bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociation Energy and Vibrational Terms of Ground-State (X 1Σg+) Hydrogen
Authors: LeRoy Robert J. , Bernstein Richard B.
Published: February 2004

In an attempt to elucidate the discrepancy between the theoretical and experimental dissociation energies of H2, accurate binding energies and term differences have been computed for the 15 vibrational levels using the Kolos and Wolniewicz clamped-nuclei potential with its various corrections. The results suggest possible interpretations of the discrepancy. In one of these an extrapolation method is introduced which combines experimental term differences with computed binding energies for the uppermost levels to yield the dissociation energy; this result is in accord with the experimental value of Herzberg and Monfils. The effect of uncertainties in the values of the natural constants is considered. Apparent inconsistencies between previously computed vibrational energies are explained.
Charge Transfer and Dissociative Charge Transfer between Rare-Gas Ions and Molecular Nitrogen
Authors: Champion R. L. , Doverspike L. D.
Published: February 2004

The reaction systems He+, Ne+, and Ar++N2 giving charge transfer and dissociative charge transfer have been studied for low collision energies. Kinetic-energy distributions and angular distributions of N2+ and N+ have been measured. In all cases the reactions proceed via a variety of endothermic or exothermic channels. Product ions are found to be scattered at large c.m. scattering angles, but do not appear to be isotropic in the c.m. system. For dissociative charge transfer, the data are consistent with a two-step process involving first an electron transfer followed by predissociation. It is emphasized that these low-energy charge-transfer reactions are not conceptionally different from ion–molecule (reactive) collisions.
Neutron-Diffraction Study of Ice Polymorphs. II. Ice II
Authors: Finch E. D. , Rabideau S. W. , Wenzel R. G. , Nereson N. G.
Published: February 2004

Neutron-diffraction spectra have been obtained at 80°K for polycrystalline D2O ice II prepared from ice Ih at 3 kbar and 195°K. An analysis of the 55 peaks in the 2θ diffraction angle between 10° and 57.3° has been made. The results of this study are in accord with the existence of a proton-ordered arrangement for this ice as proposed by Kamb. Ice II has a rhombohedral unit cell with lattice parameters aR = 7.743 ± 0.002 Å, αR = 113.09 ± 0.03°. Indexed on the triply primitive hexagonal cell, ice II has the parameters aH = 12.920 ± 0.003 Å and cH = 6.234 ± 0.002 Å at 80°K. The transformation of ice V to ice II by decompression at −35°C, an alternate way of forming ice II, has been observed to be slow. Although the O–O–O bonds in ice II are appreciably distorted from the tetrahedral arrangement found in ice Ih, the computer-refined position parameters for the deuterons indicate that the D–O–D angles are 106° ± 3°. These results support the existence of bent hydrogen bonds in ice II.
Neutron-Diffraction Study of Ice Polymorphs. III. Ice Ic
Authors: Arnold G. P. , Finch E. D. , Rabideau S. W. , Wenzel R. G.
Published: February 2004

Neutron-diffraction spectra have been recorded at 80°K for polycrystalline D2O cubic ice (Ic) obtained by the transformation of the high-pressure ices II, V, and IX. Spectral differences were found among these ice Ic preparations, principally in the degree of asymmetry of the (111) cubic reflection and in the extent of line broadening. The cubic (111) peak of greatest symmetry was obtained by the transformation of ice II. The line broadening appears to be associated with the small particle sizes of the ice Ic produced by the transformation of the high-pressure ices under controlled-temperature conditions. Peak centroids for scattering angles between 15° and 62° give a weighted cubic lattice parameter, a0 = 6.353 ± 0.001 Å. As revealed by an analysis of neutron-diffraction line intensities, attempts to induce the formation of ice Ic by the quenching of aqueous ferrous chloride or potassium chloride solutions in liquid nitrogen were unsuccessful.
Bending Frequencies and New Dimer Modes in the Far-Infrared Spectra of Transition-Metal Dihalides
Authors: Thompson Kenneth R. , Carlson K. Douglas
Published: February 2004

This article describes measurements and analysis of the infrared absorption spectra of monomer and dimer species of the transition-metal dihalides in the region of 520–40 cm−1. Experiments have been carried out on MnCl2, FeCl2, CoCl2, NiCl2, and NiBr2 isolated in argon matrices at 4°K. For the monomers, the following ν2 bending frequencies (cm−1) have been observed: 83 (MnCl2), 88 (FeCl2), 94.5 (CoCl2), 85 (NiCl2), and 69 (NiBr2). In addition to these, the ν3 frequencies have been observed. These data are compatible with a linear structure, but in connection with other information, suggest that a simple valence force field is an inadequate description of the normal vibrations. For the dimers, the following frequencies (cm−1) have been observed and assigned on the basis of a bridge-bond structure of D2h symmetry: ν12 = 436.2 (Fe2Cl4), 432.5 (Co2Cl4), 440 (Ni2Cl4); ν11 = 289 (Co2Cl4); ν10 = 110 (Fe2Cl4); ν9 = 323 (Co2Cl4).
Contribution to the Thomas–Fermi Theory
Authors: Tietz T.
Published: February 2004

