bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffraction-derived spontaneous polarization in lithium sulfate monohydrate at 80 and 298 K
Authors: Karppinen M. , Liminga R. , Kvick Å. , Abrahams S. C.
Published: July 2002

The principal axes of the sulfate ion minimum-equipotential contact surface with respect to its charge center, and the charge and electric dipole moment distribution in the sulfate ion and in the water molecule, are derived for Li2SO4⋅H2O on the basis of previously determined net atomic charges [J. Chem. Phys. 85, 5221 (1986)]. The sulfate ion has a nonzero dipole moment of about 2.3×10−30 C m at 80 K that rotates 9° in the crystallographic a,c plane between 298 and 80 K. The dipole moment of the water molecule increases from about 6.7×10−30 C m at 298 K to 7.8×10−30 C m at 80 K, as it rotates 3.6° in the b,c plane. The total spontaneous polarization at 298 K is calculated to be 3.19(62)×10−2 C m−2, and at 80 K it is 4.37(51)×10−2 C m−2, along the positive direction of the polar axis. The sense and magnitude of the resulting calculated 1.18×10−2 C m−2 polarization change compare well with the experimental value of 1.04×10−2 C m−2 measured on heating through the same temperature interval. Determination of the absolute polarity of a Li2SO4⋅H2O crystal shows that the  p2-pyroelectric and the d22-piezoelectric coefficients are both negative, indicating that similar dipole rotations occur in each, respectively, on heating and under tensile stress.
Theoretical study on electronic structures of AuF6 and its anions
Authors: Miyoshi Eisaku , Sakai Yoshiko
Published: July 2002

SCF–MO and CI calculations are carried out for AuFq−6 (q=0, 1, 2, and 3) using a model potential method in which the major relativistic effects are incorporated. The calculated electron affinity of AuF6 is 9.56 eV, which is very close to Bartlett’s estimation of 10±0.5 eV. The calculated values for the equilibrium Au–F distance and the totally symmetric vibrational frequency of AuF−6 agree well with experimental ones. The AuF−6 anion has the lowest minimum energy in the AuFq−6 (q=0, 1, 2, and 3) sequence, and the AuF3−6 anion is higher in energy by ∼4 eV than AuF−6 and AuF2−6 at their equilibrium positions. The effects of adding a d-type polarization function on F and an f-type polarization function on Au are discussed.
Conformational statistics of random polyelectrolyte chain in theta region and transition to collapse: Qualitative study
Authors: Qian Caibao , Kholodenko Arkady L.
Published: July 2002

We study conformational properties of the polyelectrolyte chain with prescribed Bernoullian distribution of positive and negative charges on its backbone. For the case when the total charge of the chain is almost zero, the charge fluctuations are of special importance leading to the collapse transition qualitatively different from that expected for the uncharged homopolymers. The present case of collapse transition directly demonstrates the biological significance of nonuniformly charged polymers.
Probing of conformational relaxation processes of proteins by frequency labeling of optical states
Authors: Köhler W. , Friedrich J.
Published: July 2002

We show how optical frequency labeling at cryogenic temperatures can be used to probe in a most direct way protein substates and to elucidate the nature of the structural relaxation processes involved.
Diffusion-controlled reactions. II. Further bounds on the rate constant
Authors: Torquato S. , Rubinstein Jacob
Published: July 2002

We apply variational principles derived earlier by us to obtain rigorous upper and lower bounds on the rate constant k associated with diffusion-controlled reactions among static, reactive traps. We derive a phase-interchanged interfacial-surface lower bound on k and a new type of lower bound which we term a ‘‘void’’ bound. Among other results, we compute the phase-interchanged interfacial-surface bound on k for a distribution of fully penetrable (i.e., randomly centered) spheres in which the trap region is the space exterior to the spheres (the ‘‘holes’’ between the spheres) and diffusion occurs in the sphere region. We also derive an upper bound on k for identical spherical traps (randomly or periodically arranged), and evaluate it for a simple cubic lattice.
A lattice model describing hydrogen bonding in polymer mixtures
Authors: Painter Paul C. , Graf John , Coleman Michael M.
Published: July 2002

A Flory lattice model is used to obtain an expression for the free energy of mixing polymers that hydrogen bond. The derivation is based on the determination of the probability that a mixture of the nonhydrogen bonded chains would spontaneously occur in a configuration equivalent to the hydrogen bonded system. Our result has three parts. The usual contribution to the free energy from mixing covalent chains, a contribution from the hydrogen bonds that is equal to the result that would be obtained if the segments were not covalently linked, and a ‘‘correction’’ term that accounts for the excess entropy of mixing introduced by the second term. A consequence of this model is that there should be no dependence of the equilibrium constants describing hydrogen bonding upon covalent chain length.
Population transfer between molecular vibrational levels by stimulated Raman scattering with partially overlapping laser fields. A new concept and experimental results
Authors: Gaubatz U. , Rudecki P. , Schiemann S. , Bergmann K.
Published: July 2002

The feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on- or near-resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins earlier than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two-level systems, off-resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.
Cross sections and product kinetic energy analysis of H2O+–H2O collisions at suprathermal energies
Authors: Lishawa C. Randal , Dressler Rainer A. , Gardner James A. , Salter Richard H. , Murad Edmond
Published: July 2002

