bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoemission study of direct photomicromachining in poly(vinylidene fluoride)
Authors: Morikawa Eizi , Choi Jaewu , Manohara Harish M. , Ishii Hisao , Seki Kazuhiko , Okudaira Koji K. , Ueno Nobuo
Published: July 2002

Direct pattern transfer onto poly(vinylidene fluoride) was achieved by using x-ray photons from a synchrotron radiation source. Quadrupole mass spectrometry and ultraviolet photoemission spectroscopy, combined with ab initio molecular orbital calculations, were employed to investigate the mechanism of direct photomicromachining. The mass spectrometry identified H2, F, and HF as the etched products, with no carbon containing species being detected. The changes in photoemission spectra due to photodegradation were analyzed by comparison with ab initio molecular orbital calculations. This analysis indicated that a high degree of conjugation is generated in the degraded polymer due to the loss of fluorine atoms. It is concluded that the mechanism of direct photomicromachining is ascribable to the shrinking of the irradiated polymer region due to defluorination and the generation of conjugation.
Spin-dependent electron transport in NiFe/GaAs Schottky barrier structures
Authors: Hirohata A. , Xu Y. B. , Guertler C. M. , Bland J. A. C.
Published: July 2002

Photoexcitation at the Schottky barrier formed between 5 nm thick Ni80Fe20 films and both n+- and p−-type GaAs(100) substrates with doping density in the range 1023⩽n(p)⩽1025 m−3 was investigated using circularly polarized laser light. A helicity-dependent photocurrent dependent upon the magnetization configuration of the film and the Schottky barrier height was detected. The results provide evidence of spin-dependent electron transport through the NiFe/GaAs interface and show that the Schottky barrier height controls the spin-dependent current passing from the semiconductor to the ferromagnet.
Crystal structure and dielectric relaxation studies of the [N(CH3)3H]3Sb2(1−x)Bi2xCl9 mixed crystals
Authors: Bator G. , Jakubas R. , Zaleski J. , Mróz J.
Published: July 2002

The influence on ferroelectric properties of a partial substitution of antimony by bismuth in the [N(CH3)3H]3Sb2Cl9 (TMACA) crystals are investigated by dielectric and x-ray diffraction methods. The dielectric relaxation in the [N(CH3)3H]3Sb2(1−x)Bi2xCl9 (TMACAB) mixed crystals (x=0, 0.07, 0.33, 0.38, and 1) is studied in the frequency range between 20 Hz and 1 GHz and temperature range between 170 and 375 K. The ferroelectric properties of mixed crystals are preserved for the Bi concentration less than 0.33. The increase of the Bi concentration distinctly influences the phase transition temperature but only weakly the dynamical properties of those crystals. The dielectric response of the ferroelectric TMACAB crystals is characterized by a presence of two relaxation modes. One of them behaves critically and the other one is only thermally activated. The disappearance of ferroelectric properties is connected with the essential change of crystal structure of mixed compounds. The anionic sublattice of nonferroelectric TMACAB (x>0.33) (R3̄c space group) instead of two-dimensional layers is composed of the isolated Sb2(1−x)Bi2xCl9 bioctahedral units.
Pulsed optoacoustic characterization of layered media
Authors: Paltauf G. , Schmidt-Kloiber H.
Published: July 2002

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6–8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelectric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields optimal conditions for direct measurements of the distribution of absorbed energy and the absorption coefficient in the medium. Far field measurements are advantageous for detecting and imaging layer boundaries at large depths in the sample.
Structure factors of polydisperse systems of hard spheres: A comparison of Monte Carlo simulations and Percus–Yevick theory
Authors: Frenkel D. , Vos R. J. , de Kruif C. G. , Vrij A.
Published: July 2002

We present Monte Carlo (MC) simulations of the structure factors of polydisperse hard-sphere fluids. The simulations were carried out for 108 particles and packing fractions up to φ=0.5. The size distribution of the particles was chosen randomly from a log-normal distribution. The MC results are compared with predictions obtained using Percus–Yevick approximation. It is found that for all but the highest densities and the highest polydispersities studied, the Percus–Yevick approximation provides a satisfactory description of the MC data.
Analysis of deuterium nuclear magnetic resonance line shapes in anisotropic media
Authors: Wittebort R. J. , Olejniczak E. T. , Griffin R. G.
Published: July 2002

Methods for the analysis of motional effects on 2H NMR solid state line shapes are described. Several simple models of anisotropic motions in solids are presented from which line shapes and relaxation experiments are calculated. Using these methods order parameters of fast limit spectra can be explicitly evaluated. These methods are extended to include analysis of intermediate exchange spectra in terms of specific motional models. The effects of differential T2’s arising from exchange broadening on the line shape in powders are described. Methods for the calculation and analysis of T1 anisotropy in partially relaxed spectra of powders for both fast and intermediate exchange regimes are presented.
Thermochromism in poly(3-hexylthiophene) in the solid state: A spectroscopic study of temperature-dependent conformational defects
Authors: Salaneck W. R. , Inganäs O. , Thémans B. , Nilsson J. O. , Sjögren B. , Österholm J.-E. , Brédas J. L. , Svensson S.
Published: July 2002

