bluuebunny/crossref_metadata_2025 huggingface dataset:
A new multipole expansion method for the calculation of the electrostatic part of lattice energies. Application to NaBr⋅2H2O
Authors: Herzig P. , Neckel A.
Published: February 2003

In this paper a new method is described for the calculation of the electrostatic interaction energy of an infinite lattice of nonoverlapping, but otherwise arbitrary, charge distributions. The method is based on a multipole expansion of the electrostatic interaction energy of two nonoverlapping charge distributions. The summation over the infinite lattice is performed using a procedure given by Nijboer and de Wette. As an example, the method is applied to the calculation of the electrostatic part of the lattice energy of monoclinic NaBr⋅2H2O. The charge distributions of the water molecules in the crystal are represented approximately by the multipole moments of the gaseous molecule. The electrostatic energy in the crystal is found to be 913.8 kJ mol−1. Using a simple point charge model, the Madelung parameter can be represented by Ma=2.8697 + 3.9011 qH − 0.8665 q2H, where qH is the charge on the hydrogen atom.
Repulsive potentials derived from beam scattering of Rb+ and Cs+ ions by rare gas atoms
Authors: Inouye H. , Noda K. , Kita S.
Published: February 2003

Integral cross sections of elastic scattering through effective LAB angles greater than 5×10−3 rad have been measured for collisions of Rb+ and Cs+ ions in the LAB energy range 500–4000 eV with room temperature rare gas atoms (He through Xe). Repulsive potentials in the range 0.5–10 eV derived from the results for the ion–atom pairs can be all expressed by exponential formulas. The potentials for Rb+–Ar, –Kr, and –Xe are in excellent agreement with the results derived from the mobility experiments by Gatland et al., but agreement between the two experiments for Cs+–Ar, –Kr, and –Xe are poor. The potential values calculated by Ishikawa et al. on the basis of the Gordon–Kim electron gas model are generally 20%–25% lower than the results of the present work.
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o investigation of the structure, vibrational frequencies, and intensities of HO2 and HOCl
Authors: Komornicki Andrew , Jaffe Richard L.
Published: February 2003

The infrared spectral intensities for HOC1 and HO2 have been calculated using a new ab initio technique. Theoretical results for the geometries, vibrational frequencies, and the dipole moments of these species are also reported. All of the calculations were performed at the SCF level using near Hartree–Fock quality basis sets. Our results for the molecular geometries and the vibrational frequencies are in good agreement with available experimental data. We believe that our computed intensities are accurate to at least 50%. Our results should be helpful in attempts to determine the stratospheric abundance of HOC1 and HO2 by in situ infrared spectroscopic measurements.
Sub-Doppler laser spectroscopy of the NaK molecule
Authors: Eisel D. , Zevgolis D. , Demtröder W.
Published: February 2003

Excitation of NaK molecules in a collimated molecular beam by a single mode tunable argon laser with sub-Doppler resolution allows selective population of definite vibrational rotational levels (v′, J′) in the D 1Π-state. The resultant fluorescence consists of a singlet spectrum in the 470–570 nm range and a triplet spectrum in the 625–715 nm range. The analysis of the triplet fluorescence, which consists of discrete lines and of a modulated continuum, allows the determination of the a 3Σ+ potential and yields very precise values for the dissociation energies in the a 3Σ+ and X 1Σ+ states, which are measured as De(3Σ+) =204±4 cm−1 and De(1Σ+) =5269±6 cm−1. For some excitation wavelengths a fluorescence spectrum with discrete lines and with a double-modulated continuum is observed which is assigned to ’’ Condon internal diffraction’’ bands terminating close below and above the dissociation limit of the X 1Σ+ state. Its relevance for the determination of the upper and lower potential curves is discussed.
On the derivation of rate equations for collisionless molecular multiphoton processes
Authors: Mukamel Shaul
Published: February 2003

Starting with the complete molecular Liouville equation and making use of the Mori–Zwanzig projection operator formalism we derive generalized rate equations which describe the collisionless dynamics of level populations in the quasicontinuum of molecular multiphoton processes. We define precisely the complete molecular information required for the description of these processes which is expressed in the form of a hierarchy of intramolecular dipole correlation functions. We then discuss the general conditions under which the equations attain the form of simple rate equations and show how the complete molecular information reduces considerably in this case to three relevant quantities per transition: an integrated Rabi frequency, a dephasing time and ratios of statistical weights of the levels.
Theory of resonance Raman scattering by doubly degenerate modes
Authors: Zgierski M. Z. , Pawlikowski M.
Published: February 2003

A model is presented that allows calculation of excitation profiles, depolarization ratios, and resonance Raman spectra of doubly degenerate (e-type) modes. It consists of a doubly degenerate excited (E-type) electronic state and a nondegenerate excited A- or B-type electronic state. The sources of Raman scattering by the e-type modes are pseudo-Jahn–Teller (Herzberg–Teller) interactions between these excited states and Jahn–Teller and/or Renner–Teller interaction within the E-type electronic state. Molecules of trigonal, pentagonal, and hexagonal symmetries are investigated for different coupling schemes and different energy separations between the two excited states. The results are presented in the form of three-dimensional graphs for the fundamental and the first two overtones together with the corresponding absorption spectrum. The model also allows one to follow changes in resonance Raman scattering produced by trigonal distortion of molecules possessing a triply degenerate electronic state in resonance with the incident light. The case of resonance scattering in the region of a forbidden nondegenerate state which is likely to occur in hexagonal molecules such as benzene is also discussed.
Photoelectron spectra of AsF5-doped polyacetylenes
Authors: Salaneck W. R. , Thomas H. R. , Duke C. B. , Paton A. , Plummer E. W. , Heeger A. J. , MacDiarmid A. G.
Published: February 2003

