bluuebunny/crossref_metadata_2025 huggingface dataset:
Superselectivity and solvation forces of a two component fluid adsorbed in slit micropores
Authors: Somers Susan A. , McCormick Alon V. , Davis H. Ted
Published: July 2002

We use the grand canonical Monte Carlo simulation technique to calculate adsorption of mixtures in molecularly narrow slit pores immersed in a two-component bath of spherical molecules that are different in size. The composition of the pore fluid oscillates strongly with the pore width. The oscillations reflect the differing ability of each molecule to pack as layers in the pore. Even in pores wide enough to admit both components, this difference in packing ability leads to a shut-out of the smaller component. Trends in the calculated solvation force agree with both experiment and theory.
Vibrational ab initio calculations and spectra of C–H bonds of trimethylboron
Authors: Manzanares I Carlos , Blunt Victor M. , Peng Jingping
Published: July 2002

Visible laser photoacoustic and near-IR spectra of the overtones of the C–H stretches of (CH3)3B in the gas phase are reported. Two bands are assigned to nonequivalent methyl C–H bonds. The interaction of an empty 2p orbital of the boron atom with the C–H bonds of a methyl group changes the strength of the C–H bonds during the internal rotation. The most intense, higher energy absorption band in each overtone region is assigned to the CH bonds in the molecular plane (C–H∥) and the least intense, lower energy absorption band to the CH bonds out of the molecular plane (C–H⊥). To interpret the experimental results, overtone transitions are described in terms of the local mode model. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to particular transitions. Ab initio molecular orbital calculations were also performed. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartree–Fock level using the 3–21G and 6–31G* split valence basis set. Several geometries were calculated. The minimum energy corresponds to a geometry in which the CH3 groups are aligned with a single CH bond in the molecular plane (C3h symmetry). Another geometry in which two CH3 groups are aligned with a C–H bond in the plane of the molecule and one CH3 group is aligned with a C–H bond perpendicular to the molecular plane is found to be the saddle point. A third geometry in which each CH3 group has a single CH bond aligned perpendicular to the molecular plane (C3v symmetry) is higher in energy than the first two and does not correspond to a minimum. Calculations were performed on the deuterated molecule (CHD2)B(CD3)2 using the most stable C3h conformation and the conformation of the saddle point. In this way, the isolated C–H∥ and C–H⊥ bond lengths, the corresponding C–H stretching force constants, and the vibrational frequencies were obtained.
Dynamics of molecular surface diffusion: Energy distributions and rotation–translation coupling
Authors: Dobbs Kerwin D. , Doren Douglas J.
Published: July 2002

Surface diffusion rates have been simulated using classical molecular dynamics in a model of CO adsorbed on Ni(111). This paper describes the energy distribution among adsorbate modes at the transition state, energy relaxation after crossing the transition state, and correlations among adsorbate modes near the transition state. The adsorbate bending (frustrated rotation) mode is strongly coupled to lateral translational motion. This molecular mode provides an important source of energy for reaching the transition state to diffusion, and an important frictional force that dissipates excess lateral translational energy. In this model, the molecular bending mode is a more important source (and sink) of lateral translational energy than the surface at short times. This result is interpreted as a consequence of directional bonding to the surface, and it should be generally important in surface diffusion of chemisorbed molecules.
On the use of acceptance ratio methods in free energy calculations
Authors: Ferguson David M.
Published: July 2002

Molecular dynamics free energy simulations have been performed on three solvated systems to examine the potential benefits of Bennett’s acceptance ratio (AR) method [Bennett, J. Comput. Phys. 22, 245 (1976)], as compared to standard statistical perturbation techniques (FEP). The AR sampling equation has been implemented along with double-ended FEP sampling to calculate the relative solvation free energy of neon/nothing, neon/TIP3P-water and phenol/benzene in periodic TIP3P water. A histogram method is also introduced to calculate the overlap in the double-ended samples 〈E1−E0〉0 and 〈E0−E1〉1 from the density functions, p0(ΔE) and p1(ΔE), allowing window increments (δλ) to be correlated with % overlap and accuracy in the calculations. This relatively new concept is based on the fact that accuracy in both AR and FEP methods can be shown to depend on the overlap. The results indicate that the AR methodology has some advantages for calculating free energies when sample sizes are limited and minimal overlap exists in the ensemble samples at each window, mainly in reductions to the statistical errors. However, as more extensive simulations revealed, if the overlap in the density functions is increased (through reductions in window sizes), the simple FEP averages and variances approach near parity with the AR ‘‘best estimate’’ results, indicating that the methods are effectively the same; with the caveat of adequate sampling.
Platinum-induced morphology and reactivity changes on W(111)
Authors: Dong C. Z. , Shivaprasad S. M. , Song K.-J. , Madey T. E.
Published: July 2002

The structure and CO chemisorption properties of a model bimetallic system Pt on W(111) have been studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and thermal desorption spectroscopy (TDS). The results reveal that upon annealing, the Pt/W(111) system undergoes a variety of structural transformations, with the most striking being the reconstruction of the W(111) substrate to form pyramidal facets with {211} orientations. The presence or absence of facets, and size distribution of the facets, are sensitive functions of Pt coverage and annealing temperature. LEED results for faceted surfaces annealed to different temperatures correlate well with the STM data. CO has been used as a probe molecule to examine the chemisorption properties of the different structural phases. Increasing Pt coverage causes site blocking that results in reduced dissociative adsorption of CO.
Theoretical study of the highly vibrationally excited states of FHF−: Ab initio potential energy surface and hyperspherical formulation
Authors: Yamashita Koichi , Morokuma Keiji , Leforestier Claude
Published: July 2002