The reaction of H2O+ with H2O is studied using a longitudinal geometry double mass spectrometer in the collision energy range Ec.m.=0.5–25 eV. Cross sections are reported for oxonium ion (H3O+) production and the symmetric charge exchange. Isotopic substitution is used to discern the product branches, including the separation of the two channels for oxonium ion production: (i) proton transfer to the target molecule; and (ii) atom pickup by the primary ion. The largest branching ratio is observed for the charge exchange channel, where no isotope effect is detected in the investigated energy range. Proton transfer exhibits the second largest branching ratio and accounts for more than 90% of the oxonium ion production throughout the measured energy range. The proton transfer cross section is dependent on isotopic substitution, while the atom pickup channel is too weak to make a distinct statement on its isotopic behavior. Product ion energies, determined by time-of-flight measurements, are also reported for each of the three channels. These measurements show that most (>95%) of the oxonium ions are formed via a direct, spectator stripping type mechanism while a small amount of reaction products exhibit considerable internal excitation. The charge exchange secondary ions are primarily formed at near-thermal energies in the laboratory frame. Small amounts of high laboratory energy product ions are also observed which at least partly originate through the dissociation of excited oxonium ions.
Gaussian-1 theory of molecular energies for second-row compounds
Authors: Curtiss Larry A. , Jones Christopher , Trucks Gary W. , Raghavachari Krishnan , Pople John A.
Published: July 2002

The Gaussian-1 theoretical procedure is extended and tested on compounds containing second-row atoms (Na–Cl). This is a composite procedure based on ab initio molecular orbital theory, utilizing large basis sets (including diffuse-sp, double-d, and f-polarization functions) and treating electron correlation by Mo/ller–Plesset perturbation theory and by quadratic configuration interaction. Total atomization energies for a set of 24 species agree with accurate experimental data to an accuracy of better than 3 kcal/mol in most cases, SO2 being the notable exception. Similar agreement is achieved for ionization energies, electron affinities, and proton affinities. The method is used to assess experimental data for a number of other compounds having less accurate atomization energies.
Investigation of very fast motions in electrolyte solutions by far infrared spectroscopy
Authors: Guillot Bertrand , Marteau Philippe , Obriot Jacques
Published: July 2002

The far infrared spectra of solutions of a number of alkali halide salts in protic (methanol) and dipolar aprotic solvents (acetonitrile, acetone, dimethyl sulfoxide) have been investigated in the frequency range 25–650 cm−1. Special attention has been paid to detect weak bands in making the difference between the absorption spectrum of the solution and that of the pure solvent. These spectra are characterized by several absorption peaks spread all over the FIR spectral range. Introducing a chemical model of the electrolyte solution, it is shown that the time correlation function at the origin of the absorption phenomenon is governed essentially by three processes, namely, (i) the ion-induced dipole mechanism, (ii) the ionic association, and (iii) the relaxation of the solvent molecules within the ionic solvation shell. A band shape analysis, based upon the Mori theory, permits us to identify the molecular motions which give rise to specific frequency modes on the absorption spectrum. In particular, the role played by the time scale separation between the fast dynamics occuring within the first solvation shell of ionic species and the time spent to interconvert inner and outer solvation shells, is emphasized.
Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. I. Rotational and vibrational relaxation in the 2ν2 band
Authors: Lavorel B. , Millot G. , Saint-Loup R. , Berger H. , Bonamy L. , Bonamy J. , Robert D.
Published: July 2002

The 2ν2 component of the Fermi dyad ν1/2ν2 of CO2 has been studied with high-resolution stimulated Raman spectroscopy (SRS). The behavior of the band shape has been explored in a large density range: 0.2 to 50 amagat at a temperature of 295 K and 0.5 to 20 amagat at 500 K. Energy corrected sudden (ECS) and modified energy gap (MEG) laws are used to model the relaxation matrix in order to account for the collisional narrowing induced by rotational energy transfers. ECS model allows us to accurately determine the vibrational shift and width as a function of density by fitting the experimental spectra, leading to the determination of the vibrational relaxation coefficients for the 2ν2 mode. Connection is established between the present calculations of the collisionally narrowed SRS spectra based on the diagonalization of the relaxation matrix, which applies for any line overlap, and the usual spectral line shape for weak line coupling. Particular emphasis is put on the situation of strong collapse and on the concomitant predominance of the vibrational dephasing.
Mass analyzed threshold ionization spectroscopy
Authors: Zhu Langchi , Johnson Philip
Published: July 2002

The developement of a new mass selective method is reported for the determination of the optical spectra of molecular ions and for the production of state selected ions.(AIP)
Structures of carbon cluster ions from 3 to 60 atoms: Linears to rings to fullerenes
Authors: von Helden Gert , Hsu Ming-Teh , Kemper Paul R. , Bowers Michael T.
Published: July 2002

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures. Fullerenes first appear at C+30 and begin to dominate above C+45. From small to large the isomeric progression is from linear to rings to fullerenes.
Vibronic interactions in the NO3 radical
Authors: Hirota Eizi , Kawaguchi Kentarou , Ishiwata Takashi , Tanaka Ikuzo
Published: July 2002

