bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo trajectories: The reaction H + Br2 → HBr + Br
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2004

A quasiclassical trajectory study of the electronically adiabatic exothermic reaction H + Br2 → HBr + Br using 13 different semiempirical and empirical potential energy surfaces has been conducted. Detailed results are reported for 8035 reactive trajectories using Monte Carlo selection of initial conditions drawn from six different sets of distributions. For all the surfaces examined, the reaction is dominated by sideways collisions. The differential cross section for unit plane angle is sideways peaked but the differential cross section per unit solid angle peaks for molecular scattering angles (measured relative to the direction of the incident atom) in the range 156°–180°, i.e., backward scattering. There is a good correlation of scattering angle and initial orientation of the intermolecular axis, but it was not possible to use this correlation to guess a potential energy surface which yields a differential cross section in quantitative agreement with experimental results. The trajectory results, however, do indicate that a surface which has a low barrier or no barrier for collinear collisions yields too much forward scattering as compared to experiment. We find the computed angular distribution is not a strong function of atomic temperature in the range 300° to 5600°K or molecular temperature in the range 100° to 900°K. Under 300°K conditions the final state internal energy distributions are in good agreement with chemiluminescence results and at an atomic temperature of 2800°K the final translational energy distribution is in good agreement with molecular beam results. The reaction cross section is calculated to be a decreasing function of translational energy, in agrement with White and Su's interpretation of the hot atom studies; and the room temperature energy of activation is calculated to be only 0.2 kcal/mole which agrees with experiment within experimental error. The absolute values of the cross section and the room temperature rate constant are about a factor of 3 larger than experimental values. Many other details of the results are given and these may serve as a model study for the important mass combination where a light atom reacts with a heavy homonuclear diatomic molecule.
Target density dependence of molecule-molecule absolute total scattering cross sections
Authors: Dehmer P. M. , Wharton L.
Published: February 2004

The target density dependence of absolute total cross sections for LiF scattered by CO, CO2, CHF3, CH4, D2, and Ar was studied at a LiF velocity of 1.5 × 105 cm/sec and a target temperature of 297°K. The experiments were performed under conditions of high angular resolution in order to observe the full quantum mechanical cross section. For the systems LiF–CO, LiF–CO2, and LiF–CHF3 there was a substantial variation of observed cross section with density, while for LiF–CH4, LiF–D2, and LiF–Ar there was no observable variation. It is shown that the target transmission is the Laplace transform of the cross section distribution function. Information was obtained about the distribution of cross sections as a function of the internal state of LiF by fitting model distributions to the data. For the lower LiF rotational states the LiF–CO and LiF–CO3 cross sections are dominated by the dipole-quadrupole interaction and are determined at the outermost impact parameters by processes inelastic in the rotation of LiF, and inelastic and/or elastic in the targets CO of CO2; for the higher LiF rotational states the total cross section is determined mainly by elastic processes and is therefore dominated by induction and dispersion interactions. The dipole-quadrupole scattering effect, so important in microwave line broadening theories, thus has been identified in experiments of this type. For LiF–CHF3 the largest total cross sections are determined by dipole-dipole inelastic scattering. For those targets which showed no pressure dependence (Ar, D2, and CH4) the standard deviation of the cross sections was less than ∼15 percent of the mean as determined by the accuracy of the experiments and the range of target densities used. This is consistent with the symmetry of the interaction and the possible degree of anistropy. No rotational inelasticity could be inferred for these targets. A theoretical analysis of the distribution of cross sections based on dipole-quadrupole and dipole-dipole scattering processes is presented. Various fitted models for the distribution of cross sections can be evaluated in the light of these analyses.
Some molecular properties of LiH and LiD
Authors: Docken Kate Kirby , Freeman Richard R.
Published: February 2004

The results of both experiment and theory have been combined to produce some new and accurate properties of LiH and LiD. The measurement of some specific ground state hyperfine and Zeeman energies by molecular beam electric resonance (MBER) is discussed. The importance of vibrationally averaging expressions for molecular constants obtained from accurate experimental measurements as well as molecular properties obtained in theoretical ab initio calculations using well-correlated wavefunctions is emphasized. New values for the electric quadrupole moment of 7Li and D are presented: QLi = −4.26×10−26 cm2 and QD = 2.9×10−27 cm2. In addition, as a check on both experiment and theory, the molecular rotational gJ factor and spin-rotation interaction constants are calculated and compared.
Spectra of synthetic zeolites containing transition metal ions. V.* Π complexes of olefins and acetylene with Co(II)A molecular sieve
Authors: Klier K. , Kellerman R. , Hutta P. J.
Published: February 2004

The electronic spectra of the cobalt(II) ion in anhydrous cobalt(II) zeolite A in the presence of adsorbed unsaturated hydrocarbons demonstrate that olefins and acetylene are π bonded to cobalt in well defined surface complexes. Gravimetric adsorption data show that one ethylene molecule is bound by each cobalt ion with an energy of about −68 kJ mol−1. Theoretical analysis treating the unsaturated hydrocarbon as a low symmetry electrostatic perturbation to a tetrahedral Co(II) ion gives a quantitative interpretation of the band splitting in the electronic spectra. This splitting is distinguished from that occurring through the dynamic Jahn-Teller effect in purely tetrahedral Co(II) complexes. The low symmetry field contributions to the ligand field stabilization energies have been determined from the fine structure in the experimental spectra and have been found to decrease from ethylene to trans-2-butene in accord with increasing steric hindrance from substitutional methyl groups. These contributions are small, however, and do not exceed 5 kJ mol−1. The spectrum of the Co(II) zeolite A acetylene complex is similar to that of the ethylene complex but shows in addition features characteristic of charge transfer processes.
Photoionization spectrum of solvated electrons
Authors: Aulich Hubert , Nemec Ladislav , Delahay Paul
Published: February 2004

