bluuebunny/crossref_metadata_2025 huggingface dataset:
Relaxation of the continuum approximation in the theory of electrolytes. I. Formal results
Authors: Goldstein Ronald A. , Kozak John J.
Published: February 2003

In this paper, the role of the continuum approximation in the theory of electrolytes is re−examined, and a possible generalization of this approximation is considered. For several reasons (cited in the manuscript), the model of Debye and Hückel is used as the starting point in our analysis. We proceed by postulating a functional form for the dependence of the permittivity ε (r) on the distance from the central ion. When this expression for the permittivity is used within the context of Poisson−Boltzmann theory, there results a nonlinear differential equation whose analytic properties are investigated in detail; in particular, it is proved that solutions of this equation exist and are unique. By construction of the Green’s function, we obtain the associated nonlinear integral equation and solutions to this equation for a choice of parameters corresponding to an electrolyte system considered previously by Guggenheim. Our main conclusions follow from a comparison of our results with those obtained previously using the continuum approximation. We find that the relaxation of this approximation leads to a significant enhancement of the potential felt by a counterion in the immediate neighborhood of the central ion, with an attendent accelerated damping of the potential as one moves away from the central ion. A second, rather unexpected result which emerges from our study is that the computed potential is surprisingly insensitive to the explicit down−range behavior of the function postulated to describe the change in permittivity as a function of distance. This paper concludes with some remarks on future problems to be studied. In the following paper detailed ion−distribution profiles are reported, and the question of the internal consistency of the augmented Poisson−Boltzmann equation, introduced in this paper, is examined.
Two-photon dissociation of water: A new OH source for spectroscopic studies
Authors: Wang Charles C. , Davis L. I.
Published: February 2003

We have observed the phenomenon of two−photon dissociation of water using a tunable uv laser beam and the generation of hydroxyl radicals (OH) as a dissociation product. This phenomenon affords a new way for measuring the quenching rates of electronically excited OH due to nitrogen, water, and other molecules under atmospheric conditions.
Dipole properties of atoms and molecules in the random phase approximation
Authors: Martin Patrick H. S. , Henneker W. H. , McKoy Vincent
Published: February 2003

A random phase approximation (RPA) calculation and a direct sum over states is used to calculate second−order optical properties and van der Waals coefficients. A basis set expansion technique is used and no continuumlike functions are included in the basis. However, unlike other methods we do not force the basis functions to satisfy any sum−rule constraints but rather the formalism (RPA) is such that the Thomas−Reiche−Kuhn sum rule is satisfied exactly. Central attention is paid to the dynamic polarizability from which most of the other properties are derived. Application is made to helium and molecular hydrogen. In addition to the polarizability and van der Waals coefficients, results are given for the molecular anisotropy of H2, Rayleigh scattering cross sections, and Verdet constants as a function of frequency. Agreement with experiment and other theories is good. Other energy weighted sum rules are calculated and compare very well with previous estimates. The practicality of our method suggests its applications to larger molecular systems and other properties.
Isotope chemistry and molecular structure. Total deuterium isotope effects
Authors: Bigeleisen Jacob , Ishida Takanobu
Published: February 2003

A qualitative discussion is given on the relationship between the magnitude of isotope effects in polyatomic systems and bending and stretching force constants. This is followed by a general method for the calculation of the isotope effect which can be ascribed to each force constant in the general quadratic potential energy for the vibration of a polyatomic molecule. The method is applied to deuterium isotope effects in the molecules H2O, CH2O, CH4, C2H4, C2H6, and C6H6 at and above 300°K. It is found that deuterium isotope effects associated with individual internal valence force constants are nearly additive at room temperature. Strict additivity holds in the high temperature limit. Bond stretching force constants account for at least 65% of the total deuterium−to−protium reduced partition function ratio at 300°K and become even more dominant at higher temperatures. The origin of the relative importance of stretching and bending forces as a function of temperature is found in the first two terms of the finite orthogonal polynomial method of Bigeleisen and Ishida. Correlations are established between the magnitude of deuterium isotope effects in different molecules as a function of temperature with stretching and bending force constants.
Interaction between penetration sites in diffusion through thin membranes
Authors: Prager Stephen , Frisch H. L.
Published: February 2003

Two models of permeation through thin membranes are proposed: In the first, the membrane is represented as a plate with randomly distributed perforations, and the permeation rate is controlled by the rate at which penetrant diffuses through an external medium; in the second, the penetrant is considered to be adsorbed on one face of the membrane, and the rate controlling process is lateral diffusion across that face to a set of randomly distributed penetration sites. These models are treated by a self−consistent field procedure, and it appears that interaction between penetration sites influences the permeability significantly even when the number of penetration sites per unit area is quite low.
Temperature dependence of the 14N nuclear quadrupole resonance and the observation of the phase transition in cubic tetracyanoethylene
Authors: Murgich Juan , Pissanetzky Sergio
Published: February 2003

The 14N nuclear quadrupole resonance spectrum of cubic tetracyanoethylene was found. The temperature dependence was determined between 77.3 K and room temperature. The transition temperature between the cubic and the monoclinic phase was found to be 292.0±0.5 K. The values of eqQ and η for both phases are discussed considering the contributions of the crystalline electric field and the existence of specific intermolecular interactions.
Compression and bonding of ice VII and an empirical linear expression for the isothermal compression of solids
Authors: Olinger Bart , Halleck P. M.
Published: February 2003