The 1492 cm−1 band of NO3 previously reported [J. Chem. Phys. 82, 2196 (1985); 93, 951 (1990)] exhibits some features in the upper state which are difficult to understand if the band is purely vibrational, i.e., the degenerate N–O stretching band in the ground electronic state. The two most conspicuous anomalies are a finite spin-orbit coupling term which must be included in the Hamiltonian and a derived first-order Coriolis coupling constant which is smaller than the calculated value. These anomalous features are explained by the vibronic interaction with excited 2E′ electronic states.
Experimental study of the reactions of N2(A 3Σ+u) with H atoms and OH radicals
Authors: Ho Grace H. , Golde Michael F.
Published: July 2002

The reactions of N2(A 3Σ+u) with H atoms and OH radicals have been studied by the discharge-flow technique. The concentrations of the radicals were measured by resonance fluorescence and N2(A) was monitored by (A−X) emission. The rate constant of the N2(A)+H reaction was measured as (2.1±0.3)×10−10 cm3 s−1. Chemical reaction to NH+N was shown to be unimportant. The total rate constant for quenching of N2(A) by OH was measured as (1.1±0.4)×10−10 cm3 s−1. The channel leading to OH(2Σ+) has a rate constant of (1.0±0.3)×10−10 cm3 s−1. Approximately 16% of the OH(A) is formed in v′=1. The mechanisms of these two very rapid reactions are discussed.
Interligand electron transfer and transition state dynamics in Ru(II)trisbipyridine
Authors: Malone R. A. , Kelley D. F.
Published: July 2002

The interligand electron transfer (ILET) dynamics in the excited metal-to-ligand charge transfer state of RuIItrisbipyridine have been studied using picosecond time-resolved absorption polarization spectroscopy. The ILET dynamics have been studied in several solvents with widely varying relaxation times, and over the temperature range of −85 °C to room temperature. The solvents studied include acetonitrile, propanol, ethylene glycol, and glycerol. The results show that ILET is relatively fast in acetonitrile (∼47 ps), slower and nonexponential in propanol and ethylene glycol (∼30–200 ps), and fast in glycerol (∼30 ps). The glycerol results are roughly temperature independent. These results can be understood in terms of a model in which photoexcitation places the system near the ILET transition state and solvent relaxation competes with ILET in the nascent distribution. Following solvent relaxation a slower, steady state (probably transition state theory) rate is obtained.
The infrared spectrum and structure of hydrogen fluoride clusters and the liquid: Semiclassical and classical studies
Authors: Sun Huai , Watts Robert O. , Buck U.
Published: July 2002

Computer simulation methods are used to calculate the vibrational spectra of hydrogen fluoride clusters, and the liquid, in the intramolecular stretching region. A semiclassical approach, in which Monte Carlo and molecular dynamics simulations are analyzed using quantum vibrational calculations, is a considerable improvement over dipole correlation function methods. Comparisons with experimental data show that induced dipole interactions play an important role in determining spectral shifts. A simple Drude oscillator model of polarization allows the spectral shifts to be obtained correctly, but at the expense of overestimating structural correlations in the liquid.
Vibrational spectra of aniline–Arn van der Waals cations (n=1 and 2) observed by two-color ‘‘threshold photoelectron’’ [zero kinetic energy (ZEKE)-photoelectron] spectroscopy
Authors: Takahashi M. , Ozeki H. , Kimura K.
Published: July 2002

Measurements of mass-selected ion-current and threshold photoelectron spectra of jet-cooled aniline–Arn van der Waals complexes (n=1 and 2) have been carried out with a two-color resonantly enhanced multiphoton ionization (REMPI) technique using a high-resolution threshold photoelectron analyzer developed in this laboratory. From our (1+1′) REMPI experiments via the respective excited S1 states, we have obtained photoelectron spectra with well-resolved vibrational progressions due to ‘‘low-frequency van der Waals modes’’ of the cations; νvdW=16 cm−1 (n=1) and νvdW=11 cm−1 (n=2). From Franck–Condon calculations, we have assigned these low-frequency vibrations to the ‘‘van der Waals bending’’ of the cations. We have also found that the angles of the van der Waals bonds in the cations are changed by 8.2 (n=1) and 8.8 (n=2) degrees with respect to the S1 states. The adiabatic ionization potentials (Ia) of aniline and the aniline–Arn complexes (n=1 and 2) have been determined as 62 268±4 cm−1 (aniline), 62 157±4 cm−1 (n=1), and 62 049±4 cm−1 (n=2). Their shifts ΔIa are 111 cm−1 (n=1) and 219 cm−1 (n=2) with respect to aniline. Spectral shifts due to complex formation have been observed for a total of 13 ring modes of the cations.
Nonadiabatic energy corrections for the vibrational levels of the B and B′ 1Σ+u states of the H2 and D2 molecules
Authors: Wolniewicz L. , Dressler K.
Published: July 2002

The variation-perturbation method is used to calculate the nonadiabatic energy shifts of all bound vibrational levels of the B 1Σ+u and B′ 1Σ+u states of H2 and D2 from the best available ab initio wave functions. In H2 the nonadiabatic energy shifts lie between extremes of −0.31 cm−1 (B,v=0) and −5.07 cm−1 (B,v=30) and, as expected, the shifts in D2 are one half as large as the ones in H2 at the same vibrational energies. The resulting nonadiabatic vibronic term values of the B state lie ≂1.1±0.1 cm−1 above the experimental ones over a wide range of vibrational excitation energies, in H2 as well as in D2. This indicates that all nuclear-mass dependent contributions to the vibronic energies as well as the potential curve of the B state are now accounted for quite accurately. In the B′ state the remaining discrepancies with experimental term values depend markedly on vibrational excitation, and a large part of them is nuclear mass dependent, while the remaining error of the electronic energy calculation for the B′ state seems to be as small as the one for the B state.
Linear and cyclic isomers of C4. A theoretical study with coupled-cluster methods and large basis sets
Authors: Watts John D. , Gauss Jürgen , Stanton John F. , Bartlett Rodney J.
Published: July 2002