The x-ray and ultraviolet photoemission spectra of undoped polyacetylene, AsF5-doped polyacetylene, AsF5, and AsF3 are reported and compared. At a doping level near where the electrical conductivity saturates (∼11% AsF5), the As:F ratio is 1:5 and a large fraction of the arsenic–fluoride moieties are localized near the surface of the polyacetylene fibrils. The C (1s) core-level spectra indicate a charge transfer of about one electron per AsF5 molecule. The UPS spectra, the x-ray induced Auger spectra and CNDO calculations suggest, however, that the arsenic pentafluoride moieties are not simple AsF5-radical anions. At lower doping levels (0.1 to 1%) very different results are obtained.
Classical models for electronic degrees of freedom: Derivation via spin analogy and application to F*+H2→F+H2
Authors: Meyer Hans-Dieter , Miller William H.
Published: February 2003

By invoking the formal equivalence of a finite level quantum mechanical system to a quantum spin in an external field, this paper shows how a classical model can be constructed for an arbitrary finite level quantum system. For two-state and three-state cases this leads to the same classical Hamiltonian that was obtained earlier by other more heuristic methods, but it shows how to treat the general F-state case. The purpose of this overall approach is to be able to represent the electronic states in electronically nonadiabatic collision processes by a classical degree of freedom so that all degrees of freedom, electronic and heavy particle (i.e., translation, rotation, and vibration) can be treated on a consistent dynamical footing. Application of this model to the quenching of F* (2P1/2) by collisions with H2 is described, and this completely classical approach is seen to give excellent agreement with the quantum mechanical coupled channel calculations of Rebentrost and Lester.
Rates of OH radical reactions. VI. Reactions with C3H6, 1-C4H8 and 1-C5H10 at 297 K
Authors: Nip Wing S. , Paraskevopoulos George
Published: February 2003

Absolute rate constants, for the reactions of hydroxyl radicals with C3H6, 1-C4H8 and 1-C5H10 at 297±2 K, have been measured in the gas phase, using the flash-photolysis resonance-absorption technique and OH from the flash-photolysis of N2O/H2 mixtures, λ≳200 nm. The rate constants in units of cm3 mol−1 s−1 were found to be: kC3H6=1.48±0.17×1013, k1-C4H8=2.01±0.15×1013, and k1-C5H10=2.39±0.23×1013. The quoted errors are the 95% confidence limits. The approximate concentration of free radicals formed in the system has been determined in separate experiments. The maximum effect of secondary reactions of these radicals on the measured rates has been discussed and assessed to be less than 10%.
The microwave spectrum and conformation of 1,3-cycloheptadiene
Authors: Avirah T. K. , Malloy Thomas B. , Cook Robert L.
Published: February 2003

The microwave spectrum of 1,3-cycloheptadiene has been studied. The ground state and a number of excited vibrational states have been assigned and the rotational constants evaluated. A study of the Stark effect has yielded the dipole moment components for the ground state and two of the excited vibrational states. The total dipole moment for the ground state is 0.740 D. These data have been used to determine the conformation of 1,3-cycloheptadiene. The carbon skeleton is planar except for the C6 carbon atom (Cs form).
Optical phonons line broadening at the first order phase transition of (C2H5NH3)2CdCl4: A spin–phonon coupled system
Authors: Peyrard M. , Remoissenet M.
Published: February 2003

The order-disorder phase transition and the subsequent line broadening effect in the far infrared spectra of (C2H5NH3)2 Cd Cl4 is discussed. A simple model involving a spin-phonon coupling is used to explain the far infrared results. (AIP)
Microscopic capillarity approximation: Free energies of small clusters
Authors: Griffin G. L. , Andres R. P.
Published: February 2003

Free energies of crystalline microclusters containing 2⩽i⩽100 atoms as well as the bulk chemical potential and surface free energy of a microscopic fcc crystal are calculated for a model substance exhibiting a pairwise additive potential energy and only nearest neighbor interactions. These results are used to test various existing approximations for cluster free energy. An alternative treatment of small clusters is suggested. This microscopic capillarity approximation gives an improved description of the free energy of small clusters, at the expense of requiring knowledge of the free energy of the trimer.
Monte Carlo studies on the freely jointed polymer chain with excluded volume interaction
Authors: Baumgärtner A. , Binder K.
Published: February 2003

We have carried out computer simulations of the freely jointed polymer chain with an excluded volume interaction using a dynamic Monte Carlo method for chain lengths N between N=8 and N=100. The equilibrium values of end-to-end distances and radius of gyration approach the asymptotic form ∼Nν for N≳70 (scaling limit) and excluded volume parameter d/l=1.0. The scaling limit decreases to lower N for lower d/l. Here l is the length of a chain unit and d is the excluded volume distance. The exponent is ν?0.6 for d/l≠0 and ν=0.5 for d/l=0. The structure function varies as S (k) ∼q−1/ν (q=kNν) over a wide range of q. This asymptotic behavior for large N is found for N≳16 and d/l≠0. Our results are carefully compared to previous studies on the same model where other types of Monte Carlo methods are used.
Effective collision cross sections for the viscomagnetic effect in a pure gas of linear molecules
Authors: Liu W. K. , McCourt F. R. , Köhler W. E.
Published: February 2003

