontransition metals dissociate O2. This behavior is in contrast to the dissociation of H2 on Ni(100), where the 3d orbitals play a key role for the on-top dissociation.
Raman and nuclear magnetic resonance studies on the concentration dependence of orientational relaxation times of the nitrate ion in dilute aqueous solution
Authors: Adachi A. , Kiyoyama H. , Nakahara M. , Masuda Y. , Yamatera H. , Shimizu A. , Taniguchi Y.
Published: July 2002

The perpendicular orientational relaxation time (τ⊥) of the nitrate ion was determined as a function of concentration by the Raman line shape analysis (ν1 stretch) in aqueous LiNO3 and KNO3 solutions in the range of 0.2–1 M and by the nuclear magnetic resonance (NMR) T1 measurement (14N) in MNO3 (M=Li, Na, K, and Cs) solutions in the range of 0.02–1 M; the parallel orientational relaxation time (τ∥) was also determined by the NMR measurement for the solutions enriched in 17O. Both τ⊥ and τ∥ increase linearly with increasing concentration up to 1 M; the limiting values of τ⊥ and τ∥ were 1.25 and 1.9 ps, respectively. The slope increases with the surface charge density of the countercation except for Li+. These trends are predicted in very dilute solutions by the continuum theory developed in the previous paper. The anomalous behavior of Li+ was explained in terms of its strong hydration.
Localization of a polymeric manifold in quenched random media
Authors: Muthukumar M.
Published: July 2002

We present a replica calculation and obtain analytically the size of a polymer chain with excluded volume interactions in quenched random media. We show that the size of the polymer is shrunk as the density of the impurities constituting the random medium is increased. We demonstrate that this is a general phenomenon. There are three regimes depending on the strength of the impurity density v. In the weak impurity density regime the polymer obeys the self-avoiding statistics with its gyration radius R depending on its typical length L according to R∼L(D+2)/(d+2), where D is the dimension of the polymer and d is the space dimension. In the intermediate regime the polymer is in the unperturbed state where R∼L(2−D)/2. In the third regime of localization occurring for sufficiently large v, R∼v−(2−D)/[4D−(2−D)d] in the absence of three-body interactions and R∼v−1/d LD/d in the presence of strong three-body effects. We examine the effect of long-range interactions of the type w‖r‖−α between the segments of the polymer separated by r on the localization and find that the polymer collapse is suppressed for realistic impurity densities [v<wR(d−α)/2].
Rotational motion in molecular liquids
Authors: Wei Dongqing , Patey G. N.
Published: July 2002

This paper is concerned with rotational motion and the coupling between rotational and translational motion in molecular fluids. The van Hove angular space–time correlation function is expanded in a basis set of rotational invariants and some properties of the expansion are discussed. General expressions for collective reorientational time correlation functions and relaxation times are given in terms of the expansion coefficients. It is shown that this expansion allows the Kerr and Vineyard approximations which relate the distinct and self parts of the van Hove function to be cast in tractable form and Fourier–Laplace space solutions can be obtained for all frequencies and wave vectors. General solutions are given as well as explicit results for the simpler but important k=0 case. The dielectric relaxation of polar–nonpolarizable molecules is discussed and both infinite samples and the reaction field boundary conditions sometimes employed in the computer simulation of polar fluids are considered. It is shown that in the Kerr theory the frequency dependent dielectric constant ε(ω) is correctly independent of the boundary conditions applied. This is not true of the Vineyard approximation. Applying the Kerr theory and using a result for the self part of the van Hove function recently given by Caillol, we investigate the dielectric relaxation of both spherical and nonspherical diffusors. The nonspherical model is that of Berne and Pecora and couples the rotational and translational diffusion. It is found that within the Kerr approximation spherical diffusors behave as Debye dielectrics at all frequencies. However, in the nonspherical case ε(ω) can deviate substantially from the Debye result and we conclude that for highly anisotropic diffusors the coupling between rotational and translational motion can significantly contribute to the observed dielectric relaxation.
Solvent isotope effect on mobilities of potassium and chloride ions in water at high pressure. III. A high temperature study
Authors: Ueno M. , Tsuchihashi N. , Shimizu K.
Published: July 2002

