bluuebunny/crossref_metadata_2025 huggingface dataset:
A unified approach to spin polarization (CIDEP) and relaxation by Heisenberg spin exchange. II. Evaluation of the interradical distance dependence
Authors: Syage Jack A.
Published: July 2002

In paper I, a theory of Heisenberg spin exchange (HSE) was formulated to treat both electron spin polarization (CIDEP) and relaxation. In this paper (II), the expressions have been extended to consider the dependence of the exchange strength J(r) on the interradical distance r. Analytic expressions for the CIDEP enhancement E and the spin exchange frequency νex have been obtained by adopting an exponential model for J(r) and integrating over a distribution of encounter distances r based on a Brownian diffusion model. A semiempirical expression for E has also been developed by replacing the usual diffusion parameters with experimentally measurable properties that are functions of diffusion. The behavior of νex and E on the exchange strength J(r) and the steepness of the distance dependence κr0 is illustrated. The analytic results are consistent with existing numerical calculations and agree remarkably well with recent experimental measurements. The role of viscosity is treated to explain recent results for relaxation (νex) and polarization (E) measurements of (i) acetate radical anion, (ii) H and D atoms, and (iii) acetone photoreduction.
Delocalization and correlation effects in atomic systems: Solution of the Hubbard model for general spin
Authors: Whaley K. B. , Falicov L. M.
Published: July 2002

The effects of quantum statistics and atomic correlations on motion in two-dimensional periodic potentials is studied within a Hubbard model for both Bose–Einstein and Fermi–Dirac statistics. Exact eigenstates are obtained for finite clusters with periodic boundary conditions, and the extent of ground state delocalization evaluated for both statistics.
Heats of transport from diffusion thermoeffect measurements on binary liquid mixtures of carbon tetrachloride with benzene, toluene, 2-propanone, n-hexane, and n-octane
Authors: Yi S. C. , Rowley R. L.
Published: July 2002

Measurements of heats of transport are reported for binary liquid mixtures containing carbon tetrachloride and benzene, toluene, 2-propanone, n-hexane, or n-octane at 298 K and ambient pressure. Additionally, measurements were made at 308 K for carbon tetrachloride with benzene, toluene, and 2-propanone. The corresponding heat-mass Onsager coefficients are also reported, from which thermal diffusion ratios have been calculated based on Onsager reciprocity. These data will be used in a companion paper to report a modified Enskog method for correlation and prediction of heats of transport.
On the use of a modified square-well model for prediction and correlation of thermal diffusion factors in binary liquid mixtures
Authors: Yi S. C. , Rowley R. L.
Published: July 2002

Recently reported thermal diffusion factors obtained from heats of transport have been used to test the ability of existing theories to predict thermal diffusivities using model parameters regressed from pure component transport data. None of the theories tested were able to provide even qualitative agreement with experiment. However, predictions using a square-well Enskog theory, modified here for consistency with normal experimental definitions, were found to be approximately correct using pure-component square-well parameters regressed from viscosity and thermal conductivity and an assumed mixing rule for cross terms. Predicted values are very sensitive to the energy mixing rule, and correlation of experimental data is possible for some systems using this theory with one adjustable parameter in the mixing rule.
Photoemission from silver into liquid ammonia
Authors: Bennett G. T. , Coffman R. B. , Thompson J. C.
Published: July 2002

We report here part of a detailed study of pulsed photoemission into pure liquid NH3. We find that scavengers and salts are not essential to the detection of photoelectrons in NH3, in contrast to previous measurements in H2O. A six order-of-magnitude range in the quantum yield has been found. Maximum yields are near 10−2 electrons per photon. As previous workers, we find a two-slope response to the yield curve which may be interpreted in terms of two thresholds. We interpret out observations in terms of photoinjection into a band of localized states lying below the conduction band of the NH3. The bottom of the conduction band in NH3 is found to be 0.95 eV below the vacuum. The band of localized states extends approximately 0.4 eV lower. Our analysis does not support either the ″4/2’s″ or the ″5/2′s″ laws relating the yield to the excess energy above threshold.
Random spin models and chemical kinetics
Authors: Demetrius Lloyd
Published: July 2002

Lyapunov exponents constitute a class of parameters which describe the asymptotic behavior of a large class of dynamical processes in chemistry and physics. This paper gives a variational characterization of the largest Lyapunov exponent for a class of models of chemical kinetics described by products of random nonnegative matrices. We show that for this class of models the largest Lyapunov exponent satisfies an extremal principle formally identical to the minimization of the quenched free energy in random spin models. This extremal principle, which yields a computable expression for the Lyapunov exponent, implies that fluctuations in the Lyapunov exponent, due to a certain class of perturbations in the matrix elements, are determined by a macroscopic parameter which is the analog of the mean energy in random spin systems. These results characterize a class of random models in chemical kinetics that are thermodynamically stable in the sense that they possess an asymptotic limit in which analogs of the laws of equilibrium thermodynamics hold.
Anion states of η4-polyene iron tricarbonyl complexes
Authors: Olthoff J. K. , Moore J. H. , Tossell J. A. , Giordan J. C. , Baerends E. J.
Published: July 2002

