bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical absorption spectrum of KI:Sn2+. I. Experimental results
Authors: Kamishina Y. , Sivasankar V. S. , Jacobs P. W. M.
Published: February 2003

The optical absorption spectra of KI:Sn2+ were measured in the temperature range 10–300 K. The spectra were resolved into an appropriate number of skew-Gaussian components and the moments of the A-, B-, and C-bands were computed. The variation of the second moments of the A- and C-bands with temperature was fitted to the usual coth function. The temperature dependence of the splittings of the A- and C-bands has similarly been fitted to a coth function. The results show the effect of a noncubic static crystal field produced by the presence of a charge compensating cation vacancy in the vicinity of the divalent impurity ion. The analysis of the moments also provides the initial values for the parameters entering into the calculation of the absorption line shape.
Photofragmentation dynamics of CH3I at 248 and 266 nm: Vibrational distributions in the CH3(ν2) ‘‘umbrella’’ mode
Authors: Hermann Heinz W. , Leone Stephen R.
Published: February 2003

Vibrational population distributions in the ν2 out-of-plane bending mode of CH3 when produced by photofragmentation of CH3I at 266 and 248 nm are analyzed by infrared emission spectroscopy. In both cases, there is excitation of the ’’umbrella’’ mode vibration up to 10 quanta, and the peak of the excitation occurs at ν2 = 2 for both wavelengths. The vibrational distribution produced at 266 nm agrees well with molecular beam time-of-flight results. The distribution at 248 nm is only moderately enhanced, in contradiction to theoretical predictions which suggest that the peak of the distribution should be shifted to ν2 = 5. The infrared emission spectra suggest that there is no additional rotational excitation in the radical over the ambient rotational temperature of the parent CH3I. In addition, there is no detectable excitation in other vibrational modes, produced either directly in the dissociation or upon collisions.
On path integral Monte Carlo simulations
Authors: Herman M. F. , Bruskin E. J. , Berne B. J.
Published: February 2003

A Monte Carlo procedure based on a discrete point representation of the path integral for the density matrix is explored. It is found that the variance of the estimator used to evaluate the energy grows as the square root of the number of discrete points used, and is therefore to be avoided in highly quantum mechanical systems, where the number of discrete points must be large. A new energy estimator based on the virial theorem is proposed and shown to be well behaved. The main points of the paper are illustrated, using the harmonic oscillator as an example.
Parametrization of the ion–polar molecule collision rate constant by trajectory calculations
Authors: Su Timothy , Chesnavich Walter J.
Published: February 2003

A recently reported calculation by chesnavich, Su and Bowers on classical trajectory study of thermal energy ion-polar molecule capture collisions is further extended. (AIP)
Critical exponents for surface interacting self-avoiding lattice walks. I. Three-dimensional lattices
Authors: Ishinabe Takao
Published: February 2003

Self-avoiding lattice walks interacting with an interface are investigated as a model of polymer adsorption. The value of a critical exponent δa is estimated at 1.7±0.3 from the exact enumeration data up to 20 and 14 steps for the tetrahedral and simple cubic lattices, respectively; δa is defined from the dependence of the free energy of an adsorbed polymer chain on the interaction with interface. The value differs from the scaling prediction δa = 5/2 of de Gennes. An assumption involved in the scaling treatment is examined by using the enumerations of self-avoiding walks confined between two planes in order to explain the disagreement.
The experimental study of oxygen isotope exchange reaction between dissolved bicarbonate and water
Authors: Hal/as S. , Wol/acewicz W.
Published: February 2003

The rate and equilibrium fractionation factor of oxygen isotope exchange between HCO−3 and H2O were studied at 25, 35, and 45 °C. It was found that the exchange reaction is complete in 8.8 h (5τ) at 25 °C, 2 h at 35 °C, and in less than 1 h at 45 °C. The equilibrium fractionation factor as a function of temperature fits the equation 103 ln α(HCO−3–H20) = (2.92±0.07) (106/T2)−(2.66±0.18). The CO2 produced from rapidly precipitated BaCO3, to trap the HCO−3 ions maintained in isotopic equilibrium with water at CO2 directly equilibrated with the same water at 25 °C. This provides an alternate method for determining the oxygen isotopic composition of water.
Shape-resonance-induced non-Franck–Condon effects in the valence-shell photoionization of O2
Authors: Dittman P. M. , Dill Dan , Dehmer J. L.
Published: February 2003

Non-Franck–Condon effects in the valence-shell photoionization spectrum of O2 are studied using the multiple scattering model. Calculations for the 3σg, 1πu, and 1πg levels indicate that the enhanced sensitivity of dipole strengths to internuclear separation, induced by shape resonances, reduces and broadens the resonant cross section; nonresonant channels are negligibly affected. The resulting cross sections and asymmetry parameters show fair overall agreement with line and continuum source measurements.
Collision-induced transitions between electronic states B→A and C→B of laser-excited Li2 molecules
Authors: Ennen G. , Ottinger Ch.
Published: February 2003

Nonradiative transitions between the Li2 (C 1Πu, B 1Πu, A 1Σ+u) states have been studied using monochromatic laser excitation. In a cell experiment under single-collision conditions, the B–A transfer was induced by rare gas atom collisions. With argon, the cross section is 240 Å2. B→A and C→B transfer by Li atom collisions was observed with cross sections of 1200 and 330 Å2, while C→B transfer is not measurably induced by argon collisions. The final state rotational-vibrational level population distribution is very broad and non-Boltzmann.
Bead models of segmentally flexible macromolecules
Authors: Wegener William A.
Published: February 2003