Limiting molar conductances of the K+ and Cl− ions in heavy and light water have been determined at 45 °C as a function of pressure up to 2000 kgf cm−2 (1 kgf cm−2 =0.9807×105 Pa) from the measured conductances and transference numbers of KCl. The residual friction coefficients (Δζobs ) of the K+ and Cl− ions obtained by using their limiting molar conductances and the bulk viscosity of solvent are compared up to 1000 kgf cm−2 with the corresponding values (ΔζHO ) predicted by the Hubbard–Onsager dielectric friction theory. As predicted, Δζobs for the cation in H2 O is smaller than that in D2 O and slightly decreases with increasing pressure, while its value in D2 O is almost invariant in the pressure range studied. The failure of the continuum theory in D2 O indicates that the effect of the open structure of water on the cation migration cannot be neglected even at 45 °C because of stronger hydrogen bonds in D2 O than in H2 O. For the anion, on the other hand, the continuum theory shows more serious limitations: (i) Δζobs (Cl−) in H2 O is not smaller than that in D2 O even at 45 °C with a large difference at high pressures and (ii) Δζobs (Cl−) becomes negative in both types of water at high pressures. The difference in Δζ(Cl−) between theory and experiment at 45 °C, however, becomes much smaller than that at lower temperatures.
State-resolved unimolecular reactions: The vibrational overtone initiated decomposition of nitric acid
Authors: Sinha Amitabha , Vander Wal Randall L. , Crim F. Fleming
Published: July 2002

We have determined the internal state distribution of the OH fragment from the unimolecular dissociation on nitric acid initiated by excitation in the regions of the fourth (5νOH) and fifth (6νOH) overtones of the O–H stretching vibration. The two excitations correspond to average excess energies of 350 and 2700 cm−1, respectively. In the case of 5νOH excitation, where states near the threshold energy for dissociation dominate the decomposition, phase-space theory (PST) correctly predicts the observed populations of both the rotational and spin–orbit states of the OH fragment. This observation, which is consistent with there being no barrier to the unimolecular dissociation on the ground electronic state potential-energy surface, illustrates the ability of simple statistical theory to describe spin–orbit state populations, at least in near-threshold decomposition of nitric acid. In contrast, the same model does not reproduce the quantitative details of the decomposition initiated by excitation of the fifth overtone vibration, although it does reproduce the general shape of the OH rotational state distribution. The observed rotational state distribution for the 6νOH initiated decomposition is less energetic than the PST prediction, and the measured spin–orbit state population ratio P(2Π3/2)/P(2Π1/2) is larger than the calculation.
Cylindrical manifolds in phase space as mediators of chemical reaction dynamics and kinetics. I. Theory
Authors: De Leon N. , Mehta Manish A. , Topper Robert Q.
Published: July 2002

A microcanonical kinetic theory of reactions based upon the structure within phase space is developed. It is shown that the dynamics of reaction across an energetic barrier is mediated by invariant manifolds embedded in phase space that have the geometry of simple cylinders. The ideas are developed by considering molecular systems modeled by two vibrational degrees of freedom, a reaction coordinate and a ‘‘bath’’ coordinate. The kinetic theory is constructed by focusing on the dynamics between n mapping planes (‘‘n-map’’) and the ‘‘reactive island’’ (RI) structure within them. We discuss how the structure of the conformer population decay in isomerization reactions can be obtained from the RI kinetic model. Formal solutions of the kinetic equations are discussed with specific attention given towards the calculation of the isomerization reaction rate. The formal theory is developed in Paper I of this series. Numerical considerations and applications to the reaction dynamics of model molecular systems with two degrees of freedom will be given in Paper II and extension of the theory and applications to multidimensional systems will be given in Paper III.
Simulation of supercritical water and of supercritical aqueous solutions
Authors: Cummings P. T. , Cochran H. D. , Simonson J. M. , Mesmer R. E. , Karaborni S.
Published: July 2002

Molecular dynamics (MD) calculations have been performed to determine equilibrium structure and properties of systems modeling supercritical (SC) water and SC aqueous solutions at two states near the critical point using the simple point charge (SPC) potential model of Berendsen et al. for water. Both thermodynamic and dielectric properties from the simulations for pure water are accurate in comparison with experimental results even though the SPC model parameters were fitted to properties of ambient water. Details of the near-critical clustering in SC water have been predicted which have not been measured to date. MD studies have also been undertaken of systems that model sodium and chloride ions and neutral argon in SC water at the same states. The first solvation shell in SC water is observed to be similar to that in ambient water, and long-range solvation structures in SC water are similar to those observed for simple SC solvents. An excess of water molecules is observed clustering around ionic solutes which behave attractively and a deficit is observed around neutral atomic solutes which behave repulsively. These results should be helpful in developing a qualitative understanding of important processes that occur in SC water.
Large multiconfiguration Hartree–Fock calculations on the hyperfine structure of B(2P) and the nuclear quadrupole moments of 10B and 11B
Authors: Sundholm Dage , Olsen Jeppe
Published: July 2002

