bluuebunny/crossref_metadata_2025 huggingface dataset:
The 540–900 nm photodissociation of 300 K NCNO: One- and two-photon processes
Authors: Nadler I. , Pfab J. , Reisler H. , Wittig C.
Published: February 2003

The laser photodissociation of 300 K NCNO throughout the region 540–900 nm is reported, and both 1- and 2-photon processes are discussed. By monitoring CN fragments produced via the 1-photon process, we show that with photolysis wavelengths >592 nm, dissociation occurs predominantly by exciting NCNO ‘‘hot bands.’’ At shorter photolysis wavelengths, dissociation from the ground vibrational state of NCNO is observed as well, but the contributions from hot bands are still manifest in high CN rotational levels which are energetically inaccessible from the ground state (D0=48.8 kcal mol−1). Energy distributions in the CN fragments were determined for excess energies up to 1800 cm−1, and are in agreement with phase space theory calculations and a vibrational predissociation mechanism. In addition, throughout the region 620–900 nm, stepwise two-photon photodissociation proceeds using the Ã 1A″ state as a gateway, and results in rotationally and vibrationally ‘‘hot’’ CN fragments. The hot CN fragment yield vs photolysis wavelength shows peaks which correspond exactly to peaks in the NCNO absorption spectrum, allowing us to obtain high resolution spectra of the Ã 1A″← X̃ 1A′ absorption system. The one- and two-photon processes are in competition, and the latter disappears at wavelengths where one-photon photodissociation of NCNO via its ground vibrational level sets in. The nature of the electronic states involved in the one- and two-photon processes is also discussed.
1
1
3Cd and 13C chemical shielding tensors in a single crystal of cadmium (II) acetate dihydrate
Authors: Ganapathy S. , Chacko V. P. , Bryant R. G.
Published: February 2003

Both the carbon-13 and cadmium-113 chemical shielding tensors in a single crystal of cadmium (II) acetate dihydrate have been determined by solid state proton-enhanced NMR at 1.32 T. The 13C shielding tensors of a carboxyl group bound to a metal ion is determined quantitatively and unambiguously. The most shielded direction lies perpendicular to the O–C–O plane and the least shielded principal value is along C⋅⋅⋅Cd direction. The influence of carbon–cadmium dipolar interaction on the observed spectra is discussed and used to make unique assignments of the carboxyl carbon tensors. The CP-MAS spectrum exhibits resolved resonances for the two chemically distinct carboxyl carbons and these are assigned based on single crystal analysis. The methyl carbon tensors exhibit significant deviation from axial symmetry. The most shielded direction is along the C3 symmetry axis and the least shielded direction is perpendicular to the C(H3)–COO plane. It is demonstrated that the orientation dependence of 13C–1H dipolar interaction is useful in assigning 13C resonances of the two chemically similar methyl groups in the asymmetric unit. The 113Cd results show that the cadmium shielding tensor is not axially symmetric and the most shielded direction lies approximately perpendicular to the plane formed by cadmium and the two water oxygens.
Temperature dependence of the Raman OH-stretching overtone from liquid water
Authors: Monosmith W. B. , Walrafen G. E.
Published: February 2003

The first Raman overtone of the OH-stretching vibration from highly purified liquid water has been examined quantitatively in the temperature range of 20–95 °C, and at frequencies from about 5500–8000 cm−1. The overtone Raman spectra show contour shapes, depolarization ratio dispersion, and an isosbestic frequency (6960±25 cm−1, uncorrected; 6900±25 cm−1, density and refractive index corrected) which, apart from the effects of anharmonicity, tend to mimic the fundamental, including its major four-component substructure. The two components at about 7030 and 6850 cm−1 were interpreted, respectively, in terms of nonhydrogen-bonded OH, and triply hydrogen-bonded OH groups, which are common to the three-bonded H2O species, whereas two components at about 6665 and 6160 cm−1 were interpreted as intermolecular coupling components of the fully hydrogen-bonded, i.e., four-bonded H2O species. A ΔH° value of −2.5 Kcal/mol hydrogen bond was obtained from the temperature dependence of ratios of the combined component intensities of these two pairs, in agreement with results from previous Raman fundamental measurements. The feature near 6160 cm−1 was found to be better resolved in the spectrum than its fundamental analog, indicating that anharmonicity is advantageous in elucidating contour substructure.
Angular and velocity distributions of HD molecules produced by the H2–D2 exchange reaction on the stepped Pt(557) surface
Authors: Lin T. H. , Somorjai G. A.
Published: February 2003

The H2–D2 exchange reaction on Pt(557) crystal surface was investigated using a molecular beam surface scattering technique. The angular and velocity distributions of the HD product were measured at various crystal temperatures (500–1170 K). It was found that the exchange reaction occurred via interactions of adsorbed H and D atoms (Langmuir–Hinshelwood mechanism). Although the HD that is produced desorbs with a cos2 θ angular distribution, its translational energy corresponds to a temperature slightly colder than that of the substrate with the mean energy of the desorbing molecules depending on the desorption angle. As the desorbing angle increases away from the surface normal, the mean translational energy decreases. The exchange reactivity was found to be incident azimuthal angle dependent while the translational energy of the product HD is independent of the azimuthal angle of detection.
A novel liquid crystal compound with very large dielectric constants
Authors: Schad Hp. , Kelly S. M.
Published: February 2003