Photoionization cross sections are reported for solvated electrons at photon energies from 1.2–4.1 eV and the sodium species Na− (2.8–4.1 eV) in hexamethyl phosphoric triamide (HMPA). Cross sections are obtained by the recently developed method of energy distribution curve (EDC) superposition for photoelectron emission by solutions of sodium metal (5–96 mM liter−1) in HMPA. The EDC's, spectral response curves for quantum yield, and cross section curves are discussed, and photoionization mechanisms are suggested for the Na− species. Experimental methods are briefly outlined. This seems to be the first determination of cross sections for solvated electrons and the Na− species in a liquid solution of high ionic conductivity.
Compton profile and electron momentum distribution of water
Authors: Tanner Anthony C. , Epstein Irving R.
Published: February 2004

The Compton profile and electron momentum distribution of water have been calculated using a number of self-consistent-field wavefunctions of various types, including a near Hartree-Fock function. The results are found to be sensitive to the choice of basis functions. Comparison with recent experimental data shows good agreement, with the effects of improved wavefunction quality paralleling those of certain experimental corrections.
Collisional exchange of vibrational energy between CO(v = 1) and CS2: Evidence for dominant multiquantum effects
Authors: Richman Dennis C. , Millikan R. C.
Published: February 2004

Vibration to vibration energy transfer rates for the CO–CS2 system have been experimentally determined between 198 and 371 K. At 300 K, the V-V rate was found to be 1.36×104 torr−1·sec−1, orders of magnitude faster than would be expected from a system having a fundamental band center energy mismatch of 608 cm−1, if transfers to overtone and combination bands are neglected. Experimental and theoretical evidence are presented which indicate that the observed rates arise from a multiquantum process. The implications for CS2 and CS2–O2(CO) laser systems are discussed.
Use of semiempirical models for calculation of B terms in MCD spectra. II. The Pariser-Parr-Pople (PPP) model: A consequence of the pairing symmetry in π systems with alternant skeletons
Authors: Michl Josef
Published: February 2004

Within the PPP model, MCD spectra of two π-electron species which are paired in the sense of alternant symmetry are mirror images of each other. Consequences of this theorem for the understanding of MCD spectra of substituted alternant hydrocarbons are pointed out.
Charge transfer between O− ions and O2 molecules in the ground state and singlet delta excited state
Authors: Mathis R. F. , Snow W. R.
Published: February 2004

A crossed-beam apparatus has been used to study charge transfer between O− ions and O2 molecules in the ground state and in the metastable, singlet delta, excited state. The energy of the incident ions ranged from 10 to 10 000 eV. The cross section for charge transfer between O− and ground state O2 was found to have a maximum of 7.8×10−16 cm2 at about 5 keV. At the lower energies there is good agreement with the previous measurements of Snow et al. and Rutherford and Turner who also used crossed beam techniques. The cross section for charge transfer between O− and metastable excited O2 (1Δg) was found to be less than 1×10−16 cm2 over the energy range of 100 to 5000 eV. There are no other experimental measurements for comparison.
Dielectric properties of ice VII and VIII and the phase boundary between ice VI and VII
Authors: Johari G. P. , Lavergne A. , Whalley E.
Published: February 2004

The dielectric properties of H2O ice VII have been measured in a piston-cylinder cell. The limiting low-frequency permittivity implies a value of the orientational correlation parameter g that increases from 1.41 at 0°C to 1.54 at 70°C. This is about a half the corresponding value for ice I and suggests that the molecular axes are on the average more antiparallel to one another than required by the ice rules, and are consequently not fully disordered. The VI-VII and VI-VIII phase boundaries for both H2O and D2O have been measured. The entropy of the transition VII-VIII is 0.66 cal K−1 · mole−1 for H2O and 0.64 cal K−1 · mol−1 for D2O, which are 0.15 and 0.17 cal K−1 · mole−1 less than expected if ice VIII were fully ordered and ice VII were fully disordered within the limits of the ice rules. Both dielectric and phase boundary measurements therefore indicate incomplete disorder in ice VII.
The Goddard expansion and the kinetic theory for solutions of rodlike macromolecules
Authors: Abdel-Khalik S. I. , Hassager Ole , Bird R. Byron
Published: February 2004

The Goddard memory-integral expansion for the stress tensor of viscoelastic fluids differs from previous expansions in that it uses a deformation rate referred to a co-rotating reference frame. It is shown how to get the kernel functions in the Goddard series in terms of the structural parameters occurring in the kinetic theory of macromolecular solutions. That the series has much better convergence properties than previously used expansions is demonstrated by comparison with exact kinetic theory results for steady-state shear flow and elongational flow.
Silanium SiH5+. The protonation of silane and the chemistry of silanium ions studied by ion cyclotron resonance spectroscopy
Authors: Sefcik Michael D. , Henis Jay M. S. , Gaspar Peter P.
Published: February 2004