Reversible thermochromism in thin solid films of poly(3-hexylthiophene), or P3HT, has been studied using ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS, respectively). The UPS and XPS spectra, as well as previously published optical absorption spectra, are analyzed using the results of valence effective Hamiltonian (VEH) quantum chemical calculations of the electronic structure of isolated polymer chains. The analysis of the spectra indicates that at elevated temperatures thermally induced electronic localization occurs as a consequence of thermally induced conformational disorder.
Liquid to solidlike transitions of molecularly thin films under shear
Authors: Gee Michelle L. , McGuiggan Patricia M. , Israelachvili Jacob N. , Homola Andrew M.
Published: July 2002

We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very complex thin-film properties, such as the quantization of various static and dynamic properties, discontinuous or continuous solid–liquid transitions, smooth or stick–slip friction, and two-dimensional nucleation. Quantitatively, the ‘‘effective’’ viscosity in molecularly thin films can be 105 times the bulk value, and molecular relaxation times can be 1010 times slower. These properties depend not only on the nature of the liquid, but also on the atomic structure of the surfaces, the normal pressure, and the direction and velocity of sliding. We also conclude that many thin-film properties depend on there being two surfaces close together and that they cannot be understood from a consideration of a single solid–liquid interface. The results provide new fundamental insights into the states of thin films, and have a bearing on understanding boundary friction, thin-film lubrication and the stress–strain properties of solids at the molecular level.
Frequency dependent hyperpolarizabilities with application to formaldehyde and methyl fluoride
Authors: Rice Julia E. , Amos Roger D. , Colwell Susan M. , Handy Nicholas C. , Sanz Javier
Published: July 2002

Methods for the determination of the frequency dependent hyperpolarizabilities, β(−2ω;ω,ω), β(−ω;ω,0), and β(0;ω,−ω) at the self-consistent field (SCF) level of theory are discussed and compared. Two efficient alternatives are proposed; one involves determination of appropriate response vectors which arise in first-order perturbation theory at frequency ω, and the other involves solving first-order perturbation theory equations at frequency 0 (static), ω and 2ω. Neither approach involves solution of the second-order perturbation theory equations. The method is illustrated with application to formaldehyde and methyl fluoride using large one-particle basis sets. The basis set requirement for convergence of this property at the SCF level of theory is investigated. Basis sets including up to d functions on first row atoms and p functions on hydrogen incorporating diffuse functions appear to be adequate. The effect of electron correlation on the static hyperpolarizability is estimated using second-order Mo/ller–Plesset theory. Comparison is made with experimental values where available.
A complete basis set model chemistry. III. The complete basis set-quadratic configuration interaction family of methods
Authors: Petersson G. A. , Tensfeldt Thomas G. , Montgomery J. A.
Published: July 2002

The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. A family of complete basis set (CBS) quadratic CI (QCI) model chemistries is defined to include corrections for basis set truncation errors. These models use basis sets ranging from the small 6-31 G°° double zeta plus polarization (DZ+P) size basis set to the very large (14s9p4d2f,6s3p1d)/[6s6p3d2f,4s2p1d] atomic pair natural orbital basis set. When the calculated energies are compared with the experimental energies of the first-row atoms and ions and the first-row diatomics and hydrides H2, LiH, Li2, CH4, NH3, H2O, HF, LiF, N2, CO, NO, O2, and F2, two very promising new model chemistries emerge. The first is of comparable accuracy, but more than ten times the speed of the G1 model of Pople and co-workers. The second is less than one-tenth the speed of the G1 model, but reduces the root-mean-square (rms) errors in ionization potentials (IPs), electron affinities (EAs), and D0’s to 0.033 and 0.013 eV, and 0.53 kcal/mol per bond, respectively.
Eigenstates of a quantum-mechanical particle on a topologically disordered surface: H(D) atom physisorbed on an amorphous ice cluster (H2O)115
Authors: Buch V. , Czerminski R.
Published: July 2002

We examined energy levels and stationary states of a quantum-mechanical particle adsorbed on a rough and disordered surface. The specific system examined consists of an H atom (or a D atom) adsorbed on an amorphous ice cluster (H2O)115. Two kinds of stationary states of the adsorbate particle were obtained: states localized in cavities on the cluster surface, and states occupying corridors of low potential energy present on the cluster surface. Zero-point energy effects were found to be very significant; thus the ordering of ground state energies in the different cavities does not follow at all the ordering of minimum potential energies in the cavities. Significant localization was demonstrated of the calculated eigenstates within small portions of connected energetically accessible regions on the cluster surface. The localization seems to be associated with disorder in the three-dimensional potential energy surface, which includes strongly varying well depth, and sharp turns in potential contours. Phase matching of the adsorbate wave function is not easily achieved in the different parts of the potential well, the result being localization of the eigenstates.
A novel discrete variable representation for quantum mechanical reactive scattering via the S-matrix Kohn method
Authors: Colbert Daniel T. , Miller William H.
Published: July 2002