A three-dimensional description of vibrationally highly excited linear molecules is formulated in hyperspherical coordinates, based on a successive adiabatic reduction scheme. The method is applied to the low-lying and highly excited vibrational states of FHF−, a prototype of symmetric bihalide anions, which has attracted spectroscopic interest due to its peculiar vibrational anharmonicity. Ab initio potential energy surfaces (PESs) which cover the ground-state potential well of FHF− and/or its dissociation to the F−+HF channel have been obtained by using the coupled electron pair approach (CEPA) method. An hyperspherical calculation using the ab initio PES of the sixth-order Simons–Parr–Finlan analytical form has correctly reproduced the experimental fundamental frequencies. Specifically, the vibrationally highly excited FHF− above the dissociation threshold is proposed as a candidate for transition state spectroscopy (TSS) of unimolecular dissociation reactions without barrier.
The intermolecular vibrations of Ar–styrene and Ar–4-fluorostyrene complexes
Authors: Consalvo D. , Avoird A. van der , Piccirillo S. , Coreno M. , Giardini-Guidoni A. , Mele A. , Snels M.
Published: July 2002

One-color (1+1) resonance enhanced multiphoton ionization (REMPI) spectra are reported for styrene–Arn clusters with n=2, 3, and for 4-fluorostyrene–Arn clusters with n=2–5. These spectra are compared with previously recorded spectra of the mono-Ar complexes and discussed in relation to the structures of the clusters. For the mono-Ar complexes we have performed practically exact quantum calculations of the van der Waals vibrational frequencies and properties, starting from two different empirical atom–atom potentials. The intermolecular potentials are strongly anharmonic and, due to the low symmetry of these dimers, we find considerable mode mixing. As a consequence of the kinematic coupling between the Ar motion and the internal rotation, the bending frequencies depend considerably on the different rotational constants of the molecules. The order of the fundamental frequencies is the same for both dimers. For Ar–4-fluorostyrene the calculated vibrational frequencies agree well with the observed spectrum; the van der Waals side bands can thus be assigned in detail. For Ar–styrene the observed frequencies are less well reproduced, so we must conclude that the atom–atom potential used is substantially better for Ar–fluorostyrene than for Ar–styrene.
Particle exchange in the Fourier path-integral Monte Carlo technique
Authors: Chakravarty Charusita
Published: July 2002

A formalism which incorporates the effects of particle exchange in the Fourier path integral Monte Carlo framework is developed. Expressions for thermal averages of static quantities as well as the kinetic and total energy for identical-particle systems are derived. For bosons, the procedure for evaluating thermal averages is analogous to that in the discretized path integral approach. For fermion systems, it is shown that the critical ‘‘negative sign’’ problem can be substantially reduced by defining a weight function that sums over all possible permutations. The algorithm developed is applied to two types of systems, noninteracting identical particles (bosons and fermions) in a harmonic potential and a model atomic cluster of interacting, spinless bosons.
Quantum scattering calculations on H2O+H→H2+OH and isotopes: Rotational distributions and cross sections
Authors: Nyman Gunnar , Clary David C.
Published: July 2002

Quantum scattering calculations on the reactions of H atoms with H2O, D2O, and HOD have been performed at energies up to 2.7 eV. The rotating bond approximation is used. For H+H2O, this method explicitly treats the OH product rotation and H2 product vibration as well as the bending motion and a local stretch of the reactant H2O. Calculated rotational distributions of the OH and OD products are compared with several measured distributions and good agreement is found. Calculated absolute cross sections compare quite well with measured results. Also, computed differential cross sections for the OH+D2→D+HOD reaction are found to be in excellent agreement with measured ones.
Vibrational assignments in the 3p Rydberg states of acetone
Authors: Xing Xing , McDiarmid Ruth , Philis John G. , Goodman Lionel
Published: July 2002

The 2 and 3 photon resonantly enhanced multiphoton ionization spectra of the 3p Rydberg ←X̃ transitions of acetone and acetone-d6 were remeasured and reanalyzed. Seven of the eight a1 and one b1 vibrational modes were assigned. On the basis of these assigned fundamental frequencies the anomalous isotope shift of the origin of the A1 transition was rationalized, the contribution of the nonbonding electrons to the scissors force constant was demonstrated, and a location for the A1 ππ* valence excited transition was proposed.
The vacuum ultraviolet photodissociation of the chlorofluorocarbons. Photolysis of CF3Cl, CF2Cl2, and CFCl3 at 187, 125, and 118 nm
Authors: Yen Mei-wen , Johnson Philip M. , White Michael G.
Published: July 2002