The formation of the silanium ion SiH5+ and its chemistry are examined and inferences drawn concerning the structure of SiH5+. Rate constants for proton transfer from CH5+ to SiD4 and from NH2+ to SiH4 are determined to be (0.9±0.3)×10−10 and (2.5±0.5)×10−10 cm3 molecule−1·sec−1, respectively. In the reaction SiD4H+ + NH3 → SiNH2D3+ + HD there is no concomitant loss of D2. Structural and hence chemical nonequivalence of hydrogens in the silanium ion is inferred and discussed in terms of models for silanium ion structure.
Electron nuclear triple resonance of free radicals in solution
Authors: Dinse K. P. , Biehl R. , Möbius K.
Published: February 2004

Electron nuclear triple resonance (TRIPLE) experiments on free radicals in solution not only give considerable signal-to-noise improvement as compared to ENDOR, but also decrease the observed linewidth and give additional information about the number of protons contributing to a particular hfs constant. The experimental setup is described and the results are discussed in terms of a phenomenological theory.
Relativistic molecular symmetry spinors for diatomics
Authors: Oreg J. , Malli Gulzari
Published: February 2004

The general form of the four component symmetry spinors for a relativistic electron in a diatomic potential is given. A full discussion of the symmetry operations of the double group of the diatomic molecular Dirac Hamiltonian is also presented.
Quantum calculations of planar reactive H + H2. III. ``Labeled nuclei'' and angular distributions
Authors: Altenberger-Siczek A. , Light J. C.
Published: February 2004

The quantum calculation of reactive scattering of H + H2 in a plane has been modified by the utilization of a new method for projection of the initial rotational manifold onto the final one at the matching surface. This bifurcation method corresponds to the ``labeled nuclei'' calculation and yields predominant backward scattering for the angular distributions of products in accord with those expected on both theoretical and experimental grounds. Results are presented and comparisons with previous work are made.
Microdipole model for trapped electrons in nonpolar liquids and glassy solids
Authors: Feng Da-Fei , Kevan Larry , Yoshida Hiroshi
Published: February 2004

A model is developed to treat the binding energy of excess electrons in nonpolar liquids and solids such as alkanes. Interaction of the excess electron with C–H bond dipoles is considered for several geometrical arrangements. On this basis it is found that a binding energy of about 0.5 eV can be explained for trapped electrons in condensed alkanes. Although long range polarization interactions are unimportant, the electron interaction with the bond microdipoles of a net nonpolar molecule appears to be the key to understanding electron binding in alkane matrices.
Dipolar contribution to NMR spin-lattice relaxation of protons
Authors: Freeman Ray , Hill H. D. W. , Tomlinson B. L. , Hall L. D.
Published: February 2004

Dipole-dipole interactions between protons in a glucopyranose derivative have been studied by means of the nuclear Overhauser effect and by spin-lattice relaxation experiments. The Overhauser enhancement measurements utilize a new pulse technique which permits the saturating field to have any specified spectral density function so that finite frequency bands in the NMR spectrum can be irradiated. By this method, the dipolar interactions between the various pairs of protons are evaluated. In contrast, the dipolar relaxation of a chosen proton by all other protons in the sample has been determined by comparing the inversion-recovery rates after selective and nonselective 180° pulses. The relaxation measurements indicate that the relaxation in this molecule is purely dipolar in origin. The Overhauser enhancement measurements of one proton are used to determine the contribution to its relaxation from two neighboring protons and, from these results, information about the geometry of the pyranose ring is evaluated.
Magnetic rotation spectroscopy of tetraphenyl tin
Authors: Esherick Peter , Kohler Bryan E.
Published: February 2004

Magneto-optical rotary dispersion and magnetic circular dichroism studies of the lowest energy singlet to triplet transition of single crystal tetraphenyl tin which clearly demonstrate the feasibility of magnetic rotation spectroscopy on uniaxial organic molecular crystals are reported. Special considerations for these types of experiments are discussed. A simple model where only the field-induced mixing of the triplet sublevels is considered completely accounts for the observed magneto-optical activity. Symmetry assignments derived from these studies agree with those reported earlier.
Fine structure, hyperfine structure, and Stark effect in the NO A 2Σ+ state by optical radio-frequency double resonance
Authors: Bergeman T. , Zare R. N.
Published: February 2004

Using the Se I 1961 Å spectral line to pump the J = 3/2 component of the v = 3, N = 1 level of the NO A 2Σ+ state, we observe the zero-field rho-doubling hyperfine transitions by optical radio-frequency double resonance. The Stark shift of one of the resonance components is measured to determine the permanent electric dipole moment. An analysis of the resonance line shapes yields the following molecular constants: The rho-doubling constant γ = −82.91±0.03 MHz; the hyperfine parameters of the 14N nucleus b = 40.29±0.07 MHz, c = 2.25±0.11 MHz, and eqQ = −2.88±0.17 MHz; and the dipole moment μ = 1.10±0.03 D, where all errors represent three standard deviations. The value of γ is explained reasonably well by calculations of the A−X spin-orbit mixing from which we conclude that variations of γ with v and N are small. The values of the hyperfine parameters b and c are within 6% and 18% of recent CI calculations by S. Green, while the less well determined values of eqQ differ by about a factor of 2. The measured dipole moment differs considerably from the CI value, μ = 0.40±0.10 D. We speculate on possible explanations for this surprisingly large discrepancy.
Molecular orbital studies of dissociative chemisorption of first period diatomic molecules and ethylene on (100) W and Ni surfaces
Authors: Anderson Alfred B. , Hoffmann Roald
Published: February 2004

