bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagator expansions for softly coupled potentials: A model for complex reaction dynamics
Authors: Schwartz Steven D.
Published: July 2002

In this paper we investigate a new approach to reduced dimensionality descriptions of quantum mechanical systems resident in a bath. We study physical situations in which the coupling between the system and the bath is slowly varying. Our method involves an operator expansion of the Feynman propagator following the Zassenhaus theorem. From this general expansion we are able to derive an especially simple special case in which the coupling is a slowly varying function of the position operators of the system and the bath. From the approximate propagator after tracing over bath degrees, we are able to derive a short time propagator which yields both a form for efficient numerical calculation and an effective Schrödinger equation for the evolution of the system under the average influence of the bath. This theory is then applied to tunneling rearrangement in mixed crystals of benzoic acid. We find that independent of potential energy perturbations, dynamic system bath couplings increase the rate of tunneling. A central goal of this type of approach is to model the increasingly complex experimental data for large (often biological) systems.
Magnetooptic investigation of the rod to coil transition in solutions of polydiacetylene poly(4-BCMU)
Authors: Rosenblatt Charles , Rubner Michael F.
Published: July 2002

Cotton–Mouton measurements on dilute solutions of polydiacetylene poly(4-BCMU) are reported. Measurements of dΔn/dH2 vs temperature indicate that in equilibrium the red phase contains small aggregates of rods, generally numbering N<5. Measurements made at fixed temperature over three decades of time after rapid cooling from the yellow into the red phase indicate that the rod-to-coil transition is a single chain process, with rod aggregation occurring immediately thereafter.
Theory of resonance secondary emission in femtosecond laser excitation: On the connection with wave packet dynamics
Authors: Kono H. , Fujimura Y.
Published: July 2002

The resonance secondary emission (RSE) in femtosecond laser excitation is discussed in reference to the motion of the created wave packet moving on the excited state potential surface. The density matrix of emitted light for the multi-intermediate-level system is outlined, from which the emission correlation function is derived. The correlation function is put into the theoretical expression of the time-dependent ‘‘physical spectrum’’ for the Fabry–Perot interferometer (which is used in order to consider temporal and energetic resolution inherent in detection). The compact and practical expressions obtained connect the time- and frequency-resolved spectrum with the time evolution of the wave packet. Numerical results for a displaced harmonic oscillator model indicate that the time- and frequency-resolved spectrum can reveal how the wave packet created by a fs laser pulse travels on the excited potential surface if the response time 1/Γd of the photodetector satisfies the relation that Ω<Γd <∼ the Stokes shift (where Ω is the vibrational frequency). It is shown that the excited state wave function can be split into two terms, the one that adiabatically follows the temporal change in incident light (the adiabatic term) and the one that represents the effect of spectral broadening of light (the Fourier broadening term). It is only the Fourier broadening term that survives after the termination of incident light and reflects the motion of the created wave packet on the excited potential surface. In off-resonance excitation, the adiabatic term produces Raman-like emission and the Fourier broadening one produces fluorescence-like emission. In resonance excitation, these two terms are indistinguishable from each other with respect to emission frequency: for the duration of incident light, the adiabatic term offsets the Fourier broadening one, leading to a slow buildup of intensity in the time- and frequency-resolved spectrum (which is slower than the initial rise of the incident pulse profile).
Observation of phase transitions on the (111) and (100) surfaces of Si near 1000 K with He atom diffraction
Authors: Ha Jeong Sook , Greene E. F.
Published: July 2002

He diffraction has been used to study structural phase transitions on the (111) and (100) surfaces of Si. The seventh-order peaks of the Si(111)-7×7 reconstructed surface observed at room temperature first decreased with increasing temperature and then disappeared near 1140 K. A sharp decrease in the specular intensity and a sharp increase in the diffuse scattering were also observed near 1140 K, but there was no broadening of the peaks as the temperature increased. We confirm earlier evidence that the 7×7 to ‘‘1×1’’ phase transition at 1138±7 K on the Si(111) surface is an order–disorder one. The He scattering from the Si(100)-2×1 reconstructed surface stable at room temperature was also measured as the temperature increased. There was a change in the Debye–Waller slope at 930±20 K which is evidence for the structural phase transition expected from theoretical calculations and deduced from earlier experiments in this laboratory on the desorption kinetics of alkali atoms from this surface. These experiments show that He diffraction offers a useful way of studying phase transitions on semiconductor surfaces.
The charge state of Mn and defect dynamics in manganese doped KTaxNb1−xO3
Authors: Laulicht I. , Yacobi Y. , Baram A.
Published: July 2002