Photofragmentation of the chlorofluorocarbons, CF3Cl, CF2Cl2, and CFCl3, was investigated at 187, 125, and 118 nm using VUV harmonic generation techniques and (2+1) resonantly enhanced multiphoton ionization detection of ground Cl(2P3/20) and excited Cl*(2P1/20) state fragment atoms. Product translational energy and angular distributions were derived from Cl+ arrival time distributions obtained by time-of-flight mass spectrometry. Photolysis of CF3Cl at 125 and 118 nm takes place via the 4s(a1) and 4p(e) Rydberg states, respectively, and two primary fragmentation channels are observed. A ‘‘slow’’ channel with a most probable center-of-mass (c.m.) translational energy near zero is assigned to the production CF*3 radicals in the 2A′1, 2A″2, and 1E′ electronically excited states. The second Cl/Cl* fragmentation channel has a c.m. translational energy distribution peaked at Etr≥1 eV and is tentatively assigned to a sequential dissociation process in which rapid C–Cl single bond rupture is followed by a secondary fragmentation of CF*3 to CF2+F. The time-of-flight (TOF) spectra for CF2Cl2 and CFCl3 following excitation at 125 and 118 nm suggest that concerted three-body fragmentation involving the loss of two Cl/Cl* atoms is the dominant dissociation process. By contrast, photolysis of CF2Cl2 and CFCl3 at 187 nm results in structured Cl+ arrival time distributions which are used to derive translational energy distributions and asymmetry parameters. Simulations of the TOF spectra suggest the presence of three Cl/Cl* fragment channels, with the highest energy channel clearly attributable to single C–Cl bond rupture leading to internally excited molecular fragments. The contribution of sequential and simultaneous two-Cl loss processes to the low translational energy channels is also discussed.
Interaction effects on the size of a polymer chain in ternary solutions: A renormalization group study
Authors: Schäfer Lothar , Kappeler Christian
Published: July 2002

We calculate the radius of gyration of a polymer coil in a solution containing two different polymer species. The crossover scaling function is determined by renormalized perturbation theory to the order of one loop. Ternary effects manifest themselves most strongly in the crossover and lead to a breakdown of simple power law type analysis. We thus evaluate our result not only in limiting fixed point regions but also present typical examples of crossover behavior. Outside the asymptotic good solvent region ternary effects are large and very sensitive to the value of the intraspecies coupling. This may have consequences for the interpretation of neutron scattering data from mixtures of protonated and deuterated chains.
Spontaneous formation of vesicles by weakly charged membranes
Authors: Kumaran V.
Published: July 2002

The formation of vesicles by an amphiphilic bilayer at equilibrium due to spontaneous curvature and due to the difference in the charge densities in the two layers is analyzed. In the latter case, there is a decrease in the electrostatic energy when the membrane forms a vesicle with the surface having the higher charge density placed on the outside, and this could compensate for the increase in the bending energy of the membrane. If the charge densities on the two surfaces are fixed, the decrease in the electrostatic energy per vesicle is proportional to the radius of the vesicle in the Debye–Hückel approximation. If the charged molecules are permitted to flip from one surface to the other, there is an entropic increase in the free energy when there are different charge densities on the two sides, and the total decrease in the energy per vesicle is independent of the vesicle size. The distribution of vesicle radii is calculated by minimizing the total energy subject to the constraint that the area of membrane per unit volume is a constant. We find that it is possible to produce a narrow size distribution of vesicles which are much larger than the amphiphiles when the charge densities on the two sides are fixed, and there is a sharp transition from flat membranes to vesicles in this case. In the case of membranes with charges that are permitted to flip from one layer to the other and membranes stabilized by spontaneous curvature, the membrane area fraction that forms vesicles is small when the size of a vesicle is large compared to the size of an amphiphile.
Ab initio study of the H2CO–Ar complex
Authors: Sadlej Joanna , Szczȩśniak M. M. , Chal/asiński G.
Published: July 2002

The potential energy surface of the H2CO–Ar complex is calculated at the second-order Mo/ller–Plesset perturbation theory and analyzed using the perturbation theory of intermolecular forces. The equilibrium geometry (De=171 cm−1) involves a T-shape structure with the Ar atom nearly perpendicular to the C–O bond of H2CO and in the molecular plane of H2CO. The equilibrium configuration results from a minimum in the exchange repulsion. It is conceivable that H2CO undegoes a hindered internal rotation in the complex. A barrier to such a motion is estimated at 32 cm−1. The potential energy surface is very complex. The anisotropy of the surface involving the in-plane motion of Ar is very high and resembles that of the Ar–H2O complex. The anisotropy involving motion of Ar perpendicular to the molecular plane of H2CO is much weaker, and it is strikingly similar to that of the Ar–CO complex.
The effects of molecular geometry on the depolarized stimulated gain spectra of simple liquids
Authors: Friedman J. S. , She C. Y.
Published: July 2002

Depolarized Rayleigh-wing spectra of simple liquids, including CS2, chlorinated methanes, benzene and benzene derivatives have been recorded using stimulated gain spectroscopy. To adequately interpret these spectra of simple liquids, a dephasing dynamical contribution in an intermediate time scale is needed in addition to a slower Debye reorientation relaxation and a faster broad inhomogeneous oscillator. The curve-fit dynamical characteristic parameters of these simple liquids are tabulated for comparison. The microscopic molecular properties, shape anisotropy, and dominating moment of inertia are found to play an important role in determining the rates of interaction-induced dynamics; smaller shape anisotropy and smaller moment of inertia give rise to faster collision rate in the intermediate frequency regime and faster oscillation in the higher frequency regime, respectively. In the low frequency region, the shear viscosity (a macroscopic property) of the liquid and the general molecular shapes dictate reorientation diffusion; for molecules within a family, the Debye relaxation rate is faster for liquids with smaller viscosity and shape anisotropy. Halogenation of benzene breaks the symmetry of the benzene ring giving two distinct oscillator frequencies. In the case of iodobenzene, three principle molecular axes are clearly distinct; it is necessary to use two Debye relaxation rates and two broad inhomogeneous oscillators to fit its stimulated gain spectrum.
CH+H2 reaction kinetics: Temperature and pressure dependence and Rice–Ramsperger–Kassel–Marcus-master-equation calculation
Authors: McIlroy Andrew , Tully Frank P.
Published: July 2002