Using Liouville-space algebra, general expressions are derived for the three effective collision cross sections governing the viscomagnetic effect in a pure gas of linear molecules. These cross sections are presented in terms of S-matrix elements in the total-J representation since it is especially useful for the dynamical approximation procedures that have recently come into vogue. The results obtained here are briefly compared with those for diatom–atom systems and, where possible, with similar results previously obtained via other methods.
Thermoluminescence and F-center annealing in alkaline-earth fluoride crystals after reactor irradiation at low temperature
Authors: Atobe Kozo
Published: February 2003

Thermoluminescence is related to the F-center annealing stage in undoped alkaline-earth fluoride (CaF2, SrF2, and BaF2) single crystals irradiated at 20 K with reactor neutrons. The temperature of glow peak maxima are classified in four groups (P-1–4); P-1: 110–130 K, P-2: 134–152 K, P-3: 183–200 K and P-4: 201–245 K with a heating rate of 4 K/min. at temperatures ranging 77–300 K. These temperatures of peak maxima correspond well to decay stages of F-centers. F-centers play an important role as recombination centers in the TL process. The main TL emission spectrum of P-1–4 peaks consists of the intrinsic luminescence as well as the case of the x-ray emission spectrum at 77 K. It is revealed that these TL emission bands near 300 nm of P-1, 2, and 3 peaks are due to the radiative recombination of (VK+F) and (H+F) reactions, respectively.
On the question of the well depth of the He–Ar interatomic potential
Authors: Aziz R. A. , Riley P. W. , Buck U. , Maneke G. , Schleusener J. , Scoles G. , Valbusa U.
Published: February 2003

New, high resolution, differential collision cross section data for the scattering of He by Ar atoms, obtained in Göttingen, are found to be inconsistent with the predictions derived from a recently proposed He–Ar potential [M. Keil et al., J. Chem. Phys. 70, 482 (1979)] and confirm previous work by K. M. Smith et al. [J. Chem. Phys. 67, 152 (1977)]. It is shown that the He–Ar well depth is 29.4 K (accurate to 5% and 44% larger than the value of Keil et al.), independent of the parametrization of the potential. A careful analysis of the bulk properties of the He–Ar mixture also shows that the attractive part of the potential proposed by Keil et al. is too weak. A potential of the SPFD form is presented which is able to reproduce the best data for the differential cross section, the second virial coefficient, the diffusion and viscosity within the quoted experimental errors.
Molecular theory of solvated ion dynamics. II. Fluid structure and ionic mobilities
Authors: Colonomos Peter , Wolynes Peter G.
Published: February 2003

Calculations of ionic mobility based on a molecular theory of solvated ion dynamics [J. Chem. Phys. 68, 473 (1978)] are presented. We develop approximation schemes for the equilibrium averages needed in the theory. The results for ions in water are in remarkably good agreement with experiment. The importance of fluid structure, arising from the finite size of solvent molecules, is clearly illustrated.
Molecular theory of solvated ion dynamics. III. The kinetic dielectric decrement
Authors: Hubbard Joseph B. , Colonomos Peter , Wolynes Peter G.
Published: February 2003

A microscopic theory of the kinetic dielectric decrement in ionic solutions is developed. The theory clarifies the physical origin of the effect, showing that it is analogous to the electrophoretic effect, and corrects a violation of the reciprocity relations in the original continuum treatment of Hubbard and Onsager. We discuss the concept of the dielectric constant of an electrolyte solution. Calculations indicate that there are specific ion effects, in contrast with the universal behavior predicted by the continuum theory. A critical comparison with present experimental measurements is made.
Simulation of the relaxation of random-coil polymer chains by lattice models with excluded volume: The effect of bead movement rules
Authors: Kranbuehl David E. , Verdier Peter H.
Published: February 2003

Earlier work showed that excluded volume constraints lengthen the long relaxation times of lattice-model polymer chains using single-bead elementary motions by a factor proportional to about the first power of chain length. This paper reports the extension of this work to chains employing double-bead elementary motions and to chains employing random mixtures of both kinds of motion. For pure two-bead motion, the chain-length dependence of the lengthening of relaxation time is essentially identical with that previously obtained for pure single-bead motion. For models using both kinds of move, this chain-length dependence is suppressed at shorter chain lengths. However, for chains longer than about 30 beads, contrary to the prediction of Deutch and others, the lengthening of the relaxation time by excluded volume again becomes proportional to about the first power of chain length. Other studies that have not exhibited this first power dependence have either examined relaxation from highly nonequilibrium configurations or have not employed sufficiently long chains for the effect to be observed.
Magnetic properties of LiH, HF, and PH3 in its C3v and D3h structure
Authors: Keil Frerich , Ahlrichs Reinhart
Published: February 2003

Results of CHF computations for NMR shielding constants σ and magnetic susceptibilities x are reported for LiH, HF, and PH3 (C3v and D3h) as obtained from very extended GTO basis sets. The discussion of results concentrates on problems such as basis saturation and gauge invariance, especially with respect to molecules containing second row atoms such as P.
Multiphoton decomposition of hexafluoroacetone: Effects of pressure, fluence, wavelength, and temperature on the decomposition yield and C-13 and O-18 isotopic selectivity
Authors: Hackett P. A. , Willis C. , Gauthier M.
Published: February 2003