We have investigated the dielectric constants and the rotational viscosity of 4-cyano-3-fluorophenly 4-heptylbenzoate. A dielectric anisotropy of about 50 has been measured in the nematic phase and related to the absence of molecular association. This outstanding property can be exploited in the twisted nematic display effect to produce threshold voltage values below 0.4 V.
Osmotic pressure of semidilute polymer solution near the Θ point
Authors: Ito Hiroyuki , Koyama Osamu , Chiba Akio
Published: February 2003

The osmotic pressure of polystyrene in cyclohexane was measured in a polymer weight concentration range of 0.139(g/cm3) to 0.327 near the Θ point. The reduced osmotic pressure ΠM/CRT of two solutions— polystyrene in cyclohexane and polyisobutylene in benzene, near the Θ point is found to be a universal function of C/C*, where M is the molecular weight, C is the polymer weight concentration, and C* is the crossover polymer weight concentration. The osmotic pressure Π becomes proportional to C2⋅98 at high concentrations. These results support the predictions of the tricritical approach to the polymer Θ point by Daoud and Jannink.
Potential energy surfaces of MH2 (M=Co, Fe, and Cu)
Authors: Siegbahn P. E. M. , Blomberg Margareta R. A. , Bauschlicher Charles W.
Published: February 2003

The lowest low spin and high spin potential surfaces of MH2, where M is one of the metals Co, Fe, and Cu, have been investigated at the CI level. The results show that CoH2 and FeH2 are very similar to NiH2, which was investigated in an earlier study. On the low spin surface, which is chemically most interesting, H2 dissociates with little or no barrier and forms weakly bound bent complexes. The bonds are formed from sd hybrids. These results are in line with experimental findings for dissociative chemisorption of H2 on metal surfaces, where Fe, Co, and Ni are known to behave very similarly. Since sd hybrids for Cu can not be easily formed the addition of H2 to Cu leads to a high barrier, which means that on-top dissociation of H2 on a copper surface is unlikely. This is a plausible explanation of why H2 dissociates with higher barriers on copper surfaces than on iron, cobalt and nickel surfaces. In gas phase, the ground states of MH2 are linear with high spin. The ordering of these linear states is explained using very simple rules. The high barrier dissociations of the high spin states are also discussed.
Freezing of the classical two-dimensional, one-component plasma
Authors: Radloff Patricia L. , Bagchi Biman , Cerjan Charles , Rice Stuart A.
Published: February 2003

The results of a study of freezing in the two-dimensional, one-component plasma are reported. The analysis is based on a nonlinear integral equation for the singlet density in an inhomogeneous system. The existence and location of the freezing transition are obtained both from a bifurcation theory approach and by imposing equality of grand thermodynamic potentials of the liquid and crystalline phases. The value of the plasma parameter Γ≡[e2(πρ) 1/2 /kBT] corresponding to the first appearance of long-range crystalline order in the system, found by locating the bifurcation point of the nonlinear equation, is 124. The freezing transition defined by equating the grand potentials of the fluid and crystalline phases is at Γ=133. These values compare well with those found in recent experiments and computer simulations. The bifurcation approach also predicts, correctly, that the first order freezing transition occurs with zero change in volume. The entropy change on freezing predicted by the imposition of the equality of grand potentials is −0.97 kB per particle, about threefold larger than the value obtained from computer simulations. Possible sources of this discrepancy and the relationship between the bifurcation theory and thermodynamic constraint approaches are discussed.
Homogeneous nucleation rate for water
Authors: Hagen D. E. , Kassner J. L.
Published: February 2003

Homogeneous nucleation rate data for water extending over an exceptionally large domain of rate (J), supersaturation ratio (S), and temperature (T) was recently published. Because it spans a large J-S-T surface, this data constitutes a good test of nucleation theory. Here classical nucleation theory is used to analyze this data. By adjusting only the sticking coefficient, we are able to obtain a good fit between theory and experiment. It was necessary to include an increase in the water molecular density associated with the finite water compressibility.
Site selective laser spectroscopy of the insulator-to-semiconductor transition in CdF2:Er3+
Authors: Mho Sun-Il , Wright John C.
Published: February 2003

The individual Er3+ sites in CdF2:Er3+ are determined by site selective laser spectroscopy and are monitored as the crystal is changed from the insulating to the semiconducting state. The site distribution is controlled by clustering of the Er3+ dopant ions and their charge compensating fluoride interstitials (F′i). Simple, single pair sites where an Er3+ ion is locally compensated by a Fi are found to have minor importance in contrast with other fluorites. The conversion to a semiconductor state results from oxidation of the F′i that locally compensate the Er3+ clusters. No new sites are observed that arise from local compensation by electrons. Differences in the oxidation potential between the various clusters suggest differences in the numbers of Fi associated with each cluster. It is suggested that the changes in the site distribution that occur for dopants with different ionic radii account for the inability of some dopants like La3+, Ce3+, and Pr3+ to produce the semiconducting state. The observation of the dominance of clustering in CdF2 lends support to previous interpretations of the activation energies for electron transport.
Vapor condensation and the liquid state. An extension of Mayer’s theory
Authors: Gibbs J. H. , Pavlin I. , Yang A. J.
Published: February 2003