In this paper, we give a procedure for calculating one-electron eigenvalues and eigenfunctions for the Schrödinger equation with the Thomas–Fermi potential. It has been shown that the Schrödinger equation in this case gives relatively good results in comparison with the Hartree results. The obtained numerical eigenvalues of this paper are compared with the corresponding Hartree values.
Quenching Interactions Involving the First Excited States of Gd, Yb, Er, Tm and Ho in Sodium Rare-Earth Tungstates
Authors: Uitert L. G. Van , Dearborn E. F. , Grodkiewicz W. H.
Published: February 2004

Relative intensity of emission per ion and lifetime (τe) data have been obtained as a function of rare-earth (RE) ion concentration for the first excited states of Gd, Yb, Er, Tm, and Ho in Na0.5(Y, RE)0.5WO4 crystals. Excitation migration to quenching centers appears to be possible, even for rare-earth ions further apart than second-nearest (Y, RE) positions at room temperature but not at 4.2°K. A monopole–monopole-like interaction that enhances internal conversion and appears to be temperature independent is indicated also for the lower-lying first excited manifolds.
Electron Magnetic Resonance Spectrum and Magnetic Susceptibility of K4MnCl6 from 1.4° to 300°K
Authors: Swanson Terry B. , Laurie Victor W. , Duffy William
Published: February 2004

The electron magnetic resonance spectrum and magnetic susceptibility of polycrystalline K4MnCl6 have been investigated over the temperature range from 1.4° to 300°K. The susceptibility follows a Curie–Weiss behavior above 95°K with a Weiss constant θ of −7.8°K. Between 13° and 80°K, a Curie–Weiss behavior is still observed but with a θ value of −6.0°K, different than that observed in the higher temperature range. Coinciding with the susceptibility change is a resonance linewidth change from 23 G above 90°K to approximately 175 G below 70°K. The origin of these changes is not known. No evidence for a transition to a magnetically ordered state has been observed, even down to 1.4°K. Comparison of experimental and theoretical resonance linewidths suggest that the dominant superexchange interaction involves next-nearest-neighbor Mn2+ ions. This interaction is conveyed by two intervening Cl− ions. The exchange integral for the next-nearest-neighbor interaction is estimated to be 0.22°K. Results suggest that there is approximately a 0.8% probability for transfer of an electron from a Cl− ion to a Mn2+ ion.
Electronic Energy Levels of the Trivalent Lanthanide Aquo Ions. III. Tb3+
Authors: Carnall W. T. , Fields P. R. , Rajnak K.
Published: February 2004

A total of 23 experimental “free-ion” energy levels of Tb3+ (aquo) have been fit with an rms deviation of 56 cm−1. The resulting parameters are: E1 = 6021.5, E2 = 29.030, E3 = 608.54, ζ4f = 1709.5, α = 20.131, β = − 370.21, and γ = 1255.9. The latter three parameters arise from including the effects of configuration interaction in the calculation. Assignments were based upon a correlation between calculated and observed band intensities at energies up to ≈40 000 cm−1.
Electronic Energy Levels of the Trivalent Lanthanide Aquo Ions. IV. Eu3+
Authors: Carnall W. T. , Fields P. R. , Rajnak K.
Published: February 2004

The free-ion energy level scheme of Eu3+ (aquo) to ∼40 000 cm−1 has been determined from the absorption spectrum of Eu3+ in dilute acid solution at 25°. Energy-level assignments were made on the basis of correlations between calculated and observed intensities. A least-squares fit of 26 J levels, including the effects of configuration interaction, gave an rms deviation of 40 cm−1. The parameter values were E1 = 5573.0, E2 = 26.708, E3 = 557.39, ζ4f = 1326.0, α = 25.336, β = − 580.25, and γ = 1155.7.
Comparison of Molecular Structures Determined by Electron Diffraction and Spectroscopy. Ethane and Diborane
Authors: Kuchitsu Kozo
Published: February 2004

