bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulation of the hard-sphere fluid with quantum correction and estimate of its free energy
Authors: Yoon Byoung-Jip , Scheraga Harold A.
Published: July 2002

A simple Monte Carlo (MC) method is introduced to treat the quantum-mechanical hard-sphere fluid. It uses the relative two-body Slater sum (instead of the classical Boltzmann factor) as a quantum-mechanical probability in the relative two-body configurational space. The MC calculations have been carried out for the hard-sphere fluid with quantum corrections at various densities and for various thermal wavelengths. The results are consistent with the interpretation that, in the quantum-mechanical picture, hard spheres repel each other at distances slightly greater (by an amount of the order of the thermal wavelength λT) than the contact collision distance of the classical treatment. The radial distribution functions of the classical and quantum-mechanical simulations differ considerably. The relative two-body free energy has been obtained from the relative two-body Slater sum, and the total free energy has been estimated (in preliminary calculations) by using the classical expression but replacing the classical hard-sphere diameter by the effective diameter. A good analytical fit to the MC results has been obtained for the equation of state of the quantum-mechanical hard-sphere fluid; the total free energy has been obtained (in extensive calculations) by integrating over the resulting pressures and densities. The pressure, internal energy, and free energy all increase with an increase in the thermal wavelength, i.e., with increasing quantum effect. From the equation of state, the second virial coefficients at various thermal wavelengths are in good agreement with theoretical values in the literature, and the third and fourth virial coefficients are predicted. Evidence has been found for a momentum condensation; the corresponding relative two-body entropy is negative. The heat capacity due to the relative two-particle kinetic energy has been found to be proportional to T−1/2 (approximately) whereas, classically, it is zero.
State selective photodissociation dynamics of Ã state ammonia. I
Authors: Biesner J. , Schnieder L. , Schmeer J. , Ahlers G. , Xie Xiaoxiang , Welge K. H. , Ashfold M. N. R. , Dixon R. N.
Published: July 2002

The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (v′2 =0 and 1) of their Ã 1A″2 excited state has been investigated by monitoring the time-of-flight spectra of the nascent H-atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity D00 (H–NH2) (4.645±0.01 eV) and reveals that the majority of the accompanying NH2 (X̃ 2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a-inertial axis. The detailed energy disposal is sensitive to the initially excited parent vibronic (and even rovibronic) level: the NH2 (X̃) fragments resulting from photodissociation via the v′2 =1 level of NH3 (Ã) carry a higher level of excitation of the N=Ka rotational levels, which show an inverted population distribution. We also describe the results of trajectory calculations employing the recently reported [M. I. McCarthy et al., J. Chem. Phys. 86, 6693 (1987)] ab initio potential energy surfaces for the Ã and X̃ states of ammonia. These provide a detailed rationale for the experimentally observed energy disposal and highlight the massive influence on the eventual fragmentation dynamics of the conical intersection between these surfaces along the H–NH2 dissociation coordinate.
A new method for studying carbon clusters in the gas phase: Observation of size specific neutral fragment loss from metastable reactions of mass selected C+n, n≤60
Authors: Radi Peter P. , Bunn Thomas L. , Kemper Paul R. , Molchan Michele E. , Bowers Michael T.
Published: July 2002

A new method for studying the generation and structure of carbon cluster ions is described. A rotating, translating carbon rod is subjected to the focused output of an excimer laser operating at 308 nm. The resulting plume is directly sampled by a high resolution reverse geometry mass spectrometer operating between 2 and 8 kV. In this paper we report initial results for the positive cluster ions up to n=60. The mass spectra are bimodal with a minimum occurring between C+26 and C+31 in agreement with findings of other workers using related techniques. Metastable neutral loss is observed for all cluster ions with n≥5. Below C+30 the most intense neutral loss is C3 but intense C1 and C5 loss is observed for a significant number of systems and C10 and C14 for a few systems. Above C+30 the metastable loss spectra dramatically change and only C2, and a minor amount of C4 loss, is observed. The C+60 ion is slightly enhanced in the mass spectrum but its metastable reactivity is no different than any of the other C+n ions above n=30. The detailed results are interpreted in terms of stable structures of both the parent ions and neutral fragments that have been predicted theoretically.
Radiationless transitions and the quantum yield for nonresonant light scattering
Authors: de Lange Pieter J. , Drabe Karel E. , Kommandeur Jan
Published: July 2002

The quantum yield for fluorescence can be defined as the ratio of the amount of light scattered as fluorescence or as Rayleigh or Raman scattering to the amount of light removed from an incident beam. Clearly then the quantum yield should go to one for a nonresonant excitation. The conventional theory of radiationless transitons (the Bixon–Jortner model) does not yield this result. We therefore postulate an intermediate state in the radiationless pathway. Then the model does yield the desired frequency dependence of the quantum yield. It may be that the arbitrary variation of the quantum yields of organic molecules may be due to the absence or presence of such an intermediate state.
Phenomenological theory of the dynamics of polymer melts. II. Viscoelastic properties
Authors: Skolnick Jeffrey , Yaris Robert
Published: July 2002

