bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemically driven convection
Authors: Bdzil John B. , Frisch H. L.
Published: February 2003

We present the linear and nonlinear analysis of a horizontal layer of initially quiescent, simple dissociating Navier-Stokes fluid (2A?A2) in the Boussinesq approximation neglecting Dufour effect terms. To facilitate the analysis of the far from equilibrium states, the mathematically simplest free surface boundary conditions have been chosen at which either and/or the extent of dissociation or temperature are fixed. We find that in this system two classes of instability can develop. The first brought on by increasing an already adverse density gradient is similar to the conventional Bénard instability. The second class of instability is novel, and we designate them as chemical instabilities. These instabilities are not correlated to the values of the adverse density gradients and do not appear in a purely quiescent fluid. We are thus dealing with true chemically driven convection.
Charged hard spheres in dipolar hard sphere solvents. A model for electrolyte solutions
Authors: Levesque D. , Weis J. J. , Patey G. N.
Published: February 2003

The linearized and quadratic hypernetted-chain approximations are solved numerically for mixtures of charged hard spheres in dipolar hard sphere solvents. The pair correlation functions, concentration dependent dielectric constant, and thermodynamic properties are examined for several solvents and a range of ion concentrations. The ion–ion pair correlation functions contain many interesting structural features which are not present in the so-called primitive model. Solvent separated ion pairs are found to be a very important species in certain solutions.
Study of vanadium complexes in potassium oxalate monohydrate crystals
Authors: Radhakrishna S. , Chowdari B. V. R. , Salagram M.
Published: February 2003

The optical absorption of VO2+ doped K2C2O4:H2O has been studied at 300 and 77 °K. The existence of vanadyl ion in the crystal was verified by IR absorption study. Four optical peaks corresponding to d-d transitions and an intense peak at 28 900 cm−1 due to charge transfer have been recorded at 300 °K. The identity of charge transfer band was established by polarization studies. At 77 °K the band at 15 384 cm−1 showed vibronic structure with a separation ∼755 cm−1. The spectra have been interpreted in terms of both molecular orbital (MO) theory and crystal field theory. Radiation damage studies of VO2+ doped K2C2O4⋅H2O revealed the formation of VO2. The two bands due to d-d transitions in VO2 and other optical peaks due to interband transitions have been recorded for parallel and perpendicular polarizations of light. This evidence for the formation of VO2 has been discussed.
Numerical simulations of molecular multiphoton excitation models
Authors: Carmeli Benny , Schek Israel , Nitzan Abraham , Jortner Joshua
Published: February 2003

In this paper we report the results of numerical simulations of the intramolecular dynamics of a model system for multiphoton excitation of large molecules, where the low energy range is represented by a single discrete state, while the quasicontinuum is mimicked by two or three manifolds of molecular eigenstates. The random coupling model (RCM), where the radiative coupling matrix elements are assumed to be random functions of the level indices, yields conventional rate equations describing consecutive–reversible transitions for the populations with golden rule rates. In addition, numerical simulations were conducted for a constant coupling model (CCM) and for a separable random coupling model (SRCM), confirming the counterintuitive analytical results for these model systems. The time evolution of a RCM system is determined by the distribution function of the coupling elements and not by individual coupling terms, and the intramolecular dynamics is essentially determined by the lower moments (average and variance) of the distribution function. On the basis of numerical simulations we have shown that a radiative RCM, based on the molecular eigenstates, is equivalent to an intramolecular RCM founded on a zero-order molecular basis with a small number of optically active modes, random anharmonic coupling, and constant selective radiative interaction terms. Our computer experiments provide evidence for the validity of a strong coupling kinetic master equation for the RCM and suggest that random coupling is essential for the erosion of phase coherence effects in the multiphoton excitation of a molecular quasicontinuum.
Low symmetry splittings due to phase transitions in the vibronic spectrum of ReCl62− and ReBr62− doped in K2PtCl6-type crystals
Authors: Wernicke Rainer , Schmidtke Hans-Herbert
Published: February 2003

The vibronic structure of ReCl62− and ReBr62− doped in A2MX6 (A=Na, K; M=Pt, Sn, Te; X=Cl, Br) as obtained from phosphorescence spectra of microcrystals at 10 K has been investigated. Phase transitions of the host crystals to lower symmetry cause splittings of the Γ8(4A2g) electronic ground state into Kramers doublets, which are detected by splittings of the zero phonon transition and the vibronic side band. The crystal distortions also split the internal vibrational modes of the complex molecule, giving rise to a splitting pattern which is higher in symmetry than the corresponding site symmetry of the host lattice would predict. The assignment of phosphorescence bands to internal and lattice vibrations as well as to combinations and progressions is discussed.
Integral-equation theory for the radial distribution function of a hard sphere fluid in the semiclassical limit
Authors: Singh N. , Sinha S. K.
Published: February 2003