The extended Hückel molecular orbital method is used to examine interactions of Li2, B2, C2, N2, CO, NO, O2, and F2 with nine atom clusters representing W(100) and Ni(100) crystal surfaces. The following predictions are made and are corroborated by experimental facts when available: (1) A strong tendency for charge transfer between the substrate and adsorbate exists as would be expected from electronegativity differences. (2) The adsorbed molecules display a tendency to dissociate because of Coulombic repulsions and frequently because of the filling of antibonding levels as well. At the same time Li2 and F2 form ionic bonds with the surface and covalent character is also evident for the others. (3) Strong bonding interactions form between the adsorbates and surfaces for any adsorbate orientation or position. Thus a connection is made with physical theories which exclude atomic detail. Such surface homogeneity can be resolved into detail with molecular orbital methods, but the emphasis in this paper is on examining orbital interactions between adsorbates and substrates with small regard to energy changes. (4) Active sites, which are atoms on a surface which have stronger chemisorptive capability, are found on steps and corners of metal clusters. These atoms, which stick out from the bulk, gain extra electronic charge as a result of orbital orthogonality and they form stronger bonds with electrophilic adsorbates, sometimes also strengthening the diatomic bond a little. This suggests rough surface areas might initially attract electrophilic adsorbates, but that dissociation could happen in a nearby region. (5) Photoemission spectra for CO on W(100) and Ni(100) and ethylene on Ni(111) are considered in some detail. The calculations corroborate the spectra. Comments are made on spectra of N2, N2O, NO, and O2 on W(100), CO on W(110), and ethylene on W(100) and Ni(100). Bandwidths and positions are discussed. In general the extended Hückel molecular orbital method appears to be a valuable tool in surface chemisorption and catalysis studies.
High pressure studies of the electronic behavior of bianthrones
Authors: Fanselow D. L. , Drickamer H. G.
Published: February 2004

High pressure optical studies have been made on a series of bianthrones dissolved in polymethyl methacrylate (PMMA). At sufficiently high pressure at room temperature, significant occupation of the photochromic (B) state occurs, but the yield is different for the different derivatives; it is especially small for the 4,4″ dimethyl bianthrone, where steric effects are important. From data obtained at 90°C at high pressure, it was established that equilibrium yields are obtained. From the pressure coefficient of the equilibrium constant, it was possible to calculate ΔΔ̄V̄A B, the change in partial molar volume upon conversion. The photochromic (B) state has a smaller volume but also a smaller compressibility than the A state. A comparison between bianthrone in PMMA and in polystyrene (PS) demonstrates that the medium affects the yield. The shifts and half-width changes of the optical absorption peaks are used to characterize the configuration coordinates involved in optical excitation. High pressure data at low temperature (−80°C) demonstrate that the photochromic yield decreases rapidly with increasing pressure.
Dependence of impurity zero-phonon thermal broadening on vibron level in organic crystals: A manifestation of the crystal anharmonicity
Authors: Burke Francis P. , Small Gerald J.
Published: February 2004

Thermal broadening data on the zero-phonon impurity bands in the 4500 Å 1B1 ← 1A1 and 3500 Å 1A1 ← 1A1 absorption systems of 1,3-diazaazulene and azulene in a naphthalene host are reported. For both systems, a marked dependence of the thermal broadening on the final impurity vibron level of the transition is observed. The broadening for all fundamental vibron bands studied is smaller than the broadening of the origin or zero-vibron band. This fact allows one to attribute the above vibron level dependence to a specific anharmonic interaction term (a quartic mixed vibron-phonon term). It is this term which describes the dependence of the quadratic off-diagonal electron-phonon coupling on impurity vibron level. Apparently the vibron-phonon anharmonic terms which lead to temperature dependent vibron relaxation processes are negligible. We report also the unexpected observation of a dependence of zero-phonon band thermal broadening on crystal samples. No correlation between this dependence and residual widths is observed.
SCF-X αSW calculations on PH3 using a nonempirical scheme for choosing overlapping-sphere radii
Authors: Norman Joe G.
Published: February 2004

Detailed results of two SCF-X αSW calculations on PH3, one with phosphorus d-type partial waves (2) and one without (1), are presented and compared with the best Hartree-Fock calculations. Ionization energies for the 5a1, 2e, and 4a1 levels, respectively, are 10.61, 13.42, and 20.55 eV from 1 and 10.37, 13.60, and 20.22 eV from 2, in better agreement with the experimental values of 10.59 and 13.5 eV for the first two levels than any previous first-principles calculation. Total energies are −342.6114 and −342.6449 hartree from 1 and 2, respectively. Contour maps of the valence-level wavefunctions and total valence charge density are presented; phosphorus d functions are found to contribute significantly (about 15%) only to the wavefunction for the 2e level, their effect being to promote better P–H overlap. Overlapping atomic sphere radii for the above calculations were chosen by a nonempirical scheme involving satisfaction of the virial theorem and the relative sizes of spheres around each atom which enclose the atomic number of electrons in the initial X αSW molecular charge distribution. Results of several complete PH3 calculations with varying sphere radii, hydrogen α parameter, and outer sphere position, and their role in developing the scheme for choosing radii, are discussed. Overlapping of the spheres improves the quantitative agreement of the eigenvalues with experiment, but the ordering and semiquantitative spacing of the levels are insensitive to variation in any of the above parameters.
Ionizing collisions of electrons with molecular hydrogen
Authors: Shingal R. , Srivastava B. B. , Khare S. P.
Published: February 2004