The yield and isotopic selectivity of the infrared multiphoton induced decomposition of hexafluoroacetone have been studied in both parallel and focused beam geometries as a function of temperature, pressure, fluence, and photolysing wavelength. At low conversions, the chemical stoichiometry of the decomposition process is simple, CF3COCF3    nhν    CO + 2CF3. Several potential interferences are discussed but it is concluded that the yield of CO directly reflects the yield of the primary dissociation event. Pressure effects are complex and not amenable to quantitative description, however, varying parameters such as fluence and photolysing wavelength lead to predictions which give a clear qualitative picture. In particular, it is possible to distinguish parametric variations due to spontaneous dissociation and dissociation dependent upon energy pooling. Isotopic selectivity provides a powerful insight in this area. Preliminary data on the effect of temperature have been obtained at constant incident fluence and number density. Varying temperature probes the contribution of initially excited species (hot-bands). Reducing the temperature has no effect on the blue-edge of the absorption band but leads to a narrowing on the red-edge which is more marked for parallel beam geometries. The form of the wavelength dependence is in agreement with theoretical predictions. The observed carbon-13 isotopic selectivities of the dissociation are in basic agreement with those predicted from spectroscopy in that αCO is much higher than αC2F6. Very high values for this selectivity have been observed approaching αCO=1000 under extreme conditions. The equivalent selectivities for oxygen-18 are small, α?2. The hexafluoroacetone molecule is demonstrated to be a good model for study of multiphoton events and the data presented offer a powerful basis set for theoretical treatment.
Exponentiating trajectories and statistical behavior: Three dimensional K+NaCl and H+ICl
Authors: Duff James W. , Brumer Paul
Published: February 2003

The dynamic origin of statistical behavior in three dimensional bimolecular collisions is explored using the exponentiating trajectories technique; NaCl+K and H+ICl are explicitly considered. The reported results provide further evidence that statistical product distributions are associated with trajectories characterized by extensive exponential divergence of initially adjacent phase space neighbors. A technique for testing the statisticality of product distributions is introduced and its role as a numerical method for general trajectory studies of long-lived dynamics is discussed.
Pressure second virial coefficients of hydrocarbons, fluorocarbons, and their mixtures: Interactions of walks
Authors: Brostow Witold , McEachern Douglas M. , Perez-Gutierrez Salud
Published: February 2003

Molecules of some hydrocarbons and fluorocarbons have been represented by graphs, and walks on graphs considered. The walk interaction formalism developed previously for liquid state properties has been applied to gas pressure second virial coefficients of n-alkanes, n-perfluorocarbons, and alk-1-enes. Agreement between experiment and calculation within limits of the experimental accuracy has been obtained. For each class of compounds properties of the respective binary mixtures have been predicted in terms of properties of pure compounds, without the use of any combining rules; again satisfactory agreement with available experimental data has been found.
Field-induced color switching in liquid crystal displays
Authors: Schadt M.
Published: February 2003

A new electro-optical guest–host effect is described which changes its color upon the application of a voltage. It is shown that the color change depends on the voltage-induced change of the polarized absorption of dichroic dyes (guests) whose transition moments are primarily parallel (longitudinal) and perpendicular (transverse) to their respective molecular axis. The new effect does not require polarizers and can be operated at low voltages like twisted nematic displays. Its excellent brightness and contrast are much less dependent on large order parameters as compared with other guest–host effects. Analytical expressions describing the dependence of the transmission and color of the effect on dye order parameter and direction of transition moment, elastic and dielectric properties of the liquid crystal (host) matrix, viewing angle and display voltage are derived and compared with experimental results.
Low energy electron impact excitation of the b 3Σ+ state of CO
Authors: Twist J. R. , Paske W. C. , Rhymes T. O. , Haddad G. N. , Golden D. E.
Published: February 2003

The lifetime of the b 3Σ+(v′=0) level of CO has been measured using a delayed coincidence technique. Lifetime components of 51.86±0.24, 358±20, and 1500±900 ns at the 95% confidence level have been unfolded by computer fitting and extrapolating to zero pressure. In addition, the collisional quenching cross sections for the prompt decay and the first cascade component were determined to be (7.7±3.8) ×10−15 and (7.1±3.5) ×10−15 cm2 at the 95% confidence level. These results agree with spectroscopic evidence that the a′ 3Σ+(v″=32–41) levels strongly perturb the b 3Σ+(v′=0,1) levels. Discrepancies with previous lifetime results are explained.
Photodissociation of XeF2 in solid Xe and Kr
Authors: Monahan K. M. , Jones V. O. , Rehn V.
Published: February 2003

Time-resolved photoluminescence excitation (TREX) spectra of XeF2 (1%) in solid Kr and Xe were obtained by exciting with synchrotron radiation in the VUV region. The emission was due to XeF photofragments in the D and B excited states. The TREX data thus provide the first relative photodissociation yield measurements on XeF2 as a function of exciting photon energy. In addition, cage forces in solid Kr and Xe sufficiently modify one of the excited state potential surfaces to permit observation of a new band system in XeF2 which has been tentatively identified as an allowed 1Πu←1Σg transition. In solid Kr, the bands are located at 7.88, 8.07, 8.25, 8.44, 8.61, and 8.77 eV. Observation of these bands in TREX spectra provides substantial support for the linear triatomic dissociation models recently proposed by Heller and Pack.
Angular distributions and separation energies of predissociation photofragments of O2+ (b 4Σg−, v′=4, N′, F′)
Authors: Pernot C. , Durup J. , Ozenne J.-B. , Beswick J. A. , Cosby P. C. , Moseley J. T.
Published: February 2003