The problem of calculating the quantum corrections to the radial distribution function of a hard core fluid is studied, using the hypernetted-chain (HNC) integral equation theory for a semiclassical fluid. Expressions are given for the first and second quantum corrections to the RDF. The first and second quantum corrections have been evaluated for a hard sphere fluid. It is shown that these two quantum correction terms, which are of opposite nature, have significant effects in the neighborhood of the point of contact.
Evaluation by vibrational spectroscopy of the radius of the coordination sphere of ionic solutions in liquid hydrogen chloride
Authors: Huong P. V. , Desbat B.
Published: February 2003

By setting equal the interaction force constants deduced from group theory using a geometric model for solvation aggregates and that calculated from a dipole–dipole interaction model, a relation is obtained giving the radius a of the ion-coordination sphere in ionic solution, viz., a={ρ[A/(ν21−ν22)]}1/3 where ν1 and ν2 are the Raman active stretching frequencies of the solvating molecules, A is the intensity of the infrared active stretching vibration band, and ρ is a parameter depending on the symmetry of the aggregates. The method is applied to solution of chloride ion in anhydrous hydrogen chloride, where the solvation aggregate is shown to be octahedral with six solvating molecules, [Cl−, 6HC1], having a distance Cl–H⋅⋅⋅Cl− equal to 3.6 Å. This is in perfect agreement with Cl–H⋅⋅⋅Cl− distances in known hydrogen-bonded systems.
Electron gas effects in the spectroscopy of molecules chemisorbed at a metal surface. I. Theory
Authors: Maniv Tsofar , Metiu Horia
Published: February 2003

It is known that the spectroscopic properties of a chemisorbed molecule are affected by the interaction between the molecular charge density and the free electrons in the metal. Such effects have been previously discussed by using a model in which the molecule is represented by a point dipole and its interaction with the metal is treated by using macroscopic electrodynamics. We examine critically this model and remove some of its shortcomings by developing a microscopic theory in which the interaction between the molecule and the metal is treated within the random phase approximation. We discuss applications to infrared, electron energy loss and Raman spectroscopy, as well as to the computation of dispersion forces between chemisorbed molecules.
Random coupling models for intramolecular dynamics. I. Mathematical approach
Authors: Carmeli Benny , Nitzan Abraham
Published: February 2003

Intramolecular dynamics in large molecules is modeled as a problem involving random coupling between manifolds of molecular levels. The random coupling model (RCM) is based on the rapid variations observed in coupling matrix elements involving highly excited bound molecular states, and on the high density of such states encountered in large molecules. The finite time and energy scales involved in real experimental situations lead to the observation that the time evolution and spectral properties characterizing the system do not depend on the detailed arrangement of states and their coupling elements but rather on low order moments of the distribution characterizing these coupling elements. This provides the basis for an approach based on ensemble averages. The coupling V is taken as a superposition V=u+v of a smoothly varying component u=〈V〉 and a randomly varying (in state space) component v=V−〈V〉. We introduce a diagrammatic expansion and averaging method to evaluate the diadic Green’s function for problems involving absorption line shapes, and a similar approach for the evaluation of the tetradic Green’s function used in calculations of time evolution. With these methods, we study the time evolution in systems involving discrete states and quasicontinuous manifolds. The solution is relevant for multiphoton excitation of large molecules, and for intramolecular electronic and vibrational transitions. We also study the effect of random coupling in absorption line shapes involving interference between resonances or interference between a resonance and a background absorptions. The mechanism for coherence erosion resulting from the random behavior of the coupling is elucidated.
Statistical mechanics of polar fluids in electric fields
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

We have succeeded in deriving microscopically, from the statistical mechanics of a molecular fluid, the thermodynamic relations associated with the dielectric properties of a continuous dielectric medium in the presence of a static electric field, as given, e.g., by Landau and Lifshitz. We do this by extending our earlier formally exact treatment of dipolar fluids in the absence of an electric field to the case in which such a field is present. We begin by giving the dielectric tensor in terms of the pair correlation function and then show the way various thermodynamic quantities of interest are related to the field strength and chemical potentials. In the weak-field case our results are explicit and include the effects of polarizability as well as a generalization from the fluid to an anisotropic crystal. They are given in two separate developments, one of which yields directly a set of purely thermodynamic results that involve the orientation distribution only through the local number density and the polarization, the other of which involves the statistical mechanics of the orientation distribution itself. Finally, we give the mean-field results that are obtained upon letting the dipolar pair interaction (both permanent and induced) between fluid particles become infinitely weak and long ranged on the molecular scale.
Polymorphism of the crystalline methylchloromethane compounds. VII. The structures of the ordered phases of the carbon tetrahalides
Authors: Powers Roslyn , Rudman Reuben
Published: February 2003

In the ordered phase, CCl4, CBr4, and CI4 are isostructural crystallizing in a C-centered monoclinic lattice with 32 molecules per unit cell. A close examination of the results of single-crystal structure analyses of CCl4 and CBr4, reveals the exact relationship between the disordered (fcc) and the ordered phases. The molecules, which are regular tetrahedra within the limits of experimental error, lie on sites closely related to, but clearly distinguished from, the lattice points of the ordered fcc phase. The close relationship between these two phases, and the resulting similarity in lattice geometry along different directions in the crystal, leads to an apparent polymorphism between crystals that have grown along different directions. Conflicting data that have been published for ordered CF4 have been resolved and gross similarities between crystalline CF4 and the other CX4 compounds have been described on the basis of similar reduced cells.
The magnetic circular dichroism spectrum of carbon disulfide
Authors: Barth Günter , Linder Robert E. , Bunnenberg Edward , Djerassi Carl , Seamans Lloyd , Moscowitz Albert
Published: February 2003