A phenomenological theory of the nonmechanical and viscoelastic properties of polymer melts is developed. Consistent with computer simulation results [A. Kolinski, J. Skolnick, and R. Yaris, J. Chem. Phys. 86, 1567, 7164, 7174 (1987)], that fail to find evidence for reptation as the dominant mechanism of long distance motion in a melt, we assume that the long-time behavior of a chain is that of a Rouse-like chain having a number of slow moving points, each with a friction constant proportional to the degree of polymerization n. Coupled with the assumption of rubber like behavior at short times made previously by Doi and Edwards [J. Chem. Soc., Faraday Trans. 2 74, 1802 (1978)], the theory predicts that over a broad molecular weight range the shear viscosity η scales with n as approximately the 3.4 power of the molecular weight, and that η∼n3 in the infinite molecular weight limit. Furthermore, the theory rationalizes the origin of the different crossover molecular weights for the shear viscosity and the self-diffusion coefficient, D. It also accounts for the origin of the intermediate time coupling of the center-of-mass motion into the internal coordinates and for the time dependence of the single bead positional autocorrelation functions seen in previous simulations. Proceeding by analogy to Graessley [J. Poly. Sci. Poly. Phys. Ed. 18, 27 (1980)], in the large molecular weight limit, phenomenological expressions for D and η are derived and comparison is made with experiment.
A multireference CI determination of the isotropic hyperfine constants for first row atoms B–F
Authors: Feller David , Davidson Ernest R.
Published: July 2002

The requirements for obtaining converged isotropic hyperfine constants, Aiso, in first row elements B–F are investigated with extended basis set CI wave functions composed of single and double excitations from a multiconfiguration reference space. Since the restricted Hartree–Fock unpaired spin density at the nucleus is zero for these elements, correlation effects account for the entire answer. Although the wave functions computed in the present study uniformly recovered 94%–98% of the empirical correlation energy, agreement with gas phase measurements of Aiso varied from a low of 55% on B to a high of 96% on N. Extension of these findings to the B2 (3Σg ) and H2 CO+ (2B2 ) molecules is also discussed. In order for small basis sets or very lightly correlated wave functions to produce good agreement with experiment, fortuitous cancellation of error between the negative core and positive valence contributions must occur. The present findings suggest relatively large basis sets and substantial correlation recovery are necessary if reliable core/valence balance is to be achieved.
Inhomogeneous Coulomb fluids with image interactions between planar surfaces. II. On the anisotropic hypernetted chain approximation
Authors: Kjellander Roland
Published: July 2002

The physical assumptions underlying the anisotropic hypernetted chain (HNC) closure for the pair correlations in inhomogeneous fluids and the accompanying equation for the density distribution are discussed, treating the implications and limitations of the approximation. The theory is capable of incorporating image charge interactions (due to dielectric discontinuities at the surfaces) and discreteness of surface charges without further approximations. The computational implementation of the theory is described in some detail.
Energetics and dynamics in the reaction of Si+ with SiF4. Thermochemistry of SiFx and SiF+x (x=1, 2, 3)
Authors: Weber M. E. , Armentrout P. B.
Published: July 2002

The title reaction is studied using guided ion beam mass spectrometry. Absolute reaction cross sections are measured as a function of kinetic energy from thermal to 40 eV, and three endothermic product channels are observed. The dominant SiF++SiF3 channel is only slightly endothermic, while the SiF+3 +SiF and SiF+2 +SiF2 channels have much higher thresholds. The SiF+3 cross section magnitude is about half that of SiF+, while the SiF+2 cross section is an order of magnitude smaller than that of SiF+. A second feature which appears in the SiF+2 cross section is due to dissociation of SiF+3 . There is evidence that SiF+ and SiF+3 are produced via a direct mechanism. Competition between these two channels is interpreted in terms of molecular orbital correlations and qualitative potential energy surfaces. One surface is found to correlate only with the SiF+3 +SiF channel, while another correlates diabatically with this channel and adiabatically with the SiF++SiF3 channel. Competition on this latter surface has an energy dependence which is consistent with the Landau–Zener model. Reaction thresholds are analyzed to yield 298 K heats of formation for SiFx and SiF+x species. From an evaluation of these and literature values, we recommend the following values: ΔH0f(SiF+) =170.4±2.2 kcal/mol, IP(SiF)=7.54±0.16 eV, ΔH0f (SiF3)=−258±3 kcal/mol, and IP(SiF3)=9.99±0.24 eV.
Application of the pivot algorithm for investigating the shapes of two- and three-dimensional lattice polymers
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

A pivot algorithm is used to numerically investigate the shape properties of linear polymers, with and without excluded volume, in two and three dimensions. The high computational efficiency of the pivot algorithm has allowed us to obtain data which is four to eight times more accurate than our previous Brownian dynamics simulations, while employing considerably less computational resources. The accurate data for long polymer chains highlight the inadequacy of the d expansion of polymer shapes to first order only.
The reaction of iron clusters with ammonia. I. Compositions of the ammoniated products and their implications for cluster structure
Authors: Parks E. K. , Nieman G. C. , Pobo L. G. , Riley S. J.
Published: July 2002