The hyperfine parameters for B(2P) are studied using numerical multiconfiguration Hartree–Fock (MCHF) calculations. A newly developed finite-element MCHF program allowing very large configuration–interaction expansions is used. The magnetic hyperfine parameters obtained are 0.0879(28), 0.7817(11), and −0.1675(3) a.u. for the Fermi contact term, the orbital term, and the spin–dipolar term, respectively, as compared to the experimental values of 0.1016(62), 0.7783(16), and −0.1686(5) a.u. The nuclear quadrupole moments obtained by comparing computed electric field gradients with experimental nuclear quadrupole couplings constants are Q(10B)=0.084 59(24) b and Q(11B)=0.040 59(10) b.
Spin–orbit state selective formation of rare gas chlorides from three-body ionic-recombination reactions of Rg+(2P1/2,3/2)+Cl−+He at thermal energy
Authors: Tsuji Masaharu , Furusawa Makoto , Kouno Hiroyuki , Nishimura Yukio
Published: July 2002

The ArCl(C–A), KrCl(B–X,C–A,D–X), and XeCl(B–X,C–A,D–X) emissions have been observed from ionic-recombination reactions of Rg+ (Rg=Ar, Kr, or Xe) with Cl− in the flowing afterglow. Positive Rg+ ions are formed by He(23S)/Rg Penning ionization, while negative Cl− ions are produced through thermal electron attachment to CCl4. The dependence of RgCl* emission intensities on the buffer He gas pressure indicates that the excimer emissions arise from three-body reactions of Rg++Cl− +He. The spin–orbit state selectivity in the KrCl* and XeCl* formation is studied by isolating one of the spin–orbit levels of Rg+,2P1/2 or 2P3/2 . Although the Kr+(2P1/2) reaction provides the KrCl(B–X,C–A,D–X) emissions with B:C:D distribution of 0.19±0.02:0.12±0.01:0.69±0.04, only XeCl(D–X) emission is observed from the Xe+(2P1/2) reaction. The Kr+(2P3/2) and Xe+(2P3/2) reactions give the RgCl(B–X,C–A) emissions with B:C branching ratios of 0.60±0.06:0.40±0.04 for KrCl* and 0.62±0.06:0.38±0.04 for XeCl*. The high propensities for the D formation from the Rg+(2P1/2) reactions and for the B and C formation from the Rg+(2P3/2) reactions suggest that Rg+(2P1/2)+Cl− and Rg+(2P3/2)+Cl− characters are conserved well for the formation of RgCl* in the three-body ionic-recombination reactions. The relative formation rate of RgCl(D) from the Rg+(2P1/2) reaction to that of RgCl(B,C) from the Rg+(2P3/2) reaction was estimated to be 0.14±0.02 for KrCl* and 0.033±0.006 for XeCl*. The slower RgCl(D) formation rates are attributed to fast predissociation of [Rg+(2P1/2)Cl−]* intermediates into Rg*+Cl and/or Rg+Cl* than that of [Rg+(2P3/2)Cl−]* ones.
Molecular orbital theory of the 1L
b and 1L
a states of indole
Authors: Callis Patrik R.
Published: July 2002

A comprehensive study of the 1Lb and 1La excited states of indole, using a spectroscopically calibrated semiempirical molecular orbital method [the spectroscopic version of the intermediate neglect of differential overlap-configuration interaction (INDO/S-CI)], is reported. Results from two standard parametrizations and procedures are identified as giving satisfactory agreement with several experimental properties. Diagrams of MOs, π transition densities, and π density changes are provided from these two calculations. In addition, transition energies, oscillator strengths, transition moment directions, dipoles, and two-photon properties are presented for 24 additional calculations representing variations in the key parameters.
The nuclear quadrupole coupling of 10B and 11B in the carboranes B10C2H12
Authors: Lötz A. , Voitländer J.
Published: July 2002

