bluuebunny/crossref_metadata_2025 huggingface dataset:
The interphase between jellium and a hard sphere electrolyte. A model for the electric double layer
Authors: Schmickler Wolfgang , Henderson Douglas
Published: February 2003

A model for the metal/liquid electrolyte interphase is presented, in which metal is modelled as jellium, the electrolyte as an ensemble of hard spheres. An expression is derived for the interfacial capacity at the potential of zero charge. Numerical model calculations are performed for various metal/solvent systems. The model gives good results for the capacity of second and third row sp metals, and for the temperature dependence of the Hg/water interface.
Nonlinear extensions of square-gradient theory for fluid pair correlations
Authors: Klinger David J. , Fisher Michael E. , Fishman Shmuel
Published: February 2003

A nonlinear extension of the standard square-gradient theory for the pair correlation function g2(r) of a fluid is analyzed in the critical region. For spatial dimensionalities d>d> =μ+2 with μ=2/(δ−1) in which δ can be regarded as the exponent describing the critical isotherm, the critical correlations scale in classical Ornstein–Zernike fashion with η=0. For d>>d>d<=μ, there is scaling but with the nonclassical decay exponent η=d>−d. Finally, there is no true criticality for d<d< since, even if the correlation length is assumed to diverge, the compressibility remains finite. Thus although Fixman’s (1962) extension of square-gradient theory can remove the divergence of Ornstein-Zernike theory in two dimensions, it is intrinsically unsatisfactory since it entails a violation of hyperscaling for d and an unreasonable dependence of the borderline dimensionalities on δ.
New molecular dynamics simulation of a 3D fluid of Stockmayer and modified Stockmayer particles
Authors: Hesse-Bezot C. , Bossis G. , Brot C.
Published: February 2003

This paper is one of a series devoted to numerical simulations of polar fluids which do not employ periodic boundary conditions( PBC) but rather use a spherical isolated sample. In an earlier paper 2D fluids of particles interacting through 2D electrostatics were studied. We present here MD simulations using the same method for 3D fluids with the classical 3D dipolar interaction. Stockmayer particles are first considered, assuming reduced parameters which are close to those adopted by previous authors. Our results for various thermodynamic quantities are in good agreement with those of Pollock and Alder, who used MD with PBC and an Ewald summation, and of Patey et al. who used the MC method without Ewald correction. We have computed the static permittivity from the variation of the mean squared total moment of an inner sphere as a function of the relative volume of this sphere. We found ε=28, in agreement with the result of Pollock and Alder. This figure is much higher than that predicted by the simple Onsager model, and corresponds to a Kirkwood correlation factor gk=2.73. The reason can be understood from maps of average polarization and local dipole correlations. On the other hand, the LHNC and QHNC results of Patey et al. yield a still larger permittivity, which is probably erroneous, as already noted by these authors. The dynamic properties of our system have also been studied—both the dielectric absorption–dispersion vs frequency and the orientational correlation functions are in fair agreement with those of Pollock and Alder mentioned above. We have made the same studies with Stockmayer-like particles having their point dipole displaced along the symmetry axis from the Lennard-Jones center (‘‘decentered Stockmayer particles’’); the dielectric constant thus obtained, ε=11, is much closer to that of the Onsager model, for reasons that are illustrated. Finally, we show that our model fluid of ‘‘decentered Stockmayer particles’’ has reduced parameters similar to those of CH3F at 206 K. We propose a method for establishing a crude correspondence between the nonpolarizable fluids of the simulations and real fluids. Using this correspondence, our static result for the permittivity is in fair agreement with that of CH3F. However, the results for the dynamics from the MD simulation correspond to a rotational friction which is too small, because of the lack of anisotropic steric forces in the model.
Adsorption and reaction of water, oxygen, and hydrogen on Pd(100): Identification of adsorbed hydroxyl and implications for the catalytic H2–O2 reaction
Authors: Nyberg C. , Tengstål C. G.
Published: February 2003

Water adsorption on clean and oxygen covered Pd(100) has been studied by electron energy loss spectroscopy and low energy electron diffraction. On the clean surface water is associatively adsorbed and reveals hydrogen bonding. Hydroxyl is produced upon water adsorption on oxygen precovered Pd(100). The hydroxyl molecule is adsorbed with its molecular axis significantly tilted away from the surface normal. A study of the reactivity of coadsorbed hydrogen and hydroxyl yields new information on the catalytic oxidation of hydrogen on Pd(100). The following reaction path could be inferred: Oad+Had→OHad, 2OHad→Oad+H2Oad→Oad+H2Og.
A large deuterium isotope effect on molecular dipole moments
Authors: Hanson David M. , Winkler Irwin C.
Published: February 2003

Stark effect measurements were made at 4 °K on TMB-1d, and TMB-1h, isolated in durene monocrystals. (AIP)
Outer valence broken-symmetry effects within HF calculations
Authors: Ficker T.
Published: February 2003