The ground electronic states of linear and rhombic C4 have been studied by high level ab initio quantum chemical techniques. Geometries, harmonic vibrational frequencies, infrared intensities, and other quantities have been determined using 4s3p2d1f correlation consistent basis sets and coupled-cluster methods including triple excitations. The linear–rhombic isomer energy difference has been investigated with a range of basis sets, including a 5s4p3d2f1g correlation consistent set. The linear–rhombic energy difference is influenced significantly by basis set, presence of triple excitations, and the choice of reference function for the open-shell linear isomer. The effect of basis set variation is complex, but once a reasonable quality of basis set has been achieved, further extensions favor the rhombic isomer. The inclusion of triple excitations also favors the rhombic isomer. The use of a restricted Hartree–Fock reference function for the linear isomer yields higher energies at the coupled-cluster level than if an unrestricted Hartree–Fock reference function is used, thereby again favoring the rhombic isomer. The most complete calculations of this study [coupled-cluster singles and doubles with noniterative triples (CCSD(T)) with a 5s4p3d2f1g basis set] indicate that the rhombic isomer is preferred by about 1 kcal mol−1. The coupled-cluster vibrational frequencies of the linear isomer are all real, in agreement with previous work, indicating that this isomer is not bent in the gas phase. The infrared intensities of linear C4 obtained in this work differ significantly from those obtained previously with smaller basis sets and either self-consistent field theory or second-order perturbation theory. The present calculations give a dissociation energy of C4 of 433 kcal mol−1, which is close to a previous value obtained with the aid of an empirical correction, and implies that several experimental estimates of the heat of formation of C4 are unreliable. Electron detachment energies of linear C4− and electron affinities of C4 are computed with larger basis sets than previously and are in very good agreement with recent anion photoelectron data.
Electrostatic energy in the adsorption of monatomics on MgO(001) and LiF(001)
Authors: Guo Z.-C. , Bruch L. W.
Published: July 2002

The electrostatic polarization energy is evaluated for atomic hydrogen and helium in the very nonuniform fields near the LiF(001) and MgO(001) surfaces. The variational trial functions, which for helium are at the Hartree–Fock level, include configurations corresponding to many orders of multipole polarizability. The results for helium follow the trends expected from linear nonuniform response theory. The multipole series of second order perturbation theory for the polarization energy diverges for atomic hydrogen on these substrates; the calculations show comparable contributions from the first four multipole terms. The point–ion–substrate model has a charge-transfer pathology for H/MgO(001). The effective polarizability of the adatoms is calculated and the electrostatic contribution to the corrugation energy amplitude is estimated. Consequences for more polarizable inert gases are discussed.
Sequential reactions of SiD0–3+ and Si2D0–6+ with disilane
Authors: Reents W. D. , Mandich M. L. , Wang C. R. C.
Published: July 2002

Sequential reactions of SiD0–3+ and Si2D0–6+ with 10−7–10−6 Torr of disilane are described. The reactions proceed, with few exceptions, by addition of SiD2 with simultaneous loss of SiD4. The growing cluster cations decrease in reactivity with increasing size. For all reaction sequences, a terminal cluster size is reached that contains fewer than nine silicon atoms. Based on our results, we conclude that the reaction of small subsilane or disilane cations with disilane does not lead to gas phase particle formation in disilane plasmas.
Calculation of the rotational Raman spectrum of H2 dissolved in water
Authors: Hunter John E. , Taylor David G. , Strauss Herbert L.
Published: July 2002

We have carried out molecular dynamics simulations on the system of a hydrogen molecule dissolved in water or in heavy water in an attempt to understand the experimental results of Taylor and Strauss [J. Chem. Phys. 90, 768 (1989)] for the rotational Raman spectrum of H2. By extracting from the simulation the time-varying potential felt by the H2 molecule, we have computed linewidths in various theoretical limits: static, adiabatic, and Redfield. A start has been made at the nonadiabatic calculation. H2/water is a system of a quantum rotor in a classical bath, in which there is little separation of time scales. The calculations do produce the correct absolute magnitude of the linewidths without adjustable parameters. However, the details of the observed variation of the widths with J and with the change of H2O to D2O are not reproduced and are discussed.
Computer simulation of a phospholipid monolayer-water system: The influence of long range forces on water structure and dynamics
Authors: Alper Howard E. , Bassolino Donna , Stouch Terry R.
Published: July 2002