An extension of Mayer’s cluster theory beyond the gas spinodal density is discussed. Volume dependence of Mayer’s reducible cluster integrals bj(T,V) is examined and included in the discussion. It is concluded that Mayer’s first equation for the canonical partition function has a built in uniform density assumption and will give a van der Waals loop if correct bj(T,V) are used. The virial expansion is then meaningful in the liquid state as can be shown by the cumulant expansion method. An approximate but complete set of virial coefficients βk for simple fluid is constructed by supplementation of the known β1, β2, β3, β4, for the Lennard-Jones 12-6 potential with the β5, β6, ..., βN−1 given by Carnahan and Starling for the hard sphere potential. The virial series based on this set of βk gives remarkable agreement with the pressure isotherms obtained from Monte Carlo simulation of the Lennard-Jones fluid. To obtain the correct isotherms for a Lennard-Jones fluid not constrained to density homogeneity the uniform density canonical ensemble partition functions Q(T,V,N) obtained by integration of this virial series are used to construct a grand canonical ensemble partition function ∑N≥0λNQ(T,V,N). The isotherms obtained from these agree with the Monte Carlo results at densities corresponding to the homogeneous single phases of gas and of liquid, but display the proper constant pressure at the intermediate overall-average system densities appropriate to gas–liquid coexistence.
The cooperativity of nitric oxide hemoglobin ligand exchange in single crystals as studied by EPR
Authors: Doetschman D. C. , Rizos A. K. , Szumowski J.
Published: February 2003

Single crystals of human oxyhemoglobin are exposed to aqueous solutions of nitric oxide at pH 6.7 and 6 °C. The EPR spectrum of crystals at 85–100 K selected after various times of exposure to NO are recorded. The intensities of the alpha and beta subunits are determined from the c axis spectrum of the crystal. With NO in stoichiometric excess, crystals sampled early (3–200 min) display the EPR of molecules with NO bound to an average of two alpha and one beta subunit. The NO diffuses into the crystals with a diffusion rate constant of 1.5×10−9 cm2 s−1.The NO binds to the last beta subunit at a rate of 5.1×10−6 s−1.
Ultraviolet spectra of vibrationally highly excited CS2 molecules
Authors: Dove John E. , Hippler H. , Plach H. J. , Troe J.
Published: February 2003

Vibrationally highly excited CS2 molecules with energies close to the dissociation energy of the electronic ground state have been prepared by laser excitation at 308 and 351 nm. The UV spectra recorded are compared with UV spectra from CS2 molecules excited thermally in shock waves at temperatures up to 4000 K. A numerical representation of the absorption coefficients as a function of the average excitation energy of the molecules is given.
Photofragment spectroscopy of SO+
Authors: Cosby P. C.
Published: February 2003

The SO+ ion, produced by dissociative electron-impact ionization of SO2, is observed to photodissociate into S+ + O over the wavelength range of 6900–5750 Å in a series of structured bands. The bands are assigned to the b 4∑−(v=7–12)←a 4Π(v=1–8) absorption with subsequent predissociation of the b state levels. The predissociation lifetime of the v=7 level of the b state is found to lie in the range 5≤τ≤26 ps. The predissociation process is attributed to spin-orbit coupling to the 1 4∑+ state. The observed b←a band locations and photofragment kinetic energy releases are combined with existing photoelectron and emission spectroscopy data to yield improved molecular constants for the a and b states.
Direct observation of the B″ 3Πu–X 3Σ−g transition of diatomic sulfur in a supersonic free jet
Authors: Matsumi Yutaka , Munakata Toshiaki , Kasuya Takahiro
Published: February 2003

The laser-induced fluorescence (LIF) spectra of diatomic sulfur are studied in a supersonic free jet which is expanded from a high temperature nozzle source of 800 K. The lifetime separation method is advantageously combined with the LIF detection in the supersonic free jet. The weak B″ 3Πu–X 3Σ−g band is thereby observed successfully in good distinction among the strong B 3Σ−u–X 3Σ−g band system. The rotational constants and term values of the vibronic levels v′=3–18 B″ 3Πu (F1,F2) are evaluated by a rotational analysis. An inspection of the intensity distribution among the observed spectral lines indicates that the B″ 3Πu–X 3Σ−g bands borrow intensity from B″ 3Σ−u–X 3Σ−g bands through homogeneous perturbations. The anomalous behavior of molecular constants, including the irregular vibronic dependence of the spin-splitting constant of B 3Σ−u, is interpreted in terms of the perturbation from the B″ 3∏u state.
The effect of thermal vibrations on extended x-ray absorption fine structure. II
Authors: Boland John J. , Baldeschwieler John D.
Published: February 2003