The volume of ice VII was measured between 3.0 and 8.0 GPa at 298 K using a high−pressure x−ray diffraction technique. The specific volume (0.708, +0.023, −0.012 cm3/g), the isothermal bulk modulus (12.54±0.27 GPa), and the modulus’ pressure derivative (5.56±0.14) for ice VII at 298 K and zero pressure were determined using an empirical equation for isothermal compression. That equation is [P V02/(V0 − V)]1/2 = CT + ST [P (V0 − V)]1/2, where CT and ST are constants, V0 is the ambient volume, and V is the volume at pressure P. This linear relation, which describes the state of nonporous materials along their Hugoniots, is shown to characterize the isothermal compression of solids as well as does the Murnaghan equation. The zero pressure, 298 K oxygen−oxygen distance in ice VII extrapolated from the present data and a simple bonding model for the hydrogen−bonded oxygen atoms strongly support Kamb’s description of the ice VII structure as two interpenetrating ice Ic frameworks.
Excitation of fluorescence in naphthalene and azulene by electron impact
Authors: Smyth Kermit C. , Schiavone James A. , Freund Robert S.
Published: February 2003

Electron impact excitation of fluorescence in naphthalene and azulene is studied under low pressure conditions (?3×10−5 Torr). The data are obtained as (a) fluorescence spectra at a series of electron energies, and (b) optical excitation functions at several wavelengths. The naphthalene fluorescence spectrum is diffuse and exhibits an onset at the S2 spectroscopic energy, despite the fact that all emission appears to be S1 → S0. These observations are interpreted in terms of predominant S0 → S2 excitation, followed by rapid S2 → S1 internal conversion and then S1 → S0 fluorescence. For azulene S2 → S0 emission is observed which closely resembles solution results. The naphthalene and azulene fluorescence spectra exhibit no substantial variation for electron energies greater than 1 V above threshold. However, fragment emission from H, C, CH, and C2 becomes prominent above 30 eV. In addition, an unidentified continuum (or possibly many, closely spaced lines) is present throughout the 250−550 nm region for high electron energies. The excitation functions for both molecules exhibit several peaks at low electron energy (below 20 eV) and a broad peak at ∼150 eV. The low−energy peaks are assigned as due to previously unobserved resonances and to nonresonant excitation. The ∼150 eV peak is caused by continuum emission. The resonant peaks are observed to be strongest for wavelengths of maximum emission intensity. This behavior is explained in terms of preferred decay routes for the resonant states.
Extension of Koopmans’ theorem. I. Derivation
Authors: Smith Darwin W. , Day Orville W.
Published: February 2003

It is shown that the spin orbitals that diagonalize the generalized Hartree−Fock potentials [Int. J. Quantum Chem. Symp., 8, 501 (1974)] for a correlated wavefunction, together with the corresponding orbital energies, give a natural extension of Koopmans’ theorem to correlated reference states.
Extension of Koopmans’ theorem. II. Accurate ionization energies from correlated wavefunctions for closed-shell atoms
Authors: Day Orville W. , Smith Darwin W. , Morrison Robert C.
Published: February 2003

Extended Koopmans’ theorem ionization energies are presented for MC−SCF wavefunctions of the 1s2 2s2 (1S) ground state isoelectronic series, Li− through O4+, and also for some larger CI wavefunctions of Be and B+. Correlation in these reference states reduces the error in the Koopmans’ ionization energy for the 2s electron to approximately 0.01−0.08 eV (1/100−1/20 the SCF Koopmans’ error) in all states except the negative ion, for which the MC−SCF extended Koopmans’ error of 0.25 eV was of the same magnitude but of opposite sign to the SCF error. Our extended Koopmans’ energies for 1s ionization were only slightly better than the corresponding SCF values. Koopmans’ theorem does not yield a 2p ionization energy (1s2 2s2 → 1s2 2p), but the extended Koopmans’ theorem yields an ionization energy whose error is about one−third the SCF 2s error.
Calculated laboratory angular distributions of photofragments for stationary flux detector
Authors: Ormerod R. C. , Anderson W. R. , Rose T. L.
Published: February 2003

Laboratory angular distributions over the whole range from slow to fast recoil conditions are calculated for photofragments detected by a stationary flux detector. The results are compared with those obtained for the mass spectrometer density detector.
Ionizing collisions of cesium with Cl2, Br2, and I2
Authors: Tang S. Y. , Leffert C. B. , Rothe Erhard W. , Reck Gene P.
Published: February 2003

A crossed molecular beam apparatus is used to study ionizing collisions of energetic cesium atoms with Cl2, Br2, and I2. The cross sections for formation of Cs+ are reported in the near−threshold region. The experiment combines an energy resolution better than 0.1 eV (FWHM) with a deconvolution procedure. An electron affinity of 2.50 eV is deduced for all three halogens, in good agreement with previous work. In a separate experiment, with cesium energies from threshold to 350 eV, and with much poorer energy resolution, the intensity ratios X−/X−2 are obtained. The results can be reasonably explained with an electron jump model. At energies below 30 eV, the observed ratios are in agreement with two other investigators, but between 150−350 eV they are drastically different from work reported from a third. Complementary data recently reported by a fourth group are difficult to reconcile with the present results.
Radiative and nonradiative transitions in the first excited singlet state of symmetrical methyl-substituted acetones
Authors: Hansen D. Alan , Lee Edward K. C.
Published: February 2003

