bluuebunny/crossref_metadata_2025 huggingface dataset:
On Green’s function calculations of the static self-energy part, the ground state energy and expectation values
Authors: Schirmer J. , Angonoa G.
Published: July 2002

The necessity to determine the static self-energy part Σ(∞) in many-body Green’s function studies of atomic and molecular ionization introduces difficulties affecting both the accuracy and the efficiency of the method. We show how this bottleneck can be overcome under an approximation obtained by truncating the Dyson expansion for the one-particle Green’s function G(ω). Here the essential computational step consists of an inversion or a Lanczos diagonalization of constant Hermitian secular matrices associated with the dynamical self-energy part M(ω). Both methods are very practical and efficient as is demonstrated in an exemplary application to the CO molecule. The same approximation and numerical techniques apply also to the problem of extracting ground state information from G(ω), i.e., correlation energy and expectation values of single-particle operators.
The permanent dipole moment of TiN and the nuclear magnetic hyperfine structure in its X 2Σ+ and A 2Π electronic states
Authors: Simard B. , Niki H. , Hackett P. A.
Published: July 2002

The permanent dipole moment of TiN in its X 2Σ+ and A 2Σ states has been determined from the complete resolution of the first- and second-order Stark splitting of the Q21(1.5)+R2(0.5) line of the (0,0) band of the A 2Π–X 2Σ+ system. Values of 3.56±0.05 D (2σ) and 4.63±0.04 D (2σ) have been derived for the X and A states respectively, from least-squares fits to plots of Stark splitting vs electric field strength. Electric fields up to 12 kV/cm have been employed avoiding voltage breakdown. The zero-field spectrum shows resolution of the nuclear magnetic hyperfine structure of the 47TiN and 49TiN isotopes. This hyperfine structure is that of the ground X 2Σ+ state only and is shown to follow closely the coupling case bβS. The value of the Fermi contact parameter is −570 MHz which implies a 4s occupation of the 9σ molecular orbital (MO) of 72%. The results are compared with calculated and available experimental values for early first-row transition metal oxides and nitrides.
Investigation of the end-to-end distance distribution function for random and self-avoiding walks in two and three dimensions
Authors: Bishop Marvin , Clarke J. H. R.
Published: July 2002

Monte Carlo simulations employing the pivot algorithm are used to generate random and self-avoiding walks on two- and three-dimensional lattices. The moments of the end-to-end distance distribution function are calculated from the resulting configurations. It is found that the moments and the shape of the vector distribution function are in excellent agreement with the scaling form derived by des Cloizeaux.
The rheology of adhesive hard sphere dispersions
Authors: Woutersen A. T. J. M. , de Kruif C. G.
Published: July 2002

The influence of an attractive interparticle potential on the rheology of a sterically stabilized silica dispersion was investigated. Using a marginal solvent, there was an effective attraction between the particles which depended on the temperature. Three experiments in which different properties of the dispersion were probed showed that a square well model can be used to describe the temperature dependence of the pair potential. The turbidity of a dilute dispersion was measured as a function of the volume fraction and the temperature. Using dynamic light scattering techniques, the effect of the strength of the interparticle attraction on the diffusion coefficient was investigated. Furthermore, the steady shear viscosity was measured as a function of the volume fraction and the temperature. A microscopic theory for the low shear viscosity of a semidilute dispersion of adhesive hard spheres was successfully used to determine the interaction parameters. Viscosity measurement on dense suspensions showed that while the system is still in the one-phase state, temporal aggregates are formed by the interparticle forces which are disrupted by both shear and Brownian motion of the particles. The shear thinning behavior of a concentrated dispersion of adhesive hard spheres scales in a dimensionless shear stress. This group is the ratio of the forces, arising from the shear and the interparticle potential.
A computer-simulation study of hydrophobic hydration of rare gases and of methane. I. Thermodynamic and structural properties
Authors: Guillot B. , Guissani Y. , Bratos S.
Published: July 2002

A theory is proposed to study the hydrophobic hydration of rare gases and methane in water. The Ostwald absorption coefficient γ, the hydration energy ΔE, and entropy ΔS are calculated by combining large-scale molecular-dynamics simulations and test-particle methods. The convergence of calculations is checked with particular care. The structure of the first two hydration shells is analyzed in terms of appropriate pair distribution functions. The picture conveyed by this theory is compared to that provided by the early work.
Ru L-edge x-ray absorption studies of the formation of Ru–Cu bimetallic aggregates on Cu(100)
Authors: Sham T. K. , Ohta T. , Yokoyama T. , Takata Y. , Kitajima Y. , Funabashi M. , Kuroda H.
Published: July 2002