The EPR spectra of Mn impurities in KTa1−xNbxO3 crystals (X≂0.03) is studied in the 110–560 K temperature range. Intensity measurements do not support a change in the Mn charge state in the 100–140 K temperature range. The room temperature g factor and hyperfine constant values, g=2.000±0.001 and A=(84.9±0.02)×10−4 cm−1 are very similar to those of Mn2+ in KTaO3 at low temperatures. Thermally induced low-temperature change in the Mn charge state is therefore unlikely. The motional narrowing phenomenon observed at high temperatures (300 K<T<600 K) is very probably due to fast hopping of Mn2+ ions (which sit off-center on K+ sites) between three magnetic inequivalent sites. This is strongly supported by a relaxation matrix analysis of the narrowing which shows that the hopping rate can be characterized by an Arrhenius equation with a low activation energy E=(0.065±0.01) eV which is very similar to the corresponding value in Li:KTaO3, and a preexponent factor 1/τ0=4.5×1012 s−1.
Infrared spectra of 4HeH+, 4HeD+, 3HeH+, and 3HeD+
Authors: Crofton Mark W. , Altman Robert S. , Haese Nathan N. , Oka Takeshi
Published: July 2002

Isotopic species of the HeH+ molecular ion provide an excellent testing ground for studying isotopic dependence of vibration–rotation constants because of the small masses of He and H isotopes. We have observed infrared spectra of the hot band v=2←1 of HeH+ and fundamental bands of isotopic species HeD+, 3HeH+, and 3HeD+, and obtained the Dunham coefficients Ykl, and the isotopically independent parameters Ukl, ΔHekl, and ΔHkl.
Solvent effects in weak electrolytes. I. Effect of a hard sphere solvent on the sticky electrolyte model with L=σ
Authors: Rasaiah Jayendran C. , Zhu Jianjun , Lee Song Hi
Published: July 2002

The Ornstein–Zernike (OZ) equations are solved for the sticky electrolyte model (SEM) with a hard sphere solvent using the hypernetted chain (HNC) approximation for the stickiness and the mean spherical (MS) approximation for the electrical interactions. Relations among the coefficients of Baxter’s q functions and the equation for the excess internal energy are given in the MS approximation for L≤σ, where σ is the molecular diameter, and L is the distance at which the oppositely charged ions can stick. The analytical results for L=σ in the HNC/MS and PY/MS approximations are presented in detail. When the charges are switched off, the results automatically lead to those of the sticky hard sphere system; when the stickiness is turned off and the discrete solvent is changed to a continuum, the results lead correctly to those of the restricted primitive model (RPM). The thermodynamic properties of the SEM in a hard sphere solvent for L=σ are calculated and compared with the properties in a continuum solvent; special attention is paid to the derivation of the osmotic coefficient in the McMillan–Mayer system for the SEM and for the corresponding uncharged system. By switching off the charge and the stickiness, the osmotic coefficient of an isotopic solute–solvent system is also obtained. The numerical results show that the hard sphere solvent has a strong packing effect on the structural and thermodynamic properties of the electrolyte and the association of the oppositely charged ions is greatly enhanced by the hard sphere solvent. The influence of a discrete solvent on the osmotic coefficient is quite subtle: for the charged system, the solvent tends to raise the osmotic coefficient; for the sticky hard sphere system, the solvent has just the opposite effect.
On the resolution of an apparent conflict in the relation between position and momentum operators within the Hartree–Fock approximation
Authors: Harris Robert A. , Hansen Aage E. , Bouman Thomas D.
Published: July 2002

Time-dependent Hartree–Fock theory leads to two apparently quite different expressions relating matrix elements of the electronic position and momentum operators, depending upon whether a single-particle or a many-particle matrix element is used as the starting point. The two expressions are shown to be equivalent, in a complete Hartree–Fock basis set, by means of explicit use of the commutation properties of r with V and J on the one hand, and self-consistency relations and Brillouin’s theorem on the other.
Infrared spectroscopy of carbo-ions. V. Classical vs nonclassical structure of protonated acetylene C2H+3
Authors: Crofton Mark W. , Jagod Mary-Frances , Rehfuss Brent D. , Oka Takeshi
Published: July 2002

The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy.

The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed.

We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.
Solvent dynamical effects in electron transfer: Predicted influences of electronic coupling upon the rate-dielectric friction dependence
Authors: Gochev Alexander , McManis George E. , Weaver Michael J.
Published: July 2002