Radiative and nonradiative rates of symmetrically substituted, methylated acetones have been measured in the gas phase. Observed radiative lifetimes are compared to calculated radiative lifetimes obtained from the Strickler−Berg (SB) expression. The calculated values are greater by a factor of 8−9, and an empirical procedure by which the true radiative lifetime can be estimated from the SB expression and the geometry−sensitive correction parameter is given. The rates of nonradiative transitions are reduced appreciably by the α, α′ substitution of methyl groups to the C−C−CO−C−C skeleton. The steric crowding introduced by methyl group substitution is considered to be responsible for suppressing the vibrational motion of the promoting modes. In most cases, the S1 → T1 intersystem crossing predominates over the other nonradiative transitions.
Reduction of the angular divergence of a molecular beam by a coaxial beam of electrons
Authors: Bobbio S. M. , Chiou C. T. , Greene E. F. , Lambropoulos H. D.
Published: February 2003

The angular divergences of molecular beams of several species are reduced substantially if they move coaxially with an electronic beam. The flux of molecules observed arriving at a detector set on the axis of the molecular beam is increased by factors up to 103 for a beam of KCl and 102 for a beam of K. The increases become smaller as the intensity of the beams is increased. A model proposed to account for the results attributes the collimating action of the electronic beam to the decreased energy of states of the molecules and atoms in the field produced by the electronic beam.
Energy gap law in the solvent isotope effect on radiationless transitions of rare earth ions
Authors: Stein Gabriel , Würzberg Elhanan
Published: February 2003

Data on the fluorescent yields of tervalent ions of Pr, Sm, Eu, Gd, Tb, Dy, and Tm in H2O and D2O, containing perchlorate ions, and isotope effects on the fluorescence are reported. The results are interpreted in terms of the mechanisms responsible for radiationless processes in solutions of rare earth ions [Y. Haas and G. Stein, J. Phys. Chem. 75, 3668 (1971); 76, 1093 (1972)]. It is shown that along the series of the closely related rare earth ions, all of which exhibit well shielded f−f electronic transitions, the results can be correlated in terms of a single variable: the energy gap between the lowest fluorescent and highest nonfluorescent level. For several rare earth ions, the matching of this gap by a single high energy (OH or OD) vibration of one solvent molecule is the decisive factor accounting for observed yields and isotope ratios.
Extended diffusion of 1,3,5-trifluorobenzene-d3 and acetonitrile-d3 by spin–lattice relaxation
Authors: Bull T. E.
Published: February 2003

Nuclear spin−lattice relaxation times have been measured for the 19F and 2D nuclei in deuterated 1,3,5−trifluorobenzene from 450 to 570 K and for the 2D and 14N nuclei in acetonitrile−d3 from 310 to 550 K. The results have been analyzed in terms of McClung’s extended M− and J−diffusion theory for symmetric top molecules and have been found to be in good agreement with the J−diffusion model.
Microwave spectrum of propionic acid. II. Structure of c
i
s-propionic acid by double resonance modulation microwave spectroscopy
Authors: Stiefvater Otto L.
Published: February 2003

Microwave double resonance modulation has been used for the first time to observe all the isotopic species spectra that are needed for a structure determination by the microwave method. From spectra of the normal and thirteen isotopically labelled species of cis−propionic acid the substitution structure of this molecule has been determined completely. Bond lengths (in Å) are as follows; Cketo−Cmethylene = 1.509±0.002, Cmethylene−Cmethyl = 1.523±0.003, C=O = 1.210±0.001, C−O = 1.352±0.002, O−H = 0.970±0.001, (C−H)methylene = 1.098±0.002, (C−H)methyl = 1.088±0.002. The nonbonded distance between the oxygen atoms is 2.245±0.001 Å. The bond angles are (in degrees): & CCC = 112.7±0.1, & (HCH)methylene = 106.4±0.2, & (HCH)methyl = 108.6±0.2, & (CCH)methyl =110.0±0.3, & C=O = 125.8±0.2, & CC−O = 111.8±0.1 and & COH = 105.8±0.2. The plane HCH of the methylene hydrogens is tilted by 4.9±0.1° from the external bisector of the CCC−angle towards the hydroxyl oxygen atom. Spectra corresponding to the first excited state of the large amplitude, torsional vibration around the central carbon−carbon bond have been analyzed for all 14 isotopic forms, enabling the extrapolation to torsional equilibrium moments. Structure calculations based on these moments show that the torsional vibration has no significant influence on the ground state structure. For the normal and four deuterated species the effects of the three lowest vibrations could be eliminated from the ground state constants. The corresponding structure calculations suggest that the out−of−plane carbon−hydrogen bonds derived from ground state constants contain a contribution of 0.005 and 0.002 Å, respectively, for the methylene and methyl hydrogens due to the combined zero−point effects of these three vibrations. The heavy skeleton structure of propionic acid appears little affected, however.
Natural orbitals of several excited states of the He atom
Authors: Winkler P. , Porter R. N.
Published: February 2003

A method is devised by which natural orbitals of the excited states of atomic two−electron systems can be calculated from integral transform wavefunctions of the kind described previously [J. Chem. Phys. 61, 2038 (1974)]. Illustrative calculations are presented for several singlet and triplet states of He. It turns out that excited S states of the He atom can be analyzed with sufficient accuracy in terms of natural s orbitals alone. This is not true for the ground state, which is considered here mainly in order to compare some of the present results to values obtained previously by different methods.
Simple approach to the equilibrium statistical mechanics of the hard sphere fluid
Authors: Andrews Frank C.
Published: February 2003