Studies are described of the chemisorption of ammonia on isolated neutral iron clusters Fen for 2≤n≤165. Clusters are generated by laser vaporization in a continuous-flow-tube reactor, and reaction products are detected by laser-ionization time-of-flight mass spectrometry. Ammonia is found to chemisorb nondissociatively on cluster surfaces on the 1 ms time scale of these experiments. Measurements of ammonia uptake provide information on adsorption kinetics and on the number and nature of the binding sites. The ammonia binding energy is found to decrease with increasing cluster coverage. For chemically saturated clusters, the ratio of adsorbed NH3 molecules to surface iron atoms is found to decrease with increasing cluster size, going from >1/3 for small clusters to <1/3 for n>100. Ammonia chemisorption is accompanied by a large decrease in cluster ionization potentials, as much as 2 eV for saturated clusters. At sufficiently high exposure the beginning of the formation of a second, physisorbed layer of molecules is seen. Detailed measurements of product composition under different exposure conditions give evidence for numerous changes in cluster structure throughout the growth sequence from small to large clusters. Often these structural changes involve particularly stable reaction products. Evidence for the existence of metastable structures is presented. Several possibilities for cluster structure are suggested.
The submillimeter-wave spectrum of the 13CH3O radical
Authors: Momose Takamasa , Endo Yasuki , Hirota Eizi , Shida Tadamasa
Published: July 2002

The pure rotational spectrum of the isotopically substituted methoxy radical 13CH3O was observed in the millimeter- and submillimeter-wave region. The 12CH3O radical was also studied in the same wavelength region and the molecular parameters of the previous work were reexamined. It was found necessary to add a few terms for the centrifugal distortion. The hyperfine coupling constants of the carbon nucleus were determined for the first time. With reference to the rotational constants of CD3O, the rs structure of the methoxy radical was determined.
The emission spectrum of helium hydride. I. Bands near 8000 Å
Authors: Ketterle W. , Dodhy A. , Walther H.
Published: July 2002

Spectra of helium hydride were observed after neutralization of a mass-selected HeH+ beam. The molecules were produced in a fast beam, and so a special setup had to be used to avoid Doppler broadening and a careful calibration procedure had to be applied to determine the line positions and linewidths. In an earlier paper, we reported the first observation of a discrete spectrum of helium hydride, which was discovered by means of an emission band near 8000 Å. In this paper, a detailed analysis of this band for all four stable isotopic mixtures is given. For the deuterides several vibrational bands were observed, which allowed equilibrium molecular constants to be determined. These constants agree with the results of recent ab initio calculations. The similarity of these constants to those of HeH+ in the ground state confirms that the observed states are Rydberg states. Comparison of the molecular constants for different isotopic mixtures shows deviations from the Born–Oppenheimer approximation. Born–Oppenheimer breakdown parameters were derived. Both the upper and lower states show predissociation to the repulsive ground state of helium hydride. The line intensities give evidence of a dependence of the electronic transition moment on the interatomic distance.
Statistical thermodynamics of association colloids. I. Lipid bilayer membranes
Authors: Leermakers F. A. M. , Scheutjens J. M. H. M.
Published: July 2002

Step-weighted random walks (modified Markov chain statistics) combined with a self-consistent-field approximation form the basic concepts of a Flory–Huggins-type of theory to describe the lipid bilayer.1 The purpose of the present paper is to extend this model by incorporating the rotational isomeric state scheme, both for linear and branched chain molecules. Only three measurable interaction energy parameters of a Flory–Huggins-type are required, namely for the head group tail, the head group water, and the tail water contacts. In addition, the theory needs one energy parameter for the internal trans/gauche transition energy of the chain. Results of this self-consistent-field (SCF) theory are given for membranes formed by lecithin-like molecules. With respect to earlier work, more detailed insight is obtained in the behavior of the lipid bilayer above the gel to liquid phase transition temperature. Equilibrium conditions are formulated. Segment density profiles and solvent distributions are calculated. It is shown that the two apolar tails of the lecithin do not behave identically. The tail next to the head group is lifted slightly more out of the membrane than the other tail. The well-known balance of forces, responsible for membrane formation is analyzed. We found that the repulsive tail head interaction, often ignored in theories, is essential for the stability of association colloids.
Coupling schemes for atom–diatom interactions and an adiabatic decoupling treatment of rotational temperature effects on glory scattering
Authors: Aquilanti Vincenzo , Beneventi Laura , Grossi Gaia , Vecchiocattivi Franco
Published: July 2002

The quantum mechanical theory for scattering of a particle by a rigid rotor is formulated in five alternative diabatic representations, corresponding to alternative coupling schemes. Use is made of a recently introduced procedure for obtaining discrete representations by artificial quantization. In order to develop an efficient computational scheme for obtaining information on the interaction potential from atom–diatom scattering experiments, decoupling approximations are developed. An adiabatic representation in the coupled states framework is applied to the computation of integral cross sections and nonadiabatic coupling effects are analyzed. The approach provides an accurate description of the experimentally observed dependence of glory scattering from the rotational temperature of the diatom.
Metastable decay of photoionized niobium clusters: Clusters within a cluster?
Authors: Cole S. K. , Liu K.
Published: July 2002