Attachment energies of low energy electrons to Fe(CO)5 and to η4 complexes of 1,3-butadiene, 1,3-cyclohexadiene, cyclooctatetraene, and cyclobutadiene with Fe(CO3) have been determined by electron transmission spectroscopy. The spectrum of Fe(CO)5 is similar to that of Cr(CO)6, showing an anion resonance near threshold assigned to predominantly Fe3d orbitals and two resonances between 1 and 3 eV assigned to predominantly COπ* orbitals.The diene complexes show threshold features similar to Fe(CO)5, COπ* resonances around 2 eV, and one or more diene π* resonances. The resonances from the lowest π* orbitals of butadiene, cyclohexadiene, and cyclooctatetraene are little different in the free dienes and the complexes, but higher π* orbitals are substantially destabilized in the complexes, consistent with qualitative symmetry arguments. In the cyclobutadiene complex the π*3 orbital of cyclobutadiene is strongly destabilized by interaction with the Fe3d, giving a resonant feature at 1.2 eV. Dissociative attachment of electrons by the iron tricarbonyl complexes has been observed mass spectrometrically. The phenomenon is observed for electrons of energy less than 2 eV and results primarily in the loss of CO. For the cyclobutadiene complex, however, the attachment of 0 eV electrons results in a complex chemical process leading to the ejection of C2.
Solvent structure effects on electron reactivity in isomeric butanol/water mixtures
Authors: Senanayake P. Chandani , Freeman Gordon R.
Published: July 2002

The nearly diffusion controlled reaction rate constants k2 of solvated electrons with efficient electron capturers in butanol/water mixtures are examined according to the Smoluchowski, Debye, and trap-depth models. The k2 values depend on the diffusion rates of the reactants and the solvation energies of the electrons. Both these factors are affected by the rigidity of the alkyl group on the alcohol. Increased rigidity decreases the diffusion rate, which tends to decrease k2, and decreases the electron solvation energy, which tends to increase k2. Rate constants of efficient electron capturing solutes are dominated by the former, those of inefficient capturers by the latter. The free energy of activation ΔG‡13 for electron capture by an inefficient electron scavenger (solute) is empirically related to Er/ f(μ), where Er is a representative optical absorption energy of the solvated electrons (related to the solvation energy) and f(μ) is a function of the dipole moment of the scavenger. It is also related to the polarizability of the solvent molecules. Capture by an inefficient solute has a large negative entropy of activation attributed to solvent rearrangement about the reaction site.
Fractal growth in impurity-controlled solidification in lipid monolayers
Authors: Fogedby Hans C. , So/rensen Erik Schwartz , Mouritsen Ole G.
Published: July 2002

A simple two-dimensional microscopic model is proposed to describe solidifcation processes in systems with impurities which are miscible only in the fluid phase. Computer simulation of the model shows that the resulting solids are fractal over a wide range of impurity concentrations and impurity diffusional constants. A fractal-forming mechanism is suggested for impurity-controlled solidification which is consistent with recent experimental observations of fractal growth of solid phospholipid domains in monolayers.
Neutral reactions in the presence of alkali ions
Authors: Viggiano A. A. , Deakyne Carol A. , Dale F. , Paulson John F.
Published: July 2002

It has previously been shown that neutral ligands clustered to an alkali ion can react with other neutrals. In fact, the presence of the alkali ion greatly enhances the rate of the corresponding neutral–neutral reaction. Several more reactions that exhibit this phenomenon have been identified experimentally, with rate enhancements ranging from 4 to 30 orders of magnitude. Three possible explanations for the rate enhancement have been explored. Ab initio calculations have been carried out to supplement the experimental measurements and to provide further insight into the mechanism. The calculations indicate that bonding the neutral to the alkali ion causes structural rearrangements that shift the geometry of the neutral molecule toward its transition state geometry in the corresponding neutral–neutral reaction, thereby lowering the activation energy of the reaction.
Semiclassical vibrational energies and transition frequencies for a Hamiltonian with stretch–bend potential energy coupling: Application of Fourier methods
Authors: Garcia-Ayllon A. , Martens C. C. , Santamaria J. , Ezra G. S.
Published: July 2002

Semiclassical methods are applied to determine transition frequencies and vibrational energies for the two-mode HC2 stretch–bend Hamiltonian recently studied by Swamy and Hase [J. Chem. Phys. 84, 361 (1986)]. The mean action spectral approximation for calculating transition frequencies is found to give results in excellent agreement with quantum variational values. The FFT EBK method is used to calculate vibrational eigenvalues for both nonresonant and 3:1 resonant states. Both approaches give results more accurate than those reported by Swamy and Hase using the DeLeon–Heller–Miller method. The presence of stretch–bend potential coupling is found to reduce the extent of classical chaos.
Matrix effects on radiative and radiationless rates of NBr b 1Σ+ and a 1Δ trapped in solid argon
Authors: Becker A. C. , Lodemann K.-P. , Schurath Ulrich
Published: July 2002