The simple physical interpretation of the statistical mechanical expression for the reciprocal of the activity of a classical fluid is explored by using it to derive the well−known equation of state for one−dimensional fluids of hard rods. Direct extension of this derivation to three−dimensional hard spheres yields analytical equations for the activity and the pressure of the fluid branch which fit the molecular dynamics data about as well as the Padé approximant or the empirical equation of Carnahan and Starling. They represent a significant improvement over existing statistical theories, e.g., that of Percus and Yevick. The approach also yields equations which qualitatively describe the hard sphere crystalline branch.
On the relaxation of the continuum approximation in the theory of electrolytes. II. Ion distributions
Authors: Goldstein Ronald A. , Hay Patrick Fenton , Kozak John J.
Published: February 2003

In this paper, we continue our re−examination of the role of the continuum approximation in the theory of electrolytes and explore further aspects of the possible generalization of this approximation, introduced in the previous paper. In particular, we present a comparative study of the results obtained on calculating ion−distribution profiles, both for symmetrical (1−1,2−2) and asymmetrical (1−2,2−1) electrolytes. In that ion−distribution curves are extremely sensitive to small changes in the potential (computed via a generalized version of the nonlinear differential equation derived in the previous paper), more detailed information can be obtained regarding the consequences of relaxing the continuum approximation. Specifically, our studies enable us to assess the importance of changing the concentration or varying the charge on the central ion in influencing the distribution of ions about a given central ion. The overall conclusion which can be drawn from our investigation is that, relative to curves generated using the continuum approximation, ion−distribution curves generated when this approximation is relaxed tend to be more localized about the central ion and tend to exhibit an accelerated damping to zero (random distribution) as one moves away from the immediate neighborhood of the central ion. Following Bjerrum, we interpret this behavior as evidence for ’’association,’’ and from our studies we conclude that this effect is important for all electrolyte systems, and becomes more pronounced with increased concentration, or with an increase in the charge number of ionic species present in the system. Finally, we report the results of calculations performed to assess the internal consistency, with respect to charge number, of the nonlinear Poisson−Boltzmann equation, and we find that the internal consistency of this equation (and hence the associated model) is improved upon relaxation of the continuum approximation.
Triplet exciton percolation and superexchange: Naphthalene C10H8–C10D8
Authors: Kopelman R. , Monberg E. M. , Ochs F. W. , Prasad P. N.
Published: February 2003

The phosphorescence of betamethylnaphthalene doped into a naphthalene−h8/naphthalene−d8 mixed crystal has been measured. The results demonstrate that (1) dynamical exciton percolation does occur (i.e., a transition from an exciton insulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)
Reaction path properties at critical points on potential surfaces
Authors: McCullough E. A. , Silver David M.
Published: February 2003

Mathematical descriptions are developed corresponding to various definitions of the reaction path on potential energy surfaces. It is shown that ’’kinks’’ at the critical point are a natural consequence of the ’’minimum-energy’’ definition and that a ’’kinkless’’ behavior is a consequence of the ’’steepest-descent’’ definition.
Far infrared spectra of H2 in liquid Ar and memory effects in the translational bands
Authors: Buontempo U. , Cunsolo S. , Dore P.
Published: February 2003

The spectra of dilute solutions of normal hydrogen and pure parahydrogen in liquid argon have been measured in the spectral range 12–700 cm−1. From such spectra we have obtained the rototranslational profile and the pure translational band of orthohydrogen and parahydrogen dissolved in liquid argon. The low frequency part of the spectrum measured in the solutions of parahydrogen in liquid argon is dominated by the intercollisional interference effects, which are discussed and compared with those previously observed in solutions of Ne and He in liquid Ar using a simple model for the motion of the solute in the liquid.
Computation of soft x-ray transitions in gaseous CH4 and neon
Authors: Deutsch P. W. , Kunz A. Barry
Published: February 2003

Unrestricted Hartree–Fock calculations have been performed on CH4 and neon to compute threshold energies for core electron excitations in both CH4 and neon. The results compare favorably with the experimental work on CH4 [H. U. Chun, Phys. Lett. A 30, 445 (1969)], and on neon [K. Siegbahn et al., ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969); and G. Brogren, Nova Acta R. Soc. Sci. Ups. Ser. IV, No. 4, 14 (1949)]. Valence ionization calculations are reported and compared both with other theory and with diverse ESCA experiments.
PES of high temperature vapors. VII. S2 and Te2
Authors: Berkowitz J.
Published: February 2003

The 584 Å photoelectron spectra of S2 and Te2 are reported. The features of the S2 spectrum are analogous to those in the O2 spectrum, and hence the states of S2+ are assigned in corresponding fashion. However, in the spectrum of Te2 anomalous intensities are apparent. The relative intensities of the peaks assigned to 2Πg,1/2 and 2Πg,3/2 are in the ratio 10:1, whereas in O2 they are approximately equal. Some of the other anticipated peaks are either absent or very weak. An analysis is presented to account for this anomaly, based on Hund’s case-c coupling.
Zero-field magnetic resonance study on the phosphorescent triplet state of benzil
Authors: Chan I. Y. , Nelson B. N.
Published: February 2003