X-ray absorption measurements at the Ru L3 edge have been used to study the adsorption of Ru3(CO)12 on Cu(100) surfaces at submonolayer and monolayer coverages and the subsequent formation of Ru–Cu bimetallic aggregates at these surfaces. The analysis of the Ru L3 edge x-ray absorption near edge structure (XANES) reveals (a) the surface Ru–Cu bimetallic aggregates are three dimensional clusters of which the Ru atoms are in the ‘‘bulk’’ and the surface of the cluster is covered with Cu atoms, and (b) relative to pure Ru metal, Ru in the bimetallic aggregates gains d character. The chemical properties of these bimetallic surfaces and the effect of the cluster size on their electronic properties revealed by XANES as well as LEED and Auger are also reported. These results are compared with previous results of Ru3(CO)12/Cu(111). The implications of these observations are discussed.
Anharmonic vibrational properties of CH2F2 : A comparison of theory and experiment
Authors: Amos R. D. , Handy N. C. , Green W. H. , Jayatilaka D. , Willetts A. , Palmieri P.
Published: July 2002

A b initio theoretical chemistry is used to provide a complete understanding of the infrared spectroscopy of CH2F2. Second-order Mo/ller–Plesset perturbation theory (MP2) with a 631G extended basis set is used to provide a quartic expansion of the potential energy surface and a cubic expansion of the dipole surface. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for fundamentals, selected overtones, and combination bands. Effects of Fermi resonance, Darling–Dennison resonance, and Coriolis resonance are included by matrix diagonalization. Empirical (x,K) relations are used to demonstrate that the anharmonic constants for C–H are in good agreement with those determined from CH2Cl2. The local mode nature of the CH overtones is demonstrated. Important resonances are found to be (ν3,2ν4), (ν8,ν4+ν9), and (ν1,2ν2,2ν8,ν4+ν8+ν9, 2ν4+2ν9,ν3+2ν9). Rotational constants, quartic and sextic centrifugal distortion constants, vibration rotation interaction constants, and Coriolis constants are all in good agreement with the mass of experimental data. The signs of the dipole moment derivatives are in agreement with those deduced from experiment. The separate contributions to the infrared intensities from electrical, mechanical, and mixed anharmonicity are examined for fundamentals and overtones, but by far the most important effect arises from corrections due to resonant Fermi and Darling–Dennison interactions. In this way, the 2ν8, ν1 and ν6 experimental bands and their intensities are explained by assigning ν1 and ν6 as (ν1,ν4+ν8+ν9) and (ν6,2ν2) doublets, respectively. This paper therefore demonstrates that state of the art quantum chemistry can provide a complete interpretation of such spectroscopic data.
Collisional broadening, line shifting, and line mixing in the stimulated Raman 2ν2 Q branch of CH4
Authors: Millot G. , Lavorel B. , Steinfeld J. I.
Published: July 2002

Self-, argon-, and helium-broadening coefficients have been measured for 13 lines in the 2ν2 Raman Q branch of CH4 using stimulated inverse Raman spectroscopy. The linewidths clearly show the symmetry-state dependence characteristic of pressure broadening, and inelastic processes in general, involving spherical-top molecules. Pressure-induced line shifts have also been measured for these features in pure methane. The pressure-shift coefficients do not display the symmetry-state dependence found for the linewidths. By applying the Rosenkranz perturbation treatment to a pair of collisionally mixed lines, we have been able to obtain an estimate of individual state-to-state contributions to the overall linewidth.
Porous vycor glass: The microstructure as probed by electron microscopy, direct energy transfer, small-angle scattering, and molecular adsorption
Authors: Levitz P. , Ehret G. , Sinha S. K. , Drake J. M.
Published: July 2002

We provide a comprehensive analysis of the microstructure of the porous glass, vycor. Using transmission electron microscopy, small-angle x-ray scattering, molecular adsorption, and the dynamic process of direct energy transfer, a consistent picture of the mass, pore, and interfacial features of this material is presented. From a transmission-electron-microscopy image of an ultrathin section of vycor the material appears to have a homogeneous distribution of mass with no hierarchical organization. The pore interface exhibits a roughness which is probed by both small-angle x-ray scattering and molecular adsorption. The roughness has an upper cutoff of <20 Å which is not resolved in the transmission-electron-microscopy image and is shown to be unimportant to the dynamics of the direct energy transfer process. The dimensionality probed by direct energy transfer is shown to be related to interfacial geometrical crossover from two dimensional to three dimensional, which is characterized by a persistent length of the interface of 45 Å.
An ab initio molecular orbital study on adsorption at the MgO surface. I. H2 chemisorption on the (MgO)4 cluster
Authors: Sawabe Kyoichi , Koga Nobuaki , Morokuma Keiji , Iwasawa Yasuhiro
Published: July 2002