NBr in solid argon has been excited to the b 1Σ+ state with a pulsed tunable dye laser. In addition to the well-known fluorescence spectrum of the b state, infrared emission from the vibrationally relaxed a 1Δ state was detected at 1086 nm. Three major trapping sites were resolved in absorption and fluorescence excitation spectra of the b–X system. Additional sites could be identified in the excitation spectrum of the a state, which is populated via the b state by internal conversion. There is a weak site effect on the lifetime of the vibrationally relaxed b state, which varies between 40 and 30 μs. Vibrational relaxation rates in the b state show a stronger site dependence. Internal conversion from b 1Σ+ to a 1Δ accounts for <0.1% to at least 17% of the decay rate of NBr b 1Σ+, v=0. Orbitally forbidden transitions to the X10+ component of the ground state were identified 23.0 to 25.5 cm−1 on the high-energy side of the more intense a 1Δ→X21± transition of several sites. The lifetime of the a state varies between 152±12 ms in the least perturbed site and 121±15 ms in the site with the strongest a 1Δ→X10+ forbidden component. NBr trapped in double vacancies of pure fcc argon, of single stacking faults, and of multiple stacking faults (hcp pockets) in argon, can account for the sites.
Infrared diode laser spectroscopic study of the active nitrogen. The electronic transition between the lowest singlet metastable states a 1Πg–a′ 1Σ−u, 0–0 band
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The a 1Πg–a′ 1Σ−u, 0–0 band of the molecular nitrogen was observed in an electric discharge plasma of nitrogen using infrared diode laser spectroscopy at Doppler-limited resolution. Precise values were obtained for the rotational constants and the energy separation between the two electronic states. The effect of Λ-type doubling in the a 1Πg state was observed as small shifts of Q branch and P/R branch transitions to higher and lower frequencies, respectively, than those expected if Λ doubling were absent.
Observation and characterization of the CH5 radical by neutralized ion beam techniques
Authors: Selgren Susan F. , Gellene Gregory I.
Published: July 2002

The CH5(CD5) radical has been produced by charge transfer neutralization of fast ion beams using Na and Zn targets. The results obtained with Na targets are in excellent agreement with previous similar studies using alkali metal targets. In particular, no evidence for metastability was observed. Alternatively, results obtained with Zn targets show clear evidence for production of metastable states of the radical with lifetimes in the 1–10 μs range. Measurement of the kinetic energy released upon dissociation indicates that some neutral radicals produced in CH+5(CD+5)/Zn reactions have energies very close to the ionization limit. It is proposed that the observed metastability may be explained by the production of high Rydberg levels of the radical having sufficiently long radiative lifetimes.
Three-dimensional analytical model for isotope effects in the photofragmentation of triatomic molecules
Authors: Atabek O. , Bourgeois M. T. , Jacon M.
Published: July 2002

A three-dimensional analytical Franck–Condon model is presented for the interpretation of intramolecular isotope effects in the photofragmentation of a symmetric triatomic system when the dissociation occurs on a repulsive surface with a single saddle point. The role of the geometry and force constants of the ground and excited surfaces, of the intermode coupling known as the Duschinsky effect, and of the initial vibrational energy content is discussed in relation with the dissociation cross sections and branching ratios measuring the competition between different isotope fragmentation arrangements. Recent ion–ion coincidence measurements on doubly charged deuterated water cations show that the formation of H+ is five times more frequent than that of D+ in two-body fragmentations. The interpretation of such an important hydrogen–deuterium isotope effect seems to be possible by the use of this analytical model.
Rotational structure of overtone transitions of CHD3 near 13 500 cm−1
Authors: Campargue A. , Stoeckel F. , Chenevier M. , Kraiem H. Ben
Published: July 2002

The absorption spectrum of CHD3 has been recorded for the N=5 polyad near 13 500 cm−1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm−1. The rotational structure of the four observed bands has been resolved and the rotational constants given. The behavior of these constants versus the quantum number of the C–H stretching and bending modes are also discussed.
1
9F shielding anisotropy in RbCaF3
Authors: Kaliaperumal R. , Sears R. E. J. , Finch C. B.
Published: July 2002

A 19F NMR multipulse measurement of the chemical shift in a single crystal of cubic RbCaF3 gave −47.0±3 ppm as the isotropic value with respect to C6F6, and 128.7±6 ppm as the anisotropy. The shielding is accounted for by the usual diamagnetic and paramagnetic ionic overlap and covalent terms. As a result, the Ca++ −F− bond is estimated to be 98% ionic. No significant spectral changes were found when the crystal was cooled below the cubic to tetragonal phase transition temperature. Reasons for this are given.
Valence ionization spectrum and electron affinity of the open and cyclic form of S3
Authors: von Niessen W. , Tomasello P.
Published: July 2002

The valence ionization spectrum and the electron affinity are calculated for the two nearly degenerate 1 A1 states of S3, the open (C2v) and the closed (D3h) form. Electron correlation and relaxation effects are included via the Green’s function method. The theoretical ionization spectra are rather different. The D3h isomer has the lower ionization energy and the first band (at about 8.9 eV) is separated from the next band system by about 3.5 eV. The spectrum of the C2v isomer should at the ionization threshold with an ionization energy of about 9.8 eV consist of three nearly degenerate bands. The experimental value is 9.68 eV. Satellite lines are calculated to be very dominant in the case of the open form. Only the open form has an electron affinity which is calculated to be 2.07 eV (vertical value) and the adiabatic value is estimated to be 2.34 eV. This is to be compared with the experimental (adiabatic) value of 2.093 eV. The electron affinity of the other form is close to zero.
Statistical mechanics of the single occupancy system
Authors: Jhung Kyu Soo , Jhung Kwang Hwa Chung
Published: July 2002