Bead model methods developed for complex rigid macromolecules are extended to obtain diffusion coefficients for macromolecules with segmental flexibility. A suitable arrangement of beads is used to represent each segment and to account for hydrodynamic interactions between segments. Bead expressions for the set of three-dimensional tensors relating frictional drag forces and torques on one segment to translational and rotational motions of another are obtained and compared with previous expressions that neglected intersegmental hydrodynamic interactions. By specifying the nature of the flexible attachments, this set can be used to obtain the frictional resistance and diffusion tensors characterizing the body’s degrees of freedom. As an example, a bent rod is considered with a central bending region that is either rigid or swivel jointed. Although intersegmental interactions are seen to increase the translational diffusion coefficient as expected from hydrodynamic shielding, the rotational diffusion coefficients for the rigid bent rod show a more complex behavior with the interactions decreasing end-over-end motion at small bending angles and increasing it at large angles. For the flexible rod, these interactions generally increase the end-over-end motions of each rodhalf.
A quantum stochastic Fokker–Planck theory for adiabatic processes in condensed phases
Authors: Wassam William A. , Freed Jack H.
Published: February 2003

A maximum entropy approach to time-dependent phenomena is employed to construct a quantum stochastic theory for adiabatic processes in condensed phases. This theory is illustrated by considering the relative motion of a two-particle system interacting with a heat bath. We present a fully quantum mechanical phase space master equation, which is new. In the limit where the length of thermal spatial fluctuations along the relative coordinates is much greater than half the wavelength for thermal momentum fluctuations associated with relative motion, our master equation reduces to a simpler form, and the principle of detailed balance emerges. This limit is shown to be equivalent to fulfilling a ’’thermal Heisenberg uncertainty’’ relation: Δp̄Δq̄>>ℏ/2. The simpler master equation is utilized in the development of a quantum mechanical nonlinear Fokker–Planck equation. For cases in which correlations involving spatial and momentum fluxes are short ranged, the nonlinear Fokker–Planck equation reduces to the usual linear form, with ’’collision’’ and ’’streaming’’ terms that contain quantum corrections to all orders in ℏ. The relationship between the classical limit of the present theory and Brownian motion theory based on the Chapman–Kolmogorov equation is established.
The mechanism and kinetics of evaporation from laser irradiated UO2 surfaces
Authors: Magill J. , Bloem J. , Ohse R. W.
Published: February 2003

A theoretical analysis of laser evaporation experiments for the determination of the high temperture saturated vapor pressure of uranium dioxide is presented. The interaction of laser radation with the condensed phase is discussed and the timescales involved in the energy equilibration processes are given. The mechanism of evaporation is described within the framework of the Terrace–Ledge–Kink model, where it is shown that by using symmetry properties of ionic crystal lattices, the surface Madelung potentials and binding energies of ions and neutral ionic units may be determined. Results are presented for NaCl, CsCl, and UO2 surfaces. Based on these calculations, general conclusions are drawn on the nature of the evaporating species. In addition, it is shown that the multispecies evaporation from uranium dioxide is fundamentally related to the amount of atomic and electronic disorder in the surface layer. The effects of the kinetics of surface reactions on the rate of evaporation are investigated. Parallel reaction paths such as direct evaporation from kink sites are treated in addition to surface diffusion followed by desorption series reactions. Finally, the possibility of producing superheated metastable surface layers under pulsed laser heating conditions is investigated using models for the kinetics of atomic and electronic disorder in the condensed phase.
Structure and dynamics of intramolecular hydrogen bonds in carboxylic acid dimers: A solid state NMR study
Authors: Meier B. H. , Graf F. , Ernst R. R.
Published: February 2003

Direct evidence for dynamic intramolecular double proton exchange in carboxylic acid dimers in the solid is obtained from NMR measurements. For the p-toluic acid dimer, an asymmetric double minimum potential with an energy difference ΔG = 1.0 kJ/mol and an activation energy of Ea = 4.8 kJ/mol between the two tautomeric forms is found. The equilibrium position of the carboxylic proton pair at low temperature is derived from the corresponding dipolar coupling tensor. The importance of double proton tunneling at low temperature is discussed.
Photoelectron angular distributions of H2O
Authors: Truesdale C. M. , Southworth S. , Kobrin P. H. , Lindle D. W. , Thornton G. , Shirley D. A.
Published: February 2003

The partial cross section and asymmetry parameters of the 2B1, 2A1, and 2B2 ionic states of H2O+ have been measured at photon energies of 18–31 eV, by time-of-flight photoelectron spectroscopy using synchrotron radiation. The atomic character of the molecular orbitals is discussed in the interpretation of similarities and differences between the measurements for these states. Other synchrotron radiation data, resonance line measurements, and (e, 2e) dipole results are included. A multiple-scattering Xα (MSXα) calculation and a ground-state inversion method calculation are in good agreement with our cross section measurements. There is excellent agreement between experimental asymmetry parameters and those computed from the MSXα method. Stieltjes–Tchebycheff imaging model calculations suggest the presence of resonant channels. This may be the origin of weak resonance structure present in the measurements.
On the molecular theory of crystallization and formation of ordered structures in equilibrium fluids
Authors: Altenberger A. R.
Published: February 2003