The hydrogen molecule chemisorption on the low coordination or corner site of the MgO surface has been studied with the ab initio method, adopting the (MgO)4 cluster as the surface site model. The effect of basis functions and correlation have been examined. Two physisorbed complexes have been found; one is an end-on complex on the oxygen site and the other a side-on complex on the Mg site. These complexes are led to a common transition state (TS) and then to a dissociative chemisorption product. A substantial polarization of charges in the hydrogen molecule takes place at the TS, from where the dissociative chemisorption proceeds ionically. The best estimates of energies of the physisorbed complexes, the TS and the product, relative to the isolated reactants are −2, +2, and −21 kcal/mol, respectively, for this cubic cluster model. When the effect of the Madelung potential is taken into account, the chemisorbed product is further stabilized by 4 kcal/mol.
The Hessian biased force field for silicon nitride ceramics: Predictions of thermodynamic and mechanical properties for α- and β-Si3N4
Authors: Wendel John A. , Goddard William A.
Published: July 2002

A force field (MSXX) for molecular dynamics simulations of silicon nitride is derived using the Hessian biased technique from ab initio calculations on N(SiH3)3 and Si(NH2)4 clusters. This is used to model the nitrogen and silicon centers of the α and β forms of crystalline Si3N4 for prediction of crystal structures, lattice expansion parameters, elastic constants, phonon states, and thermodynamic properties. Experimental measurements on many of these important physical constants are lacking, so that these calculations provide the first reliable data on such fundamental properties of silicon nitride. This MSXX force field is expected to be useful for molecular dynamics simulations of dislocations and grain boundaries and for studying the reconstruction and energetics of clean, reduced, and oxidized surfaces.
Improved theoretical dissociation energy and ionization potential for the ground state of the hydrogen molecule
Authors: Kol/os Wl/odzimierz , Rychlewski Jacek
Published: July 2002

A new 155-term electronic wave function for the ground state of the hydrogen molecule has been selected which gives a more accurate Born–Oppenheimer potential energy curve in the range 3.2≤R≤7.2 bohr. The largest improvement 0.112 cm−1 has been found for R=4.8 bohr. The adiabatic corrections have also been calculated in the full range of R giving more accurate values. These new results have been employed to calculate the dissociation energies and ionization potentials for H2, HD, and D2. The agreement between resulting dissociation energies and recently published experimental data are excellent; the discrepancy is within the experimental error limits. For the ionization potential the agreement is fairly good, although not ideal. There is still room for improvement from both theoretical and experimental side.
An orientational perturbation theory for pure liquid water
Authors: Head-Gordon Teresa , Stillinger Frank H.
Published: July 2002

This work develops a statistical mechanical perturbation theory for understanding and quantifying the role of directional hydrogen bonding in pure water at any given temperature or density. A reference fluid has been defined with no orientational preferences, but which reproduces the short-ranged oxygen order as determined by x-ray or neutron diffraction. The orientational anisotropy can be reintroduced by perturbing the reference potential toward a fully coupled water potential; we have developed a new water model, ST4, which provides some noticeable structural improvements over its predecessor, ST2, to provide these anisotropic interactions. Monte Carlo simulations at 25 °C and 1 kg/ℓ mass density have been implemented for various values of the coupling parameter to determine the importance of directed hydrogen bonds at various strengths in dictating energetic and structural features of liquid water. We find that virtually full hydrogen bond strength is required to recover the basic structural features of liquid water. We have also evaluated and contrasted the inherent structures (potential energy minima) for the reference fluid and the ST4 model, where we find that hydrogen bonding provides significant structural rigidity to resist vibrational distortion. Furthermore, we show that the ST4 model exhibits bifurcated hydrogen bonds which only occur in local regions of high density, i.e., they are found as tetrahedral network defects. These high density clusters also include tetrahedral oxygen triplets, sometimes linearly hydrogen-bonded, which may well serve as low energy intermediates for flow processes in liquid water.
The interpretation of dielectric response measurements on colloidal dispersions using the dynamic Stern layer model
Authors: Rosen L. A. , Baygents J. C. , Saville D. A.
Published: July 2002

The standard description of electrokinetic phenomena deals with a particle whose charge is uniformly smeared over its surface and considers ion transport only within a Gouy–Chapman diffuse layer. Experimental studies with colloidal dispersions have shown that this model is not applicable to many systems. To encompass a wider class of behavior, the standard model was extended to include ion migration within the Stern layer, the region between the shear envelope and the rigid particle. Computations show that Stern layer transport increases the conductivity and dielectric response of suspensions as well as the magnitude of the ζ potential inferred from mobility measurements. Model predictions are compared with experimental measurements on two well-defined systems—colloidal silica and a polymer latex. The inclusion of surface transport processes markedly improves agreement between theory and the experimental data. For example, in situations where the standard theory underpredicts the measured dielectric increments by factors of 2 or 3, the dynamic Stern layer model yields results within 5% to 20% of the experimental data at frequencies in the kHz range.
The CO:CO2 complex in argon matrices: Experimental evidence for two conformations with spontaneous interconversion
Authors: Raducu V. , Jasmin D. , Dahoo R. , Brosset P. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