The magnetic circular dichroism spectrum of carbon disulfide vapor in the 300–380 nm region is reported and analyzed. The signs of the A terms associated with the P and R rotational branches of the triplet bands are negative and positive, respectively. These data are used in an accompanying theoretical paper (following paper in this journal) to calculate the zero field splittings in the 3A2 state of CS2.
Theoretical analysis of the magnetic circular dichroism spectrum of carbon disulfide
Authors: Seamans Lloyd , Moscowitz Albert , Linder Robert E. , Barth Günter , Bunnenberg Edward , Djerassi Carl
Published: February 2003

The upper level of the near-ultraviolet absorption system of carbon disulfide (the R system) is the B2 spin-multiplet component of a 3A2 state. A theoretical analysis of the magnetic circular dichroism spectrum of the R system shows that if, as has been assumed previously, the A1 and B1 spin-multiplet components of the 3A2 state are degenerate, they lie above the B2 component. This result is at variance with earlier conclusions. The only ordering of the spin-multiplet components which is consistent with all available data is one in which the A1 and B1 components are nondegenerate, with both of these components lying below the B2 component. The available data are insufficient to assign the relative ordering of the A1 and B1 components.
A single crystal EPR study of ground state triplet trimethylenemethane
Authors: Claesson O. , Lund A. , Gillbro T. , Ichikawa T. , Edlund O. , Yoshida H.
Published: February 2003

Trimethylenemethane, a ground state triplet, has been generated in a single crystal of methylenecyclopropane by γ irradiation at 77 K and studied by the electron paramagnetic resonance technique in the temperature range 4.2–106 K. From the observed anisotropy of the EPR spectra, the zero-field splitting constants can be determined as D=0.0248 cm−1 and ‖E‖?0.0003 cm−1, and the principal values of the proton hyperfine coupling tensor as −14, −38, and −26 MHz with an isotropic coupling of −26 MHz at 77 K. Computer simulations of some of the spectra were performed using a computer program developed for this investigation. Both the position and the intensity of the spectral lines were reproduced with a good result. The simulations gave a positive sign for the zero-field splitting constant D. The zero-field splitting is temperature dependent above ca. 20 K, probably due to an oscillating motion of the molecular plane. The hyperfine structure changes above ca. 80 K and becomes isotropic at 100 K. This change is attributed to an anisotropic rotation about the threefold symmetry axis of the molecule.
Non-Boltzmann, non-Treanor vibrational level populations of electrical discharge excited nitrogen
Authors: Campbell D. H. , Muntz E. P.
Published: February 2003

Vibrational level population distributions of electrical discharge excited nitrogen were measured to v=5 using the electron beam fluorescence diagnostic technique. Measurements were made in the free jet expansion from an electrical discharge for both pure nitrogen and nitrogen–carbon monoxide mixtures. All measured distributions were highly non-Boltzmann and also deviated from the Treanor quasisteady state distribution. A numerical matching technique was used to predict the shape of the vibrational population distribution for a given total vibrational population. The numerical results predict the shape of the distribution very well for all experimental conditions. The numerical modeling also showed that the deviation from Treanor conditions occurs because the V–V rates and electron excitation rates in the discharge are of the same order of magnitude for discharge pressures of a few Torr.
Riemann tensor theory of liquid state chain dynamics
Authors: Knauss Donald C. , Evans Glenn T.
Published: February 2003

The Riemann–Kirkwood–Fixman diffusion equation is applied to the analysis of rotational and torsional dynamics of alkane chains in liquids. It is found that the covariant metric can be calculated for chains of arbitrary length. For the three bond chain, butane, the covariant metric can be easily inverted and a diffusion equation for overall as well as internal rotation is derived. The internal torsional and overall rotational degrees of freedom are coupled by an internal angular momentum, originally proposed by Eckart. The time dependent torsion angle distribution function satisfies a diffusion equation with a nonlocal diffusion coefficient that is found to be in accord with our earlier work [J. Chem. Phys. 70, 2362 (1979)] and that of Pear and Weiner [J. Chem. Phys. 71, 212 (1979)].
The magnetic susceptibility of BH
Authors: Corcoran C. T. , Hirschfelder J. O.
Published: February 2003

The magnetic susceptibility of the BH molecule is calculated using large configuration interaction wave functions for the 1Σ+ ground electronic state. The paramagnetic contribution to the susceptibility is determined in a sum over states where the set of Π configurations is restricted to ?400. BH is again found to be paramagnetic but with magnitude substantially reduced from coupled Hartree–Fock and multiconfiguration coupled Hartree–Fock perturbation results. The best configurational interaction calculations gave values of 5.984 and −3.279 (in parts per million cgs) for the susceptibility when the gauge origin was taken to be the B and the H nuclei, respectively.
Structural relaxation time of D2O-liquid water
Authors: Endo Harumi
Published: February 2003