The nuclear quadrupole couplings of 10B and 11B in p-carborane, p-B10C2H12, were measured by the field cycling technique (nuclear quadrupole double resonance) at 77 K. The couplings are e2qQ/h(10B)=2980±40 kHz, e2qQ/h(11B)=1430±20 kHz, and η=0.49±0.03. Ab initio calculations of the electric field gradient tensors at the boron nuclei in o-, m-, and p-carborane were performed. The results based on the Gaussian basis set (9s5p/4s)/[4s2p/2s] are in good agreement with the experimental couplings for p-carborane and those for o- and m-carborane obtained earlier by us. There are no reorientational jumps of the globular carborane molecules at 77 K on the nuclear quadrupole resonance time scale. The valence bond view of bonding in the carboranes is confirmed by the good correlation of experimental quadrupole coupling constants and those derived from the valence bond structures.
Na+–Na+ and Cl−–Cl− ion pairs in water: Mean force potentials by constrained molecular dynamics
Authors: Guàrdia E. , Rey R. , Padró J. A.
Published: July 2002

Molecular dynamics simulations of Na++Na+, Na++Cl−, and Cl−+Cl− ions in dilute aqueous solution were carried out using a flexible single point charge (SPC) model for water. The resulting structural and dynamic properties are compared with experimental data and other computer simulation results. The potentials of mean force [W(r)] between the like ions were determined from constrained molecular dynamics simulations. The resulting W(r) for the Na+–Na+ ion pair is in qualitative agreement with other computer simulation findings, whereas the discrepancies are important in the case of the Cl−–Cl− ion pair. Our Cl−–Cl− mean force potential shows a moderate minimum which does not involve the unexpected strong attraction between chloride ions at short distances as predicted in earlier papers. The solvent structure around the ion pairs for separations corresponding to the maxima and minima of the W(r)’s is analyzed.
Model surface studies of metal oxides: Adsorption of water and methanol on ultrathin MgO films on Mo(100)
Authors: Wu Ming-Cheng , Estrada Cesar A. , Corneille Jason S. , Goodman D. Wayne
Published: July 2002

Model surface studies of magnesium oxide have been carried out using surface sensitive techniques. Ultrathin MgO films have been synthesized under ultrahigh vacuum (UHV) conditions by thermally evaporating Mg onto Mo(100) in the presence of oxygen. Low-energy electron diffraction (LEED) studies indicate that the MgO films grow epitaxially with the (100) face of MgO oriented parallel to Mo(100). The MgO films, prepared under optimum synthesis conditions, have essentially one-to-one stoichiometry, are nearly free from pointlike surface defects, and have properties essentially identical to those of bulk, single-crystal MgO. Adsorption of water and methanol onto the MgO films has been studied using high-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption (TPD). In order to circumvent the difficulty associated with intense multiple surface optical phonon (Fuchs–Kliewer modes) losses, a new approach to acquisition of HREELS data has been demonstrated. This new approach enables the direct observation of weak loss features due to excitation of the adsorbates without serious interference from multiple phonon losses. Our HREELS studies show that water and methanol undergo heterolytic dissociation, leading to the formation of hydroxyl and methoxy species, respectively.
On the correlation energy of graphite
Authors: Stoll Hermann
Published: July 2002

The correlation energy per atom of a graphite layer is determined by means of increments obtained in ab initio calculations for aromatic molecules with various numbers of correlated localized σ and π orbitals. The resulting correlation contribution to the cohesive energy, evaluated with a double-zeta plus polarization basis set is 0.050 a.u. per C atom, which is smaller by 0.007 a.u. than the corresponding value for diamond.
Multidimensional hydrogen tunneling dynamics in the ground vibrational state of the ammonia dimer
Authors: Loeser Jennifer G. , Schmuttenmaer C. A. , Cohen R. C. , Elrod M. J. , Steyert D. W. , Saykally R. J. , Bumgarner R. E. , Blake G. A.
Published: July 2002

We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric-dipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy-level diagrams for the Ka=0 and Ka=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the ‘‘donor’’ and ‘‘acceptor’’ roles of the monomers, internal rotation of the monomers about their C3 symmetry axes, and quite unexpectedly, ‘‘umbrella’’ inversion tunneling.

In the Ka=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In Ka=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm−1, respectively. In the A+E states, the measured 20.5 cm−1 splitting can result from a difference in ‘‘donor’’ and ‘‘acceptor’’ internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that ‘‘donor–acceptor’’ interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six-dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.