bluuebunny/crossref_metadata_2025 huggingface dataset:
Polydiacetylenes in solution; charged rods and neutral coils
Authors: Zacher Robert A.
Published: July 2002

We report experimental results on several polydiacetylenes and a polythiophene in solution which demonstrate a change in charge state associated with solvatochromic and thermochromic transitions. The planar conformations are charged and the nonplanar conformations are neutral. When fields of ≊600 V/cm were applied to solutions of polymer in chloroform–hexane mixtures or toluene (i.e., the polymer is in the extended state), the polymers plated out on one electrode. We have measured the current as a function of time associated with the deposition of the polymer to determine the quantity of charge per polymer. The time dependence of the current had two regimes which can be associated with two types of charged material in the solution: the charged polymer, which plates out, and an unidentified charge transfer species which does not plate out. Polydiacetylenes 4BCMU and 3BCMU in solutions of chloroform/hexane became positively charged. Poly-3-hexyl-thiophene also became positively charged. PDA-9PA and PDA-92NA became negatively charged. We found a charge of 3×10−4 e/monomer for 4BCMU, 7×10−3 e/monomer for 3BCMU, and 4×10−2 e/monomer for PDA-9PA. Using this effect, we can demonstrate that both rod and coil segments coexist on the same polymer chain in the intermediate stages of the rod–coil transition. We suggest a mechanism which can qualitatively account for this unusual behavior. We also describe novel experiments in which an electric field was used to drive the polymers across a boundary between two nonmiscible solvents. When the polymer crosses the boundary, it changes color. The possibility of using the plating effect for precise fractionation by electrophoresis is suggested.
Comment on: ‘‘Chaos in the Showalter–Noyes–Bar–Eli model of the Belousov–Zhabotinskii reaction’’
Authors: Györgyi László , Field Richard J.
Published: July 2002

The recent numerical work of Lindberg et al. convincingly demonstrates that chemical chaos in a continuous flow, stirred tank reactor (CSTR) can be reproduced by a spatially homogeneous, accurate model of the kinetics of the Belousov–Zhabotinskii(BZ) reaction. However, some problems remain. The chaos in this model and two others, one using an accurate model of the chemical kinetics in conjunction with spatial inhomogeneity resulting from the finite CSTR mixing time and the other using a flawed model of the BZ chemical kinetics, results from coupling of two cycles coexisting within the complex dynamic model. The second cycle in the case of the homogeneous models involves a product of the main chemical limit cycle which is present at a high average concentration. In the Lindberg et al. model this product is assumed to be HOBr. It is clear, however, that a large [HOBr] does not accumulate in the real system because of its rapid reaction with Br−. We suggest that while the Lindberg et al. results are clearly important, this process still needs to be accounted for. Furthermore, the rate parameter values used by Lindberg et al. are not those currently thought to be correct, and the chaos disappears if the accurate rate constant values are used. We discuss why this is so. It is further argued that the Lindberg et al. results do not eliminate the possibility that at least part of the experimentally observed CSTR chaos results from effects related to incomplete mixing.
Phase measurements of optical second harmonic generation on Au(111) during thallium underpotential deposition
Authors: Koos D. A. , Richmond G. L.
Published: July 2002

Phase measurements are used for the first time to monitor the resonant contribution from an adsorbate to the anisotropic second harmonic response from a metal electrode. Changes in the phase of the optical second harmonic response from Au(111) are measured during electrochemical deposition of thallium. The phase of the signal at coverages of one and two monolayers of thallium relative to the adsorbate free surface is delayed by 180° and 258°, respectively. Possible sources for the observed nonlinearities are discussed.
O(1D) lifetime shortening in SF6 matrix
Authors: Mohammed Hassan H.
Published: July 2002

Using a xenon resonance lamp emitting photons at 8.4 eV, CO2 isolated in an SF6 matrix was studied. O(1D) was formed during the photolysis of the matrix sample at 4.5 K. The continuum emission which corresponds to the CO2 (Ã 3B2→X̃ 1∑+g ) transition was also observed. The lifetime of this continuum was measured and found to be 451±25 ms. The lifetime of the O(1D) level was measured for the first time in the SF6 matrix and found to be 780±10 ms. This shortening of the O(1D) level lifetime is attributed to the external heavy atom effect, which enhances the spin–orbit coupling induced by the matrix.
Magneto-optic spectra of the A 2Σ+–X 2Πi transition of OH
Authors: Takubo Y. , Miyata Y. , Amemiya K. , Yamamoto M.
Published: July 2002

We investigated the resonant magneto-optic spectra of the electronic band A 2Σ+–X 2Πi of the OH radical. The theoretical analysis was made under the condition that OH radicals were subjected to an externally applied magnetic field, which was oriented in a direction longitudinal (the Faraday configuration) or transverse (the Voigt configuration) to the optical path. Light interacting with the OH radicals was assumed to be elliptically polarized. The polarization change caused by the magnetically induced optical anisotropy of the OH radical was calculated as a function of the frequency and the ellipticity of the incident light, the orientation of the polarization analyzer, and the number density of the OH radicals. Experiments were carried out in the Voigt configuration using a narrow-band tunable dye laser as a light source. The observed profiles of the Voigt-effect signal for the rotational lines of the various branches agreed with theoretical predictions.
Fourier transform spectroscopy analysis of the 3d-triplet complex visible emission spectra of H2
Authors: Alikacem A. , Larzillière M.
Published: July 2002