Molecular dynamics simulations of water above a lipid (dimyristoylphosphatidylcholine) monolayer show that the structure and dynamics of the water are quite sensitive to the treatment of the long range forces. To obtain a quantitative estimate of the perturbations introduced into the water behavior by truncation of the potential, we calculate various structural and dynamical properties of the water for simulations employing different cutoffs. When potential cutoffs on the order of 10–14 Å are employed, the properties of water near the monolayer exhibit perturbations of up to 20%–25% that extend 30–35 Å into the water. Employing cutoffs on the order of 30 Å leads to water behavior that is essentially uniform above the monolayer. We also present a quantitative description of the difference in the force experienced by individual atoms in simulations employing different cutoffs. Despite the significant difference in the water properties between simulations employing 14 Å and effective 30 Å cutoffs, the forces on the individual atoms differ by only ∼1%. It was also found that the motion of the lipids did not significantly affect the perturbations in the water properties. Simulations with shorter cutoffs in which the lipid atomic point charges were set to zero showed no perturbation in water structure. Water adjacent to a repulsive wall employed to maintain the system density also exhibited different behavior with effective 30 Å cutoffs than with 14 Å cutoffs.
Shapes of flexible vesicles at constant volume
Authors: Baumgärtner A.
Published: July 2002

A three-dimensional vesicle model on the cubic lattice with fixed surface area N and at constant volume V is analyzed using a Monte Carlo method and finite size scaling. On the entire range of volumes accessible to the vesicles, the mean square radius of gyration obeys a scaling law <R2≳∼Nνf(VN−3ν/2), where ν≊4/5. The scaling function f(x) has a minmum at the crumpling point xc≊0.177, separating the regimes of branched conformations V∼N (x≤xc) and expanded conformations V∼N3/2 (x≥xc). At the crumpling point Vc≊0.177N3ν/2, the conformations are crumpled with <R2≳c≊0.146Nν. The shapes of vesicles are spherically symmetric in the inflated regime, whereas oblate in the branched regime. The crossover is characterized by the asymmetry parameter A, which exhibits an effective power law <A≳≊(15x)−3.0. A crumpling parameter is introduced characterizing the increasing roughness of the vesicles at decreasing volume. The scaling of the mean local curvature is discussed as well.
Mechanism of bifurcation along the reaction path: An application in the case of thioformaldehyde
Authors: Taketsugu Tetsuya , Hirano Tsuneo
Published: July 2002

The mechanism of bifurcation along the reaction path on an adiabatic potential surface is discussed in terms of the second-order Jahn–Teller (SOJT) effect. From the viewpoint of this SOJT effect, bifurcation occurs due to the mixing of the nearest nontotally symmetric electronic excited state into the electronic ground state through a molecular vibration belonging to the same nontotal symmetry. Thus, the bifurcating region should be limited where the excited state comes close to the ground state. In order to demonstrate our SOJT theory of bifurcation, we calculated intrinsic reaction coordinates (IRC) of thioformaldehyde under Cs symmetry constraint for the dissociation reaction to H2+CS and for the isomerization reaction to thiohydroxycarbene (HCSH). Energies of the lowest excited electronic state (A″) were also calculated along the IRC. These calculations were performed with 6–31G** basis sets at three different computational levels: (I) HF level, (II) SDCI level for HF geometries, and (III) full valence complete active space self-consistent field (CASSCF) level. As is expected from our theory, bifurcations observed for both dissociation and isomerization reactions at the HF level disappear at the CASSCF level due to the larger energy separation between ground and excited states. For the isomerization reaction, while the previous calculations at the HF level predicted that the transition state and IRC should have C1 symmetry, the transition state and IRC are proved to have Cs symmetry at the CASSCF level as the result of the disappearance of bifurcation.
Path-integral Monte Carlo study of a lithium impurity in para-hydrogen: Clusters and the bulk liquid
Authors: Scharf Daphna , Martyna Glenn J. , Klein Michael L.
Published: July 2002

Simulation studies using the path-integral formulation of quantum statistical mechanics are reported for single atomic lithium impurities in bulk liquid para-hydrogen and in clusters, Li(p-H2)n, with n=12, 13, 32, 33, and 34. Over the range of temperatures studied in the clusters (T=2.5–6.0 K), the lithium impurity is found to reside outside or at the surface of the clusters. Nevertheless, perturbations of the structure are observed in comparison to neat para-hydrogen clusters. The solvation energy of the lithium in the bulk liquid and subcritical gas (T=14–25 K) is found to be slightly positive. In both the clusters and the liquid, the inhomogeneously broadened dipole spectrum of the lithium atom was calculated using the radial fast Fourier transform Lanczos method. In the clusters, the spectra exhibit a main absorption band near the unperturbed atomic Li value and a second, asymmetric band shifted to the blue. The latter can be identified as the p orbital oriented radially towards the cluster, while the main band is composed of two p orbitals oriented parallel to the cluster surface. The spectrum in the liquid, while broader, has a more symmetric line shape. The ionization spectrum of the lithium atom in the clusters was also obtained. These spectra have finite width and a band center that progressively red shifts from the atomic value as the number of para-hydrogen molecules increases. The predicted trends should be amenable to testing using molecular beam experiments.
Pulsed field ionization threshold photoelectron spectroscopy with coherent vacuum ultraviolet: NO+(a 3Σ+ v=0,1,2)←NO(X 2Π1/2)
Authors: Kong W. , Rodgers D. , Hepburn J. W.
Published: July 2002