The photoinduced metastable ion fragmentation of niobium clusters has been found to closely correlate with the laser vaporization source conditions. The experimental evidence indicates the clusters agglomerate in the supersonic expansion. These agglomerated clusters exhibit rather unique behavior for a transition metal system. Both evaporation and fission fragmentation are observed. Two color MPI studies reveal the existence of long lived neutral intermediate states. Also, the kinetic energy release is extremely small, <10 meV, and independent of the excitation energy. The concept of clusters within a cluster, i.e., agglomerated clusters, is introduced to rationalize many experimental findings.
Spectroscopic properties of 41 electronic states of Pd2
Authors: Balasubramanian K.
Published: July 2002

Complete active space MCSCF (CASSCF) followed by multireference singles+doubles CI (MRSDCI) and relativistic configuration interaction (RCI) calculations are carried out on electronic states of Pd2. Forty-one electronic states are found below the 8000–9000 cm−1 region and their spectroscopic properties are calculated. The dissociation energy of the lowest state with respect to the 3D atoms is calculated to be 2.1 eV. Relativistic and spin–orbit effects are found to be significant for the electronic states of Pd2.
Molecular beam studies of weak interactions for open-shell systems: The ground and lowest excited states of rare gas oxides
Authors: Aquilanti Vincenzo , Candori Roberto , Pirani Fernando
Published: July 2002

Integral cross sections as a function of velocity for scattering of ground state oxygen atoms by the rare gases have been measured at thermal energy. Analysis of atomic sublevels by a Stern–Gerlach magnet allows a control of the relative contribution from different fine structure scattering channels. The results are analyzed using an adiabatic decoupling scheme to derive the interaction as a spherical part and an anisotropic component, from which information is obtained on the six lowest states of the rare gas oxides and on nonadiabatic coupling terms.
Density driven self-consistent field method. I. Derivation and basic properties
Authors: Cioslowski J.
Published: July 2002

An explicit Schrödinger-like equation for the Hartree–Fock density amplitude (square root of the electron density) is derived. The potential contains a classical electrostatic part as well as the exclusion potential that arises from the Fermi statistics of the electrons. The exclusion potential is explicitly determined by solving a system of equations for auxiliary functions. Basic features of the new approach are discussed and its equivalence to the standard HF method is demonstrated.
Relation between stretched-exponential relaxation and Vogel–Fulcher behavior above the glass transition
Authors: Nagel Sidney R. , Dixon Paul K.
Published: July 2002

We point out a relationship between stretched-exponential relaxation, φ(t)=φ0 exp[−(t/τ)β], and the Vogel–Fulcher temperature dependence, τ=τ0 exp[A/(T−T0)], seen in many glasses near the glass transition. Recent experiments have shown that, in some glasses, the exponent β decreases as the temperature is lowered towards T0. Under rather nonrestrictive assumptions, this temperature dependence in φ(t) leads directly to Vogel–Fulcher behavior.
Laser-induced fluorescence of the B 2Σ+–X 2Πi system of OH: Detection of v′′=8 and 9
Authors: Sappey Andrew D. , Crosley David R. , Copeland Richard A.
Published: July 2002

The previously unobserved B 2Σ+–X 2Πi electronic system of OH is detected by laser-induced fluorescence in a low-pressure flow system. OH is produced in high vibrational levels of the ground electronic state by the H+O3 reaction. Light from a frequency-doubled tunable dye laser excites the radical via the (0,8), (0,9), and (1,8) bands of the B 2Σ+–X 2Πi transistion at 225, 236, and 221 nm, respectively. The fluorescence is detected in the B 2Σ+–A 2Σ+ system in the visible and near ultraviolet. Measured emission intensities in the (0,3) to (0,9) and the (1,3) to (1,9) bands of B–A are in good agreement with previous theoretical calculations. The experimental branching ratio for emission between the B–A (0,3) and B–X (0,10) bands is about three times larger than theoretically predicted. Detection of high-lying vibrational levels in the electronic ground state, v′′=6 through 15, for experiments in collision dynamics and reaction kinetics, is feasible using the B–X system.
The spectroscopy and A state dynamics of the NeIBr van der Waals complex
Authors: Simpson William R. , Stephenson Thomas A.
Published: July 2002

The A 3Π1←X 1Σ+ laser-induced fluorescence excitation spectrum of the NeIBr van der Waals complex is reported and analyzed to extract information regarding the structure and vibrational predissociation dynamics of the complex. While no definitive geometric information regarding NeIBr is obtained, our data indicate that a linear geometry is at least plausible. The vibrational predissociation lifetimes are a strong function of A state vibrational level and range from 2.6 to 23 ps. The variation in lifetime with vibrational level is consistent with the results of previous measurements on rare gas–halogen complexes, particularly NeBr2.
Use of the discrete variable representation in the quantum dynamics by a wave packet propagation: Predissociation of NaI(1Σ+) →NaI(0+)→Na(2S)+I(2P)
Authors: Choi Seung E. , Light J. C.
Published: July 2002