An investigation of the triplet 3d-complex of H2, the g(3d) 3Σ+g, i(3d) 3Πg, and j(3d) 3Δg and the nearby h(3s) 3Σ+g electronic states, has been performed. The 3s,3d→c (2p) 3Π−u visible emission spectra have been obtained with a low pressure discharge lamp and recorded with a Fourier transform spectrometer. Transition wave numbers and rotational energy levels are given for v=0–3 of the four states. An effective Hamiltonian is used to fit the data, which shows the rotational breakdown of the Born–Oppenheimer approximation due to the L-uncoupling effect. The eigenvectors obtained from this model provide considerable insight into the nature of the 3d-complex. Furthermore, the model shows the mixing between the 3d states and the h(3s) 3Σ+g state which is particularly strong for lower v values. Molecular constants, coupling parameters, and term values are also given.
Molecular dynamics study of crystalline F2: Velocity autocorrelation functions and thermodynamic properties
Authors: Halac E. B. , Gamba Z.
Published: July 2002

Molecular dynamics simulation techniques have been applied to study the monoclinic and cubic phases of cyrstalline F2. Dynamical properties are analyzed by calculating the velocity autocorrelation functions and their associated power spectra. From the resulting phonon densities of states, thermodynamic properties are derived and compared with experimental data.
Calculation of the free energy of solids from the energy distribution function
Authors: Phillpot S. R. , Rickman J. M.
Published: July 2002

We outline a new methodology for the calculation of the Helmholtz and the Gibbs free energies from a cumulant analysis of the internal-energy and enthalpy distribution functions obtained from a Monte Carlo simulation at a single temperature. It is validated from a comparison of the free energies calculated for a perfect crystal by using this approach with the free energy calculated from a temperature-integration scheme. The new method allows the calculation of the free energy at all temperatures up to melting from a single Monte Carlo simulation at one temperature only. Thus, it is approximately 3–6 times more computationally efficient than a temperature- integration scheme. By comparing the two methods for an inhomogeneous system containing grain boundaries, we investigate the effects of the distribution of local strain on the free energy; for the superlattice of twist grain boundaries studied here, the effects are found to be small.
An efficient formulation and implementation of the analytic energy gradient method to the single and double excitation coupled-cluster wave function: Application to Cl2O2
Authors: Rendell Alistair P. , Lee Timothy J.
Published: July 2002

The analytic energy gradient for the single and double excitation coupled-cluster (CCSD) wave function has been reformulated and implemented in a new set of programs. The reformulated set of gradient equations have a smaller computational cost than any previously published. The iterative solution of the linear equations and the construction of the effective density matrices are fully vectorized, being based on matrix multiplications. The new method has been used to investigate the Cl2O2 molecule, which has recently been postulated as an important intermediate in the destruction of ozone in the stratosphere. In addition to reporting computational timings, the CCSD equilibrium geometries, harmonic vibrational frequencies, infrared intensities, and relative energetics of three isomers of Cl2O2 are presented. The relative energies of the three isomers are further investigated using large atomic natural orbital basis sets in conjunction with the CCSD(T) method, which includes a perturbational estimate of connected triple excitations. The peroxide form of Cl2O2 is predicted to be the lowest energy isomer with the ClClO2 form lying 5.1±3.5 kcal/mol higher in energy.
The Ne–O2 potential energy surface from high-resolution diffraction and glory scattering experiments and from the Zeeman spectrum
Authors: Beneventi L. , Casavecchia P. , Pirani F. , Vecchiocattivi F. , Volpi G. G. , Brocks G. , van der Avoird A. , Heijmen B. , Reuss J.
Published: July 2002

The full anisotropic potential energy surface for the Ne–O2 van der Waals molecule has been obtained from the simultaneous analysis of total differential cross section data with well resolved diffraction oscillations and of absolute total integral cross section data with glory structure measured in Perugia, of the Zeeman spectrum measured in Nijmegen, and of transport coefficients taken from literature. The anisotropy of the interaction is obtained from the Zeeman spectrum using both numerical and analytical approaches for the energy levels, and from the quenching of the diffraction oscillations in the total differential cross section within the infinite-order-sudden approximation. The latter approximation is practically exact under the experimental conditions, as previously shown for the similar Ne–N2 system by comparison with exact close-coupling scattering calculations. The derived potential energy surface represents a dramatic improvement with respect to a previous approximate estimate.
The density of liquid sulfur near the polymerization temperature
Authors: Zheng K. M. , Greer S. C.
Published: July 2002