A new theory of crystallization and formation of ordered phases in equilibrium fluids, based on the analysis of the asymptotic behavior of the pair correlation function, is presented. The criteria of formation of ordered structures are shown to be equivalent to the limits of internal, thermodynamic stability of the system. Comparison with the earlier approaches to the problem of phase transitions (liquid–solid and isotropic fluid–ordered liquid crystal) is given.
The molecular structure of dimolybdenum tetra-acetate
Authors: Kelley M. H. , Fink M.
Published: February 2003

The molecular structure of anhydrous dimolybdenum tetra-acetate Mo2(O2CCH3)4 in the gas phase at 190 °C, was determined from the diffraction of 54 keV electrons. Two common refinement techniques, involving the use of constrained and unconstrained molecular geometries, were used and found to give comparable results for the major structure parameters. The structure parameters include distances for the Mo–Mo and Mo–O bonds of 2.079(3) and 2.108(3) Å, respectively, and a Mo–Mo–O bond angle of 92.2(2)°. The Mo–Mo bond determined by x-ray diffraction on a single crystal is longer by about 0.01 Å, demonstrating the distorting effects on the quadruple metal bond of axial coordination in the crystal geometry. The effects of intramolecular multiple scattering were also investigated and found to depend critically on the approximation used to account for the effects of molecular vibrations on the multiple scattering corrections. This indicates that a more careful treatment of the vibrational averaging is required.
Photoionization of Ar2 at high resolution
Authors: Dehmer P. M.
Published: February 2003

The relative photoionization cross section of Ar2 was determined at a resolution of 0.07 Å in the wavelength region from 800 to 850 Å using a new photoionization mass spectrometer that combines a high intensity helium continuum lamp with a free supersonic molecular beam source. In the region studied, the photoionization cross section is dominated by autoionization of molecular Rydberg states, and the structure is diffuse owing to the combined effects of autoionization and predissociation. The molecular photoionization spectrum is extremely complex and shows little resemblence either to the corresponding atomic spectrum (indicating that the spectrum of the dimer is not simply a perturbed atomic spectrum) or to the molecular absorption spectrum at longer wavelengths. The regular vibrational progressions seen at longer wavelengths are absent above the first ionization potential. Detailed spectroscopic analysis is possible for only a small fraction of the observed features; however, vibrational intervals of 50–100 cm−1 suggest that some of the Rydberg states have B 2Π3/2g ionic cores. A comparison of the absorption and photoionization spectra shows that, at wavelengths shorter than ∼835 Å, many of the excited states decay via mechanisms other than autoionization
The dissociation dynamics of state selected metastable aniline ions by single and multiphoton ionization
Authors: Baer Tomas , Carney Thomas E.
Published: February 2003

The lifetimes with respect to dissociation of state selected aniline ions have been investigated by photoion–photoelectron coincidence. Lifetimes between 0.3 and 100 μs have been found when the aniline ion is prepared in energies ranging from 13.6 to 12.6 eV, respectively. These lifetimes are in good agreement with those of a recent study by Proch, Rider, and Zare, who observed metastable aniline ions in a UV multiphoton ionization (MPI) study in which three photons were required to reach the metastable ion energy range. By contrast, our visible MPI study of aniline shows no evidence for long-lived ions. This paradox is resolved by postulating that under the UV MPI conditions, a radiationless transition to the ground electronic state allows the 13 eV ions time to dissociate slowly, while under the much higher photon intensities needed in the visible MPI study, rapid up pumping of the excited ions prevents radiationless transitions. A lower limit for the radiationless transition rate of 2×108 s−1 is derived.
Rotational excitation in D2–CO collisions
Authors: Andres J. , Buck U. , Meyer H. , Launay J. M.
Published: February 2003

In a crossed molecular beam experiment total differential cross sections and time of flight distibutions of the scattered particles have been measured for D2+CO at 87.2 meV collision energy. The energy loss spectra, which do not exhibit single rotational transitions, were transformed to the c.m. system so that not only positions but also amplitudes could be compared with calculations. The comparison shows that the spectra are dominated by two rotational rainbow peaks, one at large final rotational states j′ due to the CO excitation on the C side of the molecule and one with larger probability at small j′ due to the excitation on the O side of CO. The positions and the amplitudes of the former peak have been reproduced by the calculation using the ab initio potential [D. R. Flower, J. M. Launay, E. Kochanski, and J. Prissette, Chem. Phys. 37 355 (1979)]; however, the amplitudes of the latter peak are overestimated but can be corrected by a smaller slope of the anisotropy of the potential. The parameters of the fitted isotropic potential ε = 5.73 meV and Rm = 3.88 Å were found to deviate from the prediction. The calculations have been performed by treating D2 in the jz conserving coupled states approximation and CO in the infinite order sudden approximation.
Radiative lifetimes and quenching rate coefficients for directly excited rotational levels of OH (A 2Σ+,v′=0)
Authors: McDermid I. Stuart , Laudenslager James B.
Published: February 2003

A narrow bandwidth pulsed dye laser has been used to excite OH X 2Πi radicals to the A 2Σ+ state by pumping in the (0,0) vibrational band around 308 nm. The radiative lifetimes of specific (K′,J′) rotational levels in v′ = 0 were measured at low pressures ⩽1 mTorr which gave a mean lifetime τ0=0.721±0.009 μs (2σ). Electronic quenching rate constants for important atmospheric species N2, O2, H2O, and also for H2 were measured for a range of initially excited rotational levels. A strong dependence of this rate constant on the initially excited level was found for N2, and less markedly for O2, with the rate constant tending to increase for the lowest rotational levels K′⩽3. The implications of these measurements of radiative and quenching rates for state selected rotational levels of OH A, v′=0 to the laser-induced fluorescence detection of atmospheric OH are discussed.
Resolvent operator approach to many-body perturbation theory. II. Open shells
Authors: Banerjee Ajit , Mukherjee Debashis , Simons Jack
Published: February 2003