A novel discrete variable representation (DVR) is introduced for use as the L2 basis of the S-matrix version of the Kohn variational method [Zhang, Chu, and Miller, J. Chem. Phys. 88, 6233 (1988)] for quantum reactive scattering. (It can also be readily used for quantum eigenvalue problems.) The primary novel feature is that this DVR gives an extremely simple kinetic energy matrix (the potential energy matrix is diagonal, as in all DVRs) which is in a sense ‘‘universal,’’ i.e., independent of any explicit reference to an underlying set of basis functions; it can, in fact, be derived as an infinite limit using different basis functions. An energy truncation procedure allows the DVR grid points to be adapted naturally to the shape of any given potential energy surface. Application to the benchmark collinear H+H2→H2+H reaction shows that convergence in the reaction probabilities is achieved with only about 15% more DVR grid points than the number of conventional basis functions used in previous S-matrix Kohn calculations. Test calculations for the collinear Cl+HCl→ClH+Cl reaction shows that the unusual dynamical features of heavy+light-heavy reactions are also well described by this approach. Since DVR approaches avoid having to evaluate integrals in order to obtain the Hamiltonian matrix and since a DVR Hamiltonian matrix is extremely sparse, this DVR version of the S-matrix Kohn approach should make it possible to deal with more complex chemical reactions than heretofore possible.
Mechanical instability in ice I
h. A mechanism for pressure-induced amorphization
Authors: Tse John S.
Published: July 2002

The phenomenon of pressure-induced crystalline →amorphous phase transformation in ice Ih has been studied with constant-pressure molecular dynamics calculations. It was found that although the temperature dependency of the transformation pressure resembles that of the extrapolated melting curve, the actual mechanism is due to a mechanical instability in the water framework. The elastic moduli calculated at the transition show a violation of Born criteria for mechanical stability.
A critical test of the diagonal variation-perturbation technique for the partition function of anharmonic oscillators
Authors: Witschel W. , Murmann K. , Taubmann G.
Published: July 2002

The diagonal variation-perturbation technique to fourth order is applied to the partition function of asymmetric anharmonic oscillators. Their constants are taken from spectroscopic constants of diatomic molecules. In contrast to thermodynamic perturbation theory the series does not diverge for very low and high temperatures over a wide range of anharmonicities, but it oscillates slightly around the second order result. The approximation is excellent, differing only 10−3 to 10−4 from the exact numerical results. This technique can be extended to systems of coupled anharmonic oscillators with polynomial anharmonicity and interactions.
Potential energy surfaces of substituted anilines: Conformational energies, deuteration effects, internal rotation, and torsional motion
Authors: Disselkamp R. , Im H. S. , Bernstein E. R.
Published: July 2002

Mass resolved excitation spectra (MRES) are presented for a series of substituted anilines including 2- and 3-methylaniline, 2- and 3-ethylaniline, 2-aminobenzyl amine, and 2-aminobenzyl alcohol. The observed spectra show the following phenomena: nearly free internal rotation of the methyl substituent in the S1 state; long vibrational progressions attributed to C–C, C–N, and C–O side chain torsional motion; an inequivalence of the two amino hydrogens for both ring and side chain amino groups as determined from the spectra of deuterated species; and the existence of two conformers for 2-aminobenzyl alcohol. Semiempirical and ab initio calculations are performed on these systems to aid in the analysis of the potential energy surfaces and in the interpretation of the experimental results.
Computer simulation study of the comparative rheology of branched and linear alkanes
Authors: Daivis Peter J. , Evans Denis J. , Morriss Gary P.
Published: July 2002

We use nonequilibrium molecular dynamics simulations to investigate the rheology of branched and linear alkanes in the dense fluid state. A simplified model alkane [J.-P. Ryckaert, G. Ciccotti, and H. J. C. Berendsen, J. Comput. Phys. 23, 327 (1977); J.-P. Ryckaert and A. Bellemans, Discuss. Faraday Soc. 66, 95 (1978); G. Ciccotti and J.-P. Ryckaert, Comp. Phys. Rep. 4, 345 (1986); R. Edberg, D. J. Evans, and G. P. Morriss, J. Chem. Phys. 84, 6933 (1986)] composed of sites representing CH2 and CH3 groups with fixed bond lengths and bond angles between sites is used. The model also includes dihedral potential interactions between sites separated by three carbon bonds, and truncated and shifted Lennard-Jones potential interactions between sites on different molecules as well as those separated by more than three bonds on the same molecule. The Edberg–Evans–Morriss nonequilibrium molecular dynamics algorithm [R. Edberg, D. J. Evans, and G. P. Morriss, J. Chem. Phys. 84, 6933 (1986); R. Edberg, G. P. Morriss, and D. J. Evans, J. Chem. Phys. 86, 4555 (1987)] with periodic boundary conditions was used to simulate a system of 108 13-carbon n-alkane (tridecane) molecules and the corresponding 3-armed star branched alkane (5-butyl nonane). Equilibrium and steady state Couette flow simulations were performed. Properties calculated included the viscosity, normal stress differences, radius of gyration, alignment angles, shear induced order, angular velocity and a variety of other properties related to the conformation and dynamics of the molecules. The general features of the results were in agreement with recent simulations of n-decane and n-eicosane [G. P. Morriss, P. J. Daivis, and D. J. Evans, J. Chem. Phys. 94, 7420 (1991)], but the details of the results differed for branched and linear molecules. For example, the general trend of decreasing viscosity at low shear rates followed by an increase in the viscosity at higher strain rates was once again observed, but the viscosity for the linear alkane was always less than that of the branched alkane.
Vibrational distributions of As2 in the cracking of As4 on Si(100) and Si(111)
Authors: Alstrin April L. , Smilgys Russell V. , Strupp Paul G. , Leone Stephen R.
Published: July 2002