Formal solutions to the exact BGY equations of the single occupancy model of classical anharmonic solids have been developed and approximate solutions have also been constructed on the basis of the ‘‘consistent’’ closure relation. The partition function of the system is determined by the normalization condition of the singlet function ρ1 and the total distribution function ρN through the use of the linked hierarchical relationship between distribution functions. It is explicitly proved that the free energy expression, the virial pressure, and the energy equation are all consistent at all orders of truncation approximation. A new method to calculate solid-phase entropy in Monte Carlo experiments is also proposed based on the free energy we developed and applied to inverse 12th-power systems to give a numerical example.
Molecular beam study of the interaction of atomic and molecular oxygen with methane
Authors: Liuti G. , Pirani F.
Published: July 2002

Absolute integral cross sections for collisions of CH4 with O2 molecules and of O2 molecules and O(3Pj) atoms with CH4 are measured as a function of velocity at thermal energies in a molecular beam apparatus. For the O2–CH4 interaction the glory structure observed allows an analysis in terms of an isotropic potential model and meaningful potential parameter values are obtained. For the O(3Pj)–CH4 interaction the anisotropy due to the O(3Pj) atoms causes a partial quenching of the glory amplitude. Nevertheless a realistic isotropic potential can be extracted from the glory extrema position. The present potential parameters for the O2–CH4 and the O–CH4 interactions can be used to predict realistic parameters for more complicated interactions involving O2 molecules and O atoms with other species of interest also in combustion.
Further contributions to the energy levels of a perturbed anharmonic oscillator: Application to adiabatic corrections
Authors: Hadinger Gisèle , Tergiman Y. S. , Hadinger Gerold
Published: July 2002

The investigation of further contributions to the energy levels of a perturbed anharmonic oscillator is continued. It is shown that, by using a Taylor series for several variables, the contributions to the energy levels due to an additional perturbing interaction can be directly derived from the Yun Dunham-type coefficients by means of analytic relations involving products of differential operators. Consequently, the analytic expressions for the whole set of these contributions are generated from the Yu0 coefficients alone and are easily calculated by using a computer algebraic manipulation language. As an illustrative application, the case of the adiabatic corrections for nuclear motion to the potential energy curves of diatomic molecules in 1Σ+ states, such as lithium hydrides and hydrogen chlorides, is considered.
The exact canonical equation of state for a finite hard rod gas with square-well attractive interactions
Authors: Walmsley Roger H.
Published: July 2002

The exact canonical equation of state is found in one dimension for an arbitrary number of hard rods with an attractive square-well interaction. The reduction in pressure due to the attractive interactions is suppressed at both high and low densities, while it is enhanced at intermediate densities such that the average rod separation is equal to the rod length plus the width of the well. Consideration of the limits of low temperature, low density, and large system size (thermodynamic limit) shows that the attractive interactions lead to an ambiguity in the low temperature limit which depends upon the order in which it is taken relative to the other two variables. In particular, the low temperature limit of the Takahashi–Gürsey equation of state is not valid for the canonical ensemble.
Termolecular recombination at low gas density: Strong collision, bottleneck, and exact treatments
Authors: Flannery M. R. , Mansky E. J.
Published: July 2002

On introducing the probabilities for association as a function of internal separation R and internal energy E of the associating (A–B) species the strong collision model is thoroughly investigated and compared, as a case study, with the exact treatment of termolcular ion–ion recombination at low gas densities. A bottleneck model is also investigated. Analytical expressions for the one way equilibrium energy-change rates at fixed R are provided in the Appendix.
Electron spin resonance of CrO3−4 in Na2SO4
Authors: Yu Jiang-Tsu , Yeh J. Y.
Published: July 2002

A CrO3−4 center with the z2 ground state is detected by ESR in Na2SO4 crystals grown at 80 °C, doped with CrO2−4, and gamma irradiated. A CrO3−4 center with the x2−y2 ground state, interacting with two chemically equivalent but magnetically distinguishable protons, is detected in crystals grown at 40 °C.
Static and dynamical properties of Di (Li) in MgO:Li studied by EPR: Isotope effects
Authors: Orera V. M. , Sanjuán M. L. , Abraham M. M.
Published: July 2002

The D0i (Li) defect has been formed in MgO:Li and studied by EPR. The static spin Hamiltonian parameters, obtained from the angular dependence of the spectrum at LNT, are given and compared with those of H0i (Li). A noticeable isotope effect is found in the lithium shf interaction, which is explained as a consequence of the zero-point vibration of the hydrogen atom. The temperature evolution of the spectrum was also studied, and interpreted as a motional averaging process due to the reorientation of the defect among its eight equivalent 〈111〉 sites. Using the Liouville stochastic formalism, the EPR spectra were simulated as a function of the jump frequency (ωe) and fitted to the experimental ones. This yields the temperature dependence of ωe which can be compared with that for the H0i (Li) defect. The isotope effects found, both in the activation energy and in the preexponential factor, are interpreted in the frame of the small-polaron theory.
Kinetic equation theory of atom transport in a random alloy in a superposition approximation
Authors: Moleko L. K. , Okamura Y. , Allnatt A. R.
Published: July 2002

The matter transport coefficients defined by nonequilibrium thermodynamics are calculated for the vacancy migration mechanism in a random alloy of arbitrary vacancy content. The perturbations to the atom-vacancy pair correlation functions are calculated when steady matter fluxes are created by a uniform external field. The Kirkwood superposition approximation for triplet distribution functions is used to truncate the hierarchy of kinetic equations for correlation functions in the steady state. The procedure leads very simply to several results obtained earlier by diverse methods and generalizes some of them to arbitrary vacancy content.
A multifractal description of the hydrodynamic force distribution for reaction-limited aggregates
Authors: Meakin Paul
Published: July 2002