In this paper, we develop a time-dependent approach to many-body perturbation theory for open shells based on the resolvent of the Schrödinger equation. We introduce, analogous to the closed-shell case, quantities Sij(t) = i<φi‖e−i(H−Ec)t ‖ψj>/<ψj0‖ψj>, where ψj0 and ψj are the jth unperturbed and exact functions, respectively. The ψj0’s can be expressed in terms of ’’model space’’ functions φi = Ωi+‖φ>, where the Ωi+ are appropriate creation/annihilation operator products acting on a conveniently chosen closed-shell vacuum φ. These φi’s are not necessarily degenerate with respect to the unperturbed Hamiltonian H0. Ec is the exact (correlated) energy of the vacuum φ. The Fourier transforms Sij(ω) of Sij(t) have the form Sij (ω)=<φi‖(ω +Ec−H)−1‖ψj>/<ψj0‖ψ j> and thus have poles at energy differences (Ej−Ec), i.e., relative to the exact vacuum energy. Using the time-dependent perturbation expansion of S(t), we obtain a Dyson-like equation N̄−1(ω)=N̄0–1(ω)+Σ, where N̄ is defined as N̄ij (ω) = <φi‖(ω+Ec−H)−1‖φj> and N̄0 is the corresponding unperturbed component. Knowledge of the combining coefficients Cji in ψj0=ΣJCjiφi is thus not required for finding the poles. We arrive at the Dyson-like equation by first eliminating closed diagrams and then regrouping the remaining terms in the perturbation series for S into ’’top’’ and ’’bottom’’ parts. Regrouping appropriate to the Brillouin–Wigner (BW) case together with an associated time-integration procedure yields ΣBW which consists of disconnected and ω-dependent diagrams. This is shown to yield the open-shell BW series in the Bloch–Horowitz form. An alternative regrouping procedure and use of the ’’folding technique’’ of Johnson and Baranger leads to a ΣRS which is ω-independent, Hermitian, contains connected diagrams only, and is, thus, size-consistent.
Optical activity of the f–f transitions in trigonal dihedral (D3) lanthanide(III) complexes. II. Calculations
Authors: Saxe J. D. , Faulkner T. R. , Richardson F. S.
Published: February 2003

Calculations of dipole strengths, rotatory strengths, and dissymmetry factors associated with the 4f↔4f transitions of trigonal dihedral (D3) lanthanide(III) complexes are reported. These calculations, based on a theoretical model described in the previous paper [J. Chem. Phys. 76, 1595 (1982)], represent the first attempt to quantitatively account for the chiroptical properties of lanthanide systems. As model systems, tris-terdentate chelate structures formed by oxydiacetate (ODA2−) ligands and the trivalent lanthanide ions Pr3+, Eu3+, Tb3+, and Ho3+ are considered. Calculations are reported for both isotropic and oriented samples of the Ln(ODA)33− systems. Parameters varied in the computational model include ligand geometry, ligand charges and polarizabilities, and lanthanide crystal field coefficients. The results of the calculations are applied to the interpretation of lanthanide CD/absorption and CPL/emission spectra. Special emphasis is given to the relative chiroptical strengths predicted for term-to-term transitions and to the ligand structure parameters having the greatest influence on the chiroptical properties. The calculated results demonstrate that the theoretical model correctly accounts for most of the qualitative aspects of lanthanide optical activity as observed by experiment. In a few cases, the calculations also achieve quantitative or semiquantitative agreement with experimental observation. However, the model as applied in this study is not sufficiently refined to permit reliable calculations of lanthanide chiroptical spectra at the level of crystal field component resolution.
Raman scattering: Investigation of nematic and smectic ordering
Authors: Constant Monique , Decoster Didier
Published: February 2003

Quantitative measurements of vibrational Raman depolarization ratios have been used to study molecular orientational order in nematic and for the first time, in smectic liquid crystals. Temperature dependence of the depolarization ratios of the terminal C≡N and the central biphenyl link vibrational bands of two alkylcyanobiphenyl compounds (CBn) and a cyclohexane derivative (PCH7) has been investigated extensively. Two experimental technics have been used: a conventional triple monochromator and the Raman microprobe to overcome the spurious depolarization due to the long wave director fluctuations. The orientational order parameters <P2≳ and <P4≳ were then calculated. The results have been discussed assuming repulsive forces are dominant for the CBn compounds and using the Maïer–Saupe theory in the case of PCH7.
Overtone vibrations of OH groups in fused silica optical fibers
Authors: Stone J. , Walrafen G. E.
Published: February 2003

The OH-stretching fundamental, and the first, second, and third OH overtone vibrations have been examined using pure, GeO2-doped, graded index, fused silica optical fibers having OH contents from <1 ppm to ≳103 ppm. The previously reported component substructure of the fundamental OH-stretching contour was found to be repeated in the overtones, along with a set of involving fundamental vibrations of fused silica. Several new vibrations were uncovered and assigned. Examinations of deuterated OH samples helped to verify the assignments. The spectral broadening produced by GeO2 was interpreted in terms of a wider range of environments seen by the OH oscillators, compared to pure fused silica.
The determination of translational diffusion constants in liquid crystals from pulsed field gradient double quantum spin echo decays
Authors: Martin Joel F. , Selwyn Lyndsie S. , Vold Regitze R. , Vold Robert L.
Published: February 2003