The structural relaxation times (τT,P) of D2O-liquid water were determined by employing the models of Nemethy and Scheraga (NS) and also Davis and Bradly (DB). We proposed a new model to obtaining τT,P on the basis of the interstitial model proposed by Narten et al. from the study of x-ray diffraction. The results for τT,P obtained from the proposed models were compared with one another. The relaxation times for NS and DB are not in agreement with each other over the temperature range. The value of τT,P for our model is close to that of DB at lower temperatures, but at higher temperatures the descrepancy between them increases. On the other hand, the quantity τT,P of our model is very close to a rotational correlation time (τnd) of a dipole in a D2O molecule obtained by Hindman et al. over the temperature range. The value of τT,P for the NS model is different from that of our model and also the correlation time τnd. After the work for D2O, we also applied our model to find the value of τT,P for H2O-liquid water.
Sequential two-photon absorption and singlet–singlet energy pooling in thiophosgene vapor
Authors: Clouthier D. J. , Knight A. R. , Steer R. P. , Hackett P. A.
Published: February 2003

Emission from the second excited singlet state of Cl2CS vapor has been monitored following excitation with a tightly focused, pulsed dye laser in the region of the ground state to first excited singlet and triplet absorption systems. Excitation in the triplet bands produces prompt fluorescence from S2 as a result of sequential two-photon absorption, S0→hνT1→hνS2. Excitation in the bulk of the singlet bands produces no prompt fluorescence but does produce delayed fluorescence as a result of a bimolecular singlet–singlet energy pooling process, 2S1→S2+S0.
Dissipative structures, broken symmetry, and the theory of equilibrium phase transitions
Authors: Stein D. L.
Published: February 2003

The claim that nonlinear instabilities leading to ’’dissipative structures’’ far from thermodynamic equilibrium are analogous to equilibrium phase transitions is investigated. As a representative model, the spin wave instability occurring in a driven ferromagnetic sample is studied in arbitrary dimension. Broken symmetry of the type occurring in equilibrium phase transitions is not found in any dimension, nor does there appear to be an upper critical dimension beyond which mean field theory is correct. The Bénard instability and ’’Brusselator,’’ which are known to disobey mean field theory in three dimensions, are also studied in higher dimensions; it is found here that although broken symmetry may occur in higher dimensions, again no upper critical dimension exists. Finally, we speculate under what conditions a ’’dissipative structure’’ may exhibit true broken symmetry, and a consequent generalized rigidity, in three dimensions.
19F chemical shifts in some tetrafluoroborates
Authors: Sears R. E. J.
Published: February 2003

The 19F chemical shifts are measured for cesium, silver and ammonium tetrafluoroborates in the solid state and under ambient temperature and pressure conditions. The correlations between the chemical shift and ionic radii and with cation electronegativity are discussed. (AIP)
Reply to ‘‘Comment on ‘Stable limit cycles in a two-component bimolecular reaction system’ ’’
Authors: Kádas Z. , Othmer H. G.
Published: February 2003

A reply to Tyson’s(1) comment on a previous paper regarding stable limit cycles in two-component reaction systems is presented. The dependence of diffusion constants on concentration seems to determine whether hi-are linear functions of concentrations. (AIP)
Electric dichroism spectroscopy in the vacuum ultraviolet. I. Cyclobutanone, cyclopentanone, cyclohexanone, and cycloheptanone
Authors: Causley G. C. , Russell B. R.
Published: February 2003

Electrochromism and electric linear dichroism have been used in a study of transitions to the second excited singlet of a series of simple cyclic ketones. The results of the work show that the transition under investigation is polarized perpendicular to the direction of the dipole moment in the ground state of cyclobutanone, cyclopentanone, cyclohexanone, and cycloheptanone, where the dipole moment changes for the transition to the second excited singlet state from ground state are found to be 1.44, 1.63, 4.30, and 4.92 D, respectively. The magnitude of the changes in dipole moment and mean polarizability support the perpendicular B2←A1,3sa1←nb2 molecular Rydberg assignment. The larger dipole moment changes for the larger compounds are attributed to ring delocalization of the positive core charges in this extravalent state, a situation that appears unfavorable in the small ring compounds.
On the direct energy transfer via exchange to randomly distributed acceptors
Authors: Blumen Alexander
Published: February 2003

This paper investigates the direct, incoherent energy transfer due to exchange from an excited donor to randomly distributed acceptors. Making use of a general, exact formula for the time and concentration dependent ensemble averaged decay [Blumen and Manz, J. Chem. Phys. 71,4694(1979) ] simple approximate expressions for the limiting cases, of low and high concentrations and of short and long times, also under the inclusion of the back-transfer in the pair approximation, are derived; for low acceptor concentrations and long times the results of Inokuti and Hirayama [J. Chem. Phys. 43, 1978 (1965)] generalized to arbitrary dimensions, are retrieved. The limits of validity of the different approximations are determined by a numerical evaluation of the exact formula for particular lattices of interest.
Brownian motion with an absorbing boundary
Authors: Harris S.
Published: February 2003