We have measured the density of liquid sulfur as a function of temperature near the polymerization temperature. We measure between 424 and 445 K, with a precision in density of 2×10−5 and an accuracy of 5×10−4. We see upward shifts of the density within 3 K of the polymerization temperature of about 8×10−4 with each cycle in temperature; these shifts could be due to the reaction of the sulfur with the quartz cell, or to the persistence of metastable polymeric forms of sulfur. Thus we consider the data from the first heating run to be the best, since the sample was then the purest. Our results differ from those in the literature in that, while we see a change in slope at the transition, we do not see a sharp minimum or a singularity. We compare our data to two models of the sulfur polymerization as a second order phase transition: the mean field model [A. V. Tobolsky and A. Eisenberg, J. Am. Chem. Soc. 81, 780 (1959)] and the n→0 magnet model [S. J. Kennedy and J. C. Wheeler, J. Chem. Phys. 78, 1523 (1983)]. Both models assume that the sulfur forms linear polymers which are in an ideal solution in monomeric sulfur, and that the thermal expansions of monomeric and polymeric sulfur are linear within the temperature range under consideration. For the most physically reasonable choice of thermodynamic parameters for the models, the mean field model describes the data better than does the n→0 model. The n→0 model is superior if the change of specific volume on polymerization is adjusted. One possible explanation is that sulfur belongs in the n=1 universality class, not in the n→0 class.
Laplace transform techniques in Mo/ller–Plesset perturbation theory
Authors: Häser Marco , Almlöf Jan
Published: July 2002

We discuss how the computational obstacles related to energy denominators in various schemes for electron-correlation calculations can be circumvented by a Laplace transform technique. The method is applicable to a wide variety of electronic structure calculations. We discuss in detail an algorithm for the contribution of triple excitations in fourth-order Mo/ller–Plesset perturbation theory, which grows only with the sixth power of the size of the system, as compared to conventional N7 algorithms. Special consideration is given to efficient schemes for numerical quadrature of the integrals occurring in the Laplace transformations.
Bimodal velocity distributions after ultraviolet-laser-induced desorption of NO from oxide surfaces. Experiments and results of model calculations
Authors: Mull Th. , Baumeister B. , Menges M. , Freund H.-J. , Weide D. , Fischer C. , Andresen P.
Published: July 2002

After UV-laser-induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance-enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N–O bond length as it is found, for example, in NO−. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the excited state, even though the translational and the vibrational degree of freedom of the desorbing molecules are decoupled.
Two-photon spectroscopy, Rydberg∼valence interactions, and superexcited state dissociation of HCl probed by resonance enhanced multiphoton ionization
Authors: Green David S. , Wallace Stephen C.
Published: July 2002

The 1Σ+(0+) and 3Σ−(0+) Rydberg states as well as the V 1Σ+(0+) valence state of hydrogen chloride isotopomers are studied using (2+1) resonance enhanced multiphoton ionization (REMPI) and time-of-flight mass spectrometry (TOFMS). Pulsed supersonic expansion provides sufficient rotational cooling to facilitate rotational analysis. Polarization selection in conjunction with fragment and isotope discrimination provide the first unambiguous identification of all vibrational bands from v′=3 to 32 of the V 1Σ+(0+) state. REMPI excitation spectra from the v″=0 level of the X 1Σ+(0+) state through vibrational levels of the broad V 1Σ+(0+) state suggest non-Franck–Condon transitions. Perturbations in intensity, line position, and rotational constants are consistent with the near-adiabatic nature of the short-range E 1Σ+(0+) Rydberg state and long-range V 1Σ+(0+)state. Evidence for Rydberg∼valence interaction and repulsive superexcited states is provided by the detection of strong H+ and Cl+ ion yields. The onset of a distinct Cl+ channel is detected for the first time and confirms the presence of at least two dissociation pathways at the three-photon energy and at large internuclear distance.
Electronic structure of the group V tetramers (P4–Bi4)
Authors: Zhang Hongxing , Balasubramanian K.
Published: July 2002

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference configuration interaction (MRCI) and relativistic configuration interaction (RCI) calculations, which included up to 2 million configurations, are carried out on the ground and excited states of P4–Bi4. We computed the properties of the 1A1 ground state with tetrahedral geometry as well as five excited states of triplet, singlet, and quintet multiplicities of these clusters. The computed results were used to assign the negative photodetachment spectra of Sb4− and Bi4− as well as the matrix isolated spectra of small Bi clusters. We found spin–orbit effects were quite large for Bi4. Our computations are consistent with the recent reassignment of the spectra of Bi4 observed by Bondybey and English to Bi3.
Bridge functions for models of liquid metals
Authors: Lomba E. , Alvarez M. , Stell G. , Anta J. A.
Published: July 2002

The bridge function B(r) is extracted from pair-correlation data obtained by means of extensive molecular-dynamics simulations for two model potentials for liquid metals describing respectively aluminum and cesium. Insertion of the computed bridge functions in the reference hypernetted chain equation yields a pair-correlation function which exactly matches molecular-dynamics results. A comparison between these ‘‘experimental’’ bridge functions and the hard-sphere bridge function shows clear differences in the medium- and long-range behavior for the Al model potential and even more marked discrepancies for the softer Cs model potential.
The HeH2 energy surface
Authors: Muchnick Paul , Russek Arnold
Published: July 2002