A formalism is introduced to describe the dependence of the EXAFS amplitude and phase on thermally excited vibrations. The EXAFS expression for a general three-atom system of Cs symmetry is discussed in detail. To lowest order, thermal vibrations induce a change only in the scattering path length and give rise to the standard EXAFS expression for each scattering path damped by the appropriate Debye-Waller factor. A change in scattering angle, however, is a higher order effect and gives rise to additional EXAFS terms called modification factors. There is a modification factor associated with each multiple scattering path. Within this formalism, the correlation which exists between a change in scattering path length and a change in scattering angle is explicitly calculated. The normal modes which contribute to the modification factors are discussed, and shown to be dependent on the symmetry of the system. The nature and temperature dependence of the Debye–Waller and modification factors for the three-atom system Br2O are discussed as a function of bridging angle. The modification factors are shown to make a small contribution to the total EXAFS of the system. The magnitude of this contribution is dependent on the degree of correlation that exists between changes in scattering path length and scattering angle.
Electron–ion recombination in gas mixtures of helium, nitrogen, and carbon dioxide
Authors: Littlewood I. M. , Cornell M. C. , Nygaard K. J.
Published: February 2003

A study has been conducted to determine the electron–ion recombination rates in gas mixtures of helium, nitrogen, and carbon dioxide. Measurements were made as functions of electric field strength, gas pressure, and gas composition. In gas mixtures containing only nitrogen and carbon dioxide, the rates were dependent on pressure. This dependence is interpreted as being due to the separate effects of two- and three-body recombination processes. No pressure dependence was observed for mixtures containing helium. In such mixtures, two-body recombination appears to be dominant. Moreover the addition of relatively low concentrations of helium to the discharge leads to a dramatic change in the bulk recombination rate. Two possible explanations for this observation are suggested.
Theory of geminate recombination on a lattice
Authors: Scher H. , Rackovsky S.
Published: February 2003

Central to the photogeneration process in molecular systems is the competition between the relaxation of an excited state (to the ground state) and the separation into an ion pair via charge transfer steps. We have developed the first comprehensive model of this process which depends on molecular parameters and denumerates the competition between the various rates. The model is discrete diffusion on a lattice in a combined Coulomb and external field. The model is solved exactly and is computationally straightforward. The analytic method we have used is quite general and can easily be extended to include a broad class of problems involving large numbers of (correlated) ‘‘special sites.’’ The main feature of the computation, needed to solve these problems, is the evaluation of the lattice Green’s functions in the presence of the electric field. Our solution has a larger range of applicability than the (continuum limit) Onsager theory and is more versatile in terms of initial conditions and modeling of transient phenomena. We will elaborate this approach and discuss our results for the quantum efficiency as a function of external field, temperature, and molecular concentration, η(E,T,c). We show that η(E,T,c) exhibits a diversity of behavior dependent on the relative magnitudes of inter- and intramolecular transition rates. Onsager-like behavior for η is contained in the infinite sink limit and more generally, η can change appreciably as a function of the molecular parameters even for a fixed initial separation r0. In this theory the r0 is geometry controlled, and the initial yield φ0 can be determined by the same competition of rates as control the long time yield. The short time yield is controlled by a few discrete hops and therefore follows an exponential decay as indicated by recent picosecond experiments. In general, within this framework, one can study the influence on η of such factors as dimensionality, lattice structure, disorder, tunneling transition rates, intramolecular rates, and intrinsic energy level differences.
Boundary conditions and reversibility in diffusion controlled reactions. II. A three state model for steady state evaporation of a spherical drop
Authors: Monchick Louis
Published: February 2003

A simple three state model of the reaction complex was introduced into reversible diffusion controlled reaction theory and the flux balance equations were derived for all times. The long time asymptote of this relation was used to describe the evaporation from and condensation on a spherical drop immersed in a dilute gas infinitely dilute in vapor. The molecular velocity distribution is required to satisfy the linearized steady-state Boltzmann equation with the molecular interaction taken to be Maxwell’s potential. The solution to this and the boundary condition imposed by the flux-balance equations of the three state model was approximated by a four mode ansatz. The coefficients of these modes were determined by four moment equations which could be shown to satisfy a variation principle due to Cercignani. Two of the modes corresponded to two of the components of the usual Chapman–Enskog ‘‘normal’’ solution to the problem and were found to be dominant at long distances, falling off as r−1 and r−2. The other two modes were discontinuous in velocity, and, outside a boundary layer several mean free paths thick, were found to decay as r−4 and r−3. The boundary condition at the drop surface was found to be of the usual radiation boundary condition type used in diffusion controlled reaction theory, modified by a factor dependent on mean free path and reaction probability.
Use of the memory function in a collinear atom/harmonic chain model
Authors: Mauricio F. , Velasco S.
Published: February 2003

For a collinear atom-harmonic chain model we analyze the influence of the memory function on the spectral density of the velocity autocorrelation function of the atom. A numerical analysis for different memory functions is made. The Markovian limit has been checked. For memory times smaller than the parameter which characterizes the intensity of the random force, for the limited range of parameters which has been examined, there is very little difference among the three choices of memory which have been chosen. We show that as the memory time increases noticeable differences among the obtained spectral shapes appear.
Critical behavior of uniaxial–biaxial nematic phase transitions in amphiphilic systems
Authors: Boonbrahm P. , Saupe A.
Published: February 2003