We consider the problem of determining the joint position–velocity distribution function for a Brownian particle confined by an absorbing boundary. The approach used here, which we believe to be novel in this context, involves an integral equation formulation in terms of the distribution function, for the unrestricted problem, of first turning points in the half-space that does not contain the initial source. The advantage of this formulation is that it leads to an equation of the Wiener–Hopf type, so that in principle explicit solutions can be found by standard methods. An approximate solution is found for the case of large friction which reproduces the general features of the solution found in the theory based on the use of the diffusion equation; this result illustrates the potential of our formulation with regard to obtaining an exact solution to the absorbing boundary problem.
Continuum resonance Raman scattering of light by diatomic molecules. I. The role of radiative crossings between the potentials of the dressed molecule
Authors: Atabek O. , Lefebvre R. , Jacon M.
Published: February 2003

Usual scattering theory only provides a formal result for continuum resonance Raman scattering amplitudes, leaving the task of evaluating numerically these amplitudes. We look instead, in the case of a diatomic molecule, for a system of coupled equations patterned after those of molecular collision theory which can provide them directly (i.e., numerically). These equations emphasize the role of radiative crossings between the potential curves of the dressed molecule (molecule in its initial electronic state with the incoming photon, in its intermediate state with no photon, in its final state with the outgoing photon). The channels involve the potentials of the dressed molecule plus two artificial entrance and exit channels coupled in such a way as to yield the Raman scattering amplitudes through the analysis of one of the elements of the scattering matrix. Electronic transition moments depending on the internuclear distance are easily introduced into the formalism. In order to test the numerical accuracy of this method it is possible to use an analytically exact formula valid for harmonic and linear potentials. It is shown that a three-figure accuracy is easily achieved. Comparisons are also made with the results of a semiclassical procedure based on an Airy representation of the Franck–Condon amplitudes. This method, which is used abundantly in the next paper, has the advantage of providing a very fast way of reaching the scattering amplitudes and gives an error which generally does not exceed a few percent.
Vibrational deactivation on chemically reactive potential surfaces: An exact quantum study of a low barrier collinear model of H + FH, D + FD, H + FD and D + FH
Authors: Schatz George C. , Kuppermann Aron
Published: February 2003

We study vibrational deactivation processes on chemically reactive potential energy surfaces by examining accurate quantum mechanical transition probabilities and rate constants for the collinear H + FH(v), D + FD(v), H + FD(v), and D + FH(v) reactions. A low barrier (1.7 kcal/mole) potential surface is used in these calculations, and we find that for all four reactions, the reactive inelastic rate constants are larger than the nonreactive ones for the same initial and final vibrational states. However, the ratios of these reactive and nonreactive rate constants depend strongly on the vibrational quantum number v and the isotopic composition of the reagents. Nonreactive and reactive transition probabilities for multiquantum jump transitions are generally comparable to those for single quantum transitions. This vibrationally nonadiabatic behavior is a direct consequence of the severe distortion of the diatomic that occurs in a collision on a low barrier reactive surface, and can make chemically reactive atoms like H or D more efficient deactivators of HF or DF than nonreactive collision partners. Many conclusions are in at least qualitative agreement with those of Wilkin’s three dimensional quasiclassical trajectory study on the same systems using a similar surface. We also present results for H + HF(v) collisions which show that for a higher barrier potential surface (33 rather than 1.7 kcal/mole), the deactivation process becomes similar in character to that for nonreactive partners, with v→v−1 processes dominating.
The polarizability of H2 in the triplet state
Authors: Clarke Hunt K. L. , Buckingham A. D.
Published: February 2003

Results of the first accurate ab initio calculation of the polarizability of H2 in the lowest triplet state are presented, providing the simplest quantum mechanical model for the effects of nonbonding interactions on molecular polarizabilities. When overlap is significant, the computed polarizability anisotropy is smaller than the dipole-induced–dipole value, but somewhat larger than that obtained from the original Oxtoby–Gelbart continuum–dielectric model. Two models equivalent in first order, the simple electrostatic model introduced by Clarke, Madden, and Buckingham, and the continuum–dielectric model as modified by Oxtoby, give fairly good values for the pair polarizability if an additive exchange correction is included.
Computational and experimental study of vibrational relaxation in liquids: Symmetric top molecules
Authors: Constant Monique , Fauquembergue Renaud
Published: February 2003