We investigate the reaction of CH(X 2Π) with H2 as a function of temperature in the range 240–470 K at 8.2 and 750 Torr of helium pressure and as a function of helium pressure in the range 8–750 Torr at 294 K. Methylidyne forms upon excimer-laser photolysis of CHBr3 or CHClBr2 in a slow-flow reactor and we time-resolve its concentration profile using cw laser-induced fluorescence. The title reaction proceeds through the formation of an excited methyl radical with multiple open decay channels. We observe dramatically different temperature dependencies at high and low helium bath-gas pressures. At high pressure, collisional stabilization of CH*3 to CH3 dominates the reaction mechanism and the CH-loss rate constant exhibits a negative temperature dependence over the range studied. At low pressure, the predominant product channel switches from CH3 to CH2+H as the temperature increases. We employ a Rice–Ramsperger–Kassel–Marcus-master-equation calculation to model the experimental results.
Franck–Condon factors for photodetachment from LiO−, NaO−, and KO−
Authors: Bauschlicher Charles W. , Partridge Harry , Pettersson Lars G. M.
Published: July 2002

The 1Σ+, 3Σ+, 1Π, and 3Π states of the negative ions and the 2Π and 2Σ+ states of the neutral alkali oxides are studied at high levels of theory. The calculations show that ground state of the negative ions changes from 3Π for LiO− to 1Σ+ for KO−. Although the calculations give a 3Π ground state for NaO−, we cannot rule out the possibility that the very low-lying 1Σ+ state is the true ground state. The Franck–Condon factors for photodetachment of an electron from the 1Σ+ and/or 3Π states of the negative ion are presented to help interpret photodetachment experiments. Our best results for the A 2Σ+–X 2Π separations in LiO and NaO are 2496 and 2061 cm−1, which are in excellent agreement with that deduced (2516 and 2018 cm−1) from experiment.
The influence of classical resonances on quantum energy levels
Authors: Ramachandran B. , Kay Kenneth G.
Published: July 2002

In this paper, we examine the behavior of the quantum energy levels of a coupled oscillator system as the zero-order frequencies are varied to carry the corresponding classical system through a resonance. We find that the levels exhibit a pattern that is characteristic of the resonance. This pattern consists of clusters of levels, each containing a number of curves that run roughly parallel to one another and a number of curves that undergo pairwise narrowly avoided crossings. Adiabatic switching calculations show that the ‘‘parallel’’ curves are associated with states within the classical resonance region, while the narrowly avoiding curves are associated with states that are outside this region. It is further shown that the curves describing resonance states are formed from zero-order nonresonant curves by the overlap of many avoided crossings. This reorganization of multiply intersecting lines into parallel curves reflects the classical reorganization of phase space at a resonance.
Theoretical study of the vibronic structure of the 1 1A1→1 1B2 and 1 1A1→1 3B2 electronic transitions in cyclopentadiene
Authors: Zgierski Marek Z. , Zerbetto Francesco
Published: July 2002

The geometry of the ground and excited singlet and triplet 1B2 electronic states of cyclopentadiene is optimized by ab initio (configuration interaction singles) and semiempirical (quantum chemical force field) methods. The optimized structures provide displacement parameters of the totally symmetric modes for the singlet→singlet and singlet→triplet transitions. The calculated singlet→singlet absorption spectrum for cyclopentadiene and its fully deuterated isotopomer agree very well with the observed spectra. It is demonstrated that the difference of these spectra from those of a linear polyene (butadiene) results from the strong Franck–Condon activity of the lowest frequency a1 mode which describes CCC bending. It is shown that the properties of the triplet and singlet 1B2 states differ greatly as a result of their different nature; the former being mostly covalent, whereas the latter is mostly ionic. This leads to different Franck–Condon structures of the 1 1A1→1 1B2 and 1 1A1→1 3B2 transitions. The ionic nature of the singlet 1B2 state leads to its inherent instability with respect to the out-of-plane b1 distortion.
The relativistic valence shell effective Hamiltonian method for atomic and molecular systems
Authors: Shukla Alok , Banerjee Ajit
Published: July 2002

A new ab initio method for fully relativistic (four-component Dirac many-body formalism) correlated electronic structure calculations, which makes explicit reference to valence electrons only, is presented for atomic and molecular processes in which the core electrons remain inert. The method exactly reproduces all properties (wave functions, energies, etc.) of an all electron calculation with frozen core wave functions and thus provides a great computational advantage. The method is based on constructing a valence Hamiltonian which includes a one-electron effective potential for the core–valence interactions. This interaction potential, which needs to be calculated once and for all, is exact for a chosen basis since it is constructed (all constituent local and nonlocal interactions) in matrix form in the current basis. This potential is used subsequently in valence-electron-only calculations, based on a novel algorithm, by constructing explicitly valence orbitals orthogonal to the core orbitals as linear combinations only among themselves. It has no adjustable parameters and no restrictions on the choice of basis sets. It yields considerable savings of computer time and space over the corresponding frozen core all-electron calculations. Dirac-Fock SCF calculations are presented for the ground states of closed- and open-shell atoms Li, Be, B, C, N, O, F, F−, Ne, Na+, Na, Al, and S for demonstration.
Novel semiempirical method for quantum Monte Carlo simulation: Application to amorphous silicon
Authors: Tóth Gergely , Náray-Szabó Gábor
Published: July 2002