The temperature dependence of the birefringence and magnetic field induced effects were studied on thin films of mixtures of potassium laurate, 1-decanol, and D2O. The selected mixtures have a small biaxial nematic range between two uniaxial nematics. The biaxial–uniaxial nematic phase transitions are second order and have critical properties that deviate from mean field behavior. Values of critical exponents were determined for the order parameter in the biaxial phase at the lower temperature transition to the uniaxial nematic phase Nd, and for the susceptibility exponents on both sides of this transition. The experiment gives β=0.37 or β=0.35, depending on the method of evaluation, γ=1.33 and γ′=0.81. The values for β and γ agree within error limits with the theoretical values of the x, y model but γ′ deviates.
The dissociation of H2 on the Ni(100) surface
Authors: Siegbahn P. E. M. , Blomberg Margareta R. A. , Bauschlicher Charles W.
Published: February 2003

The dissociation of H2 on the (100) surface of Ni is investigated using a cluster model. The mechanism for dissociation of H2 directly above a Ni atom has little to no barrier and involves the Ni 3d electrons; elimination of the Ni 3d interaction with the H2 increases the barrier to more than 50 kcal/mol. The dissociation at the bridge site, treated without the Ni 3d interaction, leads to a barrier of about 30 kcal/mol, leading to the conclusion that the dissociation of H2 at any site on a Ni(100) surface requires strong 3d participation. The results are quantitatively different if the Ni 4p orbitals are not included. The effects of cluster size on the results are also discussed.
General iterative calculation of moments and distribution functions of polymer chains. Application to the poly(methylphenylsiloxane)
Authors: Freire Juan J. , Rubio Ana M.
Published: February 2003

Previous iterative equations for the calculation of high moments, based on the spherical harmonic representation of rotational operators and only valid for symmetric chains in their original version, have been generalized for any real polymer chain represented by the rotational isomeric state model. From these high moments, the end-to-end distance and end-to-end vector distribution functions can be inferred. The new generalized expressions have been applied to obtain the radial distribution of the end-to-end distance W(R) of short isotactic and syndiotactic poly(methylphenylsiloxane), PMPS, chains. Then, the remarkable influence of tacticity in W(R) has been analyzed. Furthermore, a significant role of the fluctuations of rotational angles has been clearly manifested in the conformational distribution of isotactic PMPS chains.
Phase stability of weakly crosslinked interpenetrating polymer networks
Authors: Binder K. , Frisch H. L.
Published: February 2003

A phenomenological theory is formulated for chemically quenched binary interpenetrating polymer networks, considering both simultaneously crosslinked networks and sequentially crosslinked networks, as well as pseudointerpenetrating networks (where only one component is crosslinked and the other is a linear polymer). We construct free energy functionals for these systems, starting from the classical Flory theory of rubber elasticity, and study their properties as a function of the volume fraction φ of one component and of the Flory–Huggins interaction parameter v. It is suggested that in the limit of small crosslink density the contribution to the free energy due to the mutual entanglement of the networks is small and can be neglected, provided that the spatially homogeneous phase is thermodynamically stable. The stability limit (spinodals) of this phase for the various cases are derived, and also predictions are made for the initial growth of inhomogeneous modes in the unstable regions of the phase diagrams, within the framework of the linearized theory of spinodal decomposition. We also discuss how the predictions of this theory could be checked experimentally.
Morphological instability of crystal growth in nonsteady potentiostatic electrodeposition. I. The mechanism of growth of random crystals in metal deposition
Authors: Aogaki Ryoichi , Makino Tohru
Published: February 2003

The growth mechanism of irregular, roundish crystals, which commonly appear in potentiostatic deposition, was studied from the viewpoint of the growth of surface fluctuations which originate at an electrode surface. As a result, it was suggested that such irregular crystals come from the initial surface fluctuations caused by the microscopic dissolution and deposition of the electrode surface in thermodynamical equilibrium. After deriving the relationship between the spatial wave number of the fluctuation and the degree of surface irregularity (dsi), the fluctuation component with a single wave number was examined. As for actual fluctuations, since numerous components with different wave numbers are superimposed, it was necessary for graphic simulations by use of computers to be performed. However, from the analysis of only a single wave component, it was concluded that, at high cathodic overpotential, surface diffusion of adatoms exhibits a remarkable effect, viz., if there is no surface diffusion, it can be predicted that very small crystal peaks are rapidly formed owing to multiple nucleation based on the high supersaturation of adatoms; this prediction, however, is not consistent with actual observations. According to the theoretical equation obtained in the present paper, it was clearly concluded that crystal growth at high cathodic polarizations is moderated by the surface diffusion of adatoms, which are released from the concentrated nucleation sites to active lattice points, such as kinks and steps. Moreover, it was also found that the surface energy of the electrode should become constant in value at high cathodic potentials.
Morphological instability of crystal growth in nonsteady potentiostatic electrodeposition. II. Experimental examination with the image analysis of crystal morphology
Authors: Aogaki Ryoichi , Makino Tohru
Published: February 2003