Proton double quantum decay rates in presence of pulsed magnetic field gradients have been used to measure translational diffusion coefficients. The method is superior to analogous techniques based on single quantum echoes because double quantum coherence experiences effectively twice the applied field gradient. In cases where the natural double quantum decay rate is less than that of single quantum coherence, further increases in precision can be realized. The method is illustrated by measurements of solute diffusion parallel and perpendicular to the director axis in nematic and smectic liquids crystalline media.
Strain fluctuations and elastic constants
Authors: Parrinello M. , Rahman A.
Published: February 2003

It is shown that the elastic strain fluctuations are a direct measure of elastic compliances in a general anisotropic medium; depending on the ensemble in which the fluctuation is measured either the isothermal or the adiabatic compliances are obtained. These fluctuations can now be calculated in a constant enthalpy and pressure, and hence, constant entropy, ensemble due to recent develpments in the molecular dynamics techniques. A calculation for a Ni single crystal under uniform uniaxial 100 tensile or compressive load is presented as an illustration of the relationships derived between various strain fluctuations and the elastic modulii. The Born stability criteria and the behavior of strain fluctuations are shown to be related.
A semiclassical multichannel branching model for describing state-specific unimolecular decomposition and other dynamical processes in polyatomic molecular systems
Authors: Waite Boyd A. , Miller William H.
Published: February 2003

A semiclassical multichannel branching model is developed and applied to various dynamical phenomena in polyatomic molecular systems. The model is based on the reaction path Hamiltonian of Miller, Handy, and Adams [J. Chem. Phys. 72,99 (1980)] and also utilizes the semiclassical perturbation-infinite order sudden approximation of Miller and Shi [J. Chem. Phys. 75, 2258 (1981)] for describing vibrational inelasticity along the reaction path. Specific applications of the model are made to state-specific unimolecular decomposition, energy level splitting in multidimensional double-well potentials, and to reaction probabilities along reaction paths with multiple transition states.
High pressure Mössbauer study of 197Au in Cs2Au2Cl6, Cs2AgAuCl6, and AuI
Authors: Stanek Jan
Published: February 2003

Mössbauer spectra of 197Au in Cs2Au2Cl6 were studied at 1 bar (4.2 and 45 K) and at 68 kbar (2.1, 4.2, 25, 40, and 45 K). Further, 197Au spectra in Cs2AgAuCl6 were studied at 1 bar and 31 kbar (4.2 K) and in AuI at 1 bar and 45 kbar (4.2 K). The pressure-induced changes in the isomer shifts and quadrupole splittings of Cs2Au2Cl6, Cs2AgAuCl6, and AuJ suggest a positive sign of the electric field gradient at the Au+ nuclei and a negative sign at the Au+3 nuclei, i.e., contrary to previous predictions. For Au+, 5d6s rather than 6s6p hybridization is proposed. For Au3+ the positive contribution of the 5d8 free ion configuration to the electric field gradient overrides the negative contribution from 5d6s6p2 hybrids. On this basis, the chemical bonds in the cesium gold halides are concluded to be primarily ionic. The Mössbauer spectra shown that the Au+ and Au3+ ions are clearly distinguishable up to 45 K in the high pressure (68 kbar) state of Cs2Au2Cl6. Thus, the apparently equivalent Au positions and averaging of the Au valene states observed at 52 kbar with x-ray diffraction is a dynamical process with a relatively long relaxation time due to the corner sharing topology of coordination octahedra. The significant increase in the electrical conductivity with increasing pressure appears to occur via Cl− ions, and not by direct Au+–Au3+ hopping.
Critical temperatures of two-dimensional condensation in monolayers of Ar, Kr, or Xe adsorbed on lamellar halides
Authors: Millot F. , Larher Y. , Tessier C.
Published: February 2003

From sets of adsorption isotherms, critical temperatures of two-dimensional condensation have been determined for the first monolayer of Ar, Kr, or Xe adsorbed on a number of lamellar dihalides. Once properly scaled by the critical temperature of the bulk adsorbate, their value appears to be strikingly correlated to the so-called dimensional incompatibility between the adsorbate and the substrate. Their important variation (up to 40%) indicates a strong influence of the structure of the substrate on the properties of the monolayers, chiefly on those of the high density phase involved in the 2D condensation. Depending on the value of the so-called dimensional incompatibility, this phase takes on one of at least four different structures, ranging from liquidlike to lattice wise. Only in the first case does the theory give a good quantitative account of the experimental results. For the other types of structure, we present a qualitative analysis of the results, trying to single out the main factors responsible for the values taken by the critical temperatures as well as for their variation with the dimensional incompatibility.
The influence of symmetry on the conformational dependences of scalar and vectorial properties for double-rotor molecules
Authors: Maruani J. , Smeyers Y. G. , Hernández Laguna A.
Published: February 2003

We show how specific parameters of double-rotor molecules can be characterized by up to six symmetry indices depending on whether the molecule is linear, bent planar, or gauche and whether the parameter is scalar or vectorial, aggregate, mononuclear, or binuclear. These symmetry indices can be used to select the distinct, nonzero, harmonic coefficients in the limited Fourier-series expansions of the parameters as functions of the dihedral angles. Illustrative applications are given to the numerical calculations of the torsional potentials for the isomers of xylene and of the hyperfine couplings for the protons in propyl.
On the nature of the correction to the Weizsacker term
Authors: Ludeña Eduardo V.
Published: February 2003