Kinetic energy distributions have been measured for O+ photofragments resulting from transitions O2+ (a 4Πu, v″, J″, Ω″) →O2+ (b 3Σg−,v′=4,N′,F′) →O+(4S0) +O(3P), where for the first time in the study of photodissociation all relevant quantum numbers involved are specified. Both 16,16O2+ and 16,18O2+ were studied. The measurements were made using a laser-ion coaxial-beams photofragment spectrometer, a single-mode laser, and velocity tuning of the absorption wavelength. Experimental values of the anisotropy parameter β, which describes the angular distributions of the photofragments, were obtained by fitting the experimental energy distributions with calculated distributions, obtained from a Monte Carlo computer simulation of ion trajectories in the apparatus. Values of β were thus determined with an uncertainty of ±0.05 or less. They are compared with theoretical values of β obtained by a generalization to Hund’s case (b) of the theory previously developed by R. N. Zare. In a similar manner, values of the kinetic energy of separation of the photofragments in the center-of-mass system, W, are determined with an accuracy, in the most favorable case, of ±0.14 meV. The experimental W values are used, along with other spectroscopic data, to improve the bond dissociation energy values of the ground X 3Σg− state of O2, and of the a and b states of O2+. It is shown that for W<50 meV the dissociation strongly favors the O(3P2) channel over 3P1 and 3P0. This result is compared with several possible limiting-case models of the long-range couplings involved in the dissociation process, and it is concluded that the dissociation proceeds in a completely adiabatic way. Finally, further possible refinements of the present work are discussed.
A distorted wave calculation for electronic energy transfer in molecular collisions. Application to the N2(A 3Σu+)+CO(X 1Σ+) →N2(X 1Σg+) +CO(a 3Π) system
Authors: Deperasinska I. , Beswick J. A. , Tramer A.
Published: February 2003

The electronic energy transfer in collisions between two diatomic molecules at thermal energy is treated by a distorted-wave type calculation. The predominating role of short-range interactions is assumed. Potential energy curves and coupling terms have been estimated by configuration interaction between locally excited states. The method is applied to the system: N2(A 3Σu+)+CO(X 1Σ+) →N2(X 1Σg+) +CO(a 3Π). The results are compared with the available experimental data. A general study of the dependence of the rate constants and cross sections on the energy gap and temperature is given. Finally a comparison with the golden-rule model and other quantum mechanical approximate calculations is presented.
On relationships among effective potential functions: 2,5-dihydrofuran, s-tetrazine, 1,4-dioxadiene, cyclopentene, and carbon suboxide
Authors: Malloy Thomas B. , Carreira L. A.
Published: February 2003

Using Van Vleck perturbation theory, a relationship between two-dimensional potential functions in ring-puckering and ring-twisting coordinates and effective one-dimensional ring-puckering coordinates is demonstrated. It is shown that effective one-dimensional potential functions can be derived for 2,5-dihydrofuran and s-tetrazine which have essentially the same eigenvalue separations as the two-dimensional potential functions previously derived. More useful is the fact that this procedure may be reversed and two-dimensional potential surfaces have been derived for 1,4-dioxadiene and cyclopentene. In mass weighted coordinates the potential function for dioxadiene is V (Q1,Q2) =28.49 Q14+24.00 Q12 +2555.2 Q22+294.6 Q12Q22, where V is in cm−1, Q in U1/2 Å. Q1 is a mass weighted ring-puckering coordinate and Q2 is a mass weighted ring-twisting coordinate. The potential function for cyclopentene is V (Q1,Q2) =51.50 Q14−217.4 Q12+2262.4 Q22 −9.10 Q12Q22 yielding a barrier to planarity of 229 cm−1 essentially unchanged from the barrier of 232 cm−1 reported earlier from the one-dimensional potential function. This same formalism has been used to correlate the potential functions for the large amplitude bending mode, ν7, of carbon suboxide in the ground state and several excited states of other modes.
Hindered rotational energy levels of the ammonium ion in alkali halide lattices
Authors: Smith David
Published: February 2003

The hindered rotational energy levels of the ammonium ion in alkali halide lattices have been computed by assuming a ’’single approach model,’’ i.e., that one of the N–H bonds points toward the halide ion. The barrier to rotation about the threefold axis is small. Therefore, the tunneling frequencies for these solids are much larger than for other ammonium compounds. The Hamiltonian that describes the motion of the ammonium ion in alkali halide lattices is invariant under the direct product group ?×C3, where ? is the tetrahedral group of rotations around body-fixed axes and C3 the trigonal group of rotations about space-fixed axes. For high barriers to rotation, closed mathematical expressions have been derived for the librational frequencies. Experimental determination of the low temperature heat capacity or the mean square value of the nuclear angular momentum <I (I+1) ≳ is shown to provide a method for determining the barrier to rotation.
Calculation of nuclear spin–spin coupling constants in methanol molecule
Authors: Lazzeretti Paolo
Published: February 2003