The desorption dynamics of arsenic from Si(100) and Si(111) are studied by measuring the vibrational population distributions of desorbed As2 using laser-induced fluorescence. In these measurements a steady state flux of desorbing As2 is produced by continuously dosing a heated Si surface with a beam of As4 from a conventional molecular beam epitaxy oven. Measurements of the fluxes of As2 as a function of surface temperature suggest that the As2 may be kinetically formed in two distinct steps: The As4 first chemisorbs to form atoms on the surface which then recombine to desorb as dimers. However, there may also be direct dissociation of As4 at the hot surface. The vibrational populations of As2 (v″=0–3) desorbed from Si(100) are Boltzmann and indicate a vibrational temperature Tv that is nearly 350 K lower than the surface temperature Ts (Tv=790±100 K vs Ts=1140 K). The vibrational populations of As2 desorbed from Si(111) are not Boltzmann, but also have an effective vibrational temperature 400 K colder than the surface (Tv=730±100 K vs Ts=1140 K). The observed lack of accommodation in the desorption from both surfaces is discussed in terms of the possible mechanisms.
A comparison of approximate techniques for the determination of potential energy surfaces of ion–molecule charge transfer systems
Authors: Diaz B. Ramiro , Wahnon P. , Sidis V.
Published: July 2002

The (HO2)+ molecular ion is used to experiment two approximate procedures which aim at reducing the computation effort that is needed for the determination of potential energy surfaces of ion–molecule charge transfer systems. The two procedures involve configuration interaction (CI) calculations of moderate sizes and are based on diagonal corrections of the electronic Hamiltonian matrix in a basis of projected-valence bond (PVB) configuration-state functions (CSF). The PVB-CSF’s used in practice correspond to a full valence CI for each ionic or neutral partner as well as single excitations accounting for polarization and electron transfer. The diagonal corrections are of two sorts: (i) if insufficiently large orbital expansion bases are used they remove the relative ion–molecule basis set superposition error; (ii) if asymptotic energy levels of the involved neutrals or ions in their ground or valence excited states are misplaced they properly adjust these levels. When applied to (HO2)+ using a minimal or an extended orbital basis set expansion the proposed approaches yield concording results. The results also agree with the effective model potential (EMP) data of Grimbert et al. [Chem. Phys. 124, 187 (1988)] which have proved successful in the description of the H++O2 charge transfer dynamics. Comparison with fragmentary results from MRD-CI calculations by Vazquez et al. [Mol. Phys. 59, 291 (1986)] and Schneider et al. [Chem. Phys. 128, 311 (1988)] is somewhat mitigated. The method should be particularly useful for bulky ion–molecule systems.
Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. II. Electron-impact dissociative ionization
Authors: Syage Jack A.
Published: July 2002

Dissociative-ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron-impact excitation of jet-cooled NH3, hydrazine (N2H4), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time-of-flight mass detection to ensure field-free excitation and high collection efficiency for energetic ions. Cross sections for NH3 ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument-dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N2H4 and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å2 (NH3), 3.76 Å2 (N2H4), and 4.20 Å2 (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time-of-flight peak-shape analysis. Mean KE values <εt≳ are reported for all fragment ions studied. For 170-eV excitation of NH3, <εt≳ varied from 0.026 eV (NH2+) to 1.4 eV (H+). The kinetic energies for N2H4 and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH3, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N2H4 and MMH dissociative ionization based on the appearance of ions such as NH3+ and NH4+ (the latter for MMH ionization only) and the magnitude of <εt≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N2H4+.
Size selectivity in adsorptions of polydisperse hard-rod fluids in micropores
Authors: Tang Zixiang , Scriven L. E. , Davis H. T.
Published: July 2002