It is shown that the reduced first-order density matrix may be written as γ1(1, 2) = ρ1/2(1) ρ1/2(2)[1+B(1, 2)/ρ(1) ρ(2)]1/2. The function B(1, 2) is equal to −2γ2(1, 2; 1, 2) for the Hartree–Fock case so that the quotient inside the brackets is, in fact, the statistical correlation function, which describes the Fermi hole. For the general case (when correlation is included), it is shown that B(1, 2)≠−2γ2(1, 2; 1, 2); the quotient, however, is a function which describes Fermi and Coulomb correlation. It is shown that the kinetic energy density may be written as the Weizsacker term plus a correlation term given by 1/4 ρ(1)∇1 ∇2[B(1, 2)/ρ(1) (ρ(2)]2 → 1. In view of the relationship between B(1, 2) and the correlation function, the nature of the non-Weizsacker term is discussed.
Effect of an infinite plane wall on the motion of a spherical Brownian particle
Authors: Gotoh Toshiyuki , Kaneda Yukio
Published: February 2003

The motion of a spherical Brownian particle in an incompressible fluid bounded by an infinite plane wall is studied on the basis of the linearized Landau–Lifshitz equations for the fluctuating hydrodynamics. The asymptotic forms for large time t of the autocorrelation function Φdj(t) for the random force acting on the particle and the autocorrelation function Ψij(t) for its velocity are discussed for the case that the distance l between the wall and the particle is finite but much larger than the particle radius a. It is shown that Φii and Ψii fall off as t−3/2 for a2/ν≪t≪l2/ν, but for t≫l2/ν they fall as t−5/2 or t−7/2, accordingly, as the direction i is parallel or perpendicular to the wall, where ν is the kinematic shear viscosity of the fluid.
An ESR and optical study of V4+ in zircon-type crystals
Authors: Di Gregorio S. , Greenblatt M. , Pifer J. H. , Sturge M. D.
Published: February 2003

The electron spin resonance and polarized optical absorption spectra of V4+ have been studied in single crystals of ZrSiO4, HfSiO4, ThSiO4, and ThGeO4, each having the zircon structure. The principal values of g and A in ZrSiO4 and HfSiO4 are found to be anomalous and significantly different from those of ThSiO4 and ThGeO4. The optical spectra suggest that V4+ enters the tetrahedral rather than the dodecahedral site. The spectra are all similar except for the observation of strong D2d- forbidden transitions in ZrSiO4 and HfSiO4. We show that the optical and ESR data in these two crystals cannot be accounted for by conventional crystal field theory. We show that, while the dynamic Jahn–Teller effect can give a good account of the ESR data, the optical data are not consistent with this interpretation. We tentatively attribute the anomalies to dynamic local distortions of unknown origin.
Two-dimensional nuclear magnetic resonance in rotating solids: An analysis of line shapes in chemical shift-dipolar spectra
Authors: Munowitz Michael G. , Griffin Robert G.
Published: February 2003

Magic angle sample spinning is combined with two-dimensional NMR methods to obtain high resolution chemical shift-dipolar spectra. Because both interactions are inhomogeneous, the 2D spectrum consists of a set of rotational sidebands spaced at the spinning frequency in each dimension. Projection of these sidebands onto the chemical shift or dipolar axis yields either the chemical shift or the dipolar spectrum fully separated from the other interaction. The projections may be used to extract the principal values of either the chemical shift or dipolar tensor, and from the latter, internuclear distances may be calculated. In addition, it is shown that sets of dipolar sidebands, corresponding to individual chemical shift sidebands, exhibit some unusual features. In particular, in the slow spinning regime, the dipolar rotational sideband patterns are asymmetric and in some instances, individual sidebands are inverted. The physical origin of this effect is discussed, and experimental results and computer simulations are in excellent agreement. Finally, it is shown that the intensity distribution of the sidebands permits a determination of the relative orientation of the shift and dipolar tensors.
The nature of bonding in PdC
Authors: Shim Irene , Gingerich Karl A.
Published: February 2003

The nature of bonding in PdC is described through all electron ab initio calculations. The Hartree-Fock-Rotthaan formalism is employed. (AIP)
Morse-oscillator matrix elements appropriate for vibration-rotation intensities of diatomic molecules
Authors: Huffaker J. N. , Tran Loc Binh
Published: February 2003

Exact algebraic formulas are given for Morse-oscillator matrix elements 〈v‖yn‖v〉≳ for n=4, where y=1−exp[−a(r−rd). These can be used for analysis of infrared intensities in diatomic molecules when the electric dipole function is expressed as a polynomial in y.
Reorientation and pressure broadening of IR or MW lines: New results in CH3F
Authors: Brechignac Ph.
Published: February 2003

A difference-frequency laser system has been used to measure the pressure-broadening parameters of both the Stark-split components and the degenerate line for the transition R(1, 1) in the parallel band of CH3F at 2866 cm−1. The analysis of the data shows that the individual Stark components are broader than the zero-field line. This result is shown to be in agreement with the predictions of the infinite order sudden approximation based on dominant dipole–dipole interaction, and can be interpreted in terms of interference effects between overlapping lines.
The thermochemical properties of gas-phase transition metal ion complexes
Authors: Holland Paul M. , Castleman A. W.
Published: February 2003