The distribution of forces and force components exerted by the particles on the fluid in a fractal aggregate moving through a quiescent fluid can be described in terms of a fractal measure or a multifractal distribution. The distribution of normalized forces or force components Fn for aggregates of different masses (M) can be scaled onto a common curve using the scaling form ln{N[ln(Fn)]}=ln(M)g[ln(Fn)/ln(M)]. The scaling function g(x) has been determined for three dimensional reaction-limited cluster-cluster aggregates with a fractal dimensionality (D) of 2.10 using the Kirkwood–Riseman theory and has been found to be almost indistinguishable from the scaling function for diffusion limited aggregates (D≂1.78). The spectrum of singularities f(α) of strength α defined by Halsey et al. has been determined from the scaling function g(x) and from the moments of the force probability distribution. Some of the uncertainties associated with the determination of the asymptotic (M→∞) shape of f(α) from finite size simulations or experiments are explored and discussed.
Note on the finite number of interacting neighbors and the finite number of k-point effects on the total electronic energy of a metallic polymer
Authors: Cioslowski J. , Kertesz M.
Published: July 2002

Errors introduced to the Hartree–Fock exchange term in the total electronic energy by a finite number of neighbors and a finite number of k-point approximations are discussed for a model metallic polymer.
Analysis of the absorption spectrum of neodymium:sodium beta double prime alumina
Authors: Alfrey A. J. , Stafsudd O. M. , Dunn B. , Yang D. L.
Published: July 2002

The Judd–Ofelt analysis has been applied to the sigma-polarized absorption spectrum of Nd3+:NaB″alumina to determine the modified Judd–Ofelt parameters Ωσ,2, Ωσ,4, and Ωσ,6. A model for the near-neighbor environment of Nd3+ has been proposed and tested using a simplified point charge model for the crystal field. The model indicates that the observed spectrum may be explained by a large linear field term in the crystal field induced by the shift in position of conduction plane oxygen and neodymium ions away from sites of inversion symmetry.
Decay of the first triplet Kr2 excimer in the presence and absence of N2 and effects of N2 on the decay constant and excimer yield at 298 K
Authors: Firestone R. F. , Selander W. E.
Published: July 2002

The optical absorbance of Kr2 (3∑+u)(1u,0−u) at 984 nm has been measured as a function of time at 250 to 1100 Torr in pure krypton and in the presence of N2 at 800 Torr of krypton at 298 K. The decay constant is given by 2.53 (±0.19)106+194(±84)PKr s−1 in the absence of N2. The rate constant for electronic quenching by N2 is 7.65(±0.34)10−12 cm3 s−1. All observations are consistent with the formation of a long-lived ‘‘bottleneck’’ species associated kinetically with the Kr(1s2) state as proposed by Bonifield et al. The bottleneck is essentially unaffected by the presence of N2 below 10 Torr. It is suggested that the bottleneck species is a bound molecular state which correlates with one of the Kr(2p) atomic states in the separated atom limit.
Potential anisotropy and the lifetime of triatomic collision complexes
Authors: Brass O. , Schlier Ch.
Published: July 2002

In 1984, Schelling and Castleman discussed whether the lifetime of a complex formed in an ion–molecule reaction might depend on the anisotropy of the ion–molecule potential. Their model was that of a rigid rotor with spherical repulsion, but anisotropic polarizability. In this paper, we contend that models employing a rigid diatom are quite unrealistic to discuss complex lifetime, since the latter is predominantly determined by intermediate energy storage into vibration. We use, instead, a model system with three pair potentials of the Morse type and vary the equilibrium distances to generate different anisotropies of the potential. Trajectory calculations show a definite influence of this parameter both on the complex formation cross section and on lifetime. These lifetimes are one order of magnitude longer than those computed with the rigid diatom model, after the calculations have been scaled to the same masses and potential parameters. The lifetimes turn out to be roughly proportional to the period of the fastest normal mode of the intermediate molecule, and their behavior is generally in accord with RRKM theory.
An improved analysis of transient ESR signals of photoexcited triplet states: Application to chlorophyll-a in a glassy matrix
Authors: Hiromitsu Ichiro , Kevan Larry
Published: July 2002

An improved analysis is presented for the kinetic traces of the electron spin resonance (ESR) signal intensity of photoexcited triplet states during and after a rectangular light pulse. It is assumed that the mixing ratio ξp, defined by 〈1‖1〉=ξp(q‖q)+(1−ξp)(r‖r) and 〈−1‖−1〉=(1−ξp)(q‖q)+ξp(r‖r), deviates from 1/2, where ‖1〉, ‖0〉, ‖−1〉, and ‖p), ‖q), ‖r) are the spin wave functions for the triplet sublevels with and without an external magnetic field parallel to the p principal molecular axis with p, q, r=x,y,z or a cyclic permutation thereof. An X-band ESR experiment is done for the chlorophyll-a (Chla) triplet state in a methanol–methylene chloride glassy matrix at 77 K. Previous theory fits the y and z components but not the x component. This failure can be attributed to the fact that ξp≠1/2, and the improved analysis fits well for the x component as well as the y and z components with ξx=0.555±0.010.
Asymmetry in molecular ordering in the nematic phase: DMR study
Authors: Wu Bao-Gang , Ziemnicka Barbara , Doane J. William
Published: July 2002