We report pulsed field ionization spectra of an ionic electronically excited state, specifically, the NO+(a 3Σ+ v+=0,1,2)←NO(X 2Π1/2) transition. A coherent vacuum ultraviolet light source provides single photon excitation, which enables a direct measurement of the ionization potentials for the three vibrational levels — I.P.(v+=0)=126 392±1.5 cm−1, I.P.(v+=1)=127 653±1.5 cm−1, and I.P.(v+=2)=128 884±1.5 cm−1. The rotational structure of these spectra has been simulated using a standard model for rotational line strengths in a photoelectron spectrum, and this simulation shows the strong atomic p orbital character of the initial NO orbital from which the electron is photoionized. However, our data show that the rotational line strengths are strongly dependent on the final vibrational state of NO+, in disagreement with the theoretical model. Also, the spectrum for the v+=1 state of NO+ is strongly affected by the presence of a strong autoionizing Rydberg resonance at the same photon energy, which leads to a complex resonance.
The vibrational spectra of molecular ions isolated in solid neon. XI. NO+2, NO−2, and NO−3
Authors: Forney Daniel , Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:NO2 or a Ne:NO:O2 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, infrared absorptions of NO+2, NO−2, and NO−3 appear. Detailed isotopic substitution studies support the assignment of prominent absorptions to ν3 of NO+2 and NO−2 and of weak to moderately intense absorptions to the ν1+ν3 combination band of each of these species. When the contribution of anharmonicity is considered, the positions of the NO+2 absorptions are in satisfactory agreement with the values for the stretching fundamentals obtained in a recent gas-phase study of that species. When the sample is exposed to 240–420 nm mercury-arc radiation, the initially present absorptions of NO−3 trapped in sites with a small residual cation interaction diminish in intensity, and the unsplit ν3(e′) absorption of isolated NO−3 grows. The mechanism responsible for this growth in the absorption of isolated NO−3 is considered.
Mean force potential for the calcium–chloride ion pair in water
Authors: Guàrdia E. , Robinson A. , Padró J. A.
Published: July 2002

The potential of mean force for the Ca2+–Cl− ion pair in water has been calculated from constrained MD simulations assuming a rigid simple point charged (SPC/E) model for water. Unlike for monovalent ion pairs the resulting potential does not show any noticeable minimum at short distances.
In-situ x-ray reflectivity study of incipient oxidation of Pt(111) surface in electrolyte solutions
Authors: You H. , Zurawski D. J. , Nagy Z. , Yonco R. M.
Published: July 2002

Electrochemical oxidation causes the lifting of Pt atoms of the surface layer, substantiating a place-exchange mechanism. Furthermore, for a charge transfer of ≲1.7e−/Pt atom, the flat surface is recovered by reduction, while the surface is irreversibly roughened by more excessive oxidation. Roughening involves only the atoms in the top layer.
Microwave spectrum and structure of the dimethylamine dimer: Evidence for a cyclic structure
Authors: Tubergen Michael J. , Kuczkowski Robert L.
Published: July 2002

Rotational spectra have been measured for six isotopomers of the dimethylamine dimer using a Fourier-transform microwave spectrometer. No tunneling splittings were observed for either the a- or c-type rotational transitions and the spectra could be fit to a rigid rotor Hamiltonian including centrifugal distortion terms. Possible dimer structures are discussed and compared to the experimentally determined moments of inertia, dipole moments, and nuclear quadrupole coupling constants. A cyclic structure with Cs symmetry was found to best reproduce the inertial data. The structure of the dimethylamine dimer is compared to the theoretical and experimental structures for the ammonia dimer.
Multiconfigurational self-consistent field calculations of nuclear shieldings using London atomic orbitals
Authors: Ruud Kenneth , Helgaker Trygve , Kobayashi Rika , Jo/rgensen Poul , Bak Keld L. , Jensen Hans Jo/rgen Aa.
Published: July 2002

Nuclear shielding calculations are presented for multiconfigurational self-consistent field wave functions using London atomic orbitals (gauge invariant atomic orbitals). Calculations of nuclear shieldings for eight molecules (H2O, H2S, CH4, N2, CO, HF, F2, and SO2) are presented and compared to corresponding individual gauges for localized orbitals (IGLO) results. The London results show better basis set convergence than IGLO, especially for heavier atoms. It is shown that the choice of active space is crucial for determination of accurate nuclear shielding constants.
Mobilities of He+, Ne+, Ar+, and Kr+ in He gas at 4.35 K
Authors: Saito N. , Kojima T. M. , Kobayashi N. , Kaneko Y.
Published: July 2002

Mobilities of He+, Ne+, Ar+, and Kr+ have been measured in He gas at 4.35 K. An injected-ion drift tube which can be cooled by liquid helium was used. It has a Wien filter on the ion injection line and a quadrupole mass filter on the detection line so that mass identification is explicitly made. Ions are injected into the drift tube with 20 eV, and it is assured that the thermalization of ions is completed well before reaching the gate for mobility measurement. The correction for thermal transpiration in the pressure measurement was made by Takaishi–Sensui’s empirical formula. The reduced mobility K was measured against E/N, where E is the electric field strength and N is the gas number density. Then the E/N was converted to the effective temperature Teff by Wannier’s formula. The K(Teff) obtained are compared with the previous experimental and theoretical results, some of which are given in K0(Tg), where K0 is the zero field mobility and Tg is the gas temperature. The agreement between the present results and the previous experimental results is generally good at high temperature where they are available. Recommended numerical values are presented.