We propose a novel bulk phase Monte Carlo simulation technique, in which the energy is calculated by quantum mechanical methods. The semiempirical fragment self-consistent field technique applied divides the periodic simulation cell into two parts. The first is the subsystem where the important change (the random movement of an atom or molecule) takes place and the second is the environment exerting only secondary effects on the former. Expanding the electronic wave function on the basis of strictly localized molecular orbitals and/or atomic hybrid orbitals the wave function of the environment is obtained from simple coupled 2×2 secular equations. The conventional self-consistent field equations, with a perturbation term in the Fockian, have to be solved only for the subsystem. In this way the computational efforts are decreased drastically, as the dependence on the number of atoms in the environment reduces to quadratic instead of cubic or quartic as in conventional semiempirical or ab initio methods, respectively. We wrote a computer code and applied our method to amorphous silicon. Starting from a distorted tetrahedrally bonded random network model we performed Monte Carlo simulations using the fragment self-consistent field energy calculation. After equilibration we obtained distribution functions almost identical to the ones corresponding to the distortion free tetrahedrally bonded network. This finding confirms the adequacy of our method for this specific case.
Molecular dynamics study of pressure in molecular systems
Authors: Tominaga Tetsuo , Yip Sidney
Published: July 2002

In molecular dynamics simulation of molecular systems, an atomistic model is needed to describe the intramolecular effects on system properties such as pressure. An expression for computing the pressure is derived based on the virial theorem, with explicit kinetic, intra-, and intermolecular contributions. It is shown that the virial terms arising from three- or four-body forces that depend only on internal angles are zero by using the Wilson S vector technique, and that only the two-body forces appear in the pressure expression. Molecular dynamics simulations are carried out for an atomistic model of benzene with intramolecular interactions based on ab initio harmonic potentials and intermolecular interactions given by semiempirical atom–atom potentials. Calculated total pressure depends on parameters of intermolecular potentials. An intramolecular contribution to the pressure is studied for both solid and liquid phases. In solids where the molecules are compressed, the intramolecular pressure contributions are appreciable and positive, while in liquids where molecular deformations are relatively small, the contributions are small and negative. A possible further improvement of the atomistic model of benzene is discussed.
Stability of binary mixtures: Supersaturation limits of aqueous alkali halide solutions
Authors: Ursenbach C. P. , Patey G. N.
Published: July 2002

The stability of ionic binary mixtures is investigated by an integral equation method. In presenting the theory a distinction is made between primary and secondary stability criteria, and this distinction is used to clarify some misconceptions in the literature. The derived stability criteria are then applied to electrolyte solutions as well as to a simple binary mixture. In a simple mixture of hard spheres in water, both mechanical and material instabilities are found near the spinodal line along with evidence of long-range hydrophobic forces. Results for the electrolyte solutions indicate that salts with only large ions, such as CsI, and those with a smaller ion, such as Na+ or K+, behave differently near the spinodal line. CsI acts hydrophobicly, and appears to undergo demixing from the solvent, whereas NaCl and KCl, which bind the solvent more tightly, do not show clear signs of any such demixing, but do appear to become mechanically unstable. Finally, some recent results of Chen and Forstmann [J. Chem. Phys. 97, 3696 (1992)] are discussed and applied to the present systems.
Sub-Doppler high-resolution spectroscopy and the Zeeman effect of CS2 in the near ultraviolet
Authors: Nishizawa Kiyoshi , Kubo Shunji , Doi Atsushi , Katô Hajime
Published: July 2002

A single-frequency autoscan laser spectrometer operating in the ultraviolet in combination with a collimated molecular beam has been used to measure high-resolution fluorescence excitation spectra of the 6V, 10V, and 13V band systems of CS2. The Zeeman spectra at H=210 G are also measured with sub-Doppler resolution. In the 6V band system, 30 530.0, 30 529.2, and 30 530.5 bands, which are named after the wave numbers of the band origins, are identified, and all the bands are found to be Π–Π transitions from the X 1Σ+g 0110 level. The 30 530.0 band, which is the main band of the 6V band system, is found to be perturbed by nearby levels, and the deperturbation analysis is performed. Large Zeeman splittings into 2J+1 components are observed for the 30 529.2 and 30 530.5 bands. From the analysis of the Zeeman splitting and the line intensity, these bands are assigned as the transitions to the 3A2 state. Five series of lines (bands) are classified in the 10V band system, and all the bands are found to be Σ–Σ transitions from the X 1Σ+g 0000 level. Four bands are classified in the 13V band system, and all the bands are found to be Π–Π transitions from the X 1Σ+g 0110 level. The 30 900.5 and 31 199.1 bands are, respectively, the main bands of the 10V and 13V band systems.
The pure rotational spectrum of CaNC
Authors: Scurlock C. T. , Steimle T. C. , Suenram R. D. , Lovas F. J.
Published: July 2002