The recently proposed semiempirical expression by Jhanwar et al. for ionization cross section per unit energy range has been utilized to calculate the energy spetrum of secondary electrons, total ionization cross sections, stopping power, and mean energy expended per ion pair for the collision of electrons with hydrogen molecules in the energy range of 50 eV–10 keV. Satisfactory agreement with the available experimental data and theoretical results has been obtained.
Application of the higher order modified WKB method to the Lennard-Jones potential
Authors: Wald S. S. , Lu P.
Published: February 2004

We solve the phase shifts of Lennard-Jones potential at low energy by the modified WKB approximation in order to demonstrate the validity of the modified WKB approximation. We choose to solve exactly a model three-turning-point problem. The differences between the potentials of the actual and the model problem are taken care of by WKB type perturbations. We report here the detailed calculations to the order of ℏ2. Although the model potential is somewhat different from the actual potential (except that they have an equal number of turning points), the higher order terms do contribute to such an extent that the agreement of the phase shifts with the numerical values is satisfactory even near the maximum of the potential where the conventional WKB method is known to fail.
Rotational energy distribution in a nozzle beam
Authors: Kukolich S. G. , Oates D. E. , Wang J. H. S.
Published: February 2004

The fraction of beam molecules in J = 0, J = 1, and J = 2 rotational states of OCS was measured as a function of pressure behind a nozzle beam source. The fractional populations were obtained from the intensities of microwave emission and absorption signals observed with a beam maser spectrometer. Total beam intensities were measured also. Large deviations from a Boltzmann rotational energy distribution were observed. The ``effective rotational excitation temperature'' obtained from fractional population was near 15°K for J = 0, 40°K for J = 1, and 170°K for J = 2 with 150 Torr nozzle pressure.
CN photodissociation and predissociation chemical lasers: Molecular electronic and vibrational laser emissions
Authors: West Gary A. , Berry Michael J.
Published: February 2004

Molecular electronic [CN*(A2Π3/2, v′ = 0) → CN(X2Σ+, v″ = 0,1,2)] and vibrational [CN† (X2Σ+, v′) → CN(X2Σ+, v′ − 1), v′ = 5,4,3,2] cyanide radical laser emissions have been observed following photodissociative and predissociative fragmentation of cyanide parents [HCN, ClCN, BrCN ICN, (CN)2, CH3NC, CF3CN, C2F5CN] at λ ≥ 1550 Å. The observed transitions and their relative gain coefficients and quenching behaviors are used in connection with spectroscopy and fluorescence experiments to formulate a state-to-state photochemical reaction mechanism, viz., RCN lim →ℏ ωR+CN*(A2Πi,v′=0 primarily),CN*(A2Πi,v′=0)+M→ CN†(X2Σ+,v″=4 primarily)+M,in which nascent photochemical products are formed with high electronic population inversion [NCN*(A)/NCN(X) ≫ 1] and no vibronic inversion within the A2Πi manifolds (i.e., v = 0 is the principal product vibronic state). Near-resonant collision-induced intersystem crossing is shown to preferentially populate v = 4 within the X2Σ+ electronic manifold. Electronic and vibronic population inversions are treated via state-to-state adiabatic correlation diagrams and a simple dynamical model for reaction product vibronic state distributions. Anomalous laser behavior in the BrCN photodissociation system is rationalized via an intramolecular near-resonant E → E′ energy transfer scheme.
Exchange energy in the Thomas-Fermi model
Authors: Handler George S.
Published: February 2004

The concept of exchange energy as it arises in Thomas-Fermi theory is examined. It is found that the imposition of Hermitian symmetry upon the single particle Thomas-Fermi density matrix changes the single particle exchange energy density operator in a drastic way.
Vacuum ultraviolet absorption spectrum of the van der Waals molecule Xe2. I. Ground state vibrational structure, potential well depth, and shape
Authors: Freeman D. E. , Yoshino K. , Tanaka Y.
Published: February 2004

The vacuum ultraviolet absorption spectrum of the van der Waals molecule Xe2 has been photographed at high resolution. Ground state vibrational spacings ΔG(v+½) with v = 0–9 have been extracted from the vibronic structure of two electronic transitions. An extrapolation procedure based on an assumed inverse sixth power long-range potential reveals that the ground electronic state of Xe2 supports 25 or 26 bound vibrational levels. Spectroscopic values for the well depth, 195.5 cm−1, and zero-point energy, 10.4 cm−1, are compared to results from recent molecular beam scattering experiments. The shapes of the spectroscopic and scattering potentials are examined by means of Rydberg-Klein-Rees plots of well depth vs well width.
Energy distributions of field emitted electrons from tungsten in the presence of adsorbed CO
Authors: Young Peter L. , Gomer Robert
Published: February 2004