Broken-symmetry effects accompanying symmetry unrestricted HF calculations have been studied with the outer valence hole state ⋅⋅⋅ (3σg)1 (1πu)4 1 2∑g of N+2 at various N–N distances. Some deviations from the corresponding effects in the inner valence and core regions of the respective molecular ion have been found.
Theoretical studies of tunneling processes in three-body exchange reactions of van der Waals rare gas dimers
Authors: Turner Randy A. , Raff L. M. , Thompson Donald L.
Published: February 2003

Rare gas atom–diatom collisions of Ar+Ar2, Xe+Ar2, Kr+Xe2, Kr+Ne2, and Kr+NeAr have been investigated to determine the importance of tunneling processes in exchange and dissociation reactions involving van der Waals molecules. Reaction cross sections, angular distributions, and product-energy distributions have been computed using Monte Carlo quasiclassical trajectories. The effect of tunneling through the rotational barrier upon these quantities has been computed using WKB methods. The results show that metastable diatomic products with energies above the classical dissociation limit, but below the rotational barrier, play a significant role in the dynamics of both exchange and dissociation reactions. Lifetime distributions of such metastable dimers illustrate their importance in crossed molecular beam studies of rare gas systems. The WKB calculations indicate that a significant, and possibly measurable, number of the metastables dissociate by tunneling before they would reach the detector in a molecular beam experiment. Close agreement has been found between these calculations and statistical state-counting calculations of metastable product dimer lifetimes. Experiments are suggested that might permit the direct observation of tunneling in these systems.
Two color photoionization spectroscopy of jet cooled aniline: Vibrational frequencies of the aniline X̃ 2B1 radical cation
Authors: Smith Mark A. , Hager James W. , Wallace Stephen C.
Published: February 2003

Photoionization-efficiency spectra of free jet expansion cooled aniline in the 1B2 excited state have been obtained by the technique of two color photoionization spectroscopy. The adiabatic ionization potential of rotationally cooled (<5 K) aniline is determined to be 62 265±18 cm−1. By pumping a variety of vibronic transitions, photoionization-efficiency curves are obtained from discrete vibrational levels in the 1B2 state. The spectra generally show a strong propensity for vertical (Δv=0) ionization. As a result, it has been possible to determine the vibrational frequencies of the ground state aniline 2B1 radical cation with good precision for six vibrational modes. Vibrational progressions observed in the threshold curves support these assignments as well as allowing the observation of two additional modes in the ion which do not manifest themselves in the Ã(1B2)←X̃(1A1) electronic spectrum. The magnitude of these vibrational frequencies compared to those known for the ground and first excited state of aniline suggest that the 2B1 ionic state is more rigidly planar than the 1B2 excited state.
Localized Gaussian wave packet methods for inelastic collisions involving anharmonic oscillators
Authors: Skodje Rex T. , Truhlar Donald G.
Published: February 2003

We examine several methods of implementing the time-dependent Gaussian wave packet method of Heller for collinear atom–molecule collisions involving anharmonic vibrators. We show that reasonably accurate results can be obtained with a procedure involving uncoupled frozen Gaussians with phases evaluated along classical trajectories. Although this method is much more accurate than the standard quasiclassical trajectory method, it involves about the same computational effort.
Bimolecular gas-phase reactions in the Kramers theory of chemical kinetics
Authors: Larson R. S. , Kostin M. D.
Published: February 2003

An expression for the rate constant is obtained for a bimolecular gas−phase reaction via an extended version of the Kramers theory of chemical kinetics. The multicomponent Fokker–Planck equation is transformed into a suitable set of relative coordinates that facilitated the derivation. The derived expression for the rate constant shows explicitly how the rate of reaction depends on the potential energy of interaction between the reactants, the temperature of the system, the friction constant of the surrounding molecules of the heat bath, and the masses of the reactants.
Electronic structure of Cl2 from 5 to 15 eV by electron energy loss spectroscopy
Authors: Spence David , Huebner R. H. , Tanaka H. , Dillon M. A. , Wang R.-G.
Published: February 2003

We have obtained the first high resolution electron energy loss spectra for molecular chlorine over the extended energy-loss range 5.5–14.5 eV at scattering angles between 3° and 9° for an incident electron energy of 200 eV. Spectra obtained at small scattering angles correspond closely to optical absorption spectra, the most prominent features arising from excitation of optically allowed Rydberg states, with other smaller features due to excitation of forbidden states. Our larger angle spectra show additional structures corresponding to excitation of other forbidden states and reveal a previously unobserved band. These angular scattering measurements explain some apparent differences between synchrotron radiation photoabsorption and threshold electron impact excitation spectra, and indicate some structures in the latter measurements to be mislabeled due to the presence of ‘‘hot bands.’’
Time resolved studies of naphthalene mixed crystals. Fractal and Euclidian behaviors of the migration kinetics
Authors: Evesque P. , Duran J.
Published: February 2003