Deuterium nuclear magnetic resonance (2H-NMR) was performed on the rigid perdeuterated fluorene segment of the compound 2-fluorenyl-4′-tetradecyloxy benzoate (FLOC 14-d9) to unambiguously measure the order parameter U which reflects the degree of asymmetric ordering of the molecule in the nematic liquid crystalline phase. Measurements of U showed this parameter to be finite at all temperatures in the nematic phase indicative of the uniaxial phase U2 described in the Allender–Lee generalized Landau theory. Mixtures of FLOC 14-d9 with compounds consisting of molecules more cylindrical in shape were also studied to examine their effect on the parameter U. Other parameters such as the degree of order S and the orientation of the principal axes of the Maier–Saupe order matrix were measured and are discussed.
Hydrodynamic effects in polymer solutions. I. Friction coefficients in steady elongational flow
Authors: López de Haro M. , Pérez-Madrid A. , Rubí J. M.
Published: July 2002

The hydrodynamic drag on the spherical beads of a dilute suspension of multibead–spring–rod chains immersed in a Newtonian solvent in steady elongational flow is calculated with the method of induced forces. Hydrodynamic interactions between the beads cause the drag to be anisotropic in space while the friction tensors exhibit a nonanalytic dependence on the velocity gradient when nonlinear terms in the quasistatic Navier–Stokes equation are considered. A discussion of the differences with the phenomenological form usually adopted in polymer dynamics is given and potential applications of the results in rheology are highlighted.
Electron scattering distributions as a probe of adsorbate orientation: CO and N2
Authors: Nagano S. , Luo Z.-P. , Metiu H. , Huo W. M. , McKoy V.
Published: July 2002

We calculate the differential cross section for elastic electron scattering by adsorbed CO and N2 . The dependence of this cross section on the azimuthal angle of detection depends on the molecular orientation and can be used to differentiate between a molecule lying flat on the surface, a tilted molecule, and an upright one. For molecules lying flat on the surface the orientation of the molecular axis and of the C and O atoms with respect to the plane of incidence of the electron can be determined.
Sound propagation in water–ethanol mixtures at low temperatures. II. Dynamical properties
Authors: D’Arrigo G. , Paparelli A.
Published: July 2002

Measurements of the ultrasonic absorption coefficient over the entire concentration range of ethanol aqueous solutions in the frequency range 10–250 MHz and for a temperature interval extending from +30 to −40 °C are presented. Large peaks, which increase noticeably as temperature and frequency decrease, are observed in the attenuation vs composition plot. A detailed analysis of these results and of the velocity dispersion data shows that the frequency spectra are better accounted for by the concentration fluctuations theory of Romanov and Solov’ev (RS) which assumes a distribution of relaxation times rather than by the quasichemical theories which consider single or double relaxation processes. We also show that the absorption and velocity expressions as given by RS can be put in such a form to allow the experimental data as a function of concentration, temperature, and frequency to be displayed on two single ‘‘universal’’ plots. An analysis of the extracted RS theory parameters indicates that the large attenuations are mainly related to the small mutual diffusion coefficients of the solutions. At a molecular level the observed dynamical behavior is associated to the forming and breaking up of alcohol–water aggregates whose lifetimes at the peak concentration range from about 10−9 to 10−7 s on going from higher to lower temperatures.
Statistical thermodynamics of the melting curve at high pressures
Authors: Ruffa A. R.
Published: July 2002

The thermal potential concept used in earlier publications to calculate zero pressure thermodynamic properties of solids is extended to calculations for elevated pressures. The proposed equations indicate that the theory applied to the melting process generally results in a nonlinear melting curve with the amount of curvature depending on the magnitudes of the potential parameters. Calculations for the T–P melting curve for KCl reproduce the main features of the experimental curve without any assumptions concerning the reordering of the liquid to a more densely packed arrangement. In addition, the theory makes some novel predictions concerning the behavior of some thermodynamic variables at high pressures that can be tested in principle by experiment.
Broken particle–hole symmetry in critical fluids
Authors: Goldstein Raymond E. , Parola Alberto
Published: July 2002

The quantitative validity of asymptotic particle–hole symmetry in a fluid at its liquid–vapor critical point is determined by means of the exact mapping of the fluid Hamiltonian onto that of an effective Landau–Ginzburg–Wilson model studied first by Hubbard and Schofield. A particular three-particle correlation of a reference fluid is identified as that which controls the breaking of liquid–vapor symmetry, as manifested in a linear mixing of the pure Ising-like scaling fields and a singularity in the coexistence curve diameter. The inherent smallness of the mixing coefficient in a pair-potential fluid is shown to reflect the weak density dependence of the second moment of the two-particle direct correlation function of the reference system. It is further demonstrated that three-body interactions of the Axilrod–Teller-type enhance the broken particle–hole symmetry found in a purely pairwise-additive Hamiltonian, and detailed calculations give diameter anomaly amplitudes which vary linearly with the fluid polarizability, in quantitative agreement with recent experiments.
Stimulated emission spectroscopy of van der Waals vibrational levels of glyoxal(X̃ 1Ag)⋅Ar
Authors: Frye Donald , Arias Paul , Dai Hai-Lung
Published: July 2002