The exact statistical mechanics of the one-dimensional classical hard-rod mixtures in an external field is reformulated. For any number of components and arbitrary size distribution, the problem is reduced to solving only two auxiliary functions. The component density profiles, thermodynamic properties, and correlation functions are all explicit functionals of these two functions. Application is made to study the size selectivity of a hard-rod mixture, with a uniform size distribution, confined between hard walls. Approximate theories for inhomogeneous three-dimensional fluid mixtures, based on simple generalizations of the exact results in one dimension, are discussed.
Resonance electron scattering by oriented O2, N2, and CO and application to physisorption on graphite
Authors: Hock K. M. , Palmer R. E.
Published: July 2002

In this paper, we explore the connections between resonance electron scattering by isolated and physisorbed molecules. The multiple scattering Xα method is used to calculate cross sections for electron scattering via the σ shape resonances of O2, N2, and CO, near 9, 22, and 20 eV, respectively. Special emphasis is placed on the O2 resonance, for which no previous theoretical work has been reported. In all three cases, quantitative agreement is obtained with experimental gas phase scattering results. Angular distributions are then calculated for the isolated, oriented molecules, and compared with the angular distributions recently observed in resonance scattering by O2, N2, and CO molecules oriented by physisorption on graphite. Characteristic nodes observed in each of the angular distributions are related to the calculated angular profiles, together with a previously proposed selection rule which we now formalize. This scheme allows the orientation of the molecules on the surface to be determined.
Alternative approach to stationary perturbation theory for separable problems
Authors: Fernández Francisco M.
Published: July 2002

A new implementation of stationary perturbation theory for separable quantum-mechanical problems is presented which allows the calculation of perturbation corrections for all the states simultaneously, so that the results are expressed in terms of the quantum numbers of the unperturbed system. The anharmonic oscillator, the Stark effect in a rigid rotor, and a caged harmonic oscillator are considered as illustrative examples.
Theoretical and experimental structures of vinyl chloride and vinyl bromide
Authors: Coffey Dewitt , Smith Brian J. , Radom Leo
Published: July 2002

The structures of vinyl chloride and vinyl bromide have been determined by ab initio molecular orbital calculations, and corrections applied to obtain theoretical r0 values. The theoretical r0 structures agree very well with experimental r0 structures derived from recent microwave spectral data. This contrasts with the situation for vinyl alcohol and vinyl fluoride for which significant discrepancies have been found between theoretical and experimental estimates of the CCHu angles involving the hydrogen atom geminal to the substituted center. The present results reinforce our earlier conclusion that the theoretical values for this parameter for vinyl alcohol and vinyl fluoride are more reliable than the experimental values. Difficulties in the experimental structural determinations for vinyl fluoride and vinyl alcohol are attributed to the unusual insensitivity of the experimental moments of inertia to the values of the CCHu angles, resulting in turn from the close proximity to a principal inertial axis of two of the atoms that define the CCHu angles in these molecules. In contrast, all three atoms defining the CCHu angles in vinyl chloride and vinyl bromide are far removed from the inertial axes. As a consequence, the experimental moments of inertia are sensitively dependent on the value of the CCHu angle, and theoretical and experimental structures agree well. We conclude that suggestions that substituted ethenes present unusual difficulties for ab initio structural determinations are unfounded. Sensitivity curves are suggested as valuable indicators of potential problems in experimental structural determinations.
Discrete-variational-Xα calculations of buckminsterfullerene (C60) and fulleride x-ray emission spectra
Authors: Kawai Jun , Maeda Kuniko , Takami Michio , Muramatsu Yasuji , Hayashi Takayoshi , Motoyama Muneyuki , Saito Yahachi
Published: July 2002

X-ray emission spectra of buckminsterfullerene (C60) and alkali-metal fullerides (M3C60, where M denotes alkali-metal) are calculated with the discrete-variational (DV)-Xα method. The calculated x-ray emission spectra of C60 agree well with experimental x-ray spectra. This indicates that the x-ray emission spectrum of M3C60 calculated with the same method provides a good prediction of fulleride spectra which have not been measured yet. The calculated x-ray emission spectra are proposed to be used as standards of the electron probe microanalyzer (EPMA) spectra of fullerene and related compounds for searching the existence of such compounds in carbon mixtures.
Molecular dynamic simulations of the infrared dielectric response of silica structures
Authors: Anderson D. C. , Kieffer J. , Klarsfeld S.
Published: July 2002

The molecular dynamic simulation technique was used to model the vibrational behavior of crystalline (α and β cristobalite) and amorphous silica structures. To this end a refined potential function was developed, which allows one to reproduce the correct structural geometries, the corresponding infrared spectra, and to observe a reversible phase transformation between α and β cristobalite. The complex dielectric constants in the infrared frequency range were calculated from the dipole moment time correlation functions. While idealized cristobalite exhibits the simplest spectrum with only two narrow bands, the increase of structural complexity and reduction of symmetry characteristic for the real cristobalites and amorphous silica, creates additional features in the infrared spectra. These structural changes predominantly affect the coordination of oxygen, and generate a broader spread in the normal modes characterizing the vibrations of this species. A unique method for the identification of atomic trajectories corresponding to the mechanisms responsible for individual absorption bands, which is based on the concept of Fourier transform filtering, is used for the assignment of absorption bands to atomic motions.
Rate coefficient expressions for reactions of molecules in 2Π electronic states at low temperatures
Authors: Wickham A. G. , Clary D. C.
Published: July 2002