In the accompanying paper, a theoretical equation governing unstable crystal growth under a nonsteady potentiostatic condition was derived. In the present paper, in order to examine the theory with experiments, the electrodeposition of Ag crystals on Ag electrodes was performed. SEM photographs of the Ag samples deposited during various periods of time were immediately taken for analyzing them by the technique of image analysis. As a result, it was found that the mean crystal size-deposition time curve can be calculated by means of the theory. It was then concluded that the surface diffusion coefficient takes a value in good agreement with the theoretical prediction, and that the surface energy of the electrode depends on the overpotential, i.e., it decreases with increasing overpotential, after passing about −0.2 V, however, the value becomes constant.
Study of wave function tails and reactivity from exterior electron model
Authors: Ohno Koichi , Matsumoto Setsuo , Harada Yoshiya
Published: February 2003

The exterior electron model was found to provide a means to test wave function tails of individual molecular orbitals and also to study reactivity of molecules. Demonstrative ab initio calculations were made for NH3, CO, H2O, and H2S at various levels of basis sets.
Electron-impact excitation cross sections for Na I
I
Authors: Ganas P. S. , Aryafar M. , Gately L. P.
Published: February 2003

Calculations are performed for the generalized oscillator strengths and integrated cross sections for the electron-impact excitation of NaII.(AIP)
Total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6
Authors: Chatham H. , Hils D. , Robertson R. , Gallagher A.
Published: February 2003

The total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6 have been measured for electron energies from threshold to 300 eV. Comparisons are made to earlier measurements.
Semiclassical theory of resonances in 3D chemical reactions. I. Resonant periodic orbits for F+H2
Authors: Marston C. C. , Wyatt Robert E.
Published: February 2003

Numerical methods are described for locating quasiperiodic and periodic resonant orbits in 3D atom–diatomic molecule reactions. For the quasiperiodic case, the orbits are found by minimizing the ‘‘escaping tendency’’ into reactants or products. For the periodic case, the orbit is found by adjusting the initial conditions to minimize an ‘‘aperiodicity index.’’ A resonant orbit is illustrated for the J=0 F+H2 reaction. Using only periodic orbits, the ‘‘arbitrary trajectory’’ semiclassical quantization procedure of DeLeon and Heller is used to predict the resonance energy for F+H2. The semiclassical resonance energy is in good agreement with all available quantal results.
Transformation and averaging of theoretical differential cross sections for comparison with crossed molecular beam experiments
Authors: Pack R. T.
Published: February 2003

The center of mass to laboratory transformation and averaging over beam velocity and spatial profiles, detector slits, etc., that are necessary to compare theoretical differential cross sections directly with crossed molecular beam data are presented in easily used forms. A theoretical model is developed to accurately reduce the 11-fold integral over velocities, scattering volume and detector to a fourfold integral. Example calculations show how the model can be used to optimize machine resolution and to distinguish the damping of quantum oscillations due to machine averaging from that due to the anisotropy of molecular interactions.
Macroscopic behavior in finite Brusselators: Stochastic simulation of fluctuations and ensemble statistics
Authors: Lumsden Charles J. , Rowe Glenn W.
Published: February 2003

A method of stochastic simulation is applied to the Brusselator chemical reactions and used to model the time course of chemical abundances in this system. Statistical ensembles are used to calculate the nonequilibrium probability density and the expectation values of the reactant concentrations. The time development of the expectation values is found to depend in a novel manner on the thermodynamic size of the system. The first moments of the ensemble do not remain on or near the limit cycle; in contrast to the solutions of the deterministic reaction-rate equations they spiral inward toward the center of the limit cycle in a manner determined by the thermodynamic size of the system. Previously predicted, this behavior has not heretofore been observed in an actual system or computational model. The evolution is found to exhibit multiple time scales in which there is a fast initial decay proportional to the overall time required for phase decorrelation. Mechanisms for this scaling are discussed. The general structure of the evolving ensemble is found to be related to the ‘‘weak retrace property’’ of the Brusselator, in which portions of the periodic trajectory are unable to follow the path laid down on earlier oscillations. The weak retrace phenomenon is studied for members of the statistical ensemble and a general mechanism for it proposed.
Bounding and estimation of van der Waals coefficients
Authors: Thakkar Ajit J.
Published: February 2003

Numerous bounds to the induced-dipole induced-dipole van der Waals coefficient C(6)ab are systematically compared on a self-consistent body of data pertaining to 21 atoms and molecules, and 231 pairwise interactions. Only bounds which use no more than five moments of the dipole oscillator strength distribution (DOSD) and possibly the resonance frequency are considered. The sharpest bounds in each of four groups are thus identified. It is shown that, in each of the three groups that require only integer moments of the DOSD as input, the best pair of upper and lower bounds can be supplemented by an estimate with an rms error of less than 1%. These estimates are based on generalized means. The sharpest upper and lower bound of all can both be computed on the basis of merely five moments S(−k) with 1≤k≤5. A two term effective DOSD that reproduces S(−k/4) with 7≤k≤10 is shown to provide an unbounded approximation to C(6)ab with an rms error of only 0.09%. This effective DOSD can form the basis of a very accurate combination rule.
Momentum density approach to the second energy gradient
Authors: Koga Toshikatsu , Sasaki Masaki
Published: February 2003