A composite ab initio energy surface is obtained for the HeH2 triatomic molecular system by combining earlier calculations that covered parts of the surface. The extended surface covers the full angular range of He relative to the H–H axis as well as the full range of He–H2 separations. As with all calculations thus far, the extended ab initio energy surface covers only a narrow range of H–H separations in the vicinity of 1.4 a.u., the equilibrium separation of ground state H2. In addition, a new parametric representation of the ab initio energy surface is developed in terms of simple analytic functions based on the physics underlying the three principal interaction mechanisms responsible for the energy surface, and collisional applications are discussed briefly. All of the interaction mechanisms give rise to three-body interaction terms, an understanding of which is crucial to interpreting the behavior of the energy surface. This parametric representation reproduces the ab initio calculated values with an average [root-mean-square (rms)] percentage error under 7%, and because the parametric form is based on the physics, it can be extrapolated into the hitherto unexplored region of large H–H separations. A very interesting double inversion of the force between the two H atoms caused by proximity of He is explained. The H–H force is repulsive for large distances of He from H2, weakly attractive for intermediate He distances, and strongly repulsive for small He distances.
Bond energies, ionization potentials, and the singlet–triplet energy separations of SnCl2, SnBr2, SnI2, PbCl2, PbBr2, PbI2, and their positive ions
Authors: Benavides-Garcia M. , Balasubramanian K.
Published: July 2002

Spectroscopic properties, ionization potentials, dissociation energies, and the 1A1−3B1 energy separations were determined for SnX2 and PbX2 (X=Cl,Br,I). The methods of calculations employed were complete active space self-consistent field (CASSCF) followed by multireference single+double configuration interaction (MRSDCI). The ground state for all these molecules corresponded to a 1A1 state, followed by a 3B1 state as the first excited, and the 1B1 state as the second excited state. The ground states of positive ions of these species were computed to be the 2A1 state. The 2B1−2A1 energy separations of the positive ions were computed. The 1A1−3B1 energy separations are: 60.0 kcal/mol for SnCl2, 55.5 kcal/mol for SnBr2, 47.1 kcal/mol for SnI2, 69.7 kcal/mol for PbCl2, 65.0 kcal/mol for PbBr2, and 53.8 kcal/mol for PbI2. The properties of the 1A1 ground state are Re=2.363 (Å), θe=98.4°, μe=3.739 (D), I.P.=9.72 (eV), and De=3.50 (eV) for SnCl2; Re=2.535 (Å), θe=99.7°, μe=3.400 (D), I.P.=9.34 (eV), and De=3.22 (eV) for SnBr2; Re=2.738 (Å), θe=100.9°, μe=2.863 (D), I.P.=8.62 (eV), and De=2.52 (eV) for SnI2; Re=2.542 (Å), θe=100.8°, μe=5.289 (D), I.P.=10.02 (eV), and De=3.19 (eV) for PbCl2; Re=2.684 (Å), θe=101.5°, μe=5.038 (D), I.P.=9.64 (eV), and De=3.06 (eV) for PbBr2; and Re=2.878 (Å), θe=103.6°, μe=4.277 (D), I.P.=8.82 (eV), and De=2.51 (eV) for PbI2. The general trends for SnX2 and PbX2 (X=Cl,Br,I) are discussed.
Behavior of density and refractive index in mixtures of 2,6-lutidine and water
Authors: Jayalakshmi Y. , Van Duijneveldt J. S. , Beysens D.
Published: July 2002

Density, refractive index, and their temperature derivatives are measured over a wide range of concentration, temperature, and wavelength in the partially miscible binary liquid 2,6-lutidine-water. The expected anomalies at the critical concentration are within the relative precision of the measurements (a few 10−6). The Lorentz–Lorenz formula is compared with the data in the whole concentration range. It remains valid within the regular correction factors. We present fitting functions in temperature, concentration, and wavelength.
Monte Carlo simulations of end-grafted polymer matrices under poor solvent conditions
Authors: Weinhold Jeffrey D. , Kumar Sanat K.
Published: July 2002

We report results of off-lattice Monte Carlo simulations on randomly grafted polymer layers in poor solvent. The results of the investigations show that the conformations of polymer chains in these matrices undergo a gradual transition from a mushroom to brush shape with increases in grafting density, in agreement with expectations. We also find that the location of this transition can be predicted quantitatively with a knowledge of the isolated chain dimensions. Analysis of the detailed structure of the grafted layers supports recent theories which predict that clusters will form not only at low grafting densities, but also at higher densities in which the layer covers the entire surface. The segment density profiles evolve from a monotonic decay to the shape of a step function at moderate grafting densities, but show little agreement with analytical self-consistent field theories for brushes in poor solvent. Finally, we have examined brush structures at high grafting densities and find that the segments of these chains arrange into layers parallel to the surface due to packing constraints, but do not crystallize since the model employed is too simple to permit this possibility. Consequently, we find the formation of a kinetically controlled configurational glass phase, especially for long chain brushes.
Consistency test for mixture pair correlation function integrals
Authors: Hamad Esam
Published: July 2002

A consistency test for integrals of the mixture pair correlation functions is suggested. Testing of the Percus–Yevick expressions for hard sphere mixture reveals that the compressibility pair correlation functions are consistent, but the virial and the Carnahan–Starling expressions are not.
Virial coefficients and close-packing of hard spheres and disks
Authors: Sanchez Isaac C.
Published: July 2002