Detailed studies of vibrational relaxation in pure liquid CH3I, t-butyl chloride (CH3)3CCl, t-butyl bromide (CH3)3CBr and in solutions in carbon tetrachloride and in n-hexane, over the −90 to 50°C temperature range, have been made using Raman scattering. Assuming that only phase relaxation contributes to the isotropic band shapes, the mechanism of vibrational relaxation in these liquids was first studied by investigating the characteristics of the fluctuations of the vibrational frequencies, which are caused by the intermolecular interactions. A first study allowed us to determine the correlation time of these fluctuations. The obtained values are between 0.1 and 0.5 ps and are consistent with a binary collision model. This model has been used to develop an efficient computational model to describe the experimental data relative to vibrational relaxation and orientational motions as well.
Photofragment fluorescence polarization following photolysis of HgBr2 at 193 nm
Authors: Husain J. , Wiesenfeld J. R. , Zare R. N.
Published: February 2003

When HgBr2 (2–45 mTorr) is photolyzed by the linearly polarized output of an ArF excimer laser (193 nm), the visible emission from the HgBr B 2Σ+ fragment is found to be linearly polarized. The degree of polarization is 11.9%±1.5%, in close agreement with the theoretical value of 14.3% predicted for a 1Σg+(1A1) →1B2(1Σu+) dissociative transition in HgBr2. The addition of inert gases depolarizes the HgBr* emission. A simple model, developed to calculate the average angle through which the angular momentum vector of the HgBr* fragment is tipped by each hard sphere collision, fits well the pressure dependence of the depolarization.
Reflectance and complex refractive indices in the infrared for aqueous solutions of nitric acid
Authors: Querry Marvin R. , Tyler I. L.
Published: February 2003

Near-normal incidence reflectance spectra relative to water were measured throughout the 310 to 5000 cm−1 spectral region for 0.5, 1, 2, 4, 8, and 15.7 M aqueous solutions of nitric acid. Spectral values of the complex refractive index N (ν) =n (ν)+ik (ν) of the nitric-acid solutions were computed by use of Kramers–Kronig techniques. Values of N (ν) are presented in graphical and tabular form. The spectra were further analyzed to determine the central frequencies, half-widths, and strengths of infrared bands due to intra- and intermolecular vibrational modes of liquid water and intramolecular vibrational modes of NO3−, H3O+, and HNO3.
Theory for broadband detection of ion cyclotron resonance signals
Authors: McIver Robert T. , Ledford Edward B. , Hunter Richard L.
Published: February 2003

A complete line shape theory is developed for the transient response of a new type of ion cyclotron resonance (ICR) detector circuit. The detector is basically a balanced capacitance bridge which is sensitive to the abundance of gaseous ions stored in a static magnetic ion trap. For the first time, the equations of motion of ions in the ICR analyzer cell are shown to be coupled to the circuit equations of the detector. Also, the effect of nonreactive ion–molecule collisions on line shapes and on the transient response of the detector are analyzed and shown to allow measurement of ion–molecule collisions frequencies as a function of ion translational energy. One of the most important features of the capacitance bridge detector is its broadband sensitivity to a wide range of ion cyclotron resonance frequencies. This allows a mass spectrum of ions stored in the ICR analyzer cell to be obtained by scanning the frequency ω1 of the irradiating rf electric field at a fixed magnetic field strength. The capacitance bridge circuit can serve not only as a direct replacement for marginal oscillator circuits traditionally used in ICR experiments, but also as the detector for recently developed Fourier Transform and Rapid Scan ICR experiments.
Rayleigh–Brillouin scattering and structural relaxation of polystyrene–cyclohexanes solutions
Authors: Lempert Walter , Wang C. H.
Published: February 2003

An extensive Rayleigh–Brillouin scattering study of polystyrene and cyclohexane mixtures at various temperatures and polymer concentrations has been carried out. The experimental results have been analyzed in terms of the equations of generalized hydrodynamics developed recently by Lin and Wang. Over the entire concentration range, the temperature dependence of the Brillouin frequency and linewidth data can be satisfactorily interpreted in terms of the equations in the recent theory. The pronounced effects of the Brillouin data have been related to a quantity, referred to as the effective structural relaxation time τs for the mixture. The use of the single relaxation time model is justified for Brillouin scattering. It has been found that τs is not significantly affected by the presense of polymer molecules at low polymer concentration, but it increases rapidly with increasing polymer concentration at high concentration. At low polymer concentration, the relaxation frequency τs−1 is fast compared with the sound frequency ωs; and the effects of shear viscosity and thermal relaxation due to internal degrees of freedom are not as important as that of bulk viscosity. At high polymer concentration, the τs data suggest that rapid segmental motion involving only few monomer units in the long polymer chain molecules gives the dominant contribution to the dispersion and attenuation of the hypersonic wave in the polymeric fluid.
On the supposed pressure-induced reduction of ferric salts
Authors: Lewis G. J. , Whalley E.
Published: February 2003