The predicted dependence of the bimolecular rate constant for outer-sphere electron exchange kex upon the longitudinal relaxation time τL for Debye solvents is examined numerically on the basis of a suitably combined rate formulation in order to examine the manner and extent to which the rate-solvent friction dependence, of particular experimental significance, should be sensitive to the degree of donor–acceptor electronic coupling and related factors. The treatment accounts for the contributions to kex from a spatial distribution of reactant pairs as well as for the effects of donor–acceptor interactions upon the unimolecular rate constant, ket (s−1), for each encounter geometry. The latter include the influence of electronic interactions as prescribed by the matrix coupling element H12 upon the effective frequency for adiabatic barrier crossing νn, as well as upon the electronic transmission coefficient κel (i.e., the degree of reaction nonadiabaticity). The anticipated dependence of the free-energy barrier, as well as νn and κel, upon the donor–acceptor separation is accounted for in the kex calculations. The form of the resulting log kex −log τ−1L plots exhibit, as expected, a marked dependence on the value of H12 for reactant contact, H○12. Over the τ−1L range appropriate for common solvents at ambient temperatures, ∼1×1011 to 5×1012 s−1, the log kex −log τ−1L slopes, x, are significantly (or substantially) below unity for H○12≲0.5 kcal mol−1; x decreases markedly with increasing τ−1L and/or for decreasing H○12. The approach to the solvent inertial limit (for large τ−1L) also tends to decrease x further. Numerical comparisons are made with corresponding log kex −log τ−1L plots obtained using the simple ‘‘encounter preequilibrium’’ treatment which presumes that only a fixed narrow range of encounter-pair geometries contribute to kex. While the form of the

plots are not greatly different, former more sophisticated treatment

generally yields smaller slopes. Some corresponding numerical calculations for electrochemical-exchange reactions are also included.
Reaction probabilities, cross sections, and threshold energies in the reaction of isotopically pure H atoms and n-butane
Authors: Nicholas John E. , Vaghjiani Ghanshyam L.
Published: July 2002

Reaction cross sections and threshold energies have been determined for the first time for the isotopically pure reaction, H+n-C4H10→H2+C4H9, for abstraction from both the primary and secondary C–H bonds. Reaction probabilities were measured for H atoms at selected initial energies up to 112 kJ mol−1 by a photochemical method. With these results and the data for Xe moderated systems, together with new calculations of H–Xe collision densities by a stochastic method, it was possible to evaluate the cross sections and the corresponding reaction rate coefficients. Cross sections rose from threshold energies of 31±5 and 49±5 kJ mol−1 for secondary and primary abstraction, respectively, to ∼9×10−4 nm2 per secondary C–H bond and ∼5×10−4 nm2 per primary C–H bond at around 100 kJ mol−1. These cross sections are somewhat lower than those for secondary C–H bond in propane, and primary C–H bonds in propane and in ethane, but all are considerably smaller than the total collisional cross section.
Symmetry breaking in a simple electron–phonon model with electron correlation
Authors: Zhang Q. , Silbey R.
Published: July 2002

We consider a simple model which contains both electron phonon coupling, electron transfer, and electron correlation, which can be a prototype of a mixed valence system with three nearby electronic states. We treat the adiabatic solution, simple corrections to that, and the effect of symmetry breaking terms on the solutions. We find that the adiabatic solutions in the symmetric case can show a (false) symmetry breaking which is removed in the nonadiabatic calculation. These solutions are extremely unstable with respect to the addition of a non-symmetric term in the Hamiltonian.
Computer simulation of reptation theories. II. Reptating-rope model
Authors: Öttinger Hans Christian
Published: July 2002

A simple and efficient algorithm for simulating the reptating-rope model is presented. This reptation model was developed by Jongschaap and Geurts to describe the rheological behavior of concentrated polymer solutions and melts in homogeneous flows. The reptating-rope model formally contains the well-known Doi–Edwards and Curtiss–Bird models as special cases. The stochastic reformulation of the model required for developing a simulation algorithm does not only yield a powerful and convenient tool for evaluating material functions, but it also leads to more transparent equations for the polymer dynamics and the stress tensor in the reptating-rope model. The simulation results for the viscometric functions are discussed in detail.
Valence ionization of HCl. An investigation of many-body effects
Authors: von Niessen W. , Tomasello P. , Schirmer J. , Cederbaum L. S. , Cambi R. , Tarantelli F. , Sgamellotti A.
Published: July 2002

Many-body Green’s-function calculations which employ large configurational expansions and which are accurate to third and fourth order of perturbation theory are reported for the valence ionic states of HCl. Large polarized basis sets including Rydberg functions s-, p-, and d-type symmetries have been used. Third-order calculations are sufficient in the outer valence region, but in the inner valence region where the complete breakdown of the one-particle picture of ionization is observed a fourth-order theory is necessary in conjunction with large basis sets. The synchrotron and x-ray excited spectra can be assigned nearly up to the double-ionization threshold in a very satisfactory way. No indication of strong outer valence satellite lines is found.
Classical and nonclassical forms of the vinyl cation: A coupled cluster study
Authors: Liang Congxin , Hamilton Tracy P. , Schaefer Henry F.
Published: July 2002