Using recently calculated 8th virial coefficients for hard spheres and disks, singularities in the virial expansion are reinvestigated. Using a Padé analysis, it is concluded that the singularities are associated with crystalline close-packing and not random close-packing nor with the fluid→solid phase transition. Accurate equations of state are developed for hard sphere and disk fluids. Estimates of 9th and higher virial coefficients are also given.
Transient effects in diffusion-controlled absorption by a nonuniform sink of arbitrary constitution
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

The time-dependent rate coefficient for absorption of diffusing particles by a nonuniform sink of arbitrary constitution is studied on the basis of the Smoluchowski equation. It is shown that the amplitude of the long-time tail can be expressed in terms of the asymptotic rate coefficient. The time-dependent rate coefficient is expressed as the sum of the asymptotic steady-state coefficient and an integral of purely decaying exponentials with a positive weight function. It is shown that the rate coefficient can be represented accurately over a wide range of time by an approximation to its Laplace transform, characterized by a small number of poles in the complex square root of frequency plane.
Collisional electronic quenching of OH A 2Σ (v′=0) measured at high temperature in a shock tube
Authors: Paul P. H. , Durant J. L. , Gray J. A. , Furlanetto M. R.
Published: July 2002

Rate coefficients are reported for electronic quenching of OH A 2Σ v′=0 by N2, O2, CO, CO2, NO, Ar, Kr, and Xe measured at high temperatures behind reproducible shock waves. The cross section for quenching by Ar was found to be less than 0.06 Å2. The cross sections for quenching by N2 and Kr were found to be 0.5 and 1.0 Å2, respectively. The cross sections for the remaining species were found to be of order gas kinetic. For all of the species the cross sections were found to be very weak functions of temperature from 1900 to 2300 K. The measured cross sections are compared with previous measurements at lower temperatures. The observed variation with species and with temperature is observed to be consistent with a charge-transfer model for the process.
On the relaxation of a two-level system driven by a strong electromagnetic field
Authors: Geva Eitan , Kosloff Ronnie , Skinner J. L.
Published: July 2002

A detailed and unified account of the theory of the generalized Bloch equations is presented. The equations apply to a two-level system weakly coupled to a heat bath and subject to a monochromatic rotating field of arbitrary intensity. The relaxation tensor obtained is explicitly field-dependent. The derivation is valid for general coupling to a quantum heat bath. The generalized Bloch equations are shown to be thermodynamically consistent, as opposed to the standard Bloch equations. Different limits of the generalized Bloch equations are examined and related to previous studies. The potential use of the generalized Bloch equations as a probe of the bath spectral density is demonstrated for the case of a two-level system embedded in a Debye solid.
Spin–orbit induced radiationless transitions in organometallics: Quantum simulation of the 1E→3A1 intersystem crossing process in HCo(CO)4
Authors: Daniel Chantal , Heitz Marie-Catherine , Manz Jörn , Ribbing Carl
Published: July 2002

A theoretical description of the ‘‘fast’’ (<50 ps) intersystem crossing processes occurring at critical geometries during the photodissociation of HCo(CO)4 is presented. The radiationless transitions are simulated by wave packet propagations along one-dimensional reaction coordinate on the spin–orbit coupled potential energy surfaces. The propagation are performed separately, either along the Co–H bond or along the Co–COax bond. This original approach has enabled us to understand the mechanism of desactivation of the initially populated singlet excited state in this molecule which should be considered as a model for other organometallics. We propose the following mechanism: (i) in a very short time scale (<20 fs) 40% of the system dissociates towards the primary products H+Co(CO)4 (1E), whereas the 1E→3A1 intersystem crossing along the Co–H bond elongation occurs within 50 ps; (ii) the dissociation of an axial carbonyl ligand occurs in a larger time scale (200 fs) and only 2% of the system dissociates along the Co–COax elongation. The dominant process is the 1E→3A1 intersystem crossing leading to HCo(CO)4(3A1); (iii) as soon as the lowest triplet state is populated, the system dissociates either to H+Co(CO)4 or to HCo(CO)3+COax on the 3A1 potential energy surface; (iv) the intersystem crossing process may be described as a succession of elementary transitions occurring at critical geometries or crossing points between the singlet and the triplet potential energy surfaces; (v) the efficiency of the radiationless transition is governed by the overlap of the propagated wave packet with the critical region of the coupled potential energy surfaces.
The first vibronically resolved measurement of correlated-product-state distributions in ultraviolet photodissociation: Ketene at 308 nm
Authors: Drabbels Marcel , Morgan Christopher G. , McGuire David S. , Wodtke A. M.
Published: July 2002