Time resolved experiments have been used in order to study the migration of triplet excitations at low temperature (T<4.2 K) in napthalene D8 crystals doped with 5% to 20% of naphthalene H8 containing a small amount of betamethylnaphthalene. The analysis of the time dependence of the fusion of two triplet excitations leads to clearly deomonstrate the fractal behavior of the exploration at short time and the Euclidian behavior at longer times. The spectral dimension has been measured for the smallest studied concentrations (c<12%); it ranges between 1.33 and 1 which is in very good agreement with a 3D percolation model. The time T0 at which the crossover between the fractal and Euclidian exploration occurs, has been approximately determined for the concentrations. It turns out that the 20% sample does not exhibit this fractal behavior on the studied time scale; this is also in agreement with the percolation model. We have also demonstrated that the time dependence of the fusion of two excitations gives a better insight in the energy migration processes than the time dependence of the supertrapping processes.
Proton spin-lattice relaxation in C2H4 and mixtures of C2H4 with He, A, and N2
Authors: Reddy C. P. K. , Lalita Sarkar K.
Published: February 2003

Proton spin-lattice relaxation times were measured in C2H4 and mixtures of C2H4 with He, A, and N2 as a fuction of density, composition, and temperature (300–600 K) in the region where T1∝ρ using a 30 MHz spin-echo spectrometer. The values of T1/ρ for infinite dilution of C2H4, (T1/ρ)C2H4–X were found to be proportional to T−n, where n=1.43 for C2H4–C2H4, 0.54 for C2H4–He, 1.18 for C2H4–A, and 1.25 for C2H4–N2. These data were analyzed, using Bloom–Oppenheim theory and assuming that the correlation time of the spin−rotation interaction in a J state is equal to the lifetime of a molecule in that J state, to obtain information on the spin-rotation coupling constant, up to a constant, as well as the intermolecular anisotropic interactions responsible for spin-lattice relaxation in these systems.
The visible spectrum of jet-cooled CClF2NO
Authors: Ernsting N. P.
Published: February 2003

The nπ* electronic transition of CClF2–N=O, around 680 nm, is reported in detail for the first time. The spectrum was studied at low temperatures by laser excitation of fluorescence in a pulsed supersonic free jet. A characteristic long progression is assigned to torsion around the C–N bond in the excited state. The dispersed fluorescence is compared with the absorption spectrum of the static gas at 195 K. The electronic origin is located at 7047 Å, and the sharp absorption bands at longer wavelengths are assigned to torsional hot bands. Vibrational assignments of the electronic spectra are discussed. There is evidence that the gauche conformer of the excited state and only one conformer of the ground state are mainly responsible for the red side of the excitation spectrum in a cold jet.
Orientational ordering and site structure for CCl4 in a krypton matrix
Authors: Jones Llewellyn H. , Swanson Basil I.
Published: February 2003

Infrared studies of the ν3 and ν1+ν4 modes of CCl4 trapped in krypton matrices show that the CCl4 molecules adopt two sites, one of which preserves the triple degeneracy while the other shows site symmetry splitting and a high degree of orientational ordering. At deposition temperatures above 35 K the orientational ordering of the low symmetry site is diminished as is the relative concentration of this site compared to the other. The results suggest that both cubic-close-packed and hexagonal-close-packed environments exist in these matrices. Significant differences in the ν3 vs ν1+ν4 line widths have been observed and attributed to rapid V–V energy transfer from the higher to the lower of these Fermi coupled modes.
Electron impact ionization of (N2)2: Appearance energies of N+3 and N+4
Authors: Stephan K. , Märk T. D. , Futrell J. H. , Helm H.
Published: February 2003

Using a molecular beam ionization mass spectrometer apparatus the appearance energies for the processes (N2)2+e→N+4+2e and (N2)2+e→N+3+N+2e were measured. From these measurements, the exothermicity of the ion clustering reaction N+2+N2 to form N+4 was deduced to be (0.9±0.2) eV and the lower limit for the dissociation energy of N+3 into N2(X 1∑+g)+N+(3P) was established to be (3.4±0.2) eV.
The IR study on the rotation of C≡N− ion in the triethylamine–hydrogen cyanide system at 77 K
Authors: Kanesaka Isao , Kiyokawa Yoshinobu , Kawai Kiyoyasu
Published: February 2003

The IR spectrum of the C≡N stretching region in the triethylamine–hydrogen cyanide-d1 system is reported, where there are some structures. They are analyzed on the basis of a rotation-vibration coupling model. Vibrational data is interpreted on the basis of occupancy of two different sites for the C≡N− ions.
The quantum mechanical calculation of rotational spectra. A comparison of methods for C2H2, HCN, HNC, HCO+, N2H+, CO, and N2. Predictions for HCNH+, COH+, HBO, HBNH, and HBF+
Authors: DeFrees D. J. , Binkley J. Stephen , McLean A. D.
Published: February 2003

Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of ±0.4 GHz when an empirical correction is applied to the results for C2H2, HCN, HNC, HCO+, N2H+, CO, and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J=0→1 rotational frequencies using these methods are 73.9±0.4 GHz for HCNH+, 78.6±0.4 GHz for HBO, 65.8±0.4 GHz for HBNH, and 72.1±0.4 GHz for HBF+.
Triplet correlations in the hard rod fluid: A test for topological reduction of graph-theoretic corrections to the superposition approximation
Authors: Haymet A. D. J.
Published: February 2003