Using the Gauss–Chebyshev discrete variable representation (DVR), the dissociative quantum dynamics for a wave packet evolving under the influence of the Hamiltonian for two interacting diabatic states of a diatomic molecule is calculated. The split time evolution operator method is used to obtain the solutions to the time-dependent Schrödinger equation. A specific example of the numerical calculation is shown for the predissociation process of NaI→Na(2S)+I(2P) from its first excited electronic state (0+). The numerical results are compared with the experimental observations from the femtosecond laser photofragmentation, recently reported by Zewail and co-workers.
Semiclassical quantization of the scattering from a classically chaotic repellor
Authors: Gaspard Pierre , Rice Stuart A.
Published: July 2002

The scattering of a point particle from three hard discs in a plane is studied in the semiclassical approximation, using the Gutzwiller trace formula. Using a previously introduced coding of the classical dynamics, the needed summation over the classical periodic orbits is performed. The trace function is then given in terms of Ruelle zeta functions. A semiclassical limit upper bound is obtained on the lifetimes of the scattering resonances. This bound is larger than the classical lifetime when the classical repellor is chaotic but coincides with it when the repellor is periodic. We conclude that classical chaos dramatically influences the lifetimes of the scattering resonances. Our upper bound for the resonance lifetime is compared with the results of numerical calculation of the full quantum dynamics. The distribution of the imaginary parts of the complex wave numbers of the resonances is also calculated.
Microwave electric-resonance optothermal spectroscopy of (H2O)2
Authors: Fraser G. T. , Suenram R. D. , Coudert L. H.
Published: July 2002

The microwave spectrum of (H2O)2 has been measured between 14 and 110 GHz using a newly developed electric-resonance optothermal spectrometer (EROS) described here. The reported measurements extend previous results on the a-type Ka=0–0 and 1–1 bands for the A±2 , B±2 , and E± rotational-tunneling states and include the first observations of the c-type Ka =1–0 band for the A±2 and B±2 states and the a-type Ka =0–0 band for the A±1 states. For the A±1 states an interconversion tunneling splitting of 22.6 GHz is obtained, compared to the 19.5 GHz value found previously for the Ka =0 A±2 and B±2 states.
Attenuated total reflectance infrared spectra of strongly absorbing anisotropic crystals: Orthorhombic Na2[Fe(CN)5NO]⋅2H2O
Authors: Güida J. A. , Piro Oscar E. , Castellano E. E. , Aymonino P. J.
Published: July 2002

Attenuated total reflectance (ATR) from principal faces of orthorhombic (and higher symmetry) crystals in spectral regions including strongly polar optic modes is considered. A detailed, quantitative comparison between experimental and calculated ATR infrared (IR) spectra of orthorhombic sodium nitroprusside dihydrate (SNP), Na2[Fe(CN)5NO]⋅2H2O, single crystals in the 500–3800 cm−1 region is presented. Calculated ATR spectra are generated employing the dielectric tensor of SNP obtained from Kramers–Kronig analysis of external reflectance data. The dependence of intensities, profiles, and locations of bands in the spectra upon the relative refractive index of the sample against the ATR prism and the polarization state of the reflected IR beam will be discussed. Both theoretical and experimental evidence of the coupling of the TM-polarized evanescent wave with strong in-plane-polarized optic modes at about their LO frequencies will be given with reference to the case of the strong NO stretching mode of SNP. Finally, the computed and experimental ATR results in SNP single crystals will be critically compared with powder ATR data and reported polarized IR transmittance spectra of SNP single crystals plates.
Molecular orbital and resonance Raman studies of the structures of N,N′-disubstituted indigo dyes
Authors: Abe Jiro , Nagasawa Yutaka , Takahashi Hiroaki
Published: July 2002

Molecular orbital calculations have shown that the structures of both trans and cis isomers of N,N′-disubstituted indigo dyes are considerably twisted about the central C=C bond. The twisting angle of the cis isomer is not always larger than that of the corresponding trans isomer, indicating that the large hypsochromic shift on the trans→cis isomerization of N,N′-disubstituted indigo dyes cannot be explained by the twisted structure of the cis isomer. It is shown that the hypsochromic shift can be ascribed to a larger contribution of the zwitterionic form to the resonance hybrid structure in the lowest excited singlet state S1 than in the ground state S0 .This makes the repulsion between the negative charges on the carbonyl oxygen atoms enhanced and the instability of the cis configuration compared to the trans configuration much larger in the S1 state than in the S0 state and leads to a larger vertical S1 ←S0 transition energy for the cis isomer than for the trans isomer.
The spectroscopy of Gd3+ in yttriumoxychloride: Judd–Ofelt parameters from emission data
Authors: Sytsma J. , Imbusch G. F. , Blasse G.
Published: July 2002

The luminescence of Gd3+ in YOCl is reported. From temperature dependent emission spectra and decay measurements, the Judd–Ofelt parameters Ω2 and Ω6 of Gd3+ in YOCl are obtained: Ω2=2.97×10−23 m2 and Ω6=8.65×10−24 m2. The relative consistency of Ω2 and Ω6 is confirmed by measuring the 8S7/2→6D9/2 and 6I7/2 transitions in excitation.
Electron mobility in krypton dense gas as functions of electric field strength, temperature, and density
Authors: Jacobsen Finn M. , Gee Norman , Freeman Gordon R.
Published: July 2002