The properties of the lowest excited triplet state of benzil in neat crystals have been studied with zero-field experiments combining microwave spectroscopy and optical detection. Two emitting species, originating from X traps of benzil crystals, were detected. The two X traps were found to have quite different trap depths and zero-field splittings, yet their triplet kinetics proved to be the same. One of the three spin sublevels almost totally dominates the populating and depopulating activities of the phosphorescent triplet state. A new and more accurate method is described for the determination of the steady-state population ratios. A discussion of the geometry of the benzil triplet state is given, and it is proposed that benzil assumes a trans- (near) planar configuration in the lowest triplet state. Finally, a change in the zero-field resonance spectrum is reported near a second order phase transition of benzil crystals near 80 °K.
D2–D2 total scattering cross sections in the velocity range 950–2250 m/sec
Authors: Dimoplon Gloria , Kinsey James L.
Published: February 2003

Measurements of the D2–D2 total scattering cross section are reported over the range 950–2250 m/sec in relative speed. The data were fit with a piecewise continuous potential constrained to give the correct theoretical behavior for large intermolecular separations. This potential also gives a satisfactory fit to experimental diffusion coefficients.
Effects of the multiplicity of impacts on vibration–translation energy transfer in collinear collisions
Authors: Shin Hyung Kyu
Published: February 2003

An idealized model of nonreactive collinear C–B+A collisions has been developed for the calculation of the number of impacts and amount of vibration–translation energy exchange at each impact as functions of the mass distribution and the initial collision energy. The maximum number of impacts that a single collision can have is n*=90 °/φ, where φ= tan−1 [mB(mA + mB + mC)/mAmC]1/2; for nonintegral values of 90 °/φ, n* = [90 °/φ], where the square brackets denote the algebraically largest integer which just exceeds the nonintegral value 90 °/φ. The model is applied to the collinear collisions H–F+F, F–H+F, Cl–H+Cl, Br–H+Br, I–H+Cl, and several other hypothetical systems with particular emphasis on the dependence of energy exchange on the number of impacts. Comparison with exact trajectory calculations at high collision energies shows a diminished inelasticity as the result of repeated encounters between A and B in a single collision.
Quasielastic neutron scattering study of ammonium-ion reorientations in ammonium perchlorate
Authors: Prask H. J. , Trevino S. F. , Rush J. J.
Published: February 2003

An investigation of the rotational motion of ammonium ions in NH4ClO4 is reported. Quasielastic neutron scattering measurements were made on a polycrystalline sample at temperatures between 66 and 150 °K. The shapes and widths of the quasielastic scattering peaks as a function of momentum transfer are consistent with instantaneous reorientation about the four C3 axes. Residence times are derived as a function of temperature yielding an activation energy of 2.3 kJ/mole and a frequency factor 1/τ0 of 3.7×1012 s−1.
Oscillator strength determination in LaCl3:Pr3+ by photon upconversion
Authors: Sell Jeffrey A. , Fong Francis K.
Published: February 2003

In this paper, we outline a novel method for the determination of oscillator strengths in rare-earth crystals. This method involves the observation of fluorescence resulting from a sequential two-photon excitation process. The rate equations are analyzed and a simple linear relationship between the laser flux, the fluorescent intensity, and the oscillator strength is derived. The equations are applied to LaCl3:Pr3+ and the resulting oscillator strength measured for the 3H6→3P1 transition is found to be in good agreement with the value measured by conventional absorption techniques. One advantage of the present method is the elimination of the need to determine the dopant concentration.
The calculation of isotopic partition function ratios by a perturbation theory technique
Authors: Singh Gulzar , Wolfsberg Max
Published: February 2003

The vibrational Hamiltonian of a molecule in the harmonic approximation, H = (1/2) 𝒥 (gijpipj + fijqiqj), has been divided into a diagonal part (terms with i=j) and an off-diagonal part (i≠j), which is regarded as the perturbation. The vibrational partition function of the molecule is then calculated by Schwinger perturbation theory as the partition function of the unperturbed problem, corresponding to a collection of oscillators with frequencies 2πνi′ = (fiigii)1/2, plus perturbation correction terms which are calculated to second order. With the usual assumptions of isotope effect calculations that the molecular translations and rotations are classical and separable from the vibrations, the perturbation formulation of the vibrational partition function is easily transformed into a perturbation theory formulation of (reduced) isotopic partition function ratios. If, for example, the molecular potential function is expressed in terms of the displacements of bond stretches and bond angle bends from their respective equilibrium values, the unperturbed partition function ratio corresponds to the isotope effect expected for noninteracting bond-stretch and bond-angle-bend oscillators. Detailed comparison is made for a number of molecular systems of perturbation theory calculations of partition functions and isotopic partition function ratios with exact calculations carried out by actually obtaining the normal mode vibrational frequencies of the vibrational Hamiltonian. Good agreement is found. The utility of the perturbation theory formulation resides in the fact that it permits one to look at isotope effects in a very simple manner; some demonstrations are given.
The optical activity of molecules of arbitrary size
Authors: Tobias Irwin , Brocki Thomas R. , Balazs N. L.
Published: February 2003