A new method is demonstrated which allows derivation of elements of the correlated-product-state distribution matrix in photodissociation. The method relies on quantum-state-specific, time-of-flight measurements, which were successfully carried out on CO produced in ketene photolysis at 308 nm. Such time-of-flight data exhibits well-defined peaks dictated by the CH2 internal state distribution occurring in coincidence with the chosen CO quantum state. For two rovibrational states of CO, peaks corresponding to the 1A1(0,0,0), 1A1(0,1,0), and 3B1 vibronic states of the CH2 fragment were clearly resolved.
Ni2 revisited: Reassignment of the ground electronic state
Authors: Pinegar Jacqueline C. , Langenberg Jon D. , Arrington Caleb A. , Spain Eileen M. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy was used to study jet-cooled Ni2 produced by pulsed laser ablation of a nickel target in the throat of a supersonic nozzle using argon as the carrier gas. Spectral regions previously investigated using helium as the carrier gas were reinvestigated, and the improved cooling achieved was found to suppress transitions arising from an Ω=4 state that had been thought to be the ground state. Seven new vibronic progressions were assigned, with spectroscopic constants determined for the excited states. The predissociation threshold in Ni2 was reinvestigated, and a revised value for the binding energy is given as D○0(Ni2)=2.042±0.002 eV. The ionization energy of Ni2 was found to be 7.430±0.025 eV, and from this result and the revised bond dissociation energy of the neutral, the binding energy of the cation was calculated to be D○0(Ni+2)=2.245±0.025 eV. Similarly, D○0(Ni−2)=1.812±0.014 eV is obtained using D○0(Ni2) and the electron affinities of Ni and Ni2. Twenty bands were rotationally resolved, all originating from a lower state of Ω″=0+g or 0−u which we argue is the true ground state, in agreement with ligand field and ab initio theoretical studies. The rotational analysis also yielded a ground state bond length of 2.1545±0.0004 Å for 58Ni2.
An accurate integral equation theory for hard spheres: Role of the zero-separation theorems in the closure relation
Authors: Lee Lloyd L.
Published: July 2002

We evaluate a number of current closure relations used in the integral equations for hard sphere fluids, such as the Percus–Yevick, Martynov–Sarkisov, Ballone–Pastore–Galli–Gazillo, and Verlet modified (VM) closures with respect to their abilities of satisfying the zero-separation theorems for hard spheres. Only the VM closure is acceptable at high densities (ρ∼0.7), while all fail at lower densities (lim 0<ρ<0.5). These shall have deleterious effects when used in perturbation theories, especially at low densities. To improve upon this, we propose a closure, ZSEP, that is flexible and suited to satisfying the known zero separation theorems [e.g., the ones for the cavity function y(0) and the indirect correlation γ(0), and others for their derivatives dy(0)/dr, etc.], plus the pressure consistency condition. This particular closure, after numerical solution with the Ornstein–Zernike equation, is shown to perform well at high densities (ρ∼0.9) as well as low densities (0.1<ρ<0.5) for the cavity function y(r), the pair correlation function g(r), and the bridge function B(r). Derived thermodynamic properties: pressure, isothermal compressibility, and chemical potential are also highly accurate. Comparison with available Monte Carlo data bears this out. We have formulated a ‘‘consistent’’ and accurate integral equation theory for hard spheres over a wide range of density states.
The flow diffusion nucleation chamber: A quantitative tool for nucleation research
Authors: Vohra Vivek , Heist Richard H.
Published: July 2002

We have developed a flow diffusion nucleation chamber designed to quantitatively investigate the nucleation of vapors. The design and operational characteristics of the nucleation chamber are presented and discussed. Critical supersaturation data obtained with this nucleation chamber are compared to literature data obtained using a thermal diffusion cloud chamber. The flow nucleation chamber results accurately reproduce the diffusion cloud chamber data. Results of preliminary measurements of nucleation at ambient pressure in the presence of different background gases are presented. These data suggest that the nature of the background gas may influence nucleation at ambient pressure. These data, while still of a preliminary nature, are consistent with data already published obtained at elevated pressures using a specially designed high pressure cloud chamber also in our laboratory.
A theoretical study of the ionic dissociation of HF, HCl, and H2S in water clusters
Authors: Lee Chengteh , Sosa Carlos , Planas Marc , Novoa Juan J.
Published: July 2002

The ionic dissociation of HF, HCl, and H2S in water is examined using density functional theory (DFT), Hartree–Fock (HF), and Mo/ller–Plesset theory to second order (MP2). The calculations show that HF, HCl, and H2S form fully dissociated stable clusters with four water molecules. Each cluster appears to be stabilized by the formation of six hydrogen bonds. These calculations also indicate that a minimum of four water molecules are required to form stable structures in which positive and negative ions coexist in the cluster. The hydrogen transfer between the acid and water molecules is very similar to the mechanism proposed for hydrogen transfer in water solutions. The binding energies of the hydrated hydrofluoric acid, hydrated hydrochloric acid, and hydrated hydrogen disulfide estimated with B-LYP are 37.51, 41.17, and 20.68 kcal/mol, respectively.
Numerical analysis of x-ray reflectivity data from organic thin films at interfaces
Authors: Asmussen Axel , Riegler Hans
Published: July 2002

An efficient numerical analysis procedure of x-ray reflectivity data from ultrathin organic films at interfaces is presented. It allows virtually exact modeling of reflectivity data in the entire q-vector range including the critical angle and below with minimum fit parameters and calculation effort. The electron density profile is interpreted as a linear superposition of the air/film interface, the internal film structure, and the film/substrate interface. The reflectivities at the interfaces are calculated from the accurate Fresnel reflectivities. The internal film structure is parameterized in a Fourier series and its reflectivity contribution is calculated from refraction-corrected, approximated (linearized) Fresnel reflectivities. Additionally, a new way of weighing measurement errors is introduced. It allows the selective optimization of the fit in those data sections which are considered especially significant. Finally, we show further how to determine the degree of correlation (interdependencies) between the fit parameters and how to obtain absolute errors (the significance) of all fit parameters.
The fluid structures for soft-sphere potentials via the zero-separation theorems on molecular distribution functions
Authors: Lee Lloyd L. , Ghonasgi Dhananjay , Lomba Enrique
Published: July 2002