The pure rotational spectrum of calcium isocyanide, CaNC, in its (0,0,0) X 2Σ+ vibronic state was measured using a combination of Fourier transform microwave (FTMW) and pump/probe microwave-optical double resonance (PPMODR) spectroscopy. Gaseous CaNC was generated using a laser ablation/supersonic expansion source. The determined spectroscopic parameters are (in MHz), B=4048.754 332 (29); γ=18.055 06 (23); bF=12.481 49 (93); c=2.0735 (14); and eQq0=−2.6974 (11). The hyperfine parameters are qualitatively interpreted in terms of a plausible molecular orbital descriptions and a comparison with the alkaline earth monohalides and the alkali monocyanides is given.
Ionization potentials of CH2: A comparison of the multiconfigurational spin tensor electron propagator method with benchmark full configuration interaction and large scale multireference configuration interaction calculations
Authors: Nichols Jeffrey A. , Heryadi Dodi , Yeager Danny L. , Golab Joseph T.
Published: July 2002

Using the same basis sets and geometries as were previously used in ‘‘benchmark’’ full configuration interaction (FCI) calculations we compare the multiconfigurational spin tensor electron propagator method (MCSTEP) with FCI for the vertical ionization potentials (IPs) in CH2 below 19.0 eV. Our results show that MCSTEP using a full valence complete active space MCSCF initial state accurately obtains the lowest several principal vertical ionization potentials. We also determine vertical and adiabatic IPs in CH2 with MCSTEP using larger bases and compare to accurate large scale multireference singles and doubles CI with quadruple excitations estimated via a Davidson correction.
Monte Carlo study of the adsorption layers of hard spheres near large hard spheres of varying radii
Authors: Degrève Léo , Henderson Douglas
Published: July 2002

Monte Carlo simulation results are presented for the fluid density profiles of the inhomogeneous system formed by a large colloidal hard sphere of variable radius immersed in a hard sphere solvent. The behavior of this system should give insight into the contribution of hard core exclusion effects to the structure of colloids. Such exclusion effects are believed to be the source of the oscillatory short range forces, seen in recent experiments, which are not predicted by classical theories. These profiles are reported as a function of the reduced density for inverse radii between zero and unity (in units of the solvent radius). Periodic boundary conditions and the minimum image condition were used only for the solvent hard spheres. They are not used for the large hard sphere because we wish to examine the case where the large sphere is present in zero concentration. Some error is introduced by our ad hoc treatment of the large sphere. We attempt to minimize this error by taking the number of solvent hard spheres in the unit cell to be as large as is practicable. The reliability of this method is tested by comparison of our results with hard sphere radial distribution functions obtained from conventional bulk simulations by considering the case where the large hard sphere has the same diameter as the solvent hard spheres. We also test our method by comparison of the contact values of the density profiles with interpolated values between the known exact values for the case where the colloidal hard sphere has a diameter equal to that of the solvent hard spheres and the case where the colloidal hard sphere is infinitely large. Both tests indicate that our method gives satisfactory results.
Triple intersection of H3 resonance states
Authors: Orel A. E. , Kulander K. C. , Lengsfield B. H.
Published: July 2002

In recent measurements of the cross-section for dissociative recombination of H+3 a strong enhancement feature was found which is several eV wide and centered near 9.5 eV incident electron energy. We show this peak is due to four autoionizing states of H3. We also find that three of these resonance states cross along the symmetric stretch seam of the potential energy hypersurface. Such triple intersections may be expected in any polyatomic system which is of high enough symmetry to allow degenerate molecular orbitals. Their existence can profoundly affect the nuclear dynamics on these surfaces in a manner similar to that found in the more common cases of conical intersections between two surfaces. The calculation and characterization of these states will be described.
Adiabatic rotation of effective spin. I. New insight into spin-rotational interaction
Authors: Steiner Ulrich E. , Serebrennikov Yuri A.
Published: July 2002

The effect of electron spin-rotational (SR) interaction in species with an effective spin Seff=1/2 is treated for molecular rotations representing an adiabatic perturbation to the electronic system. Based on the adiabatic rotation of effective spin (ARES) concept as well as on the transformation of the Hamiltonian to the rotating molecular frame a useful expression is obtained for the effective spin-Hamiltonian of SR interaction whereby the spin-rotational tensor is represented in terms of a tensor γ̂ (relating L+S to Seff) instead of the g tensor (related to L+geS). This relation is valid for rotating systems with arbitrary strength of spin–orbit coupling (SOC). An example is provided to demonstrate the dependence of the γ tensor on the SOC parameter. It is shown that only in the case of weak SOC, the traditional representation of the SR Hamiltonian, is equivalent to the more general ARES Hamiltonian. Furthermore, a direct relation between the ARES and the geometrical (topological phase) approach to the problem is pointed out. The ARES spin-Hamiltonian can be represented in terms of a U(2) non-Abelian gauge potential.
Compact expressions for the electron–electron distribution function for the 2S states of three-electron systems
Authors: Dressel Paul R. , King Frederick W.
Published: July 2002

The electron–electron distribution function P(rij) has been evaluated in closed form for the 2S states of three-electron systems that are described by Hylleraas-type wave functions. The function P(rij) can be reduced to the form P(rij) =∑I=13 ∑K=1gI 𝒜IKrijKe−αIrij. Numerical values of the expansion coefficients 𝒜IK, summation limits gI, and exponents αI are determined for the ground states of selected members of the lithium isoelectronic series. A discussion is given on the necessary conditions that must be imposed on the basis set in order that P(rij) be given by the analytical formula presented above. Expectation values for several moments 〈rnij〉 and 〈δ(rij)〉 are evaluated.
Effect of reagent alignment on the product state distribution in the reaction Sr+HF(v=1, j=1)→SrF(v′, j′)+H
Authors: Loesch H. J. , Stienkemeier F.
Published: July 2002