Three experiments have been done to test whether the widely reported reduction of iron compounds under combined hydrostatic and shear stresses is due to the hydrostatic stress or the shear deformation. In the first experiment, the Raman spectrum of a single crystal of potassium ferricyanide was measured under hydrostatic pressures up to 19 kbar at 22°C. The spectrum showed no evidence of reduction. In the second experiment, a single crystal of potassium ferricyanide was subjected to a hydrostatic pressure of 45 kbar and recovered at zero pressure. Its appearance and Raman spectrum were unchanged. The reported reduction occurs with a considerable hysteresis, and would have been detected if it had occurred. In the third experiment, a single crystal of potassium chloride with potassium ferricyanide dissolved in it was subjected to hydrostatic pressures up to 25 kbar, and its infrared spectrum taken after each pressurization. Pressures less than the transition pressure to the cesium chloride structure caused no detectable change in the infrared spectrum. When the pressure exceeded this pressure, reduction to ferrocyanide occurred, and more reduction occurred each time the transition occurred. There is no evidence in any of these experiments for any reduction induced by hydrostatic pressure. A good deal of the reported reduction must be due to shear. A detailed mechanism for the shear-induced reduction of potassium ferricyanide dissolved in potassium chloride is suggested.
Total Penning ionization cross sections of Cd and Zn for He(2 3S1) atoms
Authors: Fahey D. W. , Parks W. F. , Schearer L. D.
Published: February 2003

The total Penning ionization cross sections for Cd and Zn for He(2 3S1) atoms have been measured. The method used was that of Penning ion collection from a vapor cell through which a He(2 3S1) beam was passed. The beam energy was 66 meV with a velocity spread of 45%. The measured cross sections were 60(±22) and 17(±7) Å2 for the Cd and Zn targets, respectively.
Sensitivity analysis of rotational energy transfer processes to the intermolecular potential
Authors: Eno Larry , Rabitz Herschel
Published: February 2003

This paper considers the sensitivity of rotational energy transfer processes to the variation of parameters within an assumed model intermolecular potential. The following cross sections are considered here: integral state to state, pressure broadening, effective diffusion and viscosity, and final state summed integral cross sections. In order to simplify the calculation of cross sections, attention is restricted to the scattering of an atom and linear rigid rotor. Furthermore, the collision dynamics are approximated by using the infinite order sudden (IOS) method. It is shown that use of the IOS method allows for the very simple generation of first order sensitivity coefficients (i.e., the partial derivative of cross sections with respect to potential parameters). Particular attention is focused upon the sensitivities of different cross sections and combinations of cross sections to the various parameters. The first order sensitivities are also used to derive new coefficients which describe how the potential parameters correlate given a limited set of cross section measurements. These coefficients are shown to be particularly important in determining the degree to which a set of measurements is able to define various parameters of the assumed potential.
Intermolecular interaction dynamics and optical dephasing: Picosecond photon echo measurements in mixed molecular crystals
Authors: Cooper D. E. , Olson R. W. , Fayer M. D.
Published: February 2003

Picosecond optical coherence techniques are employed to study dynamical intermolecular interactions in low concentration mixed molecular crystals, i.e., pentacene in naphthalene at 1.4 K. A concentration-dependent broadening of the optical homogeneous line is observed. Stimulated photon echo measurements rule out excited state transport as the broadening mechanism. The photon echo results exhibit a linear concentration dependence and a theory is proposed that accounts for the broadening in terms of phonon-induced fluctuations in guest–guest dipole–dipole interactions. Detailed considerations indicate that the system is best described in terms of localized excitations rather than delocalized impurity band states. In the low concentration limit, there is additional broadening of the homogeneous line beyond that produced by the lifetime. A model is presented describing this broadening in terms of phonon-induced modulation of the guest–host van der Waals interactions, i.e., in terms of a fluctuating crystal shift.
The vapor phase infrared spectra of cyclohexene and 2,3-dihydropyran, 50–700 cm−1
Authors: Smithson T. L. , Wieser H.
Published: February 2003

The infrared spectrum from 50 to 700 cm−1 of cyclohexene in the vapor phase is reported and analyzed for the first time in terms of a number of very weak sequences resulting from combinations of various fundamentals with the out-of-plane skeletal bending and twisting modes. The spectrum of 2,3-dihydropyran is reinvestigated with the view of finding corresponding combination sequences not previously reported. By means of an approximate method the barriers to planarity and interconversion are estimated to be, respectively, 5826 and 2571 cm−1 for cyclohexene, and 5371 and 3310 cm−1 for 2,3-dihydropyran.
Momentum eigenfunctions in the complex momentum plane. III. Hartree–Fock functions
Authors: Huo Winifred M. , Lassettre Edwin N.
Published: February 2003

The singular points of a Hartree–Fock wave function in the complex momentum (P) plane have been located using a method developed by Lassettre [J. Chem. Phys. 64, 4375 (1976); 70, 3468 (1979)]. In the case of a closed shell Hartree–Fock function these positions are determined by the orbital energies (diagonal Lagrangian multipliers) and for an open shell restricted Hartree–Fock function they are determined by both the diagonal and off-diagonal Lagrangian multipliers. For bound state functions these singlular points all lie on the imaginary P axis and they form an infinite but discrete set, with the singular point closest to the real P axis determined by the Lagrangian multipliers associated with the most diffuse orbital. By means of conformal mappings based on the location of these singular points, a power series representation of the Hartree–Fock function in momentum space is obtained which converges not only on the real P axis but also on portions of the complex P plane. An inverse Fourier transform gives a Hartree–Fock function in coordinate space expressed in terms of a Slater basis set. The exponents appearing in this basis set are determined by the positions of singularities in the P plane. Numerical calculations on Ne and Ar using the theoretically derived exponents are presented.
Solvation forces in simple dense fluids. I
Authors: Snook I. K. , van Megen W.
Published: February 2003