Two series expansions for the triplet correlation function, which have been used previously to study three-dimensional liquids, are evaluated in a case where the exact triple correlation function is known, namely, hard rods in one dimension. These series are studied in the context of the Yvon–Born–Green (YBG) integral equation. The coefficients in the f-bond series are evaluated analytically, but the resultant corrections to the superposition approximation are minor. In contrast, the coefficients of the h-bond series, which are calculated numerically, provide an accurate approximation to the triplet correlation function for densities of interest below two-thirds of the close-packed density. The validity of the ‘‘scaling’’ approximation of the h-bond series, which has been used in theories of quantum liquids, is also examined, and these calculations are shown to be relevant to earlier studies of three-dimensional liquids.
Brillouin scattering studies of the successive phase transitions in (CH3NH3)2FeCl4
Authors: Yoshihara A. , Burr J. C. , Mudare S. M. , Bernstein E. R. , Raich J. C.
Published: February 2003

Brillouin scattering spectra are reported and analyzed for the layered perovskite compound (CH3NH3)2FeCl4. Particular attention has been given to successive phase transitions in this system: D174h ↔ D182h ⇄ D164h. It is found that the elastic constant c66 is the major contributor to instabilities in this system. A two-dimensional order parameter at the Brillouin zone boundary X point is introduced into the Landau free-energy to account for this sequence of transitions. An additional secondary instability (one-dimensional order parameter) at the Z point is used to generate the D182h ⇄ D164h phase transition: inclusion of the two order parameters circumvents the need for postulating a D102h phase between the D182h and D164h phases and for higher order temperature dependent terms in the free energy expansion. The D174h ↔ D182h phase transition is characterized by a strong Landau–Khalatnikov contribution to c66 and a large dynamical critical behavior.
Kinetic oscillations in oxidation of CO over Pt(100): A study by Rutherford backscattering, nuclear microanalysis, LEED, and work function techniques
Authors: Norton P. R. , Bindner P. E. , Griffiths K. , Jackman T. E. , Davies J. A. , Rüstig J.
Published: February 2003

The structure and composition of a Pt(100) surface have been monitored by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED, and work function (Δφ) techniques during temporal oscillations in the rate of CO oxidation. For a Δφ oscillation amplitude of 60 mV, a constant 0.46±0.06×1015 Pt atoms are out of registry with the bulk throughout an oscillation cycle. The maximum fraction of the surface that could be oscillating between the hex and (1×1) phases is thus 8%. The average CO and O coverages during the oscillations are 0.19±0.04 and 0.13±0.03 monolayers, respectively. These results are discussed in terms of a recent model for the temporal oscillations. No oscillations in Δφ have been detected in similar experiments on Pt(111).
Linear response theory of long-range solvent effects on ππ* circular dichroism of helical polypeptides
Authors: Rabenold David A. , Rhodes W.
Published: February 2003

A linear response theory is presented for the long-range solvent effect on the ππ* circular dichroism spectra of long helical polypeptides. A time dependent Hartree approximation and a partitioning technique is employed to obtain susceptibilities. Periodic boundary conditions are used to describe the polymer while the solvent is considered to be a polarizable isotropic continuum. The results of the theory are discussed in the context of applying it to an α-helical polypeptide in water. The theory is also compared to a less accurate scheme, a matrix method.
A semiclassical analysis of curvature corrections in quantal collinear reactive scattering
Authors: Pollak Eli , Römelt Joachim
Published: February 2003

The orthogonal curvilinear coordinate system defined by periodic orbits is used to analyze the quantum mechanical approximation of vibrational adiabaticity in collinear reactive scattering. We show that with this system, the quantal close coupled equations decouple at an adiabatic barrier or well. The classical adiabatic frequency of a periodic orbit is shown to be the classical analog of the quantal adiabatic frequency at a barrier or well. Finally, we find that the success of the recently formulated DIVAH theory, in quantitative prediction of quantal resonance energies, is due to the fact that the diagonal correction terms appearing in this theory serve as curvature corrections. Adding the diagonal terms in the adiabatic approximation in radial coordinates compensates to a large extent for the difference in curvature of these coordinates and the system defined by periodic orbits. Numerical examples include the H3, HMuH, and IHI systems.
Compensation effect in the dissociation of anharmonic oscillators: A model for catalysis
Authors: McCoy B. J.
Published: February 2003

The differential form of the master equation for an anharmonic oscillator is solved in a steady-state approximation to derive an expression for the rate constant for dissociation. A small increase of the anharmonicity parameter causes a decrease of activation energy and thus a very large increase in the rate constant. The energy of activation is nearly linear with the logarithm of the preexponential when the magnitude of anharmonicity varies. This compensation effect, along with the sensitivity of the rate constant to anharmonicity, suggests a relationship to catalyzed dissociation.
Measurements of absolute total cross sections for charge transfer and electron detachment of halide ions on chlorine
Authors: Huq M. S. , Scott D. , White N. R. , Champion R. L. , Doverspike L. D.
Published: February 2003