The energy difference between the classical and nonclassical forms of the vinyl cation is studied using the single and double excitation configuration interaction (CISD) and coupled cluster methods (CCSD and CCSDT-1). The basis sets employed range from double ζ to those including f functions on carbon and d functions on hydrogen. It is shown that the theoretical energy differences are very sensitive to the inclusion of higher-order polarization functions and the inclusion of triple excitations in the correlated wave functions. The results are compared with other theoretical studies in the literature and with the very recent experimental work of Crofton, Jagod, Rehfuss, and Oka. The most reliable single theoretical treatment involves the CCSDT-1 method with basis set C(10s5p2d1f/4s3p2d1f), H(5s2p/3s2p), and predicts 3.3 kcal/mol for the classical/nonclassical energy difference. Basis set extension is expected to increase this separation to about 4.0 kcal/mol, still less than the experimental value, >6 kcal/mol.
What is the lowest energy structure of the NS2 molecule?
Authors: Yamaguchi Yukio , Xie Yaoming , Grev Roger S. , Schaefer Henry F.
Published: July 2002

A paramagnetic (neutral) species, thought to be the NS2 molecule, has recently been detected by Amano using millimeter wave spectroscopy in a discharge of a mixture of N2 and CS2. Detailed theoretical studies on the possible structures of an NS2 molecule, however, had not previously been reported. This study aims to find the most probable structure for the NS2 molecule and to characterize other conceivable isomers. Among many possible structures for the NS2 species, SNS bent (2A1), SNS linear (2Πu), NS2 ring (2B1), NSS bent (2A′), and SNS bent (4A2) isomers have been investigated at the self-consistent-field (SCF) and single and double excitation configuration interaction (CISD) levels of theory with double zeta plus polarization (DZ+P) and triple zeta plus double polarization (TZ+2P) basis sets. The SNS bent (2A1) isomer has been found to be the lowest energy structure among the isomers studied in this research. This bent isomer has a bond angle of about 150 deg and has a shallow double-minimum potential, the classical barrier height to linearity being 2.5 (3.0) kcal/mol at the TZ+2P-CISD(Q) level of theory.
Vibrational quantum correction for the Lennard-Jones fluid: A formalism of effective intermolecular potentials depending on mass and temperature
Authors: Yoon Byoung Jip , Jhon Mu Shik , Scheraga Harold A.
Published: July 2002

Using the Slater sum, a method is suggested for introducing a vibrational quantum correction into an intermolecular potential function. This method is useful for qualitative and quantitative formulations of mass- and temperature-dependent effective potentials, and is tested here on a Lennard–Jones fluid. When quantum effects increase, a negative two-body entropy is obtained because the particles behave cooperatively, and not independently. We have defined a ‘‘quantum’’ material as one with a negative two-body entropy. According to this definition, argon becomes a quantum material at temperatures lower than the melting point (super-cooled liquid), and liquid water exhibits quantum effects in its normal liquid temperature range.
Length dependent mobility of large DNAs in oscillating crossed electric and magnetic fields
Authors: Lumpkin O.
Published: July 2002

Mutually orthogonal, oscillating electric and magnetic fields can cause a polyion to drift through an electrolyte solvent. At low frequencies the average drift velocity is shown to be proportional to the square root of the molecular weight. For DNA about 1 cm long and E and B field amplitudes at 60 Hz of 100 V/cm and 5 kG, respectively, the estimated drift is about 2×10−5 cm/s.
Ab initiopotentials for (H2S)2, including studies of effects from third-order many-body perturbation theory and three-body nonadditivity
Authors: Woon David E. , Beck Donald R.
Published: July 2002

A b initio Hartree–Fock (HF) and many-body perturbation theory through second (E2) and third order (E3) were used to generate pair potentials for (H2S)2. Several basis sets were developed by optimizing polarization functions with the zeroth-order dipole or quadrupole polarizabilities. Approximately 30 ab initio points (fully counterpoised HF+E2) in each of two larger basis sets were fit with site–site potentials. Intermediate potentials for fewer points indicate that only about 30 points are a sufficient data base to predict energies in the vicinity of the well with an accuracy of about 10%. The best ab initio well depth produced (−0.0693 eV) lies at 94% of the only published experimental value (−0.074±0.013 eV), although arguments are presented which indicate that the true (H2S)2 minimum may lie below −0.08 eV. The role of E3 and nonadditive effects has also been explored, but they were found not to account for the major part of the remaining discrepancy between our best number and experiment.
Molecular relaxation properties studied by Rayleigh–Brillouin scattering from aqueous solutions of ammonium nitrate salts
Authors: Lee Y. T. , Schroeder J. , Choi C. S. , Frankel J.
Published: July 2002

Rayleigh–Brillouin spectra of aqueous solutions of hydroxyl ammonium nitrate (HAN) and triethanol ammonium nitrate (TEAN) were studied at atmospheric pressure over a temperature range from 205 to 298 K. The hypersonic velocity and absorption coefficient of the phonons were calculated from the spectra as a function of temperature. The absorption due to viscosity and internal energy exchange through the relaxation mechanisms were considered. We find that both mechanisms are responsible for the absorption, but at the lower temperature, the one related to internal energy dominates. The free volume model was used to estimate the activation energy. This value is quite consistent with values calculated from the relaxation time. The effects of relaxation and the water concentration on mixtures of aqueous solutions of HAN and TEAN are also discussed.
Diatomic predissociative widths and shifts from multichannel Siegert quantization and asymptotic analysis of resonance wave functions
Authors: Lefebvre R.
Published: July 2002