The present work confirms the findings in the preliminary report by Kojima et al. that the K of He+ in He has a maximum around E/N=5 Td which corresponds to Teff=18 K, and it decreases steeply below that temperature. An apprehension that the structure might be caused by clustering is discussed and denied. The maximum found in K of He+ in He is considered to be the structure predicted theoretically as a result of orbiting resonance scattering. The Ks obtained for Ne+, Ar+, and Kr+ have a typical shape of mobilities in heterogeneous gas. They have a maximum around Teff=500–1000 K and become more or less flat below 50 K. From the maximum position the well depths of the interaction potentials are estimated and compared with theoretical calculations. There is a shallow minimum of K between 50 and 100 K for Ar+ and Kr+. The Ks at the flat are a few percent larger than the polarization limit Kpol. Below Teff=6 K, Ks tend to decrease steeply. This steep decrease of K is also suggested to be an orbiting resonance scattering effect although no theoretical calculation is available.
The formulation of quantum statistical mechanics based on the Feynman path centroid density. II. Dynamical properties
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

The formulation of quantum dynamical time correlation functions is examined within the context of the path centroid variable in Feynman path integration. This study builds on the centroid-based approach to equilibrium properties developed in the companion paper. The introduction of the centroid perspective into the calculation of real time position correlation functions is outlined and an intriguing quasiclassical role for the centroid variable in real time position correlation functions is identified. This quasiclassical perspective is developed in terms of general interaction potentials, and the computational effort in implementing the method should scale with the size of the system in the same fashion as a classical molecular dynamics calculation. The centroid-based theory is also implemented in several different approaches to calculate general time correlation functions. The theoretical results are illustrated and tested by representative numerical applications.
The hyperfine structures of small radicals from density functional calculations
Authors: Eriksson Leif A. , Malkina Olga L. , Malkin Vladimir G. , Salahub Dennis R.
Published: July 2002

The isotropic and anisotropic hyperfine (hf) structures of a set of anionic, neutral and cationic radicals are investigated by means of local and nonlocal gradient-corrected density functional theory (DFT). The molecules under study are formed by H, C, N, O, F, and Cl atoms, and the hf structures are computed at both the experimental (where present) and various DFT and CI optimized geometries. The agreement with experiment and with results from previous CI or MRCI calculations is generally very satisfactory. The anisotropic hf couplings are relatively insensitive to basis set effects and functional form, whereas the isotropic hf couplings are highly dependent on the form of the nonlocal corrections to the exchange functional, particularly for heteroatoms. Using the functional by Perdew and Wang (‘‘PW86’’), an excellent agreement with experiment is obtained for all neutral and cationic radicals, whereas for the halide containing anions somewhat elongated bond lengths, and thus less accurate hf structures, are obtained.
Time-of-flight neutron diffraction study on the low temperature phases of IF7
Authors: Marx R. , Mahjoub A. R. , Seppelt K. , Ibberson R. M.
Published: July 2002

A neutron diffraction study on iodine heptafluoride has been performed using the high resolution powder diffractometer (HRPD) at the spallation source ISIS, England. The previously unknown structure of the low temperature phase IF7 (III) was solved. IF7 (III) crystallizes in the orthorhombic space group Pbab (No. 54), a=852.160(4) pm, b=884.096(4) pm, c=599.089(2) pm with 4 formula units per unit cell. The IF7 molecule is found to exhibit an almost perfect pentagonal bipyramid with an axial bond length of 179.5(2) pm and an average equatorial bond length of 184.9 pm. The I(Feq)5 unit is slightly puckered with puckering displacements of 2.7° for two of the five Feq atoms leading to a c2 symmetry for the molecule. Two order-disorder processes lead from the plastic phase IF7 (I) to the ordered phase IF7 (III). At the first phase transition at 150 K IF7 molecules align with respect to their Fax–I–Fax units. Within the second transition at 100 K they order with respect to their I(Feq)5 units. The latter can be grouped into two sets of five-membered rings with vertices pointing up and down the orthorhombic c axis, respectively. There is evidence suggesting a temperature dependence to this second ordering process; the amount of residual disorder is 5% at 77 K and 1.4% at 5 K.
The formulation of quantum statistical mechanics based on the Feynman path centroid density. IV. Algorithms for centroid molecular dynamics
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

Numerical algorithms are developed for the centroid molecular dynamics (centroid MD) method to calculate dynamical time correlation functions for general many-body quantum systems. Approaches based on the normal mode path integral molecular dynamics and staging path integral Monte Carlo methods are described to carry out a direct calculation of the force on the centroid variables in the centroid MD algorithm. A more efficient, but approximate, scheme to compute the centroid force is devised which is based on the locally optimized harmonic approximation for the centroid potential. The centroid MD equations in the latter method can be solved with the help of an iterative procedure or through extended Lagrangian dynamics. A third algorithm introduces an effective centroid pseudopotential to approximate the full many-body centroid mean force potential by effective pairwise centroid interactions. Numerical simulations for both prototype models and more realistic many-body systems are performed to explore the feasibility and limitations of each algorithm.
The discrete Bessel transform algorithm
Authors: Lemoine Didier
Published: July 2002