Absolute total cross section measurements are reported for electron detachment and for reactive scattering between the halide ions (Cl−, Br−, and I−) and chlorine gas Cl2. The charge transfer and dissociative charge transfer cross sections are found to be very large at their maxima, which occur at a relative collision energy of about 13 eV. The electron detachment cross section for I−+Cl2 is anomalously low. Some energy loss spectra are reported for I−+Cl2. They exhibit substantial inelastic scattering which is consistent with the calculated potentials of Cl2.
Detection of small polydispersities by photon correlation spectroscopy
Authors: Pusey P. N. , van Megen W.
Published: February 2003

We consider dilute suspensions of homogeneous polydisperse spherical particles for which the Rayleigh–Gans–Debye (RGD) approximation is valid. For two model particle size distributions we calculate the dependence on scattering vector Q of the average scattered intensity I(Q) and the effective diffusion coefficient De(Q) obtained from the first cumulant measured by photon-correlation spectroscopy. If the mean particle radius R̄ is large enough (≳170 nm) that the intensity form factor P(QR̄) shows at least one minimum in the accessible range of Q, we find that De(Q) exhibits a characteristic variation with Q which is very sensitive to the sample polydispersity. Under favorable conditions it should be possible to measure polydispersities (standard deviation/mean size) as small as 0.01. These theoretical considerations are supported, at least qualitatively, by experiment. We also discuss briefly the effect of relaxing the RGD approximation and the implications of this work for the more common PCS probes of polydispersity such as Laplace transformation of the light scattering correlation function.
Angle resolved photoelectron spectroscopy of the valence shells in HI and CH3I as a function of photon energy from 13 to 90 eV
Authors: Carlson Thomas A. , Fahlman Anders , Krause Manfred O. , Keller Paul R. , Taylor James W. , Whitley Timothy , Grimm Frederick A.
Published: February 2003

Angle-resolved photoelectron spectroscopy was carried out on the valence shells of HI and CH3I over a photon energy range of 13–90 eV with the use of synchrotron radiation. From these data, partial cross sections and angular distribution parameters were obtained for bands corresponding to the 1 2Π and 1 2Σ+ states of singly ionized HI and the 1 2E, 1 2A and 2 2E states of singly ionized CH3I. Calculations were carried out on HI for the same parameters with the use of the multiple scattering Xα method. The results, both experimental and theoretical, are examined in terms of the Cooper minimum. Clear evidence for minima in both the cross sections and β values are found for the lone-pair orbitals. Bonding orbitals involving the iodine 5p subshell also show minima in the energy dependence plot of the β values, but the effects are less pronounced. These results are compared with similar studies on chlorine-containing molecules. The lone-pair orbitals showed spin-orbit resolved bands in the photoelectron spectra. The behavior of these spin-orbit split states are examined for their differences in β values and branching ratios as a function of photon energy up to 35 eV.
Rotational relaxation rates for the OCS J=0–1 pure rotational transition broadened by argon and helium
Authors: Burns Mark J. , Coy Stephen L.
Published: February 2003

We have recorded time-domain coherence decay and population decay signals for the pure rotational OCS J=0–1 transition broadened by argon and by helium using pulse methods based on Stark switching of the molecular resonance frequency. The signals have been analyzed to yield the pressure dependence of the two decay rates (1/T2 and 1/T1) and their dependence on the absorber (OCS) speed. For comparison, using a potential which has been used several times in the literature for these systems, IOS (infinite order sudden approximation) scattering results are given. These calculations give substantial agreement on the ratio of the speed dependence of the relaxation rate to the rate itself, but fail to predict the rates quantitatively, especially for helium, or the relationship between the argon and the helium results. Because the IOS is expected to be accurate for these systems, the helium potential is ruled out by these data.
Electrostatic potential at the nucleus in atomic ions and relation to chemical potential
Authors: Hill S. H. , Grout P. J. , March N. H.
Published: February 2003

Numerical values are presented, at self-consistent field accuracy, for the electrostatic potential at the nucleus created by the electron cloud of atomic ions. Via the total energies of these ions, this numerical data is related to Thomas–Fermi and 1/Z expansion results. For a given element, the way the chemical potential differences arising from different degrees of ionization can be estimated is discussed, as well as the relation to the Koopmans’ ionization potential.
Electron–ion recombination in water vapor
Authors: Morgan W. L.
Published: February 2003

A Monte Carlo simulation of electron–ion recombination in the presence of water vapor has been performed. The calculations cover a range of H2O densities and are in agreement with published low density experimental results. It is found that inelastic collisions of electrons with H2O rotational and vibrational modes enhance both the two-body (dissociative recombination) and the three-body recombination rate coefficients.
Temperature dependence of solvation forces
Authors: Christenson Hugo K. , Israelachvili Jacob N.
Published: February 2003

The oscillatory solvation force measured between two molecularly smooth surfaces in a water-free nonpolar liquid shows no measurable change with temperature up to 10 °C above and (in the supercooled state) 3 °C below its melting point.
Fundamentals and mechanisms of quantum localization in multidimensional correlated systems
Authors: Hose Gabriel , Taylor Howard S. , Bai Yi Yan
Published: February 2003