Previously, rigorous equations, which relate the total energy and interatomic force of a molecular system with the electron density in momentum space, have been derived and applied to the analysis of several interatomic interactions. This approach of momentum density is extended here to the second gradient of the energy with respect to the internuclear distance. It is shown that the second energy gradient, including the harmonic force constant, is also governed by the manner of the electron momentum distribution and the guiding principle of contraction and expansion is valid. The results are illustrated for the interactions in the two lowest states (1∑+g and 3∑+u) of the H2 system.
A
b i
n
i
t
i
o MO calculation of the anharmonic force field of methyl fluoride and methyl chloride
Authors: Kondo Shigeo , Koga Yoshinori , Nakanaga Taisuke
Published: February 2003

Several levels of ab initio MO calculations have been carried out to obtain the anharmonic symmetry force field of methyl fluoride and methyl chloride. The basis sets used were 4-31G* and 6-311G** for methyl fluoride, and 4-31G* and Dunning’s double zeta basis set augmented with polarization functions for H, C, and Cl atoms for methyl chloride. Also the effect of the configulation interaction was explored by using the Mo/ller–Plesset second-order perturbation theory. For each case, the molecular energy was calculated for about 230 or more different displacements to determine 55 force constants; 12 quadratic, 38 cubic, and 5 quartic force constants. The remarkable result concerned with the cubic force constants is that although the diagonal stretching force constants decrease on going from lower to higher levels of calculation, the differences between the off-diagonal force constants for different levels of calculation employed here are almost negligibly small. In fact even the force constants obtained through HFSCF calculation for 4-31G* basis set were found to be able to reproduce quite well the observed values of the spectroscopic constants of these molecules. Also a close similarity of the cubic and quartic force constants between the two molecules is noted.
Calculation of the deuterium electric field gradient in LiD
Authors: Pietrovito Anthony J. , Hameka Hendrik F. , Zeroka Daniel
Published: February 2003

We report a theoretical result for the deuterium electric field gradient of the LiD molecule. The calculation is based on an LCAO MO SCF function proposed by Ransil. First we evaluate the expectation value of the DEFG operator with respect to the wave function. Then we derive a corrected value by means of a variation–perturbation method. We find that the zero–order value of qa(el) is 0.1615 and that the corrected value is 0.1644; the experimental value is 0.169±1%. The agreement between theory and experiment is satisfactory, especially since the theoretical value is derived for a fixed nuclear separation and the experimental value refers to a vibrating molecule.
Effect of pressure on molecular conformations. V. The internal rotation angle of 1,2-dichloroethane by infrared spectroscopy
Authors: Ikawa S. , Whalley E.
Published: February 2003

The effect of pressure on the infrared spectrum of 1,2-dichloroethane in 2-methylbutane solution has been measured up to 36 kbar at 22 °C using a diamond anvil cell. The sample pressure was measured by the frequency shift of the 764 cm−1 band of 2-methylbutane, which was itself calibrated by the frequency shift of the ν3 vibration of the nitrite ion dissolved in sodium bromide. The effect of pressure on the internal rotation angle of the gauche form has been determined from the relative intensity of the two carbon–chlorine stretching vibrations by applying a simple model. It decreases at the rate 2.7° kbar−1 in the range 0–5 kbar, which is close to the 2.6° kbar−1 that was obtained by applying the same model to the published relative Raman intensity of the two C–Cl stretching vibrations. Above 5 kbar, the ratio of the intensities of the two vibrations is constant up to 30 kbar within the experimental uncertainty. This suggests that in the range 5–30 kbar a decrease in the internal rotation angle causes the Cl–C–C angles to open, because of the chlorine–chlorine repulsion, by enough to keep the angle between the C–Cl bonds approximately constant. The volume change at the trans–gauche transformation has been determined as −2.9±0.4 cm−3 mol−1 from the relative integrated intensities of the gauche CH2 rocking and trans CH2 wagging vibrations as a function of pressure. The value obtained agrees within experimental errors with the value which was obtained in a similar way from earlier Raman spectra.
The jet-cooled electronic spectrum of acetaldehyde and deuterated derivatives at rotational resolution
Authors: Noble Marcus , Lee Edward K. C.
Published: February 2003

The laser induced fluorescence excitation spectrum (Ã 1A″←X̃ 1A′) of jet-cooled CH3CHO, CH3CDO, CD3CHO, and CD3CDO at 0.5–2 K have been recorded up to E′vib ≂1500 cm−1. The electronic origin is unequivocally assigned for CH3CHO (29 771 cm−1), CH3CDO (29 820 cm−1), CD3CHO (29 756 cm−1), and CD3CDO (29 808 cm−1). Three Franck–Condon active vibrations in the Ã A″ state are identified: ν10 the in-plane C–C–O bend; ν′14 the out-of-plane aldehyde hydrogen bend; and ν15 the methyl torsion. Evidence is seen for anharmonic coupling of ν′14 and ν15. The height of the threefold barrier to methyl rotation (∼660 cm−1) is fairly insensitive to deuterium substitution as is the frequency of ν′10. An analysis of the rotational structure reveals that all of three possible band polarizations are present in this spectrum, i.e., A, B, and C type. The polarization of some bands is apparently anomalous. The C=O stretch (ν4) could not be assigned due to a high degree of congestion above Evib =1000 cm−1, although it is expected to have a strong Franck–Condon intensity.
Quantum mechanical treatment of circular intensity differential scattering: The elastic process
Authors: Keller David , Bustamante Carlos , Tinoco Ignacio
Published: February 2003