Energy distributions were measured on the (110), (100), (111), and (210) planes under various conditions of CO adsorption. Resonances were seen for the low temperature layer at −7 to 7.5 eV below vacuum on all planes except (100). A comparison of the widths and maximum amplitudes of the resonances shows good agreement with the theory of field emission from adsorbate covered surfaces. On heating, the resonances disappeared at ≤350 K corresponding to conversion of the low temperature states to beta-precursor states. Readsorption on beta or beta-precursor layers led to resonances at −7 eV on (210) and (111), but not on (110) or (100). The resonances are interpreted as arising from interaction of CO–π* orbitals with tungsten d-t2g orbitals in adsorption via the C atom on single W atoms. Disappearance of the resonances on heating is interpreted as activated rearrangement to new bonding configurations-for instance, bridge bonding via C-sp2 orbitals. The intensity of the resonances increases initially with coverage but, on all planes except (210), goes through a maximum and vanishes for θ<1. This is interpreted as interference of neighboring CO molecules, resulting in the squeezing out of charge from the labile-π orbitals responsible for the resonance. The details of intensity vs θ curves on different planes support this model. On the (100) plane, a resonance at −6.9 eV appeared at 600 K and is attributed to W–O bonding. A comparison of the temperature behavior of this resonance for O and CO covered emitters leads to the conclusion that CO on the (100) plane is rearranged but not dissociated up to at leat 820 K.
Potential model of dye molecule from measurements of the photocurrent in monolayer assemblies
Authors: Schoeler U. , Tews K. H. , Kuhn H.
Published: February 2004

A semitransparent Al layer is coated with several arachidate layers, one monolayer of quinquethienyl (mixed with arachidate), and some layers of arachidate; the arrangement is contacted with mercury. A bias voltage is applied and the photocurrent is measured as a function of the applied voltage and the temperature for different positions of the dye within the layer assembly. The photocurrent depends on the detailed assemblage, demonstrating the possibility of constructing photoelectric devices based on a planned molecular architecture. The results are in agreement with a simple theoretical model. A reasonably detailed description is obtained for the shape of the potential profile of the excited π electron of the dye responsible for the photocurrent.
Properties of binary tin chalcogenides determined by Mossbauer spectroscopy
Authors: Fano Vincenzo , Ortalli Ida
Published: February 2004

The parameters of the resonance absorption of the 23.8 keV γ quanta emitted by 119+Sn were measured in the temperature range of 78 to 300°K for the chalcogenides of tin: SnS, SnSe, and SnTe. Measurements of the width at half-height, quadrupole interactions, and chemical isomer shifts are used to analyze the properties of the chemical bonds and the structures of the investigated compounds. The temperature dependence of the probability of resonance absorption fa is correlated with the phonon spectrum of these semiconductors. The behavior of the fa and Γexp values and the appearance of an electric field gradient in SnTe at low temperature suggest the existence of a phase transition at about 160°K.
Anharmonic force field of ethylene
Authors: Machida Katsunosuke , Tanaka Yumiko
Published: February 2004

A simple anharmonic force field of ethylene is proposed. It includes the principal cubic and quartic stretching constants derived from appropriate Morse functions for individual bonds and two other higher-order constants. The quadratic part consists of general valence type constants that show the characteristics of the Pulay-Meyer Set II force field. The perturbation method including a general treatment of the first- and the second-order resonances has been used, and about 190 vibrational frequencies of ethylene and six deuterated ethylenes have been calculated to agree fairly well with the experimental values. The proposed potential gives also reasonable values of the Coriolis coupling constants, the centrifugal distortion constants, and the 13C-isotope shifts. The signs of the compliance constants are consistent with the assumption of monotone changes of internal coordinates on the dehydrogenation into acetylene and on the dissociation into two methylene radicals.
Formulation of molecular forces and their analytical calculation
Authors: Srebrenik Shalom , Weinstein Harel , Pauncz Ruben
Published: February 2004

A general definition of forces in a molecular system is considered, with the Hellmann-Feynmann relation representing the special case for the Born-Oppenheimer approximation. Analytic formulas are given for the calculation of forces within the fixed nuclei approximation. Derived from the analytic formulation of electrostatic potential, this calculation procedure remains valid for other average values of the form 〈r−|n|〉.
Magnetic parameters of p2,4, dt22,4, and d2 configurations in axial symmetric crystal fields
Authors: Lagendijk Ad
Published: February 2004

The energies and wavefunctions of a p2,4 configuration under the influence of a crystal field possessing axial symmetry are calculated. If the crystal field parameter Δ is smaller than, or of the order of, the Slater-Condon parameter F2, the 3P wavefunctions have the lowest energies. In that case one finds, depending on the sign of Δ, that either an orbital doublet or an orbital singlet has the lowest energy. For the latter, the fine structure parameters, the g values, and the hyperfine parameters are calculated. Using an irreducible tensor formalism, it is shown that the results for p2,4 can be used to find the interactions within dt22,4. Tetragonal as well as trigonal symmetry are treated and it is found that the hyperfine interactions differ substantially for both cases. Finally, the interactions within 3F(d2) under the influence of a crystal field possessing tetragonal symmetry are treated.
Uncoupled Hartree-Fock perturbation theory and the Pople-Schofield approximation
Authors: Amos A. T. , Crispin R. J.
Published: February 2004