Analytical rate coefficient expressions are derived at low temperatures for the fast reactions of a 2Π molecule with each of a structureless ion, a 2Π molecule, and a 1Σ molecule. The procedure, based on classical capture theory, involves perturbation theory and simple long-range electrostatic potential terms for the various interactions. The open-shell nature of the reactant molecules is fully considered. The predictions made are contrasted with those quoted previously for the equivalent reactions involving 1Σ molecules and interesting differences in the temperature dependencies of the rate coefficient are highlighted.
Nuclear magnetic resonance study of deuterium gas
Authors: Bogdan M. , Jeffrey K. R. , Armstrong R. L.
Published: July 2002

Measurements of the nuclear spin longitudinal relaxation time in deuterium gas through the region of the characteristic relaxation time minimum are reported at 55.6 MHz at 293, 220, and 171 K. The data are discussed in the context of the kinetic theory-memory equation approach of McCourt et al. [Can. J. Phys. 53, 2449 (1975)]. The data are adequately described solely in terms of the relaxation of the ortho species and neglecting any contribution from inelastic collisions.
Angular distributions of sputtered zirconium atoms
Authors: Whitaker T. J. , Li Aijun , Jones P. L. , Watts R. O.
Published: July 2002

Ion beams with energies in the range 1–2 keV are used to sputter neutral Zr atoms from a polycrystalline surface. Laser induced fluorescence detection is used to obtain angular distributions of sputtered neutrals as a function of ion impact direction, ion mass, ion energy, and spin–orbit state of the exiting atoms. About 40% of the sputtered atoms are excited. Angular distributions depend weakly on ion mass and energy. The angular distributions are fitted well by a modified form of the Roosendaal and Saunders model of sputtering.
A new perspective on quantum time correlation functions
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

A theoretical analysis suggests that the path centroid variable in Feynman path integration occupies a central role in the behavior of the real time position autocorrelation function. Based on this analysis, an intriguing quasiclassical perspective on quantum correlation functions emerges.
Electron spin resonance investigation of Sc+2 in neon matrices and assignment of its ground electronic state as X 4Σ−. Comparison with theoretical calculations
Authors: Knight Lon B. , McKinley Allan J. , Babb Robert M. , Hill Devon W. , Morse Michael D.
Published: July 2002

The discandium radical cation, Sc+2, has been isolated in neon matrices at 4 K and studied by electron spin resonance (ESR) spectroscopy and theoretical methods. It was produced by the x-irradiation of neon matrix samples containing neutral Sc2 which was formed by trapping the products generated from the pulsed laser vaporization of scandium metal. The experimental and theoretical findings indicate that Sc+2 has a 4Σ−g electronic ground state compared to an X 5Σ−u state for the neutral discandium radical. The large decrease in the 45Sc hyperfine interaction (A tensor) going from Sc2 to Sc+2 provides direct experimental information concerning the types of valence molecular orbitals that are involved in these diatomic radicals. The neon matrix magnetic parameters for 45Sc+2 are g∥≊2.00, g⊥=1.960(1), ‖A∥‖=28(6), and ‖A⊥‖=26.1(3) MHz; the D value (zero field splitting) was 15 381(3) MHz. Ab initio configuration interaction (CI) calculations of the nuclear hyperfine interactions yielded results in reasonable agreement with the experimental observations.
Local quantum chemistry. Implementation of the local space approximation at the ab initio Hartree–Fock level
Authors: Robins Kathleen A. , Kirtman Bernard
Published: July 2002

The local space approximation (LSA) method for embedding a finite cluster into its surroundings is implemented at the ab initio restricted Hartree–Fock level of theory. Our procedures for handling the initial combination of fragments in a way that takes full account of overlaps, and for obtaining the transformed two-electron integrals used in the local space self-consistent field (SCF) treatment, are presented in detail. Preliminary tests on small systems give results similar to those obtained in previous semiempirical calculations; in every case, the smallest physically meaningful local space yields an energy within about 1 kcal/mol of the ‘‘exact’’ full space value.
Martynov–Sarkisov integral equation for the simple fluids
Authors: Sarkisov G. , Tikhonov D. , Malinsky J. , Magarshak Yu.
Published: July 2002

Results of approximate integral equation calculations both for the square-well and Lennard-Jones fluids in the supercritical region are reported, and a comparison with Percus–Yevick and hypernetted chain theories is made. A relation between the effective diameter of particles and values of the mean force potential along the line of vanishing of solutions has been found.
Atomic orbital basis set optimization for ab initio calculations of molecules with hydrogen atoms in strong magnetic fields
Authors: Kappes U. , Schmelcher P.
Published: July 2002