We present a class of closures specifically designed to satisfy the zero-separation theorems for the correlation functions y(r) (the cavity function), γ(r)=h(r)−C(r) (the indirect correlation), and B(r) (the bridge function) at coincidence r=0 for soft-sphere pair potentials. The rationale is to ensure the correct behavior of these correlation functions inside the core r<σ. Since the coincidence theorems implicate the thermodynamic properties of the bulk fluid: the isothermal compressibility, the internal energy and the chemical potentials, we can hopefully enforce consistency between the structure and thermodynamic properties. We solve the Ornstein–Zernike equation for the Lennard-Jones molecules where plentiful Monte Carlo data are available for testing. It turns out that not only consistency is achieved, we also obtain accurate structures: the pair correlation function g(r), the cavity function, and the bridge function for wide ranges of fluid states (0.72<T*<1.5, ρ*<0.9). Comparison with MC data attests to the accuracy. The closure of the zero-separation type (ZSEP), is sufficiently robust and flexible to ensure not only fulfillment of the zero-separation theorems but also pressure consistency. Success with the Lennard-Jones potential implies its applicability to other similar soft-sphere potentials.
Photoelectron investigations and density functional calculations of anionic Sbn− and Bin− clusters
Authors: Gausa M. , Kaschner R. , Seifert G. , Faehrmann J. H. , Lutz H. O. , Meiwes-Broer K.-H.
Published: July 2002

We recorded photoelectron spectra of antimony Sb−N=2–9 and of bismuth clusters Bi−N=2–9 with a photon energy of 4.03 eV, as well as of Bi−N=2–21 with a photon energy of 5.0 eV. The experimentally determined photoelectron thresholds and peak positions of Sb−N=2–5 and Bi−N=2–5 are compared with the results of ab initio density-functional (LCAO) calculations. The agreement between the experimental thresholds and the calculated adiabatic electron affinities, as well as between the first maxima in the spectra and the calculated vertical detachment energies is fair to good for the antimony clusters and qualitative for the bismuth systems. For the calculation of the ionization (detachment) energies we determined for the neutral and anionic clusters the most stable structures by LCAO calculations. In particular, the tetramer cluster anions have a ‘‘roof’’ structure, while the negatively charged pentamers are planar rings [with similarities to the (C5H5)− anion]; positive and negative trimers are nonlinear. Furthermore, the ionization energies and affinities of larger antimony and bismuth clusters are discussed qualitatively and compared to jellium calculations of Seidl and Brack.
The dynamics of predissociating high Rydberg states of NO
Authors: Bixon M. , Jortner Joshua
Published: July 2002

In this paper we present a theoretical study of the predissociation dynamics of the nf(N+=2) (with the principal quantum numbers n=40–95) and the np(N+=0) (n=70–125) Rydberg series of NO, which exhibit a marked lifetime dilution (lengthening) at n≳65 for the f series and at n≳116 for the p series [M.J.J. Vrakking and Y. T. Lee, J. Chem. Phys. 102, 8818 (1995)]. The multichannel effective Hamiltonian with several doorway (for excitation) and escape (for decay) states was constructed using experimental information on the quantum defects and on the decay width constants incorporating both intramolecular coupling and exterior electric field coupling between high Rydbergs. The analysis of the intramolecular Rydberg electron–core dipole long range coupling (HR-D) in conjunction with the energy gaps between proximal pairs of energy levels, which are subjected to appropriate selection rules, reveals that (i) for low l(≤3) core-penetrating Rydbergs only a small number of accidental near-resonances are exhibited, and (ii) for high l(≳3) nonpenetrating Rydbergs the electron-core dipole coupling decreases fast with increasing l, i.e., (HR-D)∝l−7. The general characteristics of the high l(≳3) manifold establish a bottleneck effect, which precludes intramolecular l mixing, implying that high Rydberg lifetime dilution effects can be induced only by exterior electric field coupling (HSTARK). Parameter-free multichannel effective Hamiltonian calculations were conducted under narrow-band excitation conditions, which interrogate the electric field induced mixing in the energetic vicinity of the doorway state. The electric field induced l mixing model accounts semiquantitatively for the electric field dependence of the energy-resolved line shapes of the nf(N+=2) series and for the n and electric field dependence of the lifetimes of the nf(N+=2) and the np(N+=0) series. Accidental near-resonant simultaneous intramolecular and electric field coupling np(N+=0)↔HR-Dn′d(N+=1)↔HSTARKn′ l(≥3)(N+=1) for two sets of proximal states n=92, n′=80 and n=95, n′=82, result in mediated-sequential mixing, which is manifested by slow decay times below the onset of effective electric field mixing by weak (F0≂0.04–0.08 V/cm) stray electric fields.
Fast beam photodissociation spectroscopy and dynamics of the vinoxy radical
Authors: Osborn David L. , Choi Hyeon , Mordaunt David H. , Bise Ryan T. , Neumark Daniel M. , Rohlfing Celeste McMichael
Published: July 2002