Gas phase ion clustering reactions of the form A+(L)n+L⇄A +(L)n+1 were studied using high pressure mass spectrometry. Equilibrium constants together with enthalpy, entropy, and free energy changes were determined for stepwise clustering reactions of water about the Ag+ ion with n=0 to5 and the Cu+ ion with n=2 to4, ammonia about the Ag+ and Cu+ ions from n=1 and n=2 to4, respectively, and pyridine about the Ag+ and H+ ion from n=2 and n=1 to  3, respectively. These results, together with those of other studies, show evidence for the existence of structure in some cluster ions. Considerations of bonding and entropy, together with comparisons with the results of ion chemistry in the liquid phase, suggest that such structures can be attributed to several types of cluster ion systems, including gas phase analogs of traditional ’’coordination complexes’’ known in the aqueous phase.
Infrared band intensities of formaldehyde and formaldehyde-d2
Authors: Nakanaga Taisuke , Kondo Shigeo , Saëki Shinnosuke
Published: February 2003

The infrared band intensities of formaldehyde and formaldehyde-d2 have been measured in the gas phase using an FT–IR spectrometer. The effect of the polymerization of the sample molecule was minimized to measure the correct intensities. When the bands overlapped each other, their individual intensities have been determined by a band simulation calculation taking account of the Coriolis interactions. The signs of the dipole moment derivatives (∂p/∂Qi) have completely been determined by the least-squares method, and the results were compared with those obtained by the molecular orbital calculation (CNDO/2) and by the analyses of the Coriolis interactions. The local intensity parameters of the CH2 group of this molecule were calculated and compared with the corresponding ones of ethylene.
Thermodynamic properties of Stockmayer molecules by Monte Carlo simulation
Authors: Yao J. , Greenkorn R. A. , Chao K. C.
Published: February 2003

Thermodynamic properties are determined for Stockmayer molecules by means of Monte Carlo simulation to establish the contribution of dipolar interaction to bulk properties. Both the canonical and the grand canonical ensembles are employed to calculate results for the supercritical fluid, and the subcritical gas and liquid.
High resolution laser spectroscopy of Cs2. II. Doppler-free polarization spectroscopy of the C 1Πu←X 1Σ+g system
Authors: Raab M. , Höning G. , Demtröder W. , Vidal C. R.
Published: February 2003

More than 2000 completely resolved rotational lines in the C 1Πu(1u)←X 1Σ+g(0+g) band-system and 200 lines in the D 1Σu(0+u)←X 1Σ+g system of Cs2 have been accurately measured with the techniques of Doppler-free polarization spectroscopy and optical–optical double resonance (OODR). The wave numbers of these lines were determined within 10−3 cm−1 using a traveling Michelson interferometer and a 120 cm confocal Fabray–Perot interferometer. The OODR technique allows the unambiguous assignment of even perturbed lines, and enables us to find different perturbations in the upper C 1Πu(1u) state. A thorough analysis of the measured data, including laser induced fluorescence lines from higher electronic states into high lying vibrational levels of the electronic ground state, yields molecular constants, potential energy curves, and Franck–Condon factors.
Characterization of some autoionization resonances in CO2 using triply differential photoelectron spectroscopy
Authors: Parr A. C. , Ederer D. L. , Dehmer J. L. , Holland D. M. P.
Published: February 2003

We report vibrationally resolved branching ratios and asymmetry parameters for two sets of autoionizing resonances in CO2 near 680 and 750 Å. These resonances were excited with monochromatized synchrotron radiation from the National Bureau of Standards storage ring and the energy and angle of ejection of the photoelectrons were analyzed. The results show striking non-Franck–Condon behavior.
The application of the hypernetted chain approximation to the electrical double layer: Comparison with Monte Carlo results for symmetric salts
Authors: Lozada-Cassou Marcelo , Saavedra-Barrera Rafael , Henderson Douglas
Published: February 2003

The hypernetted chain integral equations (HNC/MSA version) for the reduced density profiles of a model electrolyte near a charged electrode (the so-called electrical double layer problem) are solved and the resulting density and potential profiles are compared with recent Monte Carlo calculations. Good agreement is found when the bulk electrolyte direct correlation functions are calculated from the mean spherical approximation. In particular, the HNC/MSA correctly predicts that for divalent salts, the potential profile is nonmonotonic and changes sign. As a result, there is charge oscillation or a layering of charge. That is, near the electrode the ions predominately have the opposite charge as the electrode but in some situations the ions at a greater distance generally have the same charge as the electrode. In contrast, the Gouy–Chapman theory gives poor results. The Gouy–Chapman density and potential profiles are always monotonic. Thus, the widely used practice of estimating the effects of solvent or non-Coulombic interactions by subtracting the Gouy–Chapman theory results from experimental results may in certain situations lead to erroneous estimates of these effects.
Evidence for a molecular intermediate during the oxidation of aluminum by water
Authors: Szalkowski F. J.
Published: February 2003