Stimulated emission spectroscopy was used to study the electronic ground state vibration–rotation levels of van der Waals complexes generated in a supersonic beam. This technique can be used to study all the vibrational levels with nonvanishing Franck–Condon factors from the electronic excited states with 0.04 cm−1 resolution. Five van der Waals vibrational levels in the X̃ state were directly observed in the stimulated emission spectra and were assigned to the fundamental and first overtone levels of the two Ar⋅glyoxal bending modes and the stretching fundamental level. In addition, the rotational constants and the structure were determined for both the Ã and X̃ state complex, together with the vibrational term values and the widths of single rotational levels of the three glyoxal vibrations in the glyoxal⋅Ar complex, the C–C stretch v4=1, the CH wag v8=1 and the OCC bend v5=1 level.
Infrared diode laser spectroscopy of lithium hydride
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The fundamental and hot bands of the vibration–rotation transitions of 6 LiH, 7 LiH, 6 LiD, and 7 LiD were observed by infrared diode laser spectroscopy at Doppler-limited resolution. Lithium hydride molecules were produced by the reaction of the Li vapor with hydrogen at elevated temperatures. Some 40 transitions were observed and, after combined with submillimeter-wave spectra reported by G. M. Plummer et al. [J. Chem. Phys. 81, 4893 (1984)], were analyzed to yield Dunham-type constants with accuracies more than an order of magnitude higher than those published in the literature. It was clearly demonstrated that the Born–Oppenheimer approximation did not hold, and some parameters representing the breakdown were evaluated. The Born–Oppenheimer internuclear distance rBOe was derived to be 1.594 914 26 (59) Å, where a new value of Planck’s constant recommended by CODATA was employed. The relative intensity of absorption lines was measured to determine the ratio of the permanent dipole moment to its first derivative with respect to the internuclear distance: μe /[(∂μ/∂r)e re ]=1.743(86). The pressure broadening parameter Δνp /P was determined to be 6.40 (22) MHz/Torr by measuring the linewidth dependence on the pressure of hydrogen, which was about four times larger than the value for the dipole–quadrupole interaction estimated by Kiefer and Bushkovitch’s theory.
Determination of the centrifugal–distortion-induced dipole moment of SnH4 by infrared–infrared double resonance Stark spectroscopy
Authors: Ohshima Yasuhiro , Matsumoto Yoshiyasu , Takami Michio , Kuchitsu Kozo
Published: July 2002

Doppler-free infrared–infrared double resonance Stark spectrum is observed for SnH4 to determine the dipole moment induced by centrifugal distortion. A tunable diode laser is frequency modulated to generate sidebands for the double resonance. Twenty-eight double resonance signals are observed in 16 ν4 band absorption lines of the E symmetry species. The signals are assigned to the Δm=±2 resonances among first-order Stark multiplets in the ground and ν4 vibrational states. The induced dipole moment in the ground state is determined to be θxyz =4.26(12)×10−5 D from 12 signals observed in the J=5–17 rotational levels.
Exact normalization of scattering states in Liouville space
Authors: Muga J. G.
Published: July 2002

In Liouville space it is possible to obtain scattering states exactly normalized independent of volume considerations. This is achieved only through nonfactorizable Mo/ller superoperators of the kind used in kinetic equations for dilute gases. This states a difference (for plane waves) with the factorizable Mo/ller superoperator.
Selection rule in the vibrational–electron-hole pair energy transfer for molecules interacting with metal surfaces
Authors: Sebastian K. L. , Jayadev V.
Published: July 2002

In the scattering of a polyatomic molecule from the surface of a metal, there can be exchange of energy between electron-hole pair excitations of the metal and vibrations of the molecule. A possible mechanism for this is electron transfer to an (affinity) orbital of the molecule. We point out that certain vibrations would be selectively excited/deexcited in this mechanism. A similar rule exists for the damping of vibrations of molecules adsorbed on metal surfaces.
Theory of electro-optical effects in two-photon spectroscopy
Authors: Andrews David L. , Blake Nick P. , Hopkins Kevin P.
Published: July 2002

This paper develops the general theory of electro-optical effects in two-photon molecular spectroscopy. Two distinct mechanisms can play a role in these nonlinear interactions with an external static electric field. One relates specifically to polar fluids and is associated with the partial molecular alignment produced by the applied field. The resultant anisotropy produces a relaxation of symmetry restrictions on the allowed two-photon transitions. The other mechanism directly involves a nonlinear electro-optical channel and is associated with quite distinct selection rules. Both mechanisms induce changes in line intensities and can under suitable polarization conditions enable certain transitions to be ‘‘switched into’’ the spectra. Generalized rate equations are derived using standard quantum electrodynamical procedures. The results, which are applicable to any required laser beam configuration, are cast in terms of irreducible Cartesian tensors. This facilitates the elucidation of the selection rules appropriate for molecules of any given symmetry. A comprehensive tabulation of the transformation properties of the relevant molecular tensors under the operations of the common molecular point groups is also presented, paving the way for the subsequent application of the results specifically to two-photon absorption and Raman scattering processes.
Statistical thermodynamics of a fluid adsorbed onto a spherical surface. II. Scaled particle theory
Authors: Post A. J. , Glandt E. D.
Published: July 2002