The perturbed Hartree-Fock scheme is applied to rationalize the nuclear spin–spin coupling constants in methanol molecule. The theoretical predictions are partially successful, sign and relative magnitude of the various coupling constants being correctly estimated. The coupled HF method overestimates the Fermi contact contribution to total coupling constants, which is possibly due to electron correlation effects and to convergence problems of the perturbation scheme. The relative importance of the commonly neglected spin–orbit contribution is demonstrated. The dependence of computed values on the molecular configuration is discussed; the reliability of the geometric approximation to coupling constants is tested with good results for the spin–orbit contribution.
Proton anisotropic chemical shift spectra in a single crystal of hexagonal ice
Authors: Rhim W. K. , Burum D. P. , Elleman D. D.
Published: February 2003

The proton anisotropic chemical shift spectra in a single crystal of hexagonal ice are reported for the first time. (AIP)
A guide to the lifetimes of vibrationally excited van der Waals molecules: The momentum gap
Authors: Ewing George E.
Published: February 2003

A guide is offered towards understanding the lifetimes of vibrationally excited Van der Waals molecules. A correlation of the theoretically calculated lifetimes, to interpret experimental lifetimes is also given. (AIP).
Predissociations in the B 3Π+ state of I35Cl and I37Cl
Authors: Gordon Robert D. , Innes K. K.
Published: February 2003

Profiles of lines in the 3–1 and 2–0 bands of the B 3Π0+←X 1Σ+ system of I35Cl and I37Cl vapor, as well as in several fragmentary P and R branches associated with an adiabatic B′ state, have been recorded using a Fabry–Perot interferometer spectrometer. The potential curves of the B state, and of the adiabatic B′ state formed by an avoided crossing near 3.6 Å and 18 140 cm−1 between the B state and a repulsive Y O+ state, have been determined using the intermediate coupling model of Child. Linewidths in v=2 of the B state are observed to be 0.034 cm−1, independent of J and of isotopic composition; however, linewidths in v=3 are broader, corresponding to predissociative lifetimes in the subnanosecond range, and are found to oscillate as a function of J. The major features of the observed linewidth pattern can be explained in terms of analytical approximations developed by Child and co-workers, and provide evidence for a second curve crossing, near 2.94 Å and 17 975 cm−1, of the B state and an Ω=1 state.
Kinetics of order–disorder transformations in alloys. III
Authors: Gschwend Klaus , Sato Hiroshi , Kikuchi Ryoichi , Iwasaki Hiroshi , Maniwa Hideyo
Published: February 2003

Calculations of specific heat vs temperature curves corresponding to measurements with finite heating rates have been carried out for alloys under various initial conditions utilizing the pair approximation of the Path Probability method of time dependent cooperative phenomena and are compared with experiments on Mg3Cd, FeCo, and Cu3Au. New experiments on Mg3Cd suitable for comparison are also presented here. The main emphases of the comparisons are (1) the appearance of a specific heat subpeak below the main specific heat peak at Tc for well annealed alloys, (2) the shift of the subpeak by a change of the heating rate, (3) the appearance of a specific heat dip for alloys quenched from higher temperatures, and (4) the appearance of two specific heat dips for highly disordered alloys quenched from above the critical point of order–disorder. Good qualitative agreement of theory and experiment is obtained in each of these four categories.
The geometry and inversion barrier of the sulphonium ion SH3+. Dependence on basis set
Authors: Dixon D. A. , Marynick D. S.
Published: February 2003

The inversion barrier of the sulphonium ion, SH+3, has been calculated using a variety of basis sets. The dependence of the calculated barrier and geometry on basis set size and inclusion of electron correlation is studied. The best estimate of the inversion barrier, calculated with a polarized double zeta basis set and extensive configuration interaction, is 32.2 kcal/mole. Correlation effects on the barrier are small, but polarization functions are found to be crucial for an accurate calculation of the barrier.
On a fundamental difference between energy functionals based on first- and on second-order density matrices
Authors: Donnelly Robert A.
Published: February 2003

The Euler equations and kernel F[γ] of an energy functional of the first-order density matrix are compared to the corresponding quantities which result from Löwdin’s treatment of the extended Hartree–Fock equations (the latter are based on an energy functional ▪v dependent on the second-order density matrix). Comparison of the functionals Ev and ▪v, facilitated by transformation of Löwdin’s kernel to the Hermitian kernel ▪[γ] which is central to the extended Koopmans’ theorem, leads to a clarification of the fundamental difference between ionization and chemical potentials. A definition of chemical potential (electronegativity) appropriate to Hartree–Fock theory is proposed. Denoting the Fock operator by the symbol FN[γ;x′,x], this definition is μ=−χ=ℱℱdxdx′ FN[γ;x′,x] [∂γ (x,x′)/∂N]. This reduces, in the special case of a system with a single valence electron, to a measure of the Hartree–Fock electronegativity proposed originally by Mulliken and by Moffitt; namely χ=−ε−J/2, where ε is an eigenvalue of the Fock operator, and J is a Coulomb integral evaluated for the canonical valence orbital χN.
Vibrational spectroscopy of cation-site interactions in phosphate glasses
Authors: Nelson B. N. , Exarhos Gregory J.
Published: February 2003