Laser-induced fluorescence spectra of SrF from the reaction Sr+HF(v=1,j=1)→SrF(v′,j′)+H have been measured for two alignments of the molecular axis of HF favoring either end-on or side-on attacks. We find at the studied collision energies, Etr=217 and 279 meV, a significant but small influence of the alignment on the intensity of the four bandheads associated with v′=0, 1, 2, and 3.
On the perturbation of the B̃ 2A′ state of HCO by the 1 4A″ and 1 4A′ states: Surfaces of intersection and spin–orbit interactions
Authors: Manaa M. Riad , Yarkony David R.
Published: July 2002

The excited B̃ 2A′ state of HCO is studied using multiconfiguration self-consistent field/configuration interaction wave functions of 300 000 to 1 400 000 terms. Equilibrium structures for the X̃ 2A′, B̃ 2A′, 1 4A″, and 1 4A′ states are determined as is the X̃ 2A′–B̃ 2A′ electronic transition dipole moment. This dipole transition moment vector is found to make an angle of ∼+34° with the inertial a axis of the B̃ 2A′ state. The possibility of spin–orbit induced perturbation of the low-lying vibrational levels of the B̃ 2A′ state attributable to the 1 4A″ or 1 4A′ states is considered. The minimum energy points on the surface of intersection of the B̃ 2A′ and 1 4A″ states, denoted MECP(B̃ 2A′–1 4A″), and of the B̃ 2A′ and 1 4A′ states, denoted MECP(B̃ 2A′–1 4A′), are determined. MECP(B̃ 2A′–1 4A″) is located very close to the 1 4A″ equilibrium structure and is only ∼1500 cm−1 above the B̃ 2A′ minimum. At MECP(B̃ 2A′–1 4A″) the parallel and perpendicular components of the B̃ 2A′∼1 4A″ spin–orbit coupling, based on the microscopic Breit–Pauli operator, are approximately 50 and 4 cm−1, respectively. No barrier was found separating the B̃ 2A′ equilibrium structure and MECP(B̃ 2A′–4A″). MECP(B̃ 2A′–1 4A′) is close to the 1 4A′ minimum and is placed ∼2400 cm−1 above the minimum of the B̃ 2A′ state. However, the B̃ 2A′∼1 4A′ spin–orbit interaction, a perpendicular interaction, is only ∼1 cm−1. Additional points on the B̃ 2A′–1 4A″ surface of intersection and the corresponding B̃ 2A′∼1 4A″ spin–orbit interactions are determined.
Large tunneling frequencies of the symmetric tops NH3D+ and NHD+3 in crystalline fields of low symmetry
Authors: Smith David
Published: July 2002

The tunneling frequencies of the symmetric tops NH3D+ and NHD+3 in ammonium compounds have been found to be large when the site symmetry for the ammonium ion is less than tetrahedral. The rotational potential function of these symmetric tops are derived from atom–atom potentials of the Buckingham (exp-6) type. The tunneling frequencies of these symmetric tops, as well as their librational frequencies, have been computed in NH4F, NH4HF2, (NH4)2SiF6, (NH4)2GeF6, (NH4)2TiF6, NH4ReO4, NH4IO4, and NH4ClO4. In all these compounds, the computed tunneling frequencies are much larger than would have ordinarily been expected.
Spectroscopic detection and characterization of the FS2 free radical
Authors: Zhuo Q. , Karolczak J. , Clouthier Dennis J.
Published: July 2002

An extensive vibronic band system of the FS2 radical in the 700–485 nm region has been observed for the first time by laser-induced fluorescence (LIF) probing of the products of the reaction of F2 with various sulfur-containing compounds. Jet-cooled spectra were obtained by reacting fluorine with COS, H2S, or CS2 in the body of a continuous supersonic jet, just prior to expansion. Vibrational analysis of the jet spectra gives ν1′ = 768, ν2′ = 217, ν3′ = 495, ν1″ = 705, ν2″ = 293, ν3″ = 684, and T00=14 922 cm−1, all in good agreement with ab initio predictions for the FS2 free radical. A rotational analysis of the high-resolution spectrum of the 350 band shows that the transition is polarized out-of-plane, with spin splittings characteristic of a molecule with a single unpaired electron. The rotational constants and spin–rotation interaction constants were determined with good precision for the ground and excited states. By combining the rotational constants with our ab initio estimates of the S–F bond length, the following structural parameters were obtained: r(S–F)=1.651 Å, r(S–S)=1.865(5) Å, and θ(FSS)=109.1(1)° for the X̃ 2A″ state and r(S–F)=1.642 Å, r(S–S)=2.105(5) Å, and θ(FSS)=97.6(1)° for the Ã 2A′ state.
First observation and electronic structure of the diatomic platinum nitride molecule
Authors: Friedman-Hill E. J. , Field R. W.
Published: July 2002