Predissociative widths and shifts are studied for a diatomic molecule described by a Morse potential crossed by one or two repulsive exponential potentials. This defines two models with either two channels or three channels. The coupled channel equations are solved either with a complex rotated coordinate (in which case the boundary conditions are the same as for a bound state) or with explicit enforcement of Siegert outgoing wave only asymptotic boundary conditions. The widths and shifts derived from the complex quantized energies of the two channel model are compared to previous results for this model based on semiclassical, phase shift, or Green operator methods. In the weak coupling regime, significant improvement over previous phase shift based results is obtained. In the intermediate coupling case, Siegert quantization confirms the value given by the semiclassical approach to the coupling giving a resonance of zero width. This is in disagreement with the Green operator prediction. Additional support is given by an asymptotic analysis performed on the resonance wave function (A.A.R.W.) which gives widths in very good agreement with those derived from the complex quantized energies. The three channel model leads to the definition of a total width and of two partial widths. Asymptotic analysis of the three-channel resonance wave function gives directly partial widths which, in the weak width regime, sum accurately to the total width.
Temperature dependence of fluorescence from liquid butane, pentane, and hexane: Lifetimes and quantum yields
Authors: Preses Jack M. , Holroyd Richard A.
Published: July 2002

The rate constants for the decay of ultraviolet fluorescence from liquid butane, pentane, and hexane have been measured as a function of temperature between their respective freezing points and near-room temperature. The rate constants are well characterized as the sum of an activated and nonactivated term, as has been described previously. The parameters determining the rate constants are evaluated. Relative fluorescence quantum yields are also measured over the same temperature range. Normalization of the relative fluorescence quantum yields with absolute quantum yields measured previously at 195 K renders the relative determinations absolute. A comparison of the fluorescence results with the photochemical yields shows that a fraction of the initially formed upper excited states dissociates mainly into radicals, in competition with internal conversion to the fluorescing state.
The time-local Fokker–Planck equation
Authors: Rodger P. Mark , Sceats Mark G.
Published: July 2002

A time-local Fokker–Planck equation (TLFPE) is derived which accounts for memory effects in stochastic problems. This is expected to provide a computationally efficient method of modeling the phase space evolution of such systems by simple (local time) Langevin equations with Markovian fluctuating forces that are characterized by time-dependent moments; it is this explicit time dependence that describes the memory effects. The TLFPE is derived from the probability theory of non-Markovian systems as a generalization of Chandrasekar’s derivation of the Fokker–Planck equation (FPE) from the Chapman–Kolmogarov equation for Markovian systems. In this article it is applied to free particle diffusion and barrier crossing problems, and is shown to give rise to physically realistic results. Further, the form of the TLFPE suggests that the conditions required for systems to exhibit Markovian behavior are less restrictive than the Brownian criterion of separation of time scales between the fluctuating forces and the momentum response of the system. Rather, a sufficient condition is that the time-dependent moments of the TLFPE reach plateau values before the time scale of the phenomenon of interest.
Excitation cross sections for electron collisions with sulfur atoms
Authors: Ganas P. S.
Published: July 2002

An analytic atomic independent particle model potential adjusted to experimental single-particle energy levels is used to generate wave functions for the valence and excited states of sulfur. Using these wave functions in conjunction with the Born approximation and LS coupling, we calculate generalized oscillator strengths and integrated cross sections up to 5 keV for various electron impact excitations from the 1s22s22p63s23p4(3P2) ground state.
Observation of the lowest triplet state in silane by electron energy loss spectroscopy
Authors: Tanaka H. , Boesten L. , Kimura M. , Dillon M. A. , Spence D.
Published: July 2002

The electron energy loss spetra of silane was recorded at several impact energies and angles. It is concluded that the lowest triplet state of silane is formed from configuration mixing of the 4s Rydberg state with a valence state of the same symmetry.(AIP)
Time dependence of trapping and detrapping of particles by saturable traps
Authors: Richards Peter M.
Published: July 2002

The rate for trapping and detrapping of particles diffusing to saturable traps is investigated. Careful attention is paid to the fact that, for traps which can hold only one particle, the trap occupation pT must be regarded as a stochastic rather than continuous variable. It is shown that neglecting correlation between pT and the occupation p1 of particles in the trap’s neighborhood can lead to serious errors when the reaction is diffusion limited. However, an average correlation (AC) approximation, which is justified on the basis of detailed consideration of the hopping equations, is shown to be reliable in all cases. The AC is further shown to be equivalent to the McNabb–Foster equations, previously derived by heuristic arguments only, in the continuum limit. Short time effects are predicted in the diffusion-limited regime which result from the discreteness of the lattice. Results of the theory are supported by good agreement with Monte Carlo simulation data.
Reaction at traps involving two diffusing species: Application to atomic–molecular hydrogen conversion at defects
Authors: Richards Peter M. , Myers S. M.
Published: July 2002