The first-order perturbation equation in UCPT is derived in a form which reflects the essential lack of uniqueness of the perturbed orbital. The functionals leading to this equation and the other forms of the first-order equation are compared and this functional found to be best, although equivalent to the others if additional conditions are applied to their variation. The Pople-Schofield approximation, which is frequently applied to simplify the computation of the variational functional, is examined and found to be exact in the sense that H0 can be constructed to make it so. A simpler way of regarding the Pople-Schofield approximation is presented using a method due to Hirschfelder and originally designed to deal with exchange forces.
Nonequilibrium force fluctuations in an isotopically impure linear harmonic crystal
Authors: Greer William L.
Published: February 2004

A linear, harmonic, isotopically impure crystal is placed into contact with two harmonic oscillator heat baths, each at a different temperature. Force fluctuations on the ends of the crystal are studied in the nonequilibrium steady state that results from the ultimate evolution of the coupled crystal-heat baths. The exact behavior of steady state force fluctuations is obtained in general and examined in detail for periodic or randomly disordered isotopic lattices. Numerical results for the periodic crystal reveal unexpected complexity in the temporal decay of the force autocorrelation function. From exact expressions for the entire force distribution functions we show that, in general, the distribution is Gaussian and non-Markovian. However, on a coarse grained, semimicroscopic time scale (∼10−12 sec), the Gaussian force distribution is purely random and in accord with the usual assumptions about heat baths. One case of a macroscopic decay time occurs when one bath is initially at T =0°K in contact with a randomly disordered lattice.
Atomization energies and heats of formation of gaseous Au2, Tb2, TbAu, HoAu, TbAu2, and HoAu2
Authors: Kordis J. , Gingerich K. A. , Seyse R. J.
Published: February 2004

By means of Knudsen effusion mass spectrometric studies the atomization energies, D°0(kcal mol−1), and standard heats of formation, ΔH°f,298(kcal mol−1), respectively, were obtained for the following: Au2(52.9±0.5, 122.0±1.0), Tb2(30.5±6.0, 155.2±7.0), TbAu(69.2±8.0, 110.5±10), HoAu(63.0±8.0, 96.0±8.7), TbAu2(139.1±10, 128.0±11) and HoAu2(127.3±10, 100.6±10). These data were deduced from a critical assessment of the enthalpy changes of the reactions M2(g)=2M(g) and M(s,1)+M(g)=M2(g) for gold and terbium, and of the reactions LnAu(g)=Ln(g) + Au(g), LnAu(g)+Au(g)=Ln(g)+Au2(g), LnAu2(g)=Ln(g)+2Au(g) and LnAu2(g)+2Au(g)=Ln(g)+2Au2(g) for terbium and holmium. The cited standard heats of formation incorporate appropriate literature data as well as the measured reaction enthalpies. The Pauling model of a polar bond has been used for the interpretation of the measured atomization energies of the Ln–Au molecules and for predicting the dissociation energies of selected intermetallic compounds of terbium.
The collinear Cl + XY system (X,Y=H,D,T). A comparison between quantum mechanical, classical, and transition state theory results
Authors: Baer Michael , Halavee Uriel , Persky Avigdor
Published: February 2004

In this work, exact quantum mechanical (QM) and classical (CL) transition probabilities for the Cl+XY (X,Y=H,D,T) collinear system are compared. The calculations were performed using a semiempirical LEPS surface. The main features discussed are tunneling and threshold behavior for both the ground state and the first excited state of the hydrogen molecule. In the second part, kinetic isotope effects are presented and discussed. Those were calculated in three different ways, using (i) QM transition probabilities, (ii) CL transition probabilities, and (iii) transition state theory (TST-1D). Tunneling coefficients kQM/kCL and kQM/kTST were calculated and discussed. Finally, a brief comparison with experimental data was performed.
Effects of pressure on the first triplet state of anthracene
Authors: Arnold S. , Whitten W. B. , Damask A. C.
Published: February 2004

An optical study of the first triplet state of anthracene single crystals has been made under hydrostatic pressure from 1 atm to 6.8 kbar. The (0,0) level undergoes a linear red shift with pressure of −11.8 cm−1/kbar. The Davydov splitting of this level is increased by 3.4±1.0 cm−1 in going from 1 atm to 5.4 kbar. This increase in the Davydov splitting is interpreted in terms of basic triplet interactions.
Configuration interaction studies of the HeH+ molecular ion. I Singlet sigma states
Authors: Green T. A. , Michels H. H. , Browne J. C. , Madsen M. M.
Published: February 2004

The method of superposition of configurations is applied to the singlet sigma states of HeH+ which correlate to the separated atom states of principal quantum number less than or equal to 3. The calculations are carried out for internuclear separations in the ranges 0(.1)34.5, 35.0(.5)50.0 bohr. Energy eigenvalues and the expectation values of the electron coordinate along the internuclear line are discussed in this paper. Dipole transition matrix elements and radial coupling matrix elements are presented in a companion paper. The quality of the calculations is exhibited by comparisons of the length and velocity forms of the dipole matrix elements, by the Hellmann-Feynman theorem, and by the approach to atomic properties at large internuclear separations. Supplementary calculations of higher quality wavefunctions for the lowest states are also reported. These are used to help estimate the extent to which the results of the main calculations have converged toward the exact values. Comparisons with other work are given. It is shown that for internuclear separations greater than about 3 bohr a one-electron electron model potential approach is quite successful for the excited states. At smaller internuclear separations, and as the united atom limit is approached, current model potential methods cannot be trusted. In this region the model potential, being based on the separated atoms, cannot describe the actual changing charge distributions of the core electron. This work demonstrates the feasibility, for simple systems, of the ab initio structure calculations required for the quantitative description of low-energy atom-atom collisions.
Theory of vibrational relaxation in isolated molecules
Authors: Kay Kenneth G.
Published: February 2004