Thirty different binary metal oxide glasses having the metaphosphate composition have been prepared containing cations from the following groups: alkali metals, alkaline earths, transition metals, and lanthanide and actinide metals. Far infrared absorption assigned to the cation vibration in its oxygen cage has been measured for these glasses. Empirical ionic models are proposed correlating the absorption maximum with the cation mass, charge, and ionic radius. Discrepancies between the observed and predicted vibrational frequency indicate a more covalent interaction between the metal cation and the glass network. Raman intensities and vibrational frequencies of the network metaphosphate vibrations have been obtained and provide additional evidence about the cation–site interaction.
13C NMR rotating frame relaxation in a solid with strongly coupled protons: Polyethylene
Authors: VanderHart D. L. , Garroway A. N.
Published: February 2003

The validity of interpreting measured rotating frame relaxation times, T1ρC*, in terms of molecular motion is investigated for crystalline, oriented, linear polyethylene (PE) as a representative rigid organic solid with reasonably strong dipolar couplings. T1ρC* data are presented at three temperatures, −100, 28, and 100 °C and for 13C rf fields, ν1C, in the range 35<ν1C<90 kHz, for the orientation where B0 is parallel to the PE chain axes. With the exception of the T1ρC* data taken at ν1C≳80 kHz and T=100 °C, all T1ρC* data observed were determined not by molecular motion, but rather by spin–spin effects in which the spin-locked carbon system seeks to equilibrate with the proton dipolar system with a characteristic time constant TCHD(ν1C), while at the same time the proton dipolar system is equilibrating with the lattice with a time constant T1D. The TCHD values deduced from T1ρC* support existing theory which predicts that TCHD(ν1C) ∝ exp(2πν1CτD) where τD is the correlation time for dipolar fluctuations. A calculation of τD from PE crystal structure agrees very well with the experimentally determined τD of 24 μs. Further experimental proof of the importance of the spin–spin contribution to T1ρC* is demonstrated via changes in the carbon rotating frame magnetization, MSL(τ), for differing states of proton dipolar order. The question of extracting information about molecular motion from T1ρC* data is examined under the assumption that the total reduced correlation function for the local proton dipolar field at a carbon nucleus is the product of a Lorentzian reduced correlation function describing dipolar fluctuations and an exponential reduced correlation function describing molecular motion. It is shown for molecular motion in the long correlation time regime and for sufficiently large ν1C that contributions to T1ρC* from molecular motion and dipolar fluctuations are cleanly separated. Pertinent background material for the interpretation of T1ρC* data is also presented; this includes effects of sample spinning, transients in carbon and proton magnetization, and ambiguities which arise when molecular motion is fast enough to cause some averaging of the static dipolar line shape. Criteria are offered whereby one can decide on the validity of interpreting T1ρC* data in terms of molecular motion. As an example, the T1ρC* data point for ν1C=87 kHz and T=100 °C is interpreted in terms of molecular motion and the deduced correlation time, assuming a flip–flop motion of the polymer chains, is shown to be qualitatively consistent with published proton T1ρH results. Finally, 13C longitudinal relaxation time measurements are discussed as a possible alternative to T1ρC* for obtaining information about molecular motion.
Hydrogen atom in a spherical box. II. Effect on hyperfine energy of excited state admixture
Authors: Weil John A.
Published: February 2003

The exact eigenfunctions of the hydrogen atom H0 trapped in a finite sphere are used to evaluate radial averages and allow, for example, evaluation of 〈r−3〉 to yield the 2p hyperfine interaction energy. Inclusion of compressed 2p orbital in the ground state permits simultaneous fitting of the relatively large hyperfine anisotropy and the isotropic component, observed by EPR for H0 in α-quartz, with an acceptable sphere radius and an admixture coefficient a2p more reasonable (smaller) than is possible with the ordinary hydrogenic 2p orbital. The local uniform electric field required to generate the desired a2p value is estimated.
Semiclassical Monte Carlo methods
Authors: Doll J. D. , Myers L. E.
Published: February 2003

The problem of constructing quantum–mechanical corrections to classical equilibrium statistical–mechanical results is considered. These corrections (excluding interchange symmetry effects) can be computed within a classical framework if a temperature-dependent effective potential is utilized. Two approaches to the construction of this potential are investigated. The first of these is a modification of an earlier procedure by Feynman. This method is similar in spirit to the empirical Pitzer–Gwinn approximation, and in fact is utilized to derive this earlier result. The second approach involves a general Monte Carlo prescription both for the construction of the effective potential and for obtaining the ratio of the quantum and classical–mechanical partition functions.
Picosecond flash photolysis of c
i
s- and t
r
a
n
s-stilbene. Observation of an intense intramolecular charge-resonance transition
Authors: Yoshihara Keitaro , Namiki Akira , Sumitani Minoru , Nakashima Nobuaki
Published: February 2003

Extremely intense and sharp Sn←S1 absorption of cis- and trans-stilbene was observed with a picosecond flash technique. The decay of the transient absorption of trans-stilbene was exponential with one component (110±10 ps), being same as the fluorescence decay at room temperature, and thus assigned to the Sn←S1 absorption. The molar coefficient of the absorption was 7×105 M−1 cm−1 with the full width at half maximum of 1×103 cm−1. The band width had a strong temperature dependence and became only 500 cm−1 at 77 K. A theoretical calculation, using a semiempirical Hamiltonian of PPP type, was made to analyze the nature of this observed intense absorption, and was found that the character of the transition was of the charge-resonance among the ethylenic and two phenyl moieties in stilbene.
Finite perturbation–configuration interaction calculations of nuclear spin–spin coupling constants. I. The first row hydrides and the hydrogen molecule
Authors: Kowalewski Jozef , Laaksonen Aatto , Roos Björn , Siegbahn Per
Published: February 2003