The spectroscopy and photodissociation dynamics of the vinoxy (CH2CHO) radical B̃(2A″) ←X̃(2A″) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B̃ state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: (1) CH3+CO, and (2) H+CH2CO, with a branching ratio of ≈1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH3+CO, and a considerably smaller barrier for H+CH2CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the Ã(2A) or C̃(2A) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions.
Effect of pressure on hydrogen bonding in glycerol: A molecular dynamics investigation
Authors: Root Leslie J. , Berne B. J.
Published: July 2002

We report results of a molecular dynamics study of liquid glycerol at the experimental density and at a series of elevated densities corresponding in our model to pressures of up to 0.7 GPa. We find that the degree of hydrogen bonding increases with increasing pressure over the range studied, and that the width of the hydrogen bond angle distribution increases with increasing pressure. The relevance to the experimental finding by Cook et al. [R. L. Cook, H. E. King, C. A. Herbst, and D. R. Herschbach, J. Chem. Phys. 100, 5178 (1994)] that the fragility of glycerol increases with increasing pressure is discussed.
Interaction anisotropy and vibrotational excitation in proton scattering from N2(1Σg+)
Authors: Gianturco F. A. , Kumar S. , Ritschel T. , Vetter R. , Zülicke L.
Published: July 2002

Stimulated by the experimentally observed low vibrational inelasticity and the somewhat stronger rotational inelasticity in the H+–N2 collisions, we present here a quantum dynamics study of the scattering process in the framework of vibrational close-coupling rotational infinite-order sudden approximation. We have employed the recently obtained ground-state potential-energy surface of the system from which both rotational and vibrational coupling potentials can be obtained. The various computed dynamical attributes such as differential and integral cross sections, and average vibrational as well as rotational energy transfers, are analyzed in detail and compared successfully with the available experimental results.
Is there a glass transition for dense hard-sphere systems?
Authors: Robles M. , López de Haro M. , Santos A. , Bravo Yuste S.
Published: July 2002

The recent results for the contact value of the radial distribution function obtained from large-scale molecular dynamics and Monte Carlo simulations of systems of dense hard spheres [M. D. Rintoul and S. Torquato, J. Chem. Phys. 105, 9258 (1996)] are compared to those of the Padé equation of state for a hard-sphere fluid (derived from the knowledge of the first eight virial coefficients), including the metastable fluid region up to a packing fraction of ηg≃0.56, and above such packing fraction to the ones corresponding to an equation of state of the free-volume type which presents a simple pole at random close-packing. This latter equation involves the same value for the pressure at ηg as the Padé equation of state, and arises in a consistent way from the application of a rational-function approximation method to the computation of the radial distribution function of a hard-sphere fluid. The substantial improvement of the agreement with the simulation results over the one obtained with the equation of state proposed by Torquato [Phys. Rev. E 51, 3170 (1995)] suggests that the hard-sphere fluid may indeed present a glass transition at ηg.
Quantum Monte Carlo simulations of the structure in the liquid–vapor interface of BiGa binary alloys
Authors: Zhao Meishan , Chekmarev Dmitriy , Rice Stuart A.
Published: July 2002

We report the results of self-consistent quantum Monte Carlo simulations of the structure of the liquid–vapor interface of a dilute alloy of Bi in Ga. The results of the simulations are shown to be in good agreement with the results of experimental studies of the structure of the liquid–vapor interface of such an alloy reported by Lei, Huang, and Rice [J. Chem. Phys. 104, 4802 (1996), who determined the longitudinal density distribution] and by Flom et al. [Science 260, 332 (1993), who determined the transverse pair structure function].
Valence states of C2 Feynman’s way
Authors: Sorensen Thomas E. , England Walter B.
Published: July 2002

Feynman’s way is used to calculate total-energy curves for the X 1Σg+, a 3Πu, b 3Σg−, A 1Πu, c 3Σu+, 1 1Δg, 2 1Σg+, d 3Πg, C 1Πg, e 3Πg, D 1Σu+, and C′ 1Πg valence states of C2. Lewis structures are derived for each state. Average (maximum) deviations of calculated spectroscopic constants from experiment are 1.9 (4.3) pm for Re, 18 (32) kJ/mol for De, 12 (36) kJ/mol for Te, 62 (162) cm−1 for ωe, and 16 (31) kJ/mol for asymptotic excitation energies.
Polymer adsorption on planar random surfaces
Authors: Sumithra K. , Baumgaertner A.
Published: July 2002

The adsorption transitions of a single self-avoiding polymer chain at chemically heterogeneous surfaces have been investigated using Monte Carlo methods and analyzed using scaling arguments. Evidence is provided that the crossover exponent φ, characterizing the transition between the weakly and the strongly adsorbed states, depends linearly on the dilution 0.6⩽p⩽1 of adsorbing sites on the surface, φ(p)≈0.35+0.2p. The transition temperatures of chains of length N scale according to Tc(N,p)/Tc(∞,p)∼N−φ(p). In particular, we have analyzed the adsorption energies and the parallel and perpendicular components of the end-to-end distance.