Kinetics of diffusion-limited reactions are derived for the case in which there are two mobile species A and M which can convert to one another at a trap or catalysis site. At the same time the standard reaction of A getting trapped and detrapped is allowed to compete with the A⇄M reaction. Nonlinearities associated with trap saturation and the model where M is a diatomic molecule of A are included. Rate equations are derived for the bulk concentrations of A and M. Excellent agreement is obtained with simulation data when account is taken of the correlation between trap occupation and concentration immediately outside the trap. The equations are specifically designed for trapping, recombination, and dissociation of hydrogen atoms and molecules at defects; but they should have a broad range of applicability so that problems with several species and reactions can be handled with the same degree of accuracy as the Smoluchowski theory for trapping and detrapping of a single species. Results are accurate and general enough to include the nonlinearities introduced by trap saturation and trap-concentration-dependent rate constants.
Raman intensities and interference effects for thin films adsorbed on metals
Authors: Ager Joel W. , Veirs D. Kirk , Rosenblatt Gerd M.
Published: July 2002

Large oscillations in the vibrational Raman intensity have been observed for thin films adsorbed on a metal substrate as a function of film thickness. The systems studied, N2 and O2 physisorbed on Ag(111) at 14±1 K, have previously been shown to be unenhanced, i.e., the observations are not caused by the surface enhanced Raman (SERS) effect. The observed oscillations are due to multiple beam interference caused by reflections at the film boundaries of the incident laser light, and, to a lesser extent, the Raman scattered light. We extend the two-dimensional theoretical development of Moscovits and co-workers for Raman scattering in thin films to include more general three-dimensional experimental geometries. We derive expressions for the total intensity as a function of film thickness, incident laser polarization and angle, and scattered light polarization and angle. The resultant expressions are applicable to Raman scattering from any dielectric thin film adsorbed on a substrate with known optical parameters. Although complicated, the general expressions have been numerically evaluated for our experimental conditions using a commercial mathematical package on a personal computer. The calculated Raman intensities are in excellent agreement with the experimental results. The observed period of the intensity oscillation relates the film thickness to the exposure allowing the sticking probabilities to be determined. The sticking probabilities of N2 on N2 and O2 on O2 at 14±1 K are 0.84±0.2 and 0.82±0.2, respectively.
Kinetic energy distribution of excited H atoms produced through dissociative photoionization excitation of H2
Authors: Wu C. Y. Robert , Chien T. S. , Judge D. L.
Published: July 2002

The kinetic energy distributions of excited hydrogen atoms produced in the dissociative photoionization excitation of H2 by He II 304 Å photons have been obtained. A fluorescence photon–photoion coincidence technique has been utilized in the present study. A coincidence spectrum showing a broad peak with a weak shoulder corresponding to a time delay of 5.6–7.5 μs is obtained. The kinetic energy distribution of the peak is between 1.2 and 7.5 eV with a maximum at 4.2 eV indicating that this peak is attributable to the lowest energy dissociative photoionization excitation processes which correlate with H(2l)+H+. The relative partial cross sections for direct dissociative photoionization excitation through the 2pπu and 2sσg states at 304 Å are determined to be 1: 0.25.
Hole burning and luminescence spectroscopy of the site distribution and photoinduced reorientation of dihydrophenazine in single crystal matrices
Authors: von Borczyskowski C. , Prass B. , Stehlik D.
Published: July 2002

The nonplanar structure of the electronic ground state of 5,10 dihydrophenazine (PH2) is responsible for a distribution of sites of PH2 observed upon doping into molecular crystals. The change in molecular geometry of PH2 following optical excitation results in considerable stress at the respective crystalline site, which is compensated by guest reorientation. Photochemical hole burning experiments suggest that this reorientation involves the 118 cm−1 butterfly backbone vibration of PH2 as indicated by a hole width equivalent to a vibrational relaxation time of 600 fs.
Uniaxial compression induced collective tilting and distorted-hexagonal structure in Langmuir monolayers
Authors: Shin Seokmin , Rice Stuart A.
Published: July 2002

We propose a theoretical model, based on a free-energy density-functional formalism, for the uniaxial compression-induced hexagonal-to-distorted-hexagonal structural phase transition in Langmuir monolayers. It is shown that the experimentally observed distorted-hexagonal structure is associated with collective tilting, relative to the normal to the surface, of the rod-like molecules.
The use of Raman scattering to study wetting and fluid flow of binary liquids in porous media
Authors: Dierker S. B. , Dennis B. S. , Wiltzius P.
Published: July 2002