The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.
Perfect pairing valence bond generalization of self-consistent electron pair theory
Authors: Dykstra Clifford E.
Published: February 2003

A generalization of the self-consistent electron pairs (SCEP) method for correlated wave functions is presented which makes possible the use of multiconfiguration reference wave functions that have the form of a generalized valence bond, perfect pairing (GVB/PP) wave function. All singly and doubly substituted configurations relative to this reference are included, though the sets of configurations arising from a given orbital substitution are restricted to occur in the correlated wave function with the same relative importance as the corresponding configurations in the reference wave function. The computational manipulations of this new procedure are essentially the same as basic SCEP, but the generalization provides for the treatment of systems where a Hartree–Fock reference wave function is inappropriate. These would include i reactive systems that involve changes in bonding and also weakly interacting systems where higher order correlation effects are necessary.
Diffusion-controlled reactions on a two-dimensional lattice
Authors: Prager Stephen , Frisch H. L.
Published: February 2003

We discuss the kinetics of surface-catalyzed reactions occurring by adsorption from a gas phase onto a square lattice and subsequent diffusion to fixed reaction sites. In the absence of other interactions between migrating species, the restriction of lattice sites to single occupancy does not greatly complicate the problem, so that saturatuion effects can be rigorously taken into account. At very low density of active sites, the combination of saturation and surface diffusion can generate an effective rate constant which varies inversely as the absolute logarithm of the reactant concentration in the gas. Steady state results valid at all reaction site densities are given for both periodic and random distributions of active sites, in the latter case as variational inequalities placing the reaction rate between closely spaced upper and lower bounds.
Energy redistribution in low energy collisions between I2* and He
Authors: Russell Timothy D. , DeKoven Benjamin M. , Blazy Joseph A. , Levy Donald H.
Published: February 2003

The process of collision-induced energy transfer between an excited I2 molecule and a He atom has been studied in a supersonic free jet expansion. The relative rates of vibrational relaxation as well as the extent of the rotational energy transfer were measured by observing the dispersed fluorescence spectrum of the I2* molecule. Results show that the vibrational relaxation cross section is much larger in a supersonic free jet than in a hot static gas, and this indicates that the low energy colllisions prevalent in the jet are much more effective than are high energy collisions. Mechanisms for energy transfer and relaxation enhancement in cold collisions are discussed. A comparison is made between the collision-induced energy redistribution in I2* and the energy distribution in the photodissociation of the van der Waals molecule I2He. Results show that in a collision more extensive rotational energy transfer occurs, and that the branching ratio of the Δv′=−2 to the Δv′=−1 vibrational relaxation processes is much larger than in the decomposition of the I2He* van der Waals complex.
Electron mobility in supercritical ethane as a function of density and temperature
Authors: Nishikawa Masaru , Holroyd Richard A. , Sowada Ulrich
Published: February 2003

The electron mobility is reported for ethane as a function of density at various temperatures above Tc. The high pressure cell used permits measurements to 200 atm. Our analysis shows that theory is consistent with the ethane mobility results at low and intermediate densities. At densities less than 1×1021 molecules/cm3 electrons are scattered by isolated ethane molecules and the Lorentz equation is valid. At intermediate densities, μe correlates with the square of the velocity of sound, indicating that in dense fluids the adiabatic compressibility must be included. The data are consistent with a modified Cohen–Lekner equation, and the minimum in μeN observed at densities just below dc is qualitatively accounted for by changes in the adiabatic compressibility. Thus the concept of quasilocalization, suggested by others to qualitatively explain such minima, is unnecessary here. At higher densities an additional, unspecified, scattering mechanism becomes important.
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o calculations of the interaction of ions with neutral ligands. Pair potentials for Li+/ether–, Li+/thioether– and Li+/amide– systems
Authors: Corongiu G. , Clementi E. , Pretsch E. , Simon W.
Published: February 2003

The interaction of Li+ with model compounds (ethers, amides, and a thioether) of interest as building blocks in ionophores is studied by performing ab initio SCF–LCAO–MO calculations in sufficient numbers so as to be able to scan the potential energy hypersurface for the lithium ion in the field of the model compound. As previously reported for the case of Na+, atom pair potentials are obtained by fitting the SCF–LCAO–MO energies with an analytical potential of simple form. The pair potentials were found transferable, i.e., the energy minimum for the interaction of Li+ with molecules different from those used for the model calculations is properly described by using these analytical potentials. The analysis of the inter- and intramolecular atom-pair interactions indicates that the Li+/ligand binding is primarily due to the polarization of the model compound in the field of the lithium ion. The position of the minima and the corresponding interaction energies are compared with those reported by F. Hinton et al., by A. Pullman et al., and with the experimental results.