Electron mobilities μ were measured in dense gaseous krypton as a function of density normalized electric field E/n at 3.8≤n/1026 molecule m−3≤40 and 152≤T/K≤250. At each density a constant value of the mobility μ0 is attained at low E/n. At fields higher than a threshold (E/n)th μ first increased, passed through a maximum, and then decreased. In the saturated vapor nμ0 decreased with increasing (n,T) while at constant n, nμ0 increased with T. The density dependence of nμ0 is compared to the dielectric screening model of Baird [Phys. Rev. A 32, 1235 (1985)].
Tracer diffusion in concentrated colloidal dispersions. III. Mean squared displacements and self-diffusion coefficients
Authors: van Megen W. , Underwood S. M.
Published: July 2002

The incoherent dynamic structure factor Fs has been measured on concentrated dispersions of particles with interactions approximating hard spheres. The particle mean squared displacements and self-diffusion coefficients are obtained from Fs at times and scattering vectors where the Gaussian approximation (for Fs ) is expected to be reasonable.
The multiconfigurational spin tensor electron propagator method (MCSTEP): Electron affinities of Li, Na, and K
Authors: Graham Richard L. , Yeager Danny L. , Rizzo Antonio
Published: July 2002

The multiconfigurational spin tensor electron propagator approximation to the electron propagator is applied for the first time to the calculation of electron affinities. The electron affinities of Li, Na, and K are calculated as the ionization potentials of the negative ions. On average these electron affinities differ from experiment by 0.003 eV, giving the best theoretical values reported to data.
On decomposition of second-order Mo/ller–Plesset supermolecular interaction energy and basis set effects
Authors: Cybulski S. M. , Chal/asiński G. , Moszyński R.
Published: July 2002

The basis set effects on the total self-consistent field (SCF) and second-order Mo/ller–Plesset (MP2) interaction energies in the HF dimer (in the equilibrium geometry) are investigated in relation to their components: electrostatic, exchange, induction, and dispersion, calculated within the framework of intermolecular Mo/ller–Plesset perturbation theory (IMPPT). The basis set dependence of the SCF interaction energy in the HF dimer is almost exactly determined by the electrostatic contribution. The exchange, induction, and the SCF-deformation terms are found substantially less sensitive. The MP2 correlation contribution reflects primarily the basis set dependence of dispersion. However, an accurate image of the basis set dependence is reproduced only if the electrostatic-correlation term is considered as well. Other correlation contributions: the deformation- correlation and exchange terms are found to be much less sensitive to basis set effects. All these conclusions are valid only under the condition that the supermolecular interaction energies are counterpoise-corrected for the basis set superposition error and IMPPT interaction energies are calculated with the full basis set of the dimer.
Nonlinear optical properties of H2 and D2
Authors: Bishop David M. , Pipin Janusz , Rérat Michel
Published: July 2002

Accurate wave functions of the James–Coolidge type, which account for electron correlation, are used to calculate for H2 and D2 the dynamic second hyperpolarizabilities (γ) which mediate the nonlinear optical processes: dc Kerr, dc electric-field-induced second-harmonic generation and third-harmonic generation. Values are given for a range of frequencies (ω=0 to ω=0.05 a.u.) as well as for some common laser frequencies. The effects of vibration are explicitly considered. As well, values of the dynamic field-gradient polarizabilities B and C are found. The results are more accurate than those previously published.
Dispersion dipoles and dispersion forces: Proof of Feynman’s ‘‘conjecture’’ and generalization to interacting molecules of arbitrary symmetry
Authors: Hunt K. L. C.
Published: July 2002

This paper provides the first explicit, general proof that the leading-order dispersion forces between two interacting molecules result from the attraction of nuclei in each molecule to the dispersion-induced change in the electronic charge density of the same molecule. The proof given here holds for molecules of any symmetry, provided that overlap between the charge distributions is small. New sum rules for the nonlinear response tensors are also obtained, after consideration of the long-range limit. A perturbation analysis gives the dispersion-induced polarization in each molecule in terms of nonlocal, nonlinear response tensors taken at imaginary frequencies. Forces on the nuclei are computed from a reaction-field expression for the dispersion energy, in terms of polarizability densities. Recent work has shown that the derivative of the polarizability density with respect to a nuclear coordinate is linked to an integral involving the nonlinear response tensor and the dipole propagator, and this link provides the key to the proof.
Two-photon microwave transitions within a two-level system
Authors: Martinache L. , Ozier I. , Bauder A.
Published: July 2002

Two-photon pure rotational transitions in the symmetric top CF3CCH have been observed with a pulsed beam Fourier transform crossed-cavity spectrometer modified to allow the application of a static Stark field. Transient one-photon emission signals at ν0=17 267 MHz for transitions between the levels (J=3,K,M) and (2,K,M) are generated for KM≠0 by the application of intense pulses at ν0/2. It has been demonstrated that the two-photon transitions occur within an effectively isolated two-level system as a result of the first order ac Stark effect. Quantitative studies of the intensities as a function of pulse length and power show that the two-photon transition probability in the microwave region is well represented by the theoretical model used by Meerts, Ozier, and Hougen [J. Chem. Phys. 90, 4681 (1989)] to treat multiphoton transitions in a two-level system whose one-photon frequency is ≲1 MHz. A description is given of the spectrometer with emphasis on the modifications made for two-photon studies.
Negative-ion mass spectrometric study of ion pair formation in the vacuum ultraviolet. I. N2O→O−+N+2
Authors: Mitsuke Koichiro , Suzuki Shinzo , Imamura Takashi , Koyano Inosuke
Published: July 2002