The CO:CO2 complex in argon matrices is identified near the CO absorption which appears at 2138.49 cm−1, slightly shifted from the pure argon value of 2138.64 cm−1, because of the presence of CO2. It exhibits two features on each side of the CO frequency: a doublet at 2143.34 and 2143.01 cm−1 (HF A and B lines) and a narrow line at 2135.38 cm−1 (LF line); these small shifts indicate weakly complexed C–O stretching modes. When the temperature is raised from 5 to 12 K, the LF line decreases progressively and disappears at the benefit of the HF A and B lines, the total absorption intensity remaining unchanged. This effect is perfectly reversible and can be explained by an exchange between two different conformations of the CO:CO2 complex, with complexed C–O bonding shifted either towards a higher frequency (HF form), or towards a lower one (LF form). Furthermore, after a fast cooling down to any given temperature below 12 K, the intensity ratio between the two forms is time dependent; the high temperature form (HF form) converts into the LF form within a few minutes. The equilibrium value of the intensity ratio LF/HF and the rate constants for the conversion are temperature dependent. We have checked that this conversion, occurring without any temperature change, is not photoinduced.
An accurate ab initio quartic force field and vibrational frequencies for CH4 and isotopomers
Authors: Lee Timothy J. , Martin Jan M. L. , Taylor Peter R.
Published: July 2002

A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm−1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 re by 0.0015 Å. Our best estimate for re is 1.0862±0.0005 Å.
The reaction of Cs(82P) and Cs(92P) with hydrogen molecules
Authors: Huang Xin , Zhao Jianzheng , Xing Guoqiang , Wang Xuebin , Bersohn Richard
Published: July 2002

The reactions of Cs(n2PJ), n=8, 9, J=1/2, 3/2 with H2 were studied by laser induced fluorescence of the nascent product. No difference was found in the reactivity of the J=1/2 and 3/2 states. The energies available to the reaction products were 11.5 and 16.9 kcal/mol for the n=8 and 9 states, respectively. CsH was found in the v=0 and 1 states but could not be detected in any higher vibrational state. The v=0/v=1 population ratios were 1/0.33 (n=8) and 1/0.42 (n=9). The products rotational temperatures were approximately the same as the ambient temperature of the reaction cell. Thus about 90% of the available energy is released as translation. From these data the following picture is derived. The mechanism is a primarily collinear abstraction and not an insertion. The large translational energy release is caused by a sudden switching on of repulsion when the H atoms are still close to each other. The reaction involves electron transfer, but at Cs–H distances not far from the equilibrium bond length and is therefore not a harpoon reaction in the usual sense.
Rotational population distribution of KH (v=0, 1, 2, and 3) in the reaction of K(5 2P
J, 6 2P
J, and 7 2P
J) with H2: Reaction mechanism and product energy disposal
Authors: Liu Dean-Kuo , Lin King-Chuen
Published: July 2002

Using a pump–probe method, we have systematically studied the rotational distribution of KH (v=0–3) produced in the reaction of K (5P, 6P, and 7P) with H2. The resulting rotational states fit roughly a statistical distribution at the system temperature, while the vibrational populations are characterized by a Boltzmann vibrational temperature of 1800, 3000, and 3100 K for the 5p, 6P, and 7P states, respectively. These results provide evidence that the reaction follows a collinear collisional geometry. This work has successfully probed KH from the K(5P) reaction, and confirms that a nonadiabatical transition via formation of an ion-pair K+H−2 intermediate should account for the reaction pathway. The available energy dissipation was measured to be (68±4)%, (26±2)%, and (6±3)% into the translation, vibration, and rotation of the KH product, respectively. The energy conversion into vibrational degree of freedom generally increases with the principal quantum number, indicating that the electron-jump distance elongates along the order of 5P<6P<7P. The result is different from the Cs(8P,9P)–H2 case, in which the electron-jump distances were considered roughly the same. Furthermore, a relatively large distance is expected to account for highly vibrational excitation found in the KH product. According to the classical trajectory computation reported by Polanyi and co-workers, the strong instability of the H−2 bond, inducing a large repulsion energy, appears to favor energy partitioning into the translation.
Theory of the depletion force due to rodlike polymers
Authors: Mao Y. , Cates M. E. , Lekkerkerker H. N. W.
Published: July 2002