The interaction of H2O with the Al surface plays an important role in the atmospheric and aqueous corrosion of that metal. In order to clarify the oxidation mechanism involved in this process, we studied the initial stages of H2O adsorption upon the Al(100) surface using the techniques of x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and high resolution electron energy loss spectroscopy. The experiments were performed at a substrate temperature of 100 K in an attempt to stabilize any short-lived intermediate species involved in the oxidation process. The experiments show that the initial H2O interaction with the clean Al(100) surface at 100 K forms a molecular H2O species. Further H2O exposure results in the condensation of H2O multilayers (ice) upon this surface species. Heating the sample to 300 K desorbs the ice layer and forms an oxide similar to that formed in a 300 K H2O/Al(100) oxidation; no trace of H2O is detected on this surface but an oxide-stabilized OH species is present.
Photoionization of ArKr, ArXe, and KrXe and bond dissociation energies of the rare gas dimer ions
Authors: Dehmer P. M. , Pratt S. T.
Published: February 2003

The relative photoionization cross sections for the heteronuclear rare gas dimers ArKr, ArXe, and KrXe are reported at a wavelength resolution of 0.15 to 0.28 Å in the energy region between the molecular ionization threshold and the atomic 2P°1/2 ionization limit of the lighter rare gas atom. The dimer ionization potentials are 13.484±0.015 eV for ArKr, 11.968±0.012 eV for ArXe, and 11.763±0.011 eV for KrXe. Combining these values with the known values of the atomic ionization potentials and the neutral ground state dissociation energies yields values for the A 2Σ+1/2 ionic ground state dissociation energies of 0.528±0.015 eV for ArKr, 0.176±0.012 eV for ArXe, and 0.385±0.011 eV for KrXe. Molecular Rydberg structure in the region between the atomic fine structure thresholds is analyzed in terms of Rydberg series converging to the B 2Π1/2 or D 2Σ+1/2 states of the dimer ions. Molecular Rydberg structure in other regions of the spectrum is extremely complex and, in many cases, long vibrational progressions are observed that are only partially analyzable with the present resolution. Tables are presented summarizing the best experimental values of bond dissociation energies for the ground and excited states of all of the homonuclear and heteronuclear rare gas dimer ions.
Monte Carlo simulation of n-butane in water. Conformational evidence for the hydrophobic effect
Authors: Jorgensen William L.
Published: February 2003

Monte Carlo statistical mechanics simulations have been carried out for a dilute solution of n-butane in water at 25 °C and 1 atm. The intermolecular interactions were described by Coulomb and Lennard-Jones terms in the TIPS format including the TIPS2 parameters for water. The internal rotation about the central CC bond in n-butane was included using a rotational potential based on molecular mechanics (MM2) calculations. The precision of the simulation results was enhanced by preferential sampling and by umbrella sampling for the internal rotation over chopped barriers. Conformational results are also reported from a long Monte Carlo run for pure liquid n-butane using umbrella sampling. Although no condensed phase effect is found on the conformational equilibrium in pure liquid n-butane, there is a pronounced increase in the gauche population of n-butane upon transfer from the gas phase to aqueous solution. The latter finding is in near quantitative accord with the shift predicted by Pratt and Chandler from their theoretical model. It is also consistent with the basic tenet of the hydrophobic effect regarding the folding of natural and synthetic polymers in water. In addition, detailed structural results for the system are reported. Notably, the water molecules in the first shell around n-butane have normal bonding energies and hydrogen bonding profiles for bulk water. However, since their coordination numbers are low, this situation can only be achieved by greater ordering which is entropically costly. The validity of the observations is supported by the computed heat and volume of solution which are in accord with experimental data.
Temperature dependence of three-body association reactions from 45 to 400 K. The reactions N+2+2N2→N+4+N2 and O+2+2O2→O+4 +O2
Authors: Böhringer H. , Arnold F.
Published: February 2003

The temperature dependences of the three-body ion–molecule association reactions N+2+2N2→N+4+N2 (1) and O+2+2O2→O+4+O2 (2) have been studied in a recently constructed liquid-helium cooled ion drift tube in the temperature range 45–400 and 51–340 K, respectively. The results can be approximated over a wide temperature range by the expressions k1=6.8 × 10−29 (300/T)1.64 and k2=4 × 10−30 (300/T)2.93 although deviations at low temperatures are apparent. The influence of the electric drift field on the measured rate coefficients has been investigated in detail. Experimental effects which might be of importance in the low temperature measurements are discussed.
The electronic structure of CaO. II. An MCSCF/CI treatment of the low-lying 1Σ+ and 1Π states
Authors: Diffenderfer Randall N. , Yarkony David R.
Published: February 2003

State averaged multiconfiguration self-consistent field and configuration interaction methods are used to discuss the 1,2 1Σ+ states of CaO. Spectroscopic constants Re, Te, and ωe are determined. In addition, radiative transitions between the 1,2 1Σ+ states, and the 1 1Π states are discussed. The spectroscopic constants are in reasonable agreement with experiment although the predicted bond lengths are uniformly long. The calculated lifetime for the v=6 level of the A 1Σ+ state of 130×10−9 s is in good agreement with a preliminary experimental value. The lifetime of low-lying vibrational states in the A′ 1Π manifold are over an order of magnitude longer than their A state counterparts. The v=0 level of the A′ 1Π state is predicted to have a lifetime of 25×10−6 s.
A variational calculation of the binding energy of the fermionic 3He tetramer
Authors: Nakaichi-Maeda S. , Lim T. K. , Akaishi Y.
Published: February 2003

A variational calculation is performed for the (3He)4 system interacting pairwise through the Lennard-Jones He–He potential. Use of an integrodifferential Euler equation is made to determine an optimum wave function, leading to results showing that the existence of this molecule is unlikely. It is confirmed that states of the system with nonzero orbital angular momentum yield lower energies. Also, it appears that the number of 3He atoms needed to form self-bound molecules must be larger than 5.