The optical spectrum of the PtN radical has been observed directly for the first time. The strongest band system [(0,0) bandhead at 18 591 cm−1] displays an unusual ‘‘perpendicular ΔΩ=0’’ perturbation which gives rise to strong intensity cancellation effects. Although a 4Σ− ground state was expected for PtN, we have assigned the ground state as 2Πr; a 4Σ− assignment is not consistent with our observations. We have used arguments based on both molecular orbital theory and atomic-ion-in-molecule theory to explain the observed ground state symmetry in light of the observed hyperfine structure. Four electronic bands involving a total of six Hund’s case (c) ‖v,Ω〉 substrates have been rotationally analyzed, and many more bands have been observed at lower resolution. The ground state was found to have Ω=0.5 and its principal constants are Be=0.455 708(5) cm−1, αe=0.003 448 1(1) cm−1, spin–rotation constant γ0=0.061 26(7) cm−1, ωe=947.0(5) cm−1, and ωexe=5.0(5) cm−1. Two other Ω states were observed at low energies—an Ω=1.5 state at Te=2 985.665(2) cm−1, with B0=0.445 233(8) cm−1, q=1.522(5)×10−6 cm−1, ωe=901(1) cm−1, and ωexe=8(1) cm−1, and another Ω=0.5 state with Te undetermined and other constants remarkably similar to those of the ground state (B0=0.453 07(6) cm−1, γ0=0.062 02(4) cm−1, ωe=947(1) cm−1, and ωexe=5(1) cm−1).
Hydrodynamic shielding induced stability of zipping macromolecules in elongational flows
Authors: Agarwal U. S. , Mashelkar R. A.
Published: July 2002

Enhanced shear stability of associating polymers during drag reduction observed recently has been attributed to the breakage of reversible associations (e.g., hydrogen bonds) in preference to covalent bonds in the polymer backbone. A simple mechanistic analysis of a perfectly ‘‘zipped’’ assembly of fully extended bead rod chain model of two macromolecules in steady elongational flow is presented. It explains the enhanced stability as a result of (i) distribution (near the vulnerable chain center) of the drag tension into the two parallel ‘‘zipped’’ chains, and (ii) reduction of the drag force due to the enhanced hydrodynamic shielding. Some guidelines for optimum design of shear stable and effective drag reducing macromolecules have been deduced.
Critical behavior of a cationic surfactant in an aqueous salt solution
Authors: Kubota K. , Kuwahara N. , Sato H.
Published: July 2002

The critical behavior of a cationic surfactant, butyl-tetradecyldimethylammonium bromide, in an aqueous 4M–NaBr solution was investigated concerning the isothermal osmotic compressibility and long-range correlation length by means of light scattering, coexistence curve, and turbidity. The critical exponents were obtained as γ′=1.39±0.04 and ν′=0.70±0.03 together with ξ0=(2.48±0.06) nm, and β′=0.375±0.01. The temperature dependence of turbidity showed a good accordance with the result of light scattering measurement. Negative α′ was obtained from the scaling relation. The critical exponents are in good agreement with the Fisher-renormalization prediction for the ternary system (almost fully renormalized Ising exponents).
Kinetic transitions in diffusion-reaction space
Authors: Kozak John J. , Davidson Russell
Published: July 2002

In this paper we study how the strength and range of the potential function v(r) governing the down-range motion of a diffusing coreactant A with respect to a stationary target molecule B influence the efficiency of the irreversible diffusion-reaction process: A+B→C. This problem is translated into the lattice-statistical one of determining the mean walklength before trapping of the species A on a lattice of N sites of coordination ν and dimension d. Factors affecting the reaction efficiency are explored and quantified using a combination of analytical methods and numerical techniques rooted in the theory of finite Markov processes. Our results show that there exists a transition between two qualitatively different types of behavior in diffusion-reaction space, viz., a regime where the coreactant’s motion is totally correlated with respect to the target species, and a regime where the coreactant’s motion is totally uncorrelated. The transition between these two regimes is (relatively) abrupt, and we find that significant changes in the reaction efficiency can be induced by small changes in the strength and range of the correlations between coreactants, the temperature and/or the dielectric constant of the medium. This ‘‘order–disorder’’ behavior is characterized as a kinetic transition in diffusion-reaction space, and is explored as a function of system size and spatial dimension.
Velocity modulated Fourier transform emission as a plasma diagnostic and a spectroscopic tool
Authors: Hong Xichun , Miller Terry A.
Published: July 2002

The Doppler shifts of the emissions from the A–X and B–X transitions of N+2 ions in a He glow discharge are measured using the method of Fourier transform emission (FTE) spectroscopy. The magnitudes and pressure dependencies of the Doppler shifts of rovibronic lines of the two electronic transitions are distinctly different mainly due to the difference in the lifetimes of the excited states. By fitting the data to a theoretical model, the cross sections for He–N+2 collisions (both for velocity-changing and quenching collisions) are obtained. The data from the short-lived B 2Σ+u state of N+2 in the low pressure (no-collision) regime lead to the determination of the discharge electric field. The Doppler modulated spectra demonstrate that the FTE method is a useful spectroscopic tool for discriminating between ionic and neutral transitions.
Dissociative electron attachment to condensed and adsorbed halomethanes
Authors: Rowntree P. , Sanche L. , Parenteau L. , Meinke M. , Weik F. , Illenberger E.
Published: July 2002

Dissociative electron attachment (DEA) processes have been observed and characterized for submonolayer and multilayer quantities of CCl4, CDCl3, CD2Cl2, CH3Cl, and CH3Br adsorbed on metallic substrates and submonolayer quantities adsorbed onto Kr spacer layers. Anion yields (Br−, Cl−, H−, D−) are reported for incident electron energies from 0 to 12 eV; no desorbing polyatomic anion fragments were detected. The results are compared to the gas-phase DEA analogs and demonstrate the perturbations on the DEA process that are introduced by the presence of the highly polarizable environment (molecular solid+metal substrate). We also report the first observation of H−(D−) produced by DEA of halomethanes containing hydrogen (deuterium).