We determine the explicit time-dependence of a state function which initially corresponds to a molecule interacting with a single plane-polarized photon. It turns out that two conditions must be met if the polarization of the photon is to remain linear but may change its orientation. One is automatically satisfied for freely oriented molecules in the absence of damping and external fields; the other yields an expression for the optical rotatory power. This expression can be applied to molecules of any size and is therefore a generalization of the Rosenfeld equation which is valid only in the long-wavelength limit. In the other extreme, ω→∞, the optical activity also approaches zero, but in a manner quite different from the one that is sometimes discussed.
ESR investigations of triplet naphthalene in plastic hosts
Authors: Bramwell Fitzgerald B. , Laterza Marie E. , Spinner Mark L.
Published: February 2003

The thermal dependence of the zero field splittings (zfs) %of the photoexcited triplet state of naphthalene-d8 in a variety of plastic hosts of varying macroscopic rigidity has been measured between 5 and 333 K using electron spin resonance spectroscopy. The hosts employed were polycyclohexyl methacrylate (CHMA), polymethylmethacrylate (PMMA), polyvinylacetate (PVA), and polystyrene (PS). The relative macroscopic rigidity of each host matrix as determined by the degree of crosslinking and the glass transition temperature was CHMA≳PMMA≳PS≳PVA. Over the temperature range investigated, the zfs in the CHMA, PMMA, and PVA matrices were the same within experimental error but approximately 1% to 2% greater than those observed in PS. A linear variation in the root mean square zero field splitting with temperature % (∂D*/∂T) was observed in each matrix. Despite significant differences in host macroscopic rigidity, the magnitude of (∂D*/∂T) was the same in the methacrylates and PVA, and slightly less than that observed in PS. Attempts to fit the data to a linear combination of (1/T) or of exponential functions were unsuccessful. The results suggest that the guest triplet thermal dependence is influenced by classical motional processes in the host matrix cage, and that host matrix rigidity does not reflect the local environment of the guest triplet.
A computer simulation of charged particles in solution. I. Technique and equilibrium properties
Authors: Ermak Donald L.
Published: February 2003

A computer technique is presented for simulating the translational motion of ions in a liquid solution. In the model the diffusive motion of each ion is perturbed by the electrostatic force of the surrounding ions. Several polyelectrolyte systems of spherical polyions (10–50 Å in radius) and small ions (∼1 Å in radius) have been studied. For each system the polyion electrostatic shielding length and the average potential energy of each ion species was calculated. When the shielding length was sufficiently short, the computer results and the predictions of the zero polyion concentration Debye–Hückel theory were in good agreement.
A computer simulation of charged particles in solution. II. Polyion diffusion coefficient
Authors: Ermak Donald L.
Published: February 2003

A computer simulation study was conducted on the effects of the polyion–small ion electrostatic interaction upon the translational diffusion of polyions in polyelectrolyte solutions at thermodynamic equilibrium. This interaction increased the polyion diffusion coefficient and the increase was strongly dependent upon the polyion charge and size, and the small ion charge and mobility, while only slightly affected by changes in the polyion concentration. The increase was largest when there were only enough counterions to balance the charge of the polyions. Increasing the number of counterion–byion pairs per polyion decreased the polyion diffusion coefficient toward its uncharged value. The computer results were markedly different from those predicted by the self-consistent field theory of Stephen and a comparison of the two is presented.
Electron transport and ion clustering reactions in water vapor and deuterated water vapor
Authors: Wilson J. F. , Davis F. J. , Nelson D. R. , Compton R. N. , Crawford Oakley H.
Published: February 2003

The interaction of low energy electrons and negative ions with pure water vapor and deuterated water vapor has been studied in the following two experiments: (I) A time-of-flight electron swarm experiment has been used to determine longitudinal electron diffusion coefficients, DL, in H2O and D2O for electric-field-to-pressure ratios, E/P, from 1 to 25 V cm−1⋅Torr−1. The ratio of the longitudinal diffusion coefficient to mobility DT/μ for D2O is greater than that for H2O in the range of E/P from 5 to 20 V cm−1⋅Torr−1. Measurements of DT/μ for H2O are in agreement with the calculations of Lowke and Parker. Electron drift velocities have also been measured for values of E/P from 0.5 to 25 V cm−1⋅Torr−1, and the values for D2O are found to be considerably greater than for H2O in the range of E/P from 10 to 25 V cm−1⋅Torr−1. This result is justified on the basis of the energy dependence of the momentum transfer cross section for polar molecules. (II) Negative ions with masses in the range from 1 to 200 amu produced by electron swarm interactions and subsequent ion–molecule and ion–clustering reactions in H2O and D2O at room temperature have been recorded as a function of E/P from 0 to 70 V cm−1⋅Torr−1 and pressures from 0.1 to 5 Torr. The ions observed are H−(D−), OH− (OD−), and OH−⋅nH2O(OD−⋅nD2O) where n=1 to 7. The appearance of cluster ions from D2O occurred at a lower E/P than from H2O. This observation is compatible with our reported measurements of the characteristic electron energies (D/μ) for these vapors. The variation of the cluster ion distribution with pressure and E/P verifies the sequential clustering scheme suggested previously by Moruzzi and Phelps. For comparison purposes, negative ion products from electron swarm interactions with ammonia vapor are also presented. The ions observed are the primary ions H− and NH−2 and the cluster ions NH−2⋅nNH3 where n=1 to 4. These results are at variance with the two previous studies of ammonia.
Temperature and density dependence of 129Xe chemical shift in rare gas mixtures
Authors: Jameson Cynthia J. , Jameson A. Keith , Cohen Sheila M.
Published: February 2003