We present the results of the optimization of a recently established atomic orbital basis set for ab initio calculations of molecules in strong magnetic fields. The optimization is performed for the ground and many low-lying excited states of the hydrogen atom in the presence of a magnetic field of arbitrary strength. The behavior of the nonlinear variational parameters of the atomic orbitals is discussed as a function of the field strength. In order to demonstrate the suitability of our atomic orbital basis set for the calculation of molecular spectra and wave functions in a uniform magnetic field we present, as first results of a molecular calculation, electronic potential energy curves for the H+2 ion in a magnetic field.
Two-dimensional electron paramagnetic resonance spectroscopy of nitroxides: Elucidation of restricted molecular motions in glassy solids
Authors: Dubinskii Alexander A. , Maresch Günter G. , Spiess Hans-Wolfgang
Published: July 2002

The combination of concepts of two-dimensional (2D) spectroscopy with the well-known field step electron–electron double resonance (ELDOR) method offers a practical route to recording 2D ELDOR spectra covering the full spectral range needed for electron paramagnetic resonance (EPR) of nitroxide spin labels in the solid state. The 2D ELDOR pattern provides information about molecular reorientation measured in real time, the anisotropies of electron phase, and electron spin-lattice relaxation as well as nuclear spin-lattice relaxation all of which are connected with the detailed geometry of the molecular reorientation. Thus, in 2D ELDOR the same electron spin probes the motional behavior over a wide range of correlation times from 10−4 to 10−12 s. An efficient algorithm for simulating 2D ELDOR spectra is derived, based on analytical solutions of the spin relaxation behavior for small-angle fluctuations and offers a means of quantitatively analyzing experimental data. As an example, the motion of nitroxide spin labels in a liquid-crystalline side-group polymer well below its glass transition is determined as a β-relaxation process with a mean angular amplitude of 5° and a distribution of correlation times with a mean correlation time of 0.9×10−10 s and a width of 2.5 decades.
Theoretical studies of He(1S)+CH(X 2Π). I. Ab initio potential energy surfaces
Authors: Wagner Albert F. , Dunning Thom H. , Kok Randall A.
Published: July 2002

Potential energy surfaces have been determined for the A′ and A″ states of the He(1S)+CH(X 2Π) system. The interaction energies were computed using a Hartree–Fock singles and doubles CI treatment; convergence of the calculation with respect to both basis set and configuration set was investigated. The surfaces have been represented quantitatively by standard Legendre polynomial expansions and qualitatively by novel pairwise additive potentials. With the pairwise additive models, successful fits require that the centers of force be orbital based as opposed to the typical nucleus-based form. Comparison of the surfaces would suggest that the final fitted surfaces are accurate to a few tenths of a kcal/mol up to 5–10 kcal/mol. In the following paper, the dynamical consequences of the small differences between all the surfaces will be assessed by quantum dynamics calculations of cross sections and their subsequent comparison to experiment.
Pariser–Parr–Pople force field for π-electrons: Raman and infrared shifts of trans-polyacetylene
Authors: Soos Zoltán G. , Hayden Geoffrey W. , Girlando Alberto , Painelli Anna
Published: July 2002

The transfer integrals t(R) and Coulomb potential V(R) of π-electron Hamiltonians He define linear electron–phonon (e–ph) coupling constants t′(R) and V′(R) for the equilibrium structure. We generalize linear response (LR) theory for Raman and ir shifts due to π-electron delocalization in Hamiltonians with arbitrary t(R) and spin independent V(R). π-electron contributions ΔFij to the force field of trans-polyacetylene (PA) are obtained in the symmetry coordinates Si, with i=1–5, for k=0 phonons. We compare ΔFij for Hückel chains with alternating transfer integrals t(1±δ) and for Pariser–Parr–Pople (PPP) models with hydrocarbon parameters derived from π–π* spectra to a phenomenological ΔFij for trans-PA and its isotopes. An exponential rather than linear t(R) is found. The molecular PPP potential V(R) accounts quantitatively for π-electron coupling to CCC bends and for the length dependence of the Raman shifts of finite polyenes. The dominant but not exclusive π-electron coupling remains the dimerization coordinate singled out in previous treatments, with substantially larger t′(R) than V′(R) contributions in the PPP force field. We comment on extensions of LR theory to PA models with interacting π-electrons and several electronic susceptibilities.
Quantum scattering studies of vibrational excitation in collisions of NO(X 2Π) with a Ag(111) surface
Authors: Gregurick Susan , Alexander Millard H. , DePristo Andrew E.
Published: July 2002