We describe the use of Raman scattering as a technique for measuring the concentration of binary liquid mixtures inside a porous medium. The technique is demonstrated by measuring the concentration of mixtures of 2,6-lutidine and water inside the porous glass Vycor. These measurements reveal a large increase in the lutidine concentration inside the Vycor relative to that in a reservoir in contact with the Vycor. This shift is a strong function of concentration and temperature. We formulate a theory for the shift based on a Flory–Huggins model for the concentration dependence of the free energy of the liquid mixture which fits the data well. The measurements allow us to deduce the strength of the surface field h1 exerted by the internal glass surfaces on the imbibed liquid. Finally, we show that fluid flow in porous media can be effectively studied with Raman scattering by studying the diffusion of concentration gradients within the Vycor.
Dynamical properties and transport coefficients of one-dimensional Lennard-Jones fluids: A molecular dynamics study
Authors: Bazhenov Alexiev M. , Heyes David M.
Published: July 2002

The thermodynamics, structure, and transport coefficients, as defined by the Green–Kubo integrals, of the one-dimensional Lennard-Jones fluid are evaluated for a wide range of state points by molecular dynamics computer simulation. These calculations are performed for the first time for thermal conductivity and the viscosity. We observe a transition from hard-rod behavior at low number density to harmonic-spring fluid behavior in the close-packed limit. The self-diffusion coefficient decays with increasing density to a finite limiting value. The thermal conductivity increases with density, tending to ∞ in the close-packed limit. The viscosity in contrast maximizes at intermediate density, tending to zero in the zero density and close-packed limits.
Low-temperature variational approximation for the Feynman quantum propagator and its application to the simulation of quantum systems
Authors: Cao Jianshu , Berne B. J.
Published: July 2002

The low-temperature propagator of Mak and Andersen [J. Chem. Phys. 92, 2953 (1990)] allows for much more rapid convergence of Feynman path integral computer simulations of quantum systems. The effectiveness of this propagator is very sensitive to the choice of an effective frequency and the choice of this made by Mak and Andersen, although good, is not optimum. In this paper a harmonic reference system is used together with a variation principle to compute this effective frequency. Simulations show that when this is used in the low temperature propagator, the results converge much more rapidly than for other choices of the frequency. Moreover, an energy estimator is derived, which allows this effective potential to be used for the determination of the energy of the quantum system. In addition, using a cumulant expansion of the centroid density in the free particle reference system, an effective potential along with a corresponding energy estimator is derived and compared to the above.
Phase resetting dynamics for a discrete reaction–diffusion model
Authors: Chee Merk-Na , Kapral Raymond , Whittington Stuart G.
Published: July 2002

The results of a study of spatial pattern formation in a two-dimensional oscillatory reaction–diffusion system are presented. The calculations are carried out on a discrete model of the Brusselator reaction. The system responds to inhomogeneous perturbations in two different ways. For most perturbations it relaxes back to a spatially homogeneous state with a phase shift. However, special perturbations produce persistent structures which consist of spiral waves and target patterns. The nature of these spatio-temporal states is discussed.
Theory of hydration forces. Nonlocal electrostatic interaction of neutral surfaces
Authors: Leikin S. , Kornyshev A. A.
Published: July 2002

The nonlocal electrostatic contribution to the hydration (structural) repulsion of neutral hydrophilic surfaces separated by thin water layer is considered. This contribution arises from the form of the image–charge interaction of alternating positively and negatively charged polar groups attached to one of the surfaces with nonpolar background of another surface. Characteristic features of this interaction are shown to be determined by the lateral ordering of the polar groups. Similar features were found previously in generalized free-field model of the interaction. Both models are in a good quantitative agreement with the experimental data on lipid membranes, and give consistent physical explanation to observable dependences of amplitude and decay length of the hydration force on the nature and structure of the surfaces. This similarity makes it impossible to distinguish any particular mediator of the hydration force at the present state of the knowledge. At the same time it provides a quite general picture of the role of lateral structure of the surfaces in the hydration force.
Continuous time simulation of the Doi–Edwards model
Authors: Petruccione Francesco , Biller Peter
Published: July 2002

A continuous time simulation algorithm for polymer melts is applied to the Doi–Edwards model. In this model a certain tube segment disappears when it is reached by either end of the primitive chain. The probability that this segment remains at a certain time is well-known analytically. In the underlying stochastic process a unit vector is chosen with new random orientation every time the diffusion process reaches one of its boundaries. To simulate this stochastic process an algorithm is used which is very similar to one introduced for transient network polymer models. In the special case of the Doi–Edwards model one can draw random birth times for the unit vectors from the appropriate probability distribution. The deterministic equations of motion are integrated analytically. This leads to a very efficient method for the simulation of the stochastic process.We compare the simulation results with results which were already obtained by other methods.
Pressure-induced phase transition in sterically stabilized colloids
Authors: de Kruif C. G. , Schouten J. A.
Published: July 2002