Negative ion formation from N2O has been studied using synchrotron radiation in the 15–40 eV photon energy range. Only O− ion signal is observed by mass spectrometry. The onset of the photodissociation efficiency curve for O− from N2O is in good agreement with the thermochemical threshold of 15.79 eV for the formation of O−(2Pu)+N+2(X̃ 2∑+g). Several series of peaks are identified as resulting from predissociation from the Rydberg states converging to N2O+(Ã 2∑+, B̃ 2∏, and C̃ 2∑+). In particular, two Rydberg series (ndσ and ndπ) converging to the C̃ state are observed most prominently.
Ground state densities from electron propagators: Optimized Thomas–Fermi approximation for short wavelength modes
Authors: Pratt Lawrence R. , Hoffman Gary G. , Harris Robert A.
Published: July 2002

Electron densities obtained from a ground state path integral approach to density functional theory using a primitive Monte Carlo method display large statistical uncertainties when short wavelength fluctuations of the paths are considered directly. An optimized Thomas–Fermi approximation is developed to eliminate these degrees of freedom in a systematic and physically motivated fashion. Beyond improving the precision of the numerical results, this theoretical development permits a simple qualitative discussion of how the calculation pushes electron density into tunneling regions by iteratively renormalizing the Fermi wavelength appropriate to the remaining integrations which involve only long wavelength coordinates.
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o relativistic effective potentials with spin–orbit operators. IV. Cs through Rn
Authors: Ross R. B. , Powers J. M. , Atashroo T. , Ermler W. C. , LaJohn L. A. , Christiansen P. A.
Published: July 2002

A b initio averaged relativistic effective core potentials (AREP) and spin–orbit (SO) operators are reported for the elements Cs through Rn. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definitions thereby permitting the treatment of core–valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets for the lowest energy state of each atom are tabulated. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic excitation energies and SO splitting energies with all-electron relativistic values. Calculated atomic excitation energies are found to agree to 0.12 eV and SO energies to 3.4%.
Enhanced structure in polymers at interfaces
Authors: Chan Hue Sun , Wattenbarger Miriam R. , Evans D. F. , Bloomfield Victor A. , Dill Ken A.
Published: July 2002

We explore the conformations of an isolated chain molecule as it approaches an impenetrable planar surface, by exhaustive enumeration of all the three-dimensional self-avoiding flights on simple cubic lattices. We confirm the well-known results that as a chain approaches a surface, the number of accessible configurations diminishes, so there is an entropic repulsion, and the radius of gyration becomes anisotropic and the chain flattens. Our principal observation here is that proximity to a surface causes a chain molecule to develop enhanced amounts of certain forms of internal structural organization, namely, helices and antiparallel sheets, the main architectures observed in globular proteins. In addition, we compare the process of adsorption of: (i) chains that have no intrachain attraction (i.e., ‘‘open’’ chains); with (ii) chains that have strong intrachain attraction (i.e., ‘‘compact’’ chains). We confirm the well-known result that open chains undergo an adsorption transition onto the surface. However, we find that the adsorption of compact chains differs in one respect. Compact chains appear to undergo two types of transition depending on the strength of chain–surface attraction: (i) weak attraction leads to a ‘‘docking’’ transition, in which the chain adsorbs without much deformation; (ii) strong attraction leads to a ‘‘flattening’’ transition in which the chain adopts a two-dimensional ensemble of conformations on the surface.
Computer simulation of interlayer water in 2:1 clays
Authors: Skipper N. T. , Refson K. , McConnell J. D. C.
Published: July 2002

Monte Carlo computer simulation has been used to study water confined between the layers of 2:1 clay minerals. The model systems are based on natural Mg and Na smectites. The simulation cells contain one clay layer, 64 water molecules and four magnesium or eight sodium interlayer cations. These atoms and molecules interact with each other through a new set of effective pair potentials, which we discuss. The calculations are conducted in constant (N,p,T) ensembles, at T=300 K and with a uniaxial pressure, p, of 1 M Pa applied normal to the clay sheets. All the molecules, including the clay sheets, are therefore allowed to move during the simulations. The calculated equilibrium layer spacing is 14.7±0.1 Å with interlayer Mg2+ and 14.2±0.1 Å with interlayer Na+. These spacings compare with experimental values of 15.1 Å and 14.5 Å, measured for Mg and Na saturated Chambers montmorillonite, at 79% relative humidity. The corresponding densities and average potential energies of the interlayer water molecules are 1.38±0.04 g cm−3 and −17.63±0.02 kcal mol−1, respectively, for Mg smectite and 1.14±0.04 g cm−3 and −11.77±0.02 kcal mol−1, respectively, for Na smectite. We analyze and compare the interlayer structures in the two systems.
Subpicosecond predissociation dynamics of the methyl radical Rydberg 3 s state
Authors: Westre S. G. , Kelly P. B. , Zhang Y. P. , Ziegler L. D.
Published: July 2002