Pulsed Fourier transform NMR spectroscopy was used to obtain 129Xe chemical shifts in gases with densities of xenon much lower than ever before observed. At densities of 3–28 amagat, the contamination of the desired linear term in chemical shielding by contributions due to three-body or higher order interactions is completely eliminated. This allows the determination of very precise values of σ1(T) for 129Xe in xenon, krypton, and argon gas.
Resonance Raman and electronic absorption spectra of O−3 ions in γ-irradiated KClO3 and NaClO3
Authors: Bates J. B. , Pigg J. C.
Published: February 2003

Resonance Raman and electronic absorption spectra have been measured for the ozonide ion, O−3, produced in single crystals of KClO3 and NaClO3 by irradiation with γ rays. The O−3 ions are oriented in four to six symmetrically nonequivalent positions in KClO3 and appear to be oriented in two nonequivalent positions in NaClO3. Differences between the nonequivalent orientations affect both the ground and excited electronic states of O−3 as well as its ground vibrational states. The progressions of ν1 observed in the electronic spectra show that the vibrational spacing of ν1 in the excited electronic state is about 857 cm−1 as compared with the ground state spacing of about 1020 cm−1. Measurements of relative Raman intensities obtained with different exciting lines indicates that excitation near the center of a vibronic transition (0–n′) produces extra enhancement of the intensity of the nν1 vibrational transition.
Microwave spectrum of ethyl mercaptan
Authors: Schmidt R. E. , Quade C. R.
Published: February 2003

The microwave spectrum of the trans and gauche isomers of the normal and two isotopic species (CH3CH2SD and CH2DCH2SH) of ethyl mercaptan has been studied. Identification of the spectrum of the trans isomer of the normal and sym-CH2DCH2SH molecules has been extended up to J=19, enabling preliminary centrifugal distortion constants to be determined. From a and c dipole transitions full sets of rotational constants of the gauche isomer of all species have been determined. For the normal molecule they are A=28 746.37 MHz, B=5294.85 MHz, and C=4846.96 MHz. The tunneling frequency of the thiol top between its two equivalent gauche configurations has been measured directly for the normal species (1754.09 MHz), sym-CH2DCH2SH (1790.05 MHz), and CH3CH2SD (70.40 MHz). Identification has been made of excited thiol torsion satellites in both isomers. Dipole moments of all configurations for each species have been determined using second-order Stark effects. For the trans isomer of the normal molecule they are μa=1.06±0.03 D, μb=1.17±0.03 D,μc=0, μ=1.58±0.04 D; for the gauche isomer of the normal molecule they are μa=1.48±0.02 D, μb=0.19±0.10 D, μc=0.59±0.02 D, μ=1.61±0.05 D. An approximate structure, which has been fit to the data, indicates that the thiol top rotates about an axis with the CS bond tilted away from the axis by 3.16 °±0.05 °, while & CSH=95.23 °±0.50 ° and the angle of the axis with the CC bond is 111.71 °±0.10 °. The barrier to methyl internal rotation in the trans isomer was found to be 1156±30 cm−1. Splittings observed in the gauche ground state of the normal and sym-CH2DCH2SH molecules have been effectively reproduced on the basis of structural effects and tunneling. Relative intensity measurements show the gauche isomer to be lower in energy with the difference between the lowest states (trans − gauche) equal to 142±15 cm−1 for the normal species and 138±30 cm−1 for CH3CH2SD. Using the measured energy differences of the ground and excited states, the first three Fourier coefficients of the thiol torsional potential were found to be V1 = −65 cm−1, V2 = −135 cm−1, and V3 = 440 cm−1. Evidence of coupling between the methyl and thiol internal rotors was also investigated.
Excitation of nitrogen and carbon monoxide ionic emissions by He(2 3S), He+, and He+2
Authors: Piper L. G. , Gundel L. , Velazco J. E. , Setser D. W.
Published: February 2003

The N+2(B 2Σ+u–X 2Σ+g), CO+(B 2Σ+–X 2Σ+), and CO+(A 2Π–X 2Σ) emission spectra excited by reaction of He+ and He++He+2 mixtures with N2 and CO are compared with the spectra from the reaction with He(2 3S) metastable atoms. The experiments were done in a room temperature flowing afterglow apparatus. Charge exchange between He+ and N2 gave N+2(B) and a small amount of N+2(D) in addition to N+2(C); the yield of N+2(A) was insignificant. The vibrational population distribution of N+2(B) excited by He+ differed greatly from that given by the reaction with He(2 3S). No CO+ emission could be identified from the He++CO reaction. The reaction of He+2 with N2 and CO gave strong emissions from N+2(B), CO+(B), and CO+(A) with vibrational populations resembling the distributions expected for Franck–Condon transitions from neutral N2 and CO. Although the N+2(A–X) emission could not be studied in detail, N+2(A) appeared to be a product from reaction of He+2 and N2.
Experimental investigation of the thermomagnetic force in N2, CO, and NH3 gases at 300°K
Authors: Taboada J.
Published: February 2003