Scaled particle theory is applied to the prediction of the thermodynamic properties of a system of identical hard spheres adsorbed onto another sphere of different diameter. The finite nature of the system requires modification of the usual boundary conditions on the contact correlation function. The calculated activities of the adsorbate are in good agreement with Monte Carlo simulation results and with exact values for systems of few particles.
Multiphoton dissociation of a diatomic molecule: Laser intensity, frequency, and pulse shape dependence of the fragment momentum distribution
Authors: Heather Robert , Metiu Horia
Published: July 2002

We study by an exact method the infrared multiphoton dissociation of a rotationless diatomic molecule and calculate the fragment relative momentum distribution as a function of the laser intensity and frequency, using either a square or smoothly varying pulse shape. The distribution has peaks due to multiphoton transitions. The nature of the peak structure depends on the laser intensity and whether the laser frequency is comparable to (i.e., within 20%) the ν=0 to ν=1 transition frequency (ω10) of the diatomic: If it is the distribution has bands spaced by the photon energy which contain peaks due to transitions from many bound states; if the laser frequency is not comparable to ω10, the distribution consists of isolated peaks spaced by the photon energy, which result from multiphoton transitions from the ground vibrational state. Changing the pulse shape from smoothly varying to square adds additional structure to the distributions. At sufficiently high intensities (1015 W/cm2) the high momentum peaks increase in intensity and the low momentum peaks are suppressed as the laser intensity is increased (this effect is often referred to as peak switching). At high laser intensities and frequencies comparable to ω10, classical mechanical calculations of the fragment momentum distribution give a smoothed out approximation to the quantum results and display a shifting similar to peak switching. Classical mechanics is unable to reproduce the quantum results at low intensities or at frequencies not comparable to ω10.
Rate constants for quenching the Ã 2A2 state of SO+2 by atmospheric gases
Authors: Thomas Timothy F. , Dale Fred , Paulson John F.
Published: July 2002

The effect of ion source pressure on the cross sections for photodissociation of SO+2 has been measured systematically at λirr =4735 and 4795 Å. Using a Stern–Volmer treatment modified to account for the dependence of source residence time on pressure, rate constants have been measured for quenching the Ã 2A2 (v1,v2=3,3) and Ã 2A2(v1,v2=3,0) states of SO+2 by N2O, SO2, CO2, and N2. With SO2 and N2O as quenchers the rate constants range between 1.0 and 6.5 times the theoretical thermal capture rate constants (‘‘Langevin limit’’). The occurrence of several resonant and many near-resonant charge transfer processes is proposed to explain the unusually large rate constants.
Photoionization studies of (BH3)n (n=1,2)
Authors: Ruščić B. , Mayhew C. A. , Berkowitz J.
Published: July 2002

The results of photoionization mass spectrometric studies on B2H6, and BH3 (produced by pyrolysis of B2H6) are presented. The photoion yield curves of B2H+n (n=2–6) and BH+n (n=2–3) from B2H6, as well as BH+n (n=1–3) from BH3 have been obtained. It is shown that the combination of appearance potential measurements for BH+3 (B2H6) and BH+3 (BH3) yields a poor upper limit for −ΔHdimerization, 0 K (BH3) of 52.7 kcal/mol, while the combination of BH+2 (B2H6) and BH+2 (BH3) provides a better upper limit (46.6±0.6 kcal/mol) for this quantity. However, the threshold for BH+ (BH3), combined with auxiliary data, provides the best current experimental value, (34.3−39.1)±2 kcal/mol. This experimental value is in good agreement with a recent ab initio calculation, and is arrived at by using the best current estimate of ΔHf(B2H6), rather than a radically different value proposed in that paper. The ionization potential of BH3, ΔHf (BH+2), and the atomization energy of BH3 obtained experimentally are in excellent agreement with other ab initio calculations. The upper limits on heats of formation for the ionic species B2H+n (n=2–6) are obtained, and plausible structures are discussed for these species, based on the current energetics and various ab initio calculations. Finally, the fragmentation behavior of photoions from diborane is shown to have a more facile explanation by quasiequilibrium theory than by a molecular orbital picture, with the probable exception of BH+3 (B2H6).
The electronic spectrum of manganese hydride
Authors: Balfour Walter J.
Published: July 2002

The electronic spectrum of MnH is discussed with particular reference to the infrared system near 847 nm, which is assigned by rotational analysis to a 5 Σ–5 Σ transition, and the 480 nm system which is assigned to a 5 Π–5 Σ transition. The two systems have the same lower state. Bond lengths in the lower 5 Σ, the upper 5 Σ and the 5 Π states are 0.1638, 0.1655, and 0.1674 nm, respectively.
Dynamical lattice gases and inherent structure in condensed phases
Authors: Stillinger F. H.
Published: July 2002

The nature of structural order and basic kinetic processes in condensed phases theoretically can be clarified by quenching to mechanically stable particle packings, the relevant ‘‘inherent structure’’. Numerically locating these potential energy minima in realistic models for various substances is often a very demanding task. A class of many-body systems, ‘‘dynamical lattice gases,’’ is introduced for which the steepest-descent quenching is particularly simple. The resulting packings place particles on sites of a regular array, the distribution of energies of those packings can be obtained from the solution of a related Ising model, and transition states between interconvertible packings are trivial to locate. Some elementary analytical results have been obtained for vibrational free energies of the packings. Some versions of these models, notably the quarter-filled face-centered-cubic case, are well suited for study of liquid-phase supercooling and formation of low-temperature amorphous solids.