We report the results of fully quantum close-coupled studies of vibrational excitation in collisions of NO(X 2Π) with a Ag(111) surface. The interaction potentials used were an extension, to include the dependence on the NO bond distance, of the recent corrected effective medium potential energy surfaces (PES) of Depristo and Alexander. [J. Chem. Phys. 94, 8454 (1991)]. The final state rotational distributions show evidence of at least four rotational rainbows, corresponding to scattering on (and interference between) the two PES which arise when the degeneracy of the NO molecule is lifted upon approach to the surface. A strong tendency is seen to populate the lower spin–orbit manifold at low to moderate final J, which disappears as J rises beyond 30.5 and the final states are better described in Hund’s case (b). Simultaneously, there exists a propensity to populate those Λ-doublet levels in which the electronic-rotational wave function is symmetric (ΠA′) with respect to reflection of the electronic coordinates in the plane of rotation of the scattered NO molecule. This feature is similar to what has been seen experimentally.
Randomly affected monomolecular reactions
Authors: Berlin Yuri A. , Drobnitsky Dmitri O. , Kuz’min Vladimir V.
Published: July 2002

A general approach to the description of monomolecular reactions affected by random impacts has been developed on the basis of irreversible random transitions theory. Theoretical results derived allow us to account for concrete forms of stochastic affections and to evaluate statistical characteristics of reaction systems with monomolecular processes. Kinetic curves described in terms of the proposed theory provide information on certain properties of the random media. Such an approach gives a possibility to include both classical chemical kinetics and a polychromatic approximation as two limit cases. Exact solutions are given in two relevant cases in order to illustrate the applicability of the theory developed by the authors.
State selective production of phosphorus ions via multiphoton ionization of atomic phosphorus
Authors: Harbol M. R. , Appling J. R. , Goren A. C.
Published: July 2002

The (2+1) resonance-enhanced multiphoton ionization spectrum of phosphorus atoms is reported for the wavelength range 300–317 nm. Atomic phosphorus is formed by multiphoton photodissociation of phosphorus tribromide vapor. Photoelectron spectroscopy conducted at phosphorus ionization resonances produces pure populations of P+(3PJ) and P+(1D2) ions. Production of a mixture of phosphorus ionic states indicates a clear propensity to preserve the ion core electron configuration upon ionization of phosphorus. Four spin-changing transitions are assigned supported by photoelectron spectral data.
Numerical study of the frequency spectra of dilute fluids beyond the mean spherical approximation
Authors: Ho/ye J. S. , Lomba E. , López-Martín J. L.
Published: July 2002

In this paper we investigate the frequency spectra of dilute fluids (or fluidlike disordered materials) composed of particles with embedded harmonic oscillators, using a recently proposed working scheme based on systematic corrections to the mean spherical approximation. The tests are performed in the low density regime where linear approximations are known to fail badly. A detailed comparison with computer simulation results shows that use of the improved theory correctly reproduces the qualitative features of the low density spectrum, whereas at high densities the mean spherical approximation results are recovered.
Asymmetries of a curved bilayer model membrane
Authors: Baumgärtner A.
Published: July 2002

The asymmetries of a simple model bilayer membrane with uniform curvature have been investigated using Monte Carlo methods. It is found that with increasing curvature the two layers of the membranes behave differently due to pure entropic effects. This is shown by means of three quantities; the densities of lipids, the orientational orders of the lipids, and the widths of the two layers. As an important characteristic of the asymmetry of curved bilayers, it is observed that the fluctuation of the orientational order in the inner layer can increase up to four times of its value for a planar bilayer, and can be more than two times larger than the fluctuations in the outer layer. The importance of the asymmetric fluctuations across a curved bilayer membrane for protein insertion into membranes is pointed out.
Orientation motion in glassy polymers
Authors: Patashinski Alexander Z. , Ratner Mark A.
Published: July 2002

We present a theory of orientation Brownian motion of a polar particle frozen-in in a polymer glass. The theoretical problem is then one of orientation dynamics in a viscoelastic medium. The glassy material provides, at a short-time scale, an elastic force that returns the orientation to that at time t=0. On larger time scales, this privileged orientation diffuses due to thermal motion. A cascade of orientation motions at all time scales results in a stretched-exponential behavior of the nonlinear optical response function and of other moments of the angular distribution function.
High-resolution 13C nuclear magnetic resonance in alkali intercalated fullerene C60
Authors: Reichenbach J. , Rachdi F. , Luk’yanchuk I. , Ribet M. , Zimmer G. , Mehring M.
Published: July 2002

We present the results of 13C nuclear magnetic resonance (NMR) measurements on potassium and rubidium doped Buckminster fullerene C60 at room temperature. By using high-resolution magic angle spinning NMR spectroscopy we were able to identify and characterize the different, thermodynamically stable phases. Based on an analysis of the different NMR spectra, we discuss the effects of the intercalation on the molecular dynamics as well as on the electronic properties in these compounds.
Superstable nonequilibrium states
Authors: Amini Afshin , Xu Yuanhang , Schell Mark
Published: July 2002

Relaxation times were measured from the decay to period-one attractors, a period-two attractor, and a period-three attractor in two reaction processes: the voltammetric oxidations of ethanol and 1-propanol. The results provide strong evidence that a periodic state that can undergo a subharmonic bifurcation is most stable near the point where the relaxation changes from a monotone to an alternating approach. The relation between the experimental results and theory is discussed and it is concluded that the reaction processes are in what is called the strongly contracting limit.