Coupled motion in many degrees of freedom is known to often show in classical mechanics unexpectedly localized quasiperiodic motion that could not have been anticipated from the potential function. Similar phenomena is observed in quantum theory and this article discusses the fundamental physics of such localization. It is argued below that a Born–Oppenheimer type approximation leads to dynamic potentials that make the nonobvious localization obvious. Applications are mentioned for photoselective chemistry, absorption spectroscopy, and chemical mechanisms.
A study of Be2 with many-body perturbation theory and a coupled-cluster method including triple excitations
Authors: Lee Yoon S. , Bartlett Rodney J.
Published: February 2003

The unusual potential energy curve for the 1∑+g ground state of Be2 is investigated using many-body perturbation theory (MBPT) and coupled-cluster (CC) methods. The curve, which has a ∼2 kcal/mol inner minimum at ∼2.6Å and a van der Waals minimum at ∼5.0 Å, is very difficult to describe accurately with even high-level ab initio correlated methods. To resolve uncertainties in previous MBPT/CC studies, we have generalized CC theory to include effects of triple excitations. The present calculations are compared with recent full CI results to assess the relative importance of different contributions of electron correlation. MBPT (4) is found to be qualitatively correct, but to slightly exceed the correct full CI binding energy, while CC theory even with triple excitations, has no inner minimum. The latter follows from CC theory being exact for separated Be atoms (with frozen core) but to have a 1% correlation energy error in the binding region. Yet this 1% accounts for the ∼2 kcal/mol inner well. The possibility of the remaining 1% arising from connected quadruple excitations T4 is discussed.
On the rotational relaxation of a rodlike macromolecule in a concentrated solution
Authors: Fesjian Sezar , Frisch H. L.
Published: February 2003

The Onsager theory for a concentrated system of rods is amended by a free energy term due to Kramers to include the effects of an external flow. Minimizing this quasiequilibrium free energy in the limit of zero flow we obtain an expression for the concentration dependence of the rotational relaxation time τ(c) of a given rod. It is found that, for this model, τ(c) reaches about 5.7 times its dilute solution value at the isotropic-nematic transition, and is practically a constant in the anisotropic phase.
Size effects in the excited electronic states of small colloidal CdS crystallites
Authors: Rossetti R. , Ellison J. L. , Gibson J. M. , Brus L. E.
Published: February 2003

This paper reports experimental studies of the development of bulk optical properties as a function of crystallite size for the inorganic direct gap semiconductor CdS. Small crystallites are synthesized via colloidal chemical techniques, and their optical properties are studied in situ at extreme dilution. The crystallites are characterized via high resolution transmission electron microscopy. Direct images show (111) lattice planes, and establish the crystallite structures as close to those of excised fragments of bulk CdS (zinc-blende cubic). Large crystallites (> 100 Å average diameter) show an optical absorption, in colloidal solution, close to that of bulk crystalline material. However, small crystallites of 30 Å average diameter show a large blue shift (∼0.8 eV) in absorption edge (effective band gap), and an intensification of edge absorption relative to absorption at higher energy regions. These observations can be understood as quantum size effects resulting from confinement of an electron and hole in a small volume. 40 Å average size crystallites show a smaller shift (∼0.25 eV), and corresponding changes in their fluorescence, and resonance Raman excitation, spectra.
Calculation of orientational pair correlation factors with the interaction site formalism
Authors: Richardson Diane M. , Chandler David
Published: February 2003

The arrangement of the interaction site cluster series provided by the proper integral equation theory for polyatomic fluids is applied in a perturbative fashion, using solutions to the RISM equation as input, to obtain g2 factors for liquid carbon disulfide. The theoretical calculations are in good accord with experiment and simulation. The methodology appears to be rather general in its applicability and possible extensions to polar systems are described.
A non-Gaussian theory of rubberlike elasticity based on rotational isomeric state simulations of network chain configurations. II. Bimodal poly(dimethylsiloxane) networks
Authors: Curro John G. , Mark James E.
Published: February 2003

Bimodal, poly(dimethylsiloxane) (PDMS) networks containing a large mole fraction of very short chains have been shown to be unusually tough elastomers. The purpose of this investigation is to understand the rubber elasticity behavior of these bimodal networks. As a first approach, we have assumed that the average chain deformation is affine. This deformation, however, is partitioned nonaffinely between the long and short chains so that the free energy is minimized. Gaussian statistics are used for the long chains. The distribution function for the short chains is found from Monte Carlo calculations. This model predicts an upturn in the stress-strain curve, the steepness depending on the network composition, as is observed experimentally.
Dielectric enhancement and electrophoresis due to an electrochemical double layer: A uniform approximation
Authors: Chew W. C.
Published: February 2003

We present here a method to calculate the response of a charged, insulating sphere immersed in an electrolytic solution in the presence of an externally applied electric field. Due to the charge on the sphere, it acquires a layer of counterions on the outside known as the electrochemical double layer. Since this double layer is complex, the response of the sphere to an externally applied electric field is complicated. In the past, ad hoc models have been proposed to circumvent the difficulty in the analysis of this problem. Recently, we presented a rigorous analysis of this problem using the technique of matched asymptotic expansions commonly used in boundary layer theory. Our previous solution was not valid for high zeta potential that corresponded to a highly charged sphere. In this paper, we present an analysis where this restriction is removed, and incorporate the effect of induced solvent osmotic flow. Our solutions is valid for larger value of the zeta potential compared to previous works. When the zeta potential is zero, corresponding to an uncharged sphere, we reproduce the classical result of an insulating sphere in a conducting medium. When the zeta potential is high, we reproduce a correction to the result of Fixman and O’Brien. With the response of one sphere known, we can use it to calculate the dielectric response of an ensemble of spheres, for example, in a colloidal suspension.
Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production
Authors: Braun Charles L.
Published: February 2003