The thermomagnetic force is observed as a magnetic field induced change in the thermal force exerted on an object suspended in a heat-conducting rarefied polyatomic gas. A recently developed extremely sensitive stress measuring technique and higher magnetic fields have permitted the experimental investigation of the thermomagnetic force to saturation levels in the gases N2, CO, and NH3 at 300°K. We have studied the force effects for three well-defined temperature fields produced by isothermal boundaries with different geometry. The increased accuracy of the experimental results has permitted the fit of the data to a typical saturation curve. From this curve saturation values and the position on the H/p axis at which the effect reaches half-magnitude (H/p)1/2 values have been determined for the various gases under different conditions of temperature field and pressure. A comparison is made with the Senftleben–Beenakker thermal conductivity effects.
Magnetism down to 0.89 °K and the ligand field in a planar spin triplet compound of cobalt(III)
Authors: König E. , Schnakig R. , Kanellakopulos B.
Published: February 2003

The magnetism down to 0.89 °K and the optical spectra of solid KCo(3-prbi)2⋅2H2O have been studied where 3-prbi = 3-propylbiuret. The data are uniquely reproduced on the basis of a complete 3d electron calculation where B=811.5 cm−1, C=4B, ζ=580 cm−1, Dq=2223 cm−1, Dt=1002 cm−1, and Ds=4587 cm−1. The results confirm the unusual occurence of a 3B2(e4b2a1) ground state in this compound.
The Raman spectrum of (TMA)2UCl6 and (TMA)2SnCl6 crystals and of TMACl in solution
Authors: von der Ohe W.
Published: February 2003

The Raman spectrum of (TMA)2UCl6, not reported previously, is presented and the assignments of the observed bands to normal modes of vibration are discussed based on polarized room temperature scans of single crystals. Seventeen of the eighteen fundamental vibrational bands and seven nonfundamental ones are identified. The missing fundamental vibrational band, containing mainly methyl internal symmetric stretching motions, is at least three orders of magnitude weaker than another band composed of the same methyl internal motions. Five totally symmetric nonfundamental bands are resolved near 3000 cm−1. It is suggested that they interact by Fermi resonance with the highly intense totally symmetric methyl stretching vibration. Their proposed assignments imply that one of the nonfundamental bands is an overtone of a combination level, thus involving four vibrational quanta. Another one of these bands is assigned to a combination level involving three different vibrational quanta. Light scattering by three and four phonon excitations has not been reported before, except in resonance Raman scattering. An unpolarized Raman spectrum of (TMA)2SnCl6 is shown and the bands are assigned based on the close correlation with the (TMA)2UCl6 spectrum. The Raman spectrum of TMACl in solution is reexamined. Four of the frequencies reported previously could not be verified, and five bands of this spectrum, including a fundamental one, are reported here for the first time. The polarization information of the latter spectrum is completed. The crystal structures of (TMA)2UCl6 and (TMA)2SnCl6 are discussed. Four variations of the structure compatible with O5h, the space group suggested for both crystals by x-ray investigations, are described.
Field-swept and frequency-swept ELDOR and the manifestation of rapid-passage effects
Authors: Dorio Martin M. , Chien James C. W.
Published: February 2003

Field-swept and frequency-swept electron-electron double resonance (ELDOR) have been quantitatively compared as a function of temperature and of modulation frequency (ωm). At ωm ? 1 kHz the ELDOR reduction factor for field-swept and frequency-swept experiments, R (H) and R (ν)o, respectively, show the normal inverse dependence on temperature. R (H) is about 2 to 4 times greater than R (ν)o for either the ΔMI = 1 or 2 transition of inhomogeneously broadened lines of a nitroxide radical. At 100 MHz modulation, ELDOR shows rapid-passage effects. The maximum frequency-swept signal lags the modulation field by 90° and tends to decrease with the decrease of temperature at low temperature. The results are interpreted in terms of present theories of ELDOR.
Vibrational spectrum and force field of molybdenum hexafluoride
Authors: McDowell Robin S. , Sherman Ruth J. , Asprey Larned B. , Kennedy R. Craig
Published: February 2003

Infrared and Raman spectra are reported for 92MoF6 and 100MoF6 vapor (isotopic purities ≳ 97%). The infrared-active stretching fundamental ν3 exhibits partially resolved PQR structure with an asymmetric Q branch, from which the band origin ν0 can be estimated; the isotopic frequency shift for the band origin is Δν3 = 8.1±0.3 cm−1. The shift of the Q branch maximum of the other F1u fundamental is Δν4 = 3.0±0.4 cm−1. With the aid of these two isotope shifts, the general quadratic symmetry and valence force constants of MoF6 are calculated. The primary valence force constants are fr = 4.94 and fα ? 0.22 mdyn/Å. The force fields of MoF6, WF6, and UF6 are compared and discussed.
The partial-wave self-consistent-field method for diatomic molecules: Computational formalism and results for small molecules
Authors: McCullough E. A.
Published: February 2003

A method for performing Hartree–Fock calculations on diatomic molecules using partial-wave expansions in spheroidal coordinates is presented. This method makes no use of conventional basis set techniques but, instead, relies heavily on numerical integration. The computational formalism is described, and results of partial-wave calculations are compared with results from the standard matrix method. The partial-wave method is expected to be particularly useful for those expectation values which are difficult to compute with the matrix method. Although it is limited to diatomic molecules, the partial wave method is not limited to the Hartree–Fock level of approximation.
Fine structure transitions of C+ in collisions with H2
Authors: Chu Shih-I , Dalgarno A.
Published: February 2003

Close-coupling calculations are described of the cross sections for the fine-structure excitation of C+ in collisions with molecular hydrogen for energies up to 0.2 eV. A table is presented of the cooling rate coefficients for temperatures between 10–1000 °K.