The finite perturbation scheme for calculations of nuclear spin–spin couplings based on the numerical computation of the second derivative of the total energy is described and compared to the earlier approach which uses the first derivative of the spin density matrix. The method is applied, at the Hartree–Fock and configuration interaction level of approximation, to calculations of the Fermi contact contribution to proton–proton and proton–first-row-atom coupling constants in H2, HF, H2O, NH3, and CH4. The analysis and the numerical calculations prove that for the many-electron systems the finite perturbation method is superior to the conventional second order perturbation theory approach if the same type of CI expansion is used. For the hydrogen molecule the two methods have to give identical results. The finite perturbation calculations for the first row hydrides show the following: (i) Upon going from the Hartree–Fock to the CI level the contact contributions are reduced by about 50% for the proton–proton couplings and by about 25% for the proton–first-row-atom couplings. (ii) The differences in the proton–proton couplings within the series of the first row hydrides cannot be explained without including the noncontact mechanisms. (iii) For the hydrogen fluoride, the noncontact mechanisms may contribute as much as 40% of the observed coupling. (iv) When appropriate corrections for the noncontact mechanisms are included the agreement between the calculations and experiments becomes reasonably good.
The relation between simple analytic models for optical spectra of disordered solids
Authors: Klug D. D. , Whalley E.
Published: February 2003

Several analytical models for infrared and Raman spectra of disordered solids, which predict widely different frequency dependence for the intensities of infrared absorption and Raman scattering, are examined. They are shown to be special cases of a more general formalism, and the assumptions required to obtain each model are examined. The frequency dependence of the infrared and Raman intensities of the vibrations of fully disordered materials are quite different because of the inherently different short-range correlations of effective charge and permittivity. The intensity of infrared absorption of a single oscillator is independent of the frequency if the change of dipole moment is due to the motion of permanent charges that are not correlated with one another. Raman scattering is analogous to infrared absorption caused by induced charges, and for maximum disorder the intensities of infrared absorption and reduced Raman scattering by a single oscillator are both proportional to the square of the frequency. Raman scattering cannot have an intensity that is independent of the frequency, except of course for narrow bands of oscillations. The Debye–Hückel distribution of charges is discussed as an example of infrared absorption of distributions of permanent charges intermediate between random charges and random ion pairs. The Debye–Hückel distribution of, for example, ammonium fluoride dissolved in ice can be tested by measuring the absorptivity of the sound waves.
Trajectory studies of the molecular dynamics of ethyl radical decomposition
Authors: Hase William L. , Wolf Ralph J. , Sloane Christine S.
Published: February 2003

The molecular dynamics of ethyl radical decomposition has been studied by Monte Carlo classical trajectories. Hydrogen atom migration between the carbon atoms is found to be a minor path in comparison to C2H5→H+C2H4 dissociation. Strong coupling exists between the H-atom and the C2H4 moiety during the dissociation process. This coupling significantly broadens the resultant distributions in product impact parameter, orbital angular momentum, and relative translational energy and shifts them to higher values than predicted by RRKM on minimum energy path arguements. The dissociation dynamics of C2H4D→D+C2H4 are nearly identical to those for C2H5→H+C2H4. At 100 kcal/mole excitation, ethyl radical dissociation is predicted to be an intrinsically non-RRKM process. There does not appear to be a dividing surface that separates the reactant and product regions of the C2H5?H+C2H4 phase space.
Calcium molecules—A new ground state
Authors: Wyss Jerry C.
Published: February 2003

Bright laser photoluminescence is observed when calcium metal vapor is illuminated with laser light in the 620–660 nm region. The resultant molecular bands are assigned to Ca2 B 1Σu+−X 1Σg+. As observed by others in the matrix at 4–10 K, the X state must be the ground electronic state. The gas phase vibrational constants for X are found to be ω″e?73.18(±0.16), ωex″e=0.560(±0.010), and ωey″e=−1.59×10−3  (±0.17×10−3) cm−1. The dissociation energy is found to be D0?2075  (±150) cm−1. The state previously assigned as the ground state has vibrational constants which significantly disagree with those found for X and is therefore reassigned to an excited electronic level, C 1Σg+. The results of previous workers are in agreement with these revised assignments. The equilibrium vapor pressure at 1500 K of ground state calcium molecules is estimated to be ∼1 Torr.
Curvature of Arrhenius plots caused by tunneling through Eckart barriers
Authors: Pacey Philip D.
Published: February 2003

Contributions of tunneling to the heat capacity of activation, C, a measure of Arrhenius plot curvature, are calculated for processes involving the transfer of mass across Eckart potential energy barriers. A general expression is obtained for C and this is solved in series form for low temperatures. At high temperatures numerical calculations indicate that C increases with increasing u* (=hν*/kT), reaching a maximum when u* exceeds 2π. For realistic barriers, tunneling always makes a negative contribution to activation energy, which becomes half the barrier height, V*, when u* is about 2π. For fixed u*, C increases as V*/hν* increases. Values of C for Eckart barriers are compared with those resulting from other barrier shapes. For simple, gas reactions at low temperatures, the contribution to C from tunneling is much greater than that from other sources, but it is difficult to determine barrier parameters from measurements of C.