The lowest charge-transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited. Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton. The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O-34) of ion pair dissociation. This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials.
Study of collisional relaxation in NH3 by steady-state, infrared–infrared double resonance
Authors: Kuze Hiroaki , Jones Harold , Tsukakoshi Motowo , Minoh Arimichi , Takami Michio
Published: February 2003

Steady-state double-resonance technique is applied to the study of vibrational and rotational relaxation in NH3. A tunable diode laser is used to probe the population change caused by CO2 laser irradiation. Signals are observed for the mixture of 15NH3 and 14NH3, while the 15NH3 ν2 asQ(5,4) transition is pumped by the 13CO2 10 μm R(18) laser line. A theory based on rate-equation approximation is developed to elucidate the contributions from thermal effects and intramolecular relaxation in steady-state double resonance. The signals observed for 14NH3 and ortho (K=3n) 15NH3 are due to thermal effect induced by intermolecular energy transfer. The increase of vibrational temperature by laser irradiation is found to be an order of magnitude larger than increase of rotational temperature. The branching ratio of intramolecular rotational relaxation, which includes collision-induced transitions from the pumped levels and subsequent cascading processes, is determined for para (K≠3n) 15NH3 in the ground and vibrationally excited states by correcting the thermal effect.
A numerical variational method for calculating vibration intervals of bent triatomic molecules
Authors: Cropek Donald , Carney Grady D.
Published: February 2003

A new combination of known methods is used to calculate vibrational energies and wave functions for bent symmetric triatomic molecules. The algorithm is a variational procedure which uses curvilinear coordinates, a numerical basis set, and numerical quadrature. Molecules studied include H2O, O3, H2D+, H2T+, and D2T+.
Expansion methods for the Dirac equation
Authors: Gazdy Béla , Ladányi Károly
Published: February 2003

The applicability of expansion methods to the solution of the Dirac equation is investigated. Criteria are given to distinguish between acceptable and spurious solutions. A careful analysis of the computed eigenvalues as functions of the nonlinear parameters characterizing the basis functions is presented. A definition is given for the measure of the error of the wave function which is calculated by a variational method based on a minimum principle. Illustrative numerical calculations are presented for the one-particle Dirac equation involving Coulomb and modified Coulomb potentials.
Dipole moment and hyperfine properties of the ground state and the C–H excited vibrational state of HCN
Authors: Ebenstein William L. , Muenter J. S.
Published: February 2003

Radio frequency transitions have been measured in the J=1 rotational level of HCN for both the ground vibrational state and the first excited state of the C–H stretching mode using molecular beam electric resonance spectroscopy. The vibrationally excited state was populated with an infrared laser. The dipole moment and hyperfine properties for the ground and excited states are 〈μ〉000=2.985 188(3)D, 〈μ〉001=3.017 408(4)D, 〈eQq〉000=−4707.83(6) kHz, 〈eQq〉001=−4661.39(8) kHz, 〈CN〉000=10.13(2) kHz, 〈CN〉001=10.06(3) kHz, 〈CH〉000=−4.35(5) kHz, 〈CH〉=−4.21(3) kHz.
Experimental application of the new theory of slowly tumbling axially symmetric ESR hyperfine centers in amorphous samples
Authors: Lee Sook , Brown I. M. , Ames D. P.
Published: February 2003

The results of new theoretical treatments of axially symmetric ESR hyperfine centers undergoing small angle, slow orientational diffusion motion in amorphous samples are applied to a polycarbonate polymer containing nitroxide spin probes over the temperature range 77–295 K. In this experimental application, the conventional first-derivative as well as second-derivative nitroxide hyperfine spectra at X-band microwave frequencies are obtained and the motional linewidths and shifts in spectral line positions of the low-field (M=+1) and the high-field (M=−1) nitroxide hyperfine parallel-edge lines are determined from the experimental spectra. By correlating the experimental data with the theory, it is possible to determine the temperature dependences of the nitroxide tumbling motional correlation times τ, the motional narrowing of the inhomogeneous linewidths, and the nitroxide hyperfine coupling constant. The present method provides a simple and direct way of determining experimental τ values in the range ∼1×10−5 to ∼8×10−8 s for nitroxide spin probes/labels tumbling slowly around the external magnetic field direction in amorphous samples.
Energy transfer and catalytic decomposition of ammonia on rhenium at high temperatures
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