The entropic depletion force, in colloids, arises when large particles are placed in a solution of smaller ones, and sterically constrained to avoid them. In this paper, we consider a system of two parallel plates suspended in a semidilute solution of long thin rods of length L and diameter D. By numerically solving an integral equation, which is exact in the “Onsager limit” (D≪L), we obtain the depletion force between the plates. The second integral of this determines (via the Derjaguin approximation) the depletion potential between two large hard spheres of radius R, immersed in a solution of hard rods (satisfying D≪L≪R). The results for this potential are compared with our previous second order perturbation treatment [Y. Mao, M. E. Cates, and H. N. W. Lekkerkerker, Phys. Rev. Lett. 75, 4548 (1995)], as well as with newly computed third order perturbation results. There is good agreement at low and intermediate densities (which validates our numerical procedures for the integral equation) but the gradual failure of the perturbative treatments is revealed as the density increases. From the numerical results, we conclude that for typical colloidal sphere/rod mixtures, the repulsive barrier in the depletion interaction is likely to be less than the thermal energy kBT throughout the semidilute concentration range of the rods. This reverses our previous conclusion, based on second order perturbation theory, that this barrier could easily lead to kinetic stabilization of the mixture. However, it is possible that terms of order D/L (neglected here) could further modify the results. Within the present theory, the main modification to the well-known (attractive) first order interaction, as the density is raised, is by changing the depth and range of the primary attractive well.
The influence of a thin gold film on vibrational excitation of hydrogen molecules
Authors: Čadež I. , Hall R. I. , Landau M. , Pichou F. , Schermann C.
Published: July 2002

The vibrational populations have been measured for hydrogen molecules issuing from a cell containing a mixture of atomic and molecular hydrogen in the presence of an incandescent tungsten filament. The cell walls were covered with a gold film by in situ evaporation. The gold film was observed to exist in two forms, the first being transformed irreversibly into the second by annealing. The observed behavior of the vibrational populations differed from those observed previously with other metals in that the relative distributions depended on the filament temperature. This behavior was interpreted as being the result of either special characteristics of the hydrogen–tungsten interaction at the filament or the presence of a potential barrier in the reaction path for the formation of hydrogen molecules by the process of recombinative desorption.
Hyperfine coupling constants of organic radicals
Authors: Perera S. Ajith , Salemi Lynn M. , Bartlett Rodney J.
Published: July 2002

The isotropic hyperfine coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) “relaxed density’’ matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic hyperfine coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic hyperfine coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical.
Resonant ion-dip infrared spectroscopy of benzene–(methanol)m clusters with m=1–6
Authors: Pribble R. Nathaniel , Hagemeister Fredrick C. , Zwier Timothy S.
Published: July 2002

Resonant ion-dip infrared spectroscopy has been employed to record cluster-size-specific spectra of C6H6–(CH3OH)m with m=1–6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogen-bonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm−1 due to π hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm−1. The lower frequency peak is redshifted from the free monomer value by 175 cm−1 and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm−1 due to a strengthened π hydrogen bond. In benzene–(CH3OH)3, three sharp OH stretch transitions are observed at 3389, 3435, and 3589 cm−1. The comparison of these absorptions with ab initio calculations and with experiments on the pure methanol trimer leads to a structure for benzene–(CH3OH)3 which incorporates a π hydrogen-bonded methanol trimer chain, confirming the earlier assignment based on its ultraviolet spectrum. The 3589 cm−1 transition, due to the π hydrogen bond of the terminal methanol, is redshifted from the free monomer by 93 cm−1, a value approaching that of the donor methanol in methanol dimer (−107 cm−1). The lower frequency transitions in the m=3 spectrum arise from the donor–acceptor and donor OH stretches in the methanol trimer chain. The spectral characteristics change when m=4. The OH stretch transitions are all located in a region around 3320 cm−1 and are significantly broadened compared to the smaller clusters. By comparison with ab initio calculations, the methanol tetramer structure in benzene–(CH3OH)4 is deduced to be a cyclic methanol tetramer. The spectra for m=5 and 6 are slightly redshifted but similar to m=4 and point toward cyclic structures as well.
Energy switching approach to potential surfaces. II. Two-valued function for the water molecule
Authors: Varandas A. J. C.
Published: July 2002

A recently proposed energy switching scheme is used to improve the two-valued many-body expansion potential energy surface of Murrell, Carter, Mills, and Guest [Mol. Phys. 42, 605 (1981)] for H2O by merging it with the spectroscopically accurate polynomial-type form of Polyanski, Jensen, and Tennyson [J. Chem. Phys. 105, 6490 (1996)]. An attempt is also made to improve its long range forces, and Coulombic behavior at the collapsed molecular limits. The resulting ES two-valued surface has almost spectroscopic accuracy up to 13 650 cm−1, and like the original many-body expansion form may be used for studies of reaction dynamics. A brief analysis of the Σ–Π locus of conical intersection is also presented.
A tensor formulation of many-electron theory in a nonorthogonal single-particle basis
Authors: Head-Gordon Martin , Maslen Paul E. , White Christopher A.
Published: July 2002

We apply tensor methods to formulate theories of electron correlation in nonorthogonal basis sets. The resulting equations are manifestly invariant to nonorthogonal basis transformations, between functions spanning either the occupied or virtual subspaces of the one-particle Hilbert space. The tensor approach is readily employed in either first or second quantization. As examples, second-order Mo/ller–Plesset perturbation theory, and coupled cluster theory with single and double substitutions, including noniterative triples, are recast using the tensor formalism. This gives equations which are invariant to larger classes of transformations than existing expressions. Procedures for truncating these equations are discussed.
A quantum molecular dynamics study of the properties of NO+(H2O)n clusters
Authors: Ye Ling , Cheng Hai-Ping
Published: July 2002