The subpicosecond dissociation dynamics of the methyl radical 3 s Rydberg state have been examined using rotational resonance Raman spectroscopy. The rovibronic dependence of the excited state predissociation rates and lifetimes are obtained for the B̃ state origin vibrational level of CH3 and CD3. Analysis of the tunneling rates using a cubic potential barrier yields estimates for the height (2200 cm−1) and position of the barrier along the dissociation coordinate (1.38 Å). A comparison of the potential energy surface parameters for the Rydberg 3 s states of methyl radical and ammonia is presented.
Photodestruction spectra of the anionic water clusters, (H2O)−n, n=18 and 30: Absorption to the red of e−aq
Authors: Campagnola Paul J. , Lavrich David J. , DeLuca Michael J. , Johnson Mark A.
Published: July 2002

We report absolute photodestruction cross sections of mass selected, anionic water clusters (H2O)−n, n=18 and 30, in order to compare the photoabsorption of the clusters with that of the bulk hydrated electron. The cluster absorptions are similar in magnitude to that of e−aq, but red shifted by about 0.75 eV relative to 298 K e−aq and are observed to rapidly increase in the vicinity of the vertical electron detachment energies of the clusters.
Angularly resolved rotational energy transfer in highly vibrationally excited states: Na2(v=31)–Ne
Authors: Ziegler G. , Kumar S. V. K. , Rubahn H.-G. , Kuhn A. , Sun B. , Bergmann K.
Published: July 2002

The scattering of high vibrationally excited sodium molecules Na2(v=31) with Ne atoms at 180 meV collision energy is investigated in a crossed molecular beam arrangement using laser optical methods. Angularly resolved rotationally inelastic and vibrationally elastic cross sections ji→jf are measured for ji =5, 7, and 9≤jf ≤25. Pronounced rotational rainbow maxima are observed, the angular position of which indicates a significantly larger anisotropy of the interaction potential compared to that of Na2(v=0)–Ne. Except for the increase of the anisotropy the vibrational excitation has little effect on the dynamics of rotational energy transfer. Good agreement with a new semiempirical Na2–Ne potential surface V(r,R,γ) is found.
Conjugation length dependence of Raman scattering in a series of linear polyenes: Implications for polyacetylene
Authors: Schaffer H. E. , Chance R. R. , Silbey R. J. , Knoll K. , Schrock R. R.
Published: July 2002

We have measured the solid state Raman scattering spectra of a homologous series of linear polyenes, with the number of alternated double bonds varying from 3 to 12. While we find a linear dependence of the Raman shifts of resonantly coupled modes with inverse conjugation length, we have also followed the suggestion of previous work in examining the inverse square product of the several Raman frequencies as a function of the logarithm of the measured energy gap of the molecule. This provides a linear relationship, as found for trans-polyacetylene, a result which is qualitatively consistent with the amplitude mode model of Horovitz and co-workers. We also find, consistent with previous work on polyacetylene, a monotonic decrease in the ratios of oscillator strengths of the two strongest bands with conjugation length, as recently predicted by a series of molecular dynamics calculations. Suggested interpretations of a number of qualitative observations, including splitting of modes for shorter conjugation length, are offered, and the implications for the structure of trans-polyacetylene are discussed. The present work confirms that the previously measured dispersion in the Raman spectra of trans-polyacetylene is due to a distribution of conjugation lengths and brings into question some of the quantitative aspects of the amplitude mode model.
A trajectory surface-hopping study of mode specificity in the predissociation of N2O
Authors: Marks Alison J. , Thompson Donald L.
Published: July 2002

A classical trajectory surface-hopping study of the effects of initial mode excitation on the spin-forbidden predissociation in a model of N2O reveals mode specificity. Incomplete energy randomization together with significant variation in energies of the surface crossing points with molecular geometry lead to changes in the accessibility of the dissociation channel and hence the rate of reaction when different modes are excited. The variations in the predissociation rate are most marked at relatively low energies. At higher energies, although energy randomization is still incomplete, there is less dispersion in the rate constants. This is attributed to the fact that at high energies a larger percentage of the crossing points become accessible and thus the directionality of the initial motion is less relevant to the rate.
Modification of the Duchovic–Hase–Schlegel potential energy function for H+CH3↔CH4. Comparison of canonical variational transition state theory, trajectory, and experimental association rate constants
Authors: Hu Xiche , Hase William L.
Published: July 2002

The Duchovic–Hase–Schlegel analytic potential energy function for H+CH3→CH4 association is modified to have the correct asymptotic behavior at large separations. Canonical variational transition state theory calculations performed on this modified potential yield the same association transition state structures and rate constants as reported previously for the unmodified potential. This modified potential is also used in a classical trajectory calculation of the association rate constant. The canonical variational transition state theory and trajectory association rate constants agree for both H+CH3 and D+CH3. Trajectories are used to study the unimolecular decomposition of the excited methane molecules formed by association. Most of the molecules decay exponentially. However, an initial nonrandom transient constitutes ∼5%–7% of the decomposition. Trajectory rate constants, derived from the exponential decay, are compared with harmonic Rice–Ramsberger–Kassel–Marcus (RRKM) rate constants to derive anharmonic correction factors.