Single-collision energy transfer and catalytic decomposition of NH3 on polycrystalline rhenium at temperatures up to 2300 K were studied with a modulated molecular beam mass spectrometer. Internal energy excitation was determined from threshold ionization measurements. The internal energies acquired by NH3 molecules on collision with the heated filament are well correlated with their acquired translational energies. Nitrogen molecules produced in the reaction are vibrationally excited at levels compatible with the filament temperature, in contrast to the results obtained in the reaction on Pt, where significant quantities of highly vibrationally excited N2 molecules were produced. The principal source of N2 in the reaction is the recombination of chemisorbed NH radicals: NHad+NHad→N2+H2. Appearance potential measurements on NH+2 show no evidence for evolution of NH2 radicals. At high temperatures (1600–2300 K), H atoms are produced. The reaction mechanisms involved in the decomposition of NH3 are discussed.
The gas phase structure of trifluoromethylisocyanide CF3NC. An electron diffraction and microwave spectroscopy study
Authors: Christen Dines , Ramme Klaus , Haas Bernhard , Oberhammer Heinz , Lentz Dieter
Published: February 2003

The gas phase molecular structure of trifluoromethylisocyanide has been determined by the combined use of electron diffraction and microwave spectroscopy, implementing the results of a normal coordinate analysis. The following geometric parameters (rav values with 2 σ error limits) have been derived: C–F=1.324(1) Å, C–N=1.407(3) Å, N=C=1.171(3) Å, and FCF=108.8(1) °. Furthermore, dipole moment [μ=1.153(20) D] and quadrupole coupling constants [χ=1.06(3) MHz] have been determined.
Vibrational excitation of CH3F by collisions with ‘‘hot’’ hydrogen atoms
Authors: Datta Saswati , Weston Ralph E. , Flynn George W.
Published: February 2003

Infrared emission has been observed from CH3F collisionally excited by hot hydrogen atoms produced in the photolysis of H2S at 193 nm. The time-dependent emission at 2.5 (ν1+ν3, ν4+ν3), 3.3 (ν1, ν4), and 9.6 μm (ν3) has been analyzed, and both T-V and V–V energy exchange processes are observed. Relative populations of molecules excited into the ν3 and ν1, ν4 modes were determined; the population in the C–H stretching modes (ν1, ν4) is much lower than predicted by simple impulsive models of the collision process.
Sensitivity analysis of limit cycles with application to the Brusselator
Authors: Larter Raima , Rabitz Herschel , Kramer Mark
Published: February 2003

Sensitivity analysis, by which it is possible to determine the dependence of the solution of a system of differential equations to variations in the parameters, is applied to systems which have a limit cycle solution in some region of parameter space. The resulting expressions for the sensitivity coefficients, which are the gradients of the limit cycle solution in parameter space, are analyzed by a Fourier series approach; the sensitivity coefficients are found to contain information on the sensitivity of the period and other features of the limit cycle. The intimate relationship between Lyapounov stability analysis and sensitivity analysis is discussed. The results of our general derivation are applied to two limit cycle oscillators: (1) an exactly soluble two-species oscillator and (2) the Brusselator.
Equidensity and pairwise equidensity orbitals for systems of noninteracting fermions
Authors: Capitani Joseph F. , Chang Britton , Harriman John E.
Published: February 2003

Sets of orbitals are considered, in each of which all the orbitals correspond either individually or pairwise to a single density. We address the questions of whether such orbitals can themselves be a set of solutions of a single, one-particle Schrödinger equation, or whether they can be linearly equivalent to such a set. This is possible for one or two particles, but not in general for more than two particles. Equations relating density, phase and potential in the two-particle case are obtained and discussed. The special form in which the phase is an integral of the density is not in general possible, but a particular density in one dimension for which such is the case is presented. We conclude that equidensity orbitals are rarely of physical significance.
Analytic second derivatives for Renner–Teller potential energy surfaces. Examples of the five distinct cases
Authors: Lee Timothy J. , Fox Douglas J. , Schaefer Henry F. , Pitzer Russell M.
Published: February 2003

Force constants at Renner–Teller stationary points fall into five distinct categories, which are readily explored using recently developed analytic energy second derivative methods. The nature of the Renner–Teller effect has been derived using symmetry principles and many-electron perturbation theory. By considering the BH2, CH2, NH2, and CuH2 molecules, examples of all five cases are illustrated via ab initio molecular electronic structure theory.
Rates of convergence of variational calculations and of expectation values
Authors: Klahn Bruno , Morgan John D.
Published: February 2003

We present a mathematical and numerical analysis of the rates of convergence of variational calculations and their impact on the issue of the convergence or divergence of expectation values obtained from variational wave functions. The rate of convergence of a variational calculation is critically dependent on the ability of finite linear combinations of basis functions to simulate the nonanalyticities (cusps) in the exact wave function being approximated. A slow rate of convergence of the variational energy can imply that the corresponding variational wave functions will yield divergent expectation values of physical operators not relatively bounded by the Hamiltonian. We illustrate the sorts of problems which can arise by examining Gauss-type approximations to hydrogenic orbitals. Since all many-electron wave functions have cusps similar to those in hydrogenic wave functions, this simple example is relevant to variational calculations performed on atoms and molecules. Finally, we offer suggestions on what types of variational wave functions are likely to yield rapid rates of convergence for the energy and reasonable rates of convergence for physical operators such as the dipole moment operator.