The structures and dynamics of NO+(H2O)n, with n=1,2,3, have been studied using first principles Born–Oppenheimer molecular dynamics (BOMD) performed in the framework of density functional theory (DFT) with a generalized gradient approximation (GGA). The ground-state structure of NO+(H2O), in which a relatively weak bond connects NO+ and H2O, is shown to be floppy along certain degrees of freedom. When a second water molecule is added, a new solvation shell is formed via a hydrogen bond. Our investigations indicate that a third water molecule attaches to the first water molecule and completes the second solvation shell. The hydration energies are found to be 1.31, 0.87, and 0.77 eV for n=1,2,3, respectively. The vibrational spectra at room temperature are calculated for NO+, and all three hydrated clusters. Compared to an isolated NO+ ion, a redshift of 120–200 cm−1 is observed for the N–O vibrational mode in NO+(H2O)n. For n=2, new peaks, identified as O–H stretches of the first H2O molecule, appear below the O–H stretch in the second H2O molecule. The spectrum of NO+(H2O)3, which maintains the most important features in NO+(H2O)2, indicates the presence of a complete solvation shell. Our studies suggest that the BOMD method is an efficient method for finding the optimal geometry of NO+(H2O)n clusters. More importantly, BOMD simulations allow for studies of dynamical and thermodynamical properties of these clusters at finite temperature, which mimics the physical conditions in which these clusters are found in nature and in the laboratory.
Homonuclear radio frequency-driven recoupling in rotating solids
Authors: Bennett Andrew E. , Rienstra Chad M. , Griffiths Janet M. , Zhen Weiguo , Lansbury Peter T. , Griffin Robert G.
Published: July 2002

We discuss several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic-angle spinning (MAS) experiments. These include the accurate measurement of weak dipole–dipole couplings between rare spins, the behavior of dipolar trajectories in multiple spin environments, and chemical shift correlation spectroscopy via polarization exchange. To describe dipolar trajectories accurately, we adopt an approach to the simulation of these experiments which includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach is outlined for the selection of decay parameters. Dipolar trajectories are shown to be dominated by the largest couplings in multiple spin systems via comparison of two and three interacting spins. Two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei is illustrated with a uniformly N15,C13-labeled sample of the tetrapeptide achatin-II (Gly-L-Phe-L-Ala-L-Asp). In addition, a frequency-selective approach to recoupling dipolar interactions among homonuclear spins is introduced; selective approaches have possible utility in examining weak dipole–dipole couplings in the presence of strong interactions.
Molecular dynamics algorithms for path integrals at constant pressure
Authors: Martyna Glenn J. , Hughes Adam , Tuckerman Mark E.
Published: July 2002

Extended system path integral molecular dynamics algorithms have been developed that can generate efficiently averages in the quantum mechanical canonical ensemble [M. E. Tuckerman, B. J. Berne, G. J. Martyna, and M. L. Klein, J. Chem. Phys. 99, 2796 (1993)]. Here, the corresponding extended system path integral molecular dynamics algorithms appropriate to the quantum mechanical isothermal–isobaric ensembles with isotropic-only and full system cell fluctuations are presented. The former ensemble is employed to study fluid systems which do not support shear modes while the latter is employed to study solid systems. The algorithms are constructed by deriving appropriate dynamical equations of motions and developing reversible multiple time step algorithms to integrate the equations numerically. Effective parallelization schemes for distributed memory computers are presented. The new numerical methods are tested on model (a particle in a periodic potential) and realistic (liquid and solid para-hydrogen and liquid butane) systems. In addition, the methodology is extended to treat the path integral centroid dynamics scheme, [J. Cao and G. A. Voth, J. Chem. Phys. 99, 10070 (1993)], a novel method which is capable of generating semiclassical approximations to quantum mechanical time correlation functions.
Cs
(6D
3/2
)+
H
2
→CsH+H
reaction. IV. Rotationally resolved total cross sections
Authors: Cavero V. , L’Hermite J.-M. , Rahmat G. , Vetter R.
Published: July 2002

The Cs(6D3/2)+H2→CsH [X 1Σ+(v″=0,1; J″)]+H photochemical reaction has been studied in a crossed-beam experiment with electronic excitation of Cs atoms and laser-induced fluorescence detection of CsH products. The reactive cross section is measured at a collision energy of 0.09 eV: σ(6D3/2)=0.04×10−16 cm2. The rotational distributions of CsH products measured on v″=0 for 0⩽J″⩽23 and on v″=1 for 0⩽J″⩽15 are close to statistical ones with 16% of products in v″=1. A quasi-classical 3D trajectory calculation of this reaction yields both rotational and vibrational distributions of CsH products and their angular scattering probability. The reactive cross section measured with Cs(6D3/2) excitation is an order of magnitude smaller than the cross section σ(7P1/2)=0.6×10−16 cm2, which is re-evaluated in this article.
Electron-impact total ionization cross sections of CF4, C2F6, and C3F8
Authors: Nishimura H. , Huo Winifred M. , Ali M. A. , Kim Yong-Ki
Published: July 2002