A quantum mechanical derivation is presented for the circular intensity differential scattering (CIDS) of a molecule of arbitrary size. Expressions for the special case where the scattering system is composed of a collection of independent scatterers are derived and the relationship is shown between the quantum mechanical expressions and previous expressions obtained using classical electrodynamics. The differences between elastic and inelastic CIDS are then discussed, as well as the differences between the CIDS due to large and small scattering systems.
Fluorescence from the 1 1B
u state of t
r
a
n
s,t
r
a
n
s-1,3,5,7-octatetraene in a free jet
Authors: Heimbrook Lou Ann , Kohler Bryan E. , Levy Irvin J.
Published: February 2003

The 1 1Bu→1 1Ag fluorescence spectrum of trans,trans 1,3,5,7-octatetraene in a supersonic helium jet has been determined. Excitation at the 1 1Bu origin (35 549.3 cm−1) gives rise to a well resolved emission spectrum whose vibronic development is dominated by the single and double bond stretch modes of the polyene. There is no gap between the onset of the dispersed emission spectrum and origin of the previously reported fluorescence excitation spectrum. Observed band profiles are well described by the convolution of 12 cm−1 Lorentzians with the spectrometer response function. The fluorescence lifetime for excitation at the 1 1Bu origin is shorter than 2 ns; an excited state decay time of 100–300 ps follows from an analysis of the dependence of fluorescence signal on excitation power in the nonlinear regime. The rough mirror symmetry between the 0-0 excited emission and excitation spectra and the short lifetime indicate that the emission comes exclusively from the 1 1Bu state with no significant contribution from the lower lying 2 1Ag state. For excitation at the origin the overall emission yield is estimated to be 0.05–0.15.
Difference frequency spectroscopy of the fundamental band of CH(X 2Π)
Authors: Lubic Karen G. , Amano Takayoshi
Published: February 2003

Fifty-four transitions of the fundamental band of X 2Π CH have been measured using a tunable difference frequency system. A combined fit of these data, MODR, and microwave data yielded values for the band origin, the spin-orbit constant, rotational constants, spin-rotation constants, and lambda-doubling constants for both v=0 and v=1 as well as magnetic hyperfine constants for v=0.
Rotationally inelastic collisions of LiH with He: Quasiclassical dynamics of atom-rigid rotor trajectories
Authors: Metropoulos Aristophanes , Silver David M.
Published: February 2003

Rotationally inelastic cross sections for the LiH–He collision system are computed classically using a previously derived ab initio potential energy surface [D. M. Silver, J. Chem. Phys. 72, 6445 (1980)]. The LiH is in its ground vibronic state and is initially taken to be in its j=1 rotational state. The He is in its ground electronic state. The system is treated as an atom-rigid rotor interaction. The results are compared with previously computed cross sections derived from the same ab initio potential energy surface using the coupled states approximation for quantum mechanical scattering [E. F. Jendrek and M. H. Alexander, J. Chem. Phys. 72, 6452 (1980)]. The theoretical total cross sections are averaged over a temperature distribution and are then compared with experimental measurements of corresponding cross sections for a rotationally resolved LiH beam ( j=1) incident on a He gas target in thermal equilibrium at room temperature [P. J. Dagdigian and B. E. Wilcomb, J. Chem. Phys. 72, 6462 (1980)]. The agreement between classical, quantum and experimental results is discussed.
Pulsed CO2 laser induced chemistry of vanadium oxytrichloride (VOCl3)
Authors: Sausa R. C. , Ronn A. M.
Published: February 2003

Pulsed CO2 laser multiple photon excitation of VOCl3, under collisional and collision-free conditions, resulted in dissociation via its ground electronic state and in a visible fluorescence. The photodissociative pathways leading to product formation were found to be both pressure and laser fluence dependent. Low pressure and laser fluence led to the formation of VOCl2 while high pressure and laser fluence flavored VO. The prompt fluorescence was also studied as a function of pressure and laser fluence, and was shown to arise from a spontaneous one-photon radiative decay from an electronic state belonging to the VOCl2 fragment. The reaction mechanisms leading to product formation as well as models for populating a fragment’s electronic state are proposed and discussed in conjunction with present multiphoton excitation and dissociation theory. In addition, CO2 laser induced dielectric breakdown of VOCl3, in the neat and with scavenger gases, resulted in the production of various vanadium oxides. The products are finely divided particles of very large surface area and high purity.
Thermally activated crossing of a sharp potential barrier
Authors: Larson Richard S.
Published: February 2003

The methods of nonequilibrium statistical mechanics are used to determine the rate at which particles in contact with a heat bath are able to escape from a potential well over a sharp barrier. Specifically, the rate constant for the process is found by computing the smallest nonzero eigenvalue of the governing Fokker–Planck equation. Singular perturbation techniques are used to obtain approximate expressions for the appropriate eigenfunction, and substitution into a variational formula then provides the desired eigenvalue. Explicit results are presented for cases of moderate and high friction, and the latter, in particular, is in sharp disagreement with the findings of earlier investigators: it indicates that the approximation afforded by the simpler Smoluchowski equation is not valid, even if the friction constant is arbitrarily large. Semianalytical calculations of the rate constant provide support for the new theory.