Increasing the pressure on stable dispersions of sterically stabilized silica particles in toluene induces a phase transition. This transition can also be induced by lowering the temperature. The dispersion separates into two phases with different particle densities. The origin of the phase behavior cannot be explained with the classical Flory-polymer-physics model. Here we attribute the observed behavior to a ‘‘local ordering’’ or ‘‘liquid-crystalline-like phase transition’’ of the stabilizing moieties grafted to the particles. Through such an effect we can consistently explain the observed phenomena and particularly the high value of the differential quotient dp/dTtrans =+77 bars/K for the phase transition temperature at which the dispersion separates into two liquid phases of different particle concentrations.
Variational calculations on the A
g vibrational states, the automerization, and the predicted Raman spectrum of cyclobutadiene
Authors: Čársky Petr , Špirko Vladimír , Hess B. Andes , Schaad Lawrence J.
Published: July 2002

Variational calculations of vibrational levels of three Ag modes are reported for cyclobutadiene, its vicinally 13C-dilabeled derivative, and the tetralabeled derivatives 13C4H4 and C4D4. These calculations use the ab initio two-configuration GVB/4-31G potential surface for the antisymmetric CC stretch (automerization), symmetric CC stretch, and symmetric CCH bend. The three vibrational modes are strongly coupled so that one-dimensional and adiabatic multidimensional approaches are not justifiable in this case. The splitting of the ground vibrational state of di-13C -cyclobutadiene is calculated to be 4 cm−1, which implies the rate constant for automerization is k=2.4×1011 s−1 at temperatures near absolute zero. The Raman spectrum of cyclobutadiene is predicted to have two strong bands in the region of 3100–3200 cm−1. The other transitions are predicted to be of considerably lower intensity, though three of them should be strong enough to be observed. Two of the three are predicted to be accompanied by satellite lines which are due to the automerization splitting of the Ag levels. Uncertainties involved in the theoretical approach are discussed.
Self-diffusion coefficients of ions in electrolyte solutions by nonequilibrium Brownian dynamics
Authors: Raineri Fernando O. , Wood Mark D. , Friedman Harold L.
Published: July 2002

The self-diffusion coefficients of the ions in a model electrolyte solution are calculated with a novel implementation of the nonequilibrium Brownian dynamics technique. The ions are coupled to an external color field E by color charges in such a way that each ionic species as a whole is electrically neutral to E. The ion–ion forces are not directly affected by the color charges or E. The method is tested on a model of a 1 M NaCl aqueous solution without hydrodynamic interactions and the results are compared with those of a previous equilibrium simulation for the same model system. The self-diffusion coefficients of Na+ and Cl− are determined with 2%–3% accuracy and, within this margin, agree with the results of the equilibrium simulation obtained with more than twice the computational effort. Furthermore, within the range of field strengths studied, the average color flows depend linearly on E.
High resolution studies of core electron excitations in Ru3(CO)12 and the effects of metallic screening
Authors: Sondericker D. , Fu Z. , Bradley J. , Eberhardt W.
Published: July 2002

High resolution electron yield spectra of core electron excitations in condensed Ru3(CO)12 exhibit a pronounced shoulder in the C 1s→π* excitation, due to transitions into unoccupied π levels consisting of linear combinations of the 2π orbitals of the various CO molecular ligands. The O edge near edge spectrum shows a clear and pronounced σ shape resonance transition, about 2 eV lower in energy than in free CO. We develop an intuitive model, which correlates the differences in the relaxation energies associated with the various core electron excitations with changes in the metal to ligand charge transfer process. The consequences of these differences in the core hole screening process with respect to standard models relating the position of the σ shape resonance to the bond length are discussed also.
Photoionization studies of GeHn (n=2–4)
Authors: Ruscic B. , Schwarz M. , Berkowitz J.
Published: July 2002

The adiabatic ionization potential of GeH+4 (GeH4) is measured by photoionization mass spectrometry to be ≤10.53 eV and perhaps as low as 10.44 eV. This is about 0.8 eV (∼9 vibrational quanta) lower than the value reported by photoelectron spectroscopy. This result, analogous to that found for SiH+4 (SiH4), implies a marked Jahn–Teller distortion of GeH+4. The appearance potentials of GeH+2 and GeH+3 from GeH4 are ≤10.772±0.009 eV and <11.657±0.01 eV, respectively. The reaction of F atoms with GeH4 generates GeH3, GeH2 (weakly), and some atomic germanium. The adiabatic ionization potential of GeH3 is ≤7.948±0.005 eV; that of GeH2 is ≤9.25 eV. Together with auxiliary information, limits (more probable values) of the incremental bond energies, in kcal/mol, are found to be D0 (H3Ge–H) <85.5 (82±2); D0 (H2Ge–H) >56.4 (59); D0 (HGe–H) <68.9 (66); and D0 (Ge–H) >53.7 (63). These specific bond energies, when appropriately normalized, display the same pattern as the SiHn (but not the CHn) bond energies and provide a basis for estimating the corresponding SnHn bond energies.