Both theoretical and experimental electron-impact total ionization cross sections of CF4, C2F6, and C3F8 are presented. The experimental cross sections have been measured as a function of incident electron energy T from threshold to 3 keV. A parallel plate condenser type apparatus was used. The molecular polarizability for C3F8 was empirically estimated to be α=10.6 Å3±0.8 Å3. Theoretical cross sections calculated from the binary-encounter-Bethe (BEB) method, which combines a modified form of the Mott cross section and the Bethe cross section, are compared with the experimental cross sections. The BEB cross sections calculated from correlated molecular wave functions with theoretical estimates for multiple ionization are about 10% higher than the experimental data at the peak for CF4, while they are in excellent agreement with the experimental data for C2F6 and C3F8. Our analysis shows that the BEB theory implicitly includes part of neutral dissociation, such as CF4→CF3+F, and hence tends to be an upper limit to the total ionization cross section. We found that the difference between our best theory for CF4 and the present experimental cross section exhibits a remarkable similarity to the shape of the recently measured cross section for neutral dissociation, though there is no a priori reason for the similarity. Owing to the large number of bound electrons, the correlation included in our wave functions for C2F6 and C3F8 is more limited than for CF4. Hence, we believe that for these two molecules the calculated cross sections are lower than the true BEB values, in spite of the apparent excellent agreement between the theory and the experiment.
Chemical dynamics of the reaction K*(5p 2P)+H2→KH(v=0;J)+H: Electronic orbital alignment effects
Authors: Wong T.-H. , Kleiber P. D. , Yang K.-H.
Published: July 2002

We report results from scattering state spectroscopic studies of the excited state reaction K*(5p 2P)+H2→KH(v″,J″)+H. The final state resolved action spectra allow a direct measurement of essential features of the excited state potential surfaces, including regions of local maxima and minima. We observe a pronounced blue-wing–red-wing asymmetry in the reactive to nonreactive branching ratio, peaking in the neighborhood of a strong blue wing satellite. These results show that the dominant reaction pathway passes over a small activation barrier (350±100 cm−1) in Σ+-like orbital alignment. This result is consistent with an electron jump mechanism through a K+H−H ion-pair intermediate. In contrast, approach in Π-like alignment leads predominantly to nonreactive scattering. Our results suggest that a combination of steric and energetic effects determine the major quenching pathways for alkali metal atom-H2 systems.
Polymer stretching in an elongational flow
Authors: Neumann Richard M.
Published: July 2002

Using a constant “velocity gradient” ensemble approach, the average scalar end-to-end separation is calculated as a function of the gradient for an ideal Gaussian polymer chain experiencing longitudinal elongational flow under steady-state conditions. The resulting equation, based on a dumbbell model, exhibits an initial average end-to-end separation equal to the unperturbed random coil value; the separation increases monotonically with increasing velocity gradient, in agreement with recent experimental measurements and with a classical treatment based on a diffusion equation. This approach is contrasted with one based on Hooke’s law where the end separation remains zero with increasing gradient until a critical value of the gradient is reached at which point the chain suddenly expands to an appreciable fraction of its fully extended length.
Shear-induced optical anisotropy in a Langmuir monolayer: A Brewster angle reflectivity study
Authors: Cuvillier Nicolas , Mingotaud Christophe , Ikegami Keiichi
Published: July 2002

Shearing an initially isotropic Langmuir film by a rotating disk can lead to an optical anisotropy at the interface. This induced anisotropy is related to a local in-plane molecular alignment due to the shearing and was experimentally observed using a monolayer based on a liquid crystal polyacrylate. The variation of the anisotropy versus shearing was modelized and measured by Brewster angle microscopy. The results reveal that the optical property of such a monolayer is similar to that of a two dimensional nematic liquid crystal.
Principles of centerband-only detection of exchange in solid-state nuclear magnetic resonance, and extension to four-time centerband-only detection of exchange
Authors: deAzevedo Eduardo R. , Hu W.-G. , Bonagamba Tito J. , Schmidt-Rohr Klaus
Published: July 2002