We present a general discrete Bessel transform based on the Bessel functions of the first kind for any integer or half-integer order ν. This discrete Bessel transform shares a number of similitudes with the discrete Fourier transform in that we have discretized both the coordinate and momentum continuums, and since the discrete transform of order 1/2 exactly specializes to the discrete sine Fourier transform. We demonstrate that our discrete Bessel transform is comparable to the discrete Fourier transform in terms of both the accuracy and the efficiency. Indeed, our discretization procedure provides an optimal sampling grid for Bessel functions of the first kind, and the accuracy of the transform converges exponentially as the number of grid points is increased. We successfully apply the optimally discretized Bessel methodology to the harmonic oscillator in both cylindrical and spherical coordinates.
Lifetimes of Rydberg states in zero-electron-kinetic-energy experiments. I. Electric field induced and collisional enhancement of NO predissociation lifetimes
Authors: Vrakking Marc J. J. , Lee Yuan T.
Published: July 2002

Lifetime measurements are reported for high principal quantum number (n=40–125) Rydberg states of the NO molecule, prepared using state-selective double resonance excitation with a narrow bandwidth laser. The influence of (i) the application of a dc electric field, and (ii) interactions of the Rydberg states with surrounding ions, were investigated. It is demonstrated that the presence of dc electric fields can lead to enhancements in the lifetimes due to l-mixing, and the conclusion is reached that under the conditions existing in a typical zero-electron-kinetic-energy (ZEKE) photoelectron spectroscopy experiment, the Rydberg states involved are excited in a regime where l-mixing is expected to be significant. It is observed that l- and m-mixing collisional interactions provide a further mechanism to stabilize the optically prepared Rydberg states, beyond the limits which can be achieved by l-mixing alone.
Self-collisional coupling and broadening in the asymmetric rotor CHF2Cl
Authors: Cazzoli G. , Cludi L. , Cotti G. , Esposti C. Degli , Buffa G. , Tarrini O.
Published: July 2002

The line shape of 15 selected CHF2Cl rotational transitions is studied in the 59–1049 GHz frequency region. Collisional broadening parameters are measured and compared to semiclassical calculations. For some pairs of lines a large collisional coupling effect is observed and analyzed. A semiclassical theoretical model is presented allowing calculations of the coupling parameters in good agreement with measurements.
Kinetic and thermodynamic properties of the F+O2 reaction system under high pressure and low temperature conditions
Authors: Campuzano-Jost P. , Croce A. E. , Hippler H. , Siefke M. , Troe J.
Published: July 2002

The reaction F+O2+M→FOO+M(1) in the bath gases M=He, Ar, and N2 was studied by time-resolved uv absorption spectroscopy of the FOO radical at 220 nm. The experiments were performed at total pressures between 1 and 1000 bar and temperatures between 100 and 420 K. The absorption cross section of FOO was determined as σ (300 K)=1.6⋅10−17 cm21 at 220 nm. The F–O2 bond energy was derived by third law analysis of the equilibrium constant K1=9.15⋅10−24⋅T−0.45⋅exp(5990 K/T) cm3 (between 315 and 420 K) for reaction (1), being (49.8±1) kJ mol−1 (at 0 K). Limiting low pressure rate constants for the recombination reaction (1) of k1,0/[He]=3.7⋅10−33⋅(T/300 K)−1.1 cm6 s−1, k1,0/[Ar]=4.4⋅10−33⋅(T/300 K)−1.4 cm6 s−1, and k1,0/[N2]=5.8⋅10−33⋅(T/300 K)−1.7 cm6 s−1 were obtained between 100 and 373 K. The transition to a high pressure plateau of the rate constants was observed in all cases although the transition to diffusion-controlled reaction may have been superimposed at very high pressures. Correcting for this effect, a limiting high pressure rate constant of k1,∞=1.2⋅10−10 cm3 s−1 was derived between 100 and 373 K. In addition, a rate constant for the reaction F+FOO→F2+O2(2) of k2=1.4⋅10−9 exp(−2790 K/T) cm3 s−1 (340–420 K) with an uncertainty of Δk2/k2=±10% was derived. Furthermore, an equilibrium constant for the reaction F+FOO↔F2O2(3) of K3=(1.9±0.7)⋅10−15 cm3 was obtained at 340 K.
Molecular dynamics simulations of vibrational cooling and heating in isotopically substituted molecular clusters
Authors: Kim Hackjin , Dlott Dana D. , Won Youngdo
Published: July 2002

Molecular dynamics simulations of clusters containing hundreds of naphthalene molecules were used to investigate vibrational cooling and vibrational heating. The effects of isotopic substitution, modeled by changing the masses of the extended-atom C–H groups, were also studied. In vibrational cooling, a hotter molecule (300 K) is allowed to interact with a cold cluster (10 K). Pure clusters of normal, light, and heavy naphthalene molecules were cooled with roughly the same time constant (∼50 ps). However, in mixed clusters containing a normal molecule in an isotopically substituted heavy or light cluster, the normal molecule cooled much more slowly, indicating the dominant cooling mechanism in pure clusters is resonant intermolecular vibrational energy transfer. In vibrational heating studies, a cold molecule (10 K) is allowed to interact with a cluster which is much hotter (300 K) than in the vibrational cooling studies (10 K). Normal molecules in pure or mixed clusters were heated at about the same rates and those rates were about what was seen in vibrational cooling simulations. At the higher temperatures of the vibrational heating simulation, phonon-assisted intermolecular vibrational energy transfer between unlike molecules in mixed clusters occurs at rates similar to resonant transfer processes between like molecules in pure clusters.