A quantum mechanical theory of intramolecular energy transfer is presented which treats the approach to statistical equilibrium of the vibrations in isolated molecules. The theory is appropriate for nonreactive molecules which have at least 6–9 atoms and which are so highly excited that the vibrational interaction couples together many degrees of freedom at a time and causes the simultaneous exchange of a large number of quanta. A generalized master equation of the Van Hove type and a weak-coupling master equation of the Pauli type are obtained for functions related to occupation probabilities of zero-order states. The asymptotic behavior of the coarse-grained occupation probabilities is examined and molecular ergodicity is proved. Convergent infinite expansions for the probabilities are also derived. These analytical expressions make it possible to study the intramolecular dynamics for all relevant times.
Limit cycle oscillations in compartmental chemical systems
Authors: Glass Leon , Pérez Rafael
Published: February 2004

The dynamics of a chemical system which is localized in two compartments between which mass transport occurs at a rate proportional to the difference in chemical concentrations between the two compartments have been studied. Pursuing a problem posed by Smale, we have tried to find a set of reactions such that when the cells are uncoupled there is a single unique stable steady state in each compartment, but when the cells are coupled there is a single unique global limit cycle oscillator in the combined system. We have analyzed open systems far from equilibrium in which there are two reacting chemicals. A linear stability analysis in the region of the homogeneous steady state has been performed for the case in which identical reactions are occurring in both compartments. We have also studied, using both local and global techniques, a class of systems in which different reactions are occuring in each compartment. For this latter class of systems we have been able to find evidence that there can be limit cycle oscillations which require transport in order to be sustained.
Defect structure and molecular motion in the four modifications of n -tritriacontane. I. Study of defect structure in the lamellar interfaces using small angle x-ray scattering
Authors: Strobl G. , Ewen B. , Fischer E.W. , Piesczek W.
Published: February 2004

The paraffin C33H68 undergoes three solid-solid phase transitions between room temperature and the melting point. The orthorhombic modification (A) stable at low temperatures is followed by two different monoclinic forms (B: 54.5°C < T < 65.5°C; C: 65.5°C < T < 68°C) and by the ``rotator phase'' (D: 68°C < T < Tf=71.8°C). Defect structures as they occur in the three high temperature modifications were determined with the aid of small angle x-ray scattering experiments. A method is applied which permits us to decide about the presence of different types of defects after a measurement of the absolute intensities of the 00l reflections. It was found that each one of the modifications B, C, D is characterized by a specific type of disorder. One observes orientational disorder of the skeletal planes in modification B, a superposition of longitudinal shifts of the chains in modification C, and an additional introduction of intrachain defects in modification D.
Defect structure and molecular motion in the four modifications of n -tritriacontane. II. Study of molecular motion using infrared spectroscopy and wide-line nuclear magnetic resonance measurements
Authors: Ewen B. , Fischer E. W. , Piesczek W. , Strobl G.
Published: February 2004

It was possible to specify the motional mechanisms, which are effective in the high-temperature modifications (B,C,D) of n -C33H68, using NMR and ir measurements in connection with the previously reported x-ray experiments. Modification B is characterized by the occurrence of coupled rotational jumps with amplitudes of about 180 degrees. In modification C flip-flop screw motions are superimposed. Passing to modification D, intramolecular defects appear, which diffuse along the chains. At the same time the rotational component of motion changes to a more continuous and less jumplike form.
Electron paramagnetic resonance of Eu2+ in orthorhombic sites of RbCl single crystals
Authors: O J. Rubio , P E. Muñoz , S G. Aguilar
Published: February 2004

The electron paramagnetic resonance of Eu2+ in RbCl has been observed at 1.2 and 300 K. Accurate spin-Hamiltonian parameters and their previously undetermined absolute signs have been determined. Considerable disagreement between this work and one published earlier is noted.
Excess electrons and positive charge carriers in liquid ethane
Authors: Schmidt Werner F. , Bakale George , Sowada Ulrich
Published: February 2004

Measurements of excess electron drift velocity as a function of the electric field were carried out up to field strengths of 280 kV/cm at temperatures from 111 to 216°K. Low field mobilities were obtained and at higher field strength the drift velocity increased more than proportionally with the field. The data are discussed on the basis of the trapping model, and thermally activated hopping is assumed for the motion. The jumping length as a function of the temperature was obtained from the high field data. The positive charge carrier seems to be of ionic nature and the mobility follows Walden's rule.
Specific heat of (CH3)4NCdCl3 (TMCC) from 1.66 to 17.72 K
Authors: Blacklock K. , Linebarger H. F. , White H. W. , Lee K. H. , Holt S. L.
Published: February 2004

The specific heat of polycrystalline tetramethylammonium cadmium chloride (TMCC), (CH3)4NCdCl3, has been measured from 1.66 to 17.72 K. ``Effective'' Debye temperatures, varying smoothly between 194 and 248 K, were needed to describe the specific heat data.