Theoretical principles and experimental details of the centerband-only detection of exchange (CODEX) nuclear magnetic resonance (NMR) experiment for characterizing slow segmental dynamics in solids are described. The experiment, which is performed under magic-angle spinning, employs recoupling of the chemical-shift anisotropy before and after a long mixing time during which molecular reorientations may occur. By an analysis in terms of the difference tensor of the chemical shifts before and after the mixing time, the dependence on the reorientation angle is obtained analytically for uniaxial interactions, and a relation to two-dimensional exchange NMR patterns is established; the same theory can also be applied for analyzing stimulated-echo and pure-exchange NMR data. A favorable linear dependence is derived generally for small rotations, which makes the experiment suitable for detecting small-amplitude motions. Quantification is excellent because the peaks are narrow and intense, unlike the broad powder or sideband spectra that are characteristic of all previous NMR experiments for probing slow segmental rotations. We also introduce and demonstrate a four-time CODEX experiment that yields information previously obtained only in 3D (three-dimensional) and reduced 4D (four-dimensional) exchange NMR experiments, such as the number of orientational sites accessible to the mobile groups. Chemical-shift anisotropies required in the CODEX analysis of motional amplitudes can be estimated using a closely related chemical-shift recoupling experiment. The implementation of total suppression of sidebands before detection is also explained. The experiments are demonstrated on dimethylsulfone, isotactic polypropylene, and poly(methyl methacrylate), PMMA. In isotactic poly(1-butene), the signals of the amorphous and interfacial regions have been observed selectively by using pure-exchange CODEX near the glass transition. The four-time CODEX experiment confirms that in the β-relaxation process of glassy PMMA, fewer than half of the sidegroups perform jumps between two orientations.
Direct measurement of ion evaporation kinetics from electrified liquid surfaces
Authors: Gamero-Castaño M. , Fernández de la Mora J.
Published: July 2002

When concentrated solutions of NaI in formamide with electrical conductivities K larger than 1.1 S/m are electrosprayed from a Taylor cone-jet in a vacuum, ions are evaporated at substantial rates from the surface of the meniscus and the drops. This constitutes a new source of ions and nanoparticles, where the relative importance of these two contributions is adjustable. The currents of ions are measured independently from those associated with drops by a combination of stopping voltage analysis and preferential scattering in a gas background. The magnitude E of the electric field at the surface of the drops and at the apex of the cone-jet is controlled through the electrical conductivity K of the liquid and its flow rate Q through the jet. E is related through available scaling laws for Taylor cone-jets to the ratios K/Q or I/Q, where I is the current of drops emitted by the jet. Ion currents are very small or null at typical K/Q values used in the past. A relatively small initial ion current is attributed to a few particularly sharp features present, perhaps associated with small satellite drops. At still higher K/Q this first ionization source saturates, and ion evaporation from the main drops begins to dominate (E∼1 V/nm). E can then be determined with little ambiguity, and the associated ion current is also measured over a broad enough range of electric fields to determine the ionization kinetics. At still higher K/Q the ion current from the drops approaches saturation, and ion evaporation directly from the meniscus becomes dominant. The total spray current then presents the anomaly of increasing rapidly at decreasing liquid flow rate. The ion current from the meniscus can also be measured in this regime over a broad range of K/Q, with qualitative agreement with the ionization measurements from the drops. But the relation established between K/Q and E becomes suspect because ion and drop currents are now comparable. A third approach to infer the ionization rate is based on the related disappearance of Coulomb explosions of the drops above a critical K/Q. These results are congruent with the model of Iribarne and Thomson, with an activation barrier for ion evaporation equal to 1.7 eV−(e3E/4πε0)1/2.
The Courant–Hilbert solutions of the wave equation
Authors: Hillion Pierre
Published: July 2002

The Bateman transformations which generate many new solutions from a particular solution of the scalar wave equation in a homogeneous medium are discussed. The Bateman transformations are then used to classify the Courant–Hilbert solutions of the wave equation. The latter are the product of an arbitrary function F of the phase u, a solution of the characteristic equation, and of a specific attenuation factor f that does not depend on the particular form of F. It is shown that for a given phase u, f is not unique. Consequently, there exist spherical waves F(r±ct) with an attenuation factor different from r−1.
Spectral functions and zeta functions in hyperbolic spaces
Authors: Camporesi Roberto , Higuchi Atsushi
Published: July 2002

The spectral function (also known as the Plancherel measure), which gives the spectral distribution of the eigenvalues of the Laplace–Beltrami operator, is calculated for a field of arbitrary integer spin (i.e., for a symmetric traceless and divergence-free tensor field) on the N-dimensional real hyperbolic space (HN). In odd dimensions the spectral function μ(λ) is analytic in the complex λ plane, while in even dimensions it is a meromorphic function with simple poles on the imaginary axis, as in the scalar case. For N even a simple relation between the residues of μ(λ) at these poles and the (discrete) degeneracies of the Laplacian on the N sphere (SN) is established. A similar relation between μ(λ) at discrete imaginary values of λ and the degeneracies on SN is found to hold for N odd. These relations are generalizations of known results for the scalar field. The zeta functions for fields of integer spin on HN are written down. Then a relation between the integer-spin zeta functions on HN and SN is obtained. Applications of the zeta functions presented here to quantum field theory of integer spin in anti-de Sitter space–time are pointed out.