Gas-phase average structures for the ground-vibrational state (rz) for ethane and diborane have been determined by a critical comparison of the experimental results obtained from electron diffraction (average internuclear distances rg) and those obtained from high-resolution infrared and Raman spectroscopy (rotational constants Bz(α)). Experimental values have been taken from the recent literature and converted into the average structure (rzor rα0). The rg and rα0 distances determined from electron diffraction carry uncertainties less than those in the rz distances determined from rotational constants, because the latter structures are very sensitive to assumptions about the unknown isotope differences in the structures. On the other hand, the average moments of inertia from spectroscopy are much more precise than those calculated from diffraction internuclear distances. Examinations of the data have led to the following rz structures with standard errors: For C2H6, rz(C–H) = 1.0957 ± 0.002 Å, rz(C–C) = 1.5319 ± 0.002 Å, and ∠C–C–H = 111.5° ± 0.3°; for C2D6, rz(C–D) = 1.0941 ± 0.002 Å, rz(C–C) = 1.5300 ± 0.002 Å, and ∠C–C–D = 111.4° ± 0.3°; and for B2H6, rz(B–Ht) = 1.192 ± 0.01 Å, rz(B–Hb) = 1.329 ± 0.005 Å, rz(B–B) = 1.770 ± 0.005 Å, ∠Ht–B–Ht = 121.8° ± 3°, and ∠Hb–B–Hb = 96.5° ± 0.5°. It was possible to increase the resolving power of the diffraction analysis of diborane by inclusion of calculated B–H mean amplitudes.

The effective complementary use of electron-diffraction and spectroscopic data for determining reliable gas-phase structures and the relative merits of the two alternative representations of the average structure (rgand rz) have been discussed.
Theory of Atom–Diatom Collisions. II
Authors: Kouri Donald J.
Published: February 2004

A previous study of atom–diatom collisions based on the Feshbach projection-operator formalism is utilized in further considering the effects of closed channels on the open-channel wavefunctions. The study is restricted to situations where the only even j open channel is j = 0. The Q-projected Hamiltonian consists of a part QH0Q which is diagonal in the YJjlM(r̂, r̂′) basis and a part QVQ which is not diagonal. Because of this, the closed-channel kernel 1 / (E – QHQ) is not diagonal in this basis. A nondiagonal representation of 1 / (E – QHQ) is obtained and the order of magnitude of the effects of the off-diagonal coupling on resonance positions, widths, and shifts is estimated. Criteria for determining the number of resonances which need to be included is examined. Additionally, the order of magnitude of the effect on resonance positions of off-diagonal coupling of near to far resonances is shown to be smaller than that between coupled near resonances. A brief study of the system Xe–H2 is made to examine the magnitude of the effect on the resonance positions of changes in the interaction potential as compared to that due to off-diagonal coupling. The resonance cross sections, positions, level widths, and shifts depend on the asymmetry factor a and should be sensitive to the short-range portion of the interaction. The direct cross section will be essentially independent of the asymmetry a (for j = 0 → j = 0 case).
Heat Capacity and Thermodynamic Properties of Plutonium Dioxide
Authors: Kruger Owen L. , Savage Howard
Published: February 2004

Changes in the heat content of PuO2 were measured from 192° to 1400°K with an isothermal drop calorimeter. The enthalpy curve, which was a smooth function of temperature, is described by the equation HT − H298 = − 8468 + 22.18T + 1.040 × 10−4T2 + 4.935 × 105T−1. The first derivative of this equation gave the heat-capacity relation, Cp = 22.18 + 2.080 × 10−4T−4.935 × 105T−2. At 298°K this equation agrees with previously reported low-temperature measurements made with an adiabatic calorimeter. These high- and low-temperature data were combined to obtain standard-state heat content, heat capacity, entropy, and Gibbs free energy functions to 1800°K. Measurements by other investigators show that PuO2 has a temperature-independent magnetic susceptibility below 1000°K. This behavior explains the comparatively low entropy of 16.34 cal mole−1·deg−1 for stoichiometric PuO2 at 298°K, which is somewhat greater than the value found for ThO2 and considerably less than the values for UO2 and NpO2.
Crystal Structure of Tetramethylammonium Sulfate Tetrahydrate
Authors: McLean W. J. , Jeffrey G. A.
Published: February 2004

Tetramethylammonium sulfate tetrahydrate is orthorhombic, Pnam, with a = 15.438, b = 13.832, c = 7.923 Å at −26°C. The structure contains ordered cations in channels formed by hydrogen-bonded chains of water molecules cross linked by interaction with the sulfate ions which are disordered.
Light Scattering and Hydrodynamic Properties of Polymer Chains with Excluded Volume Effects
Authors: Sharp Phillip A. , Bloomfield Victor A.
Published: February 2004

This paper considers the implications of the Domb–Gillis–Wilmers distribution function PN(R) = CNRδexp−(R / σ)δ for end-to-end distances R in polymer chains of N segments with excluded volume effects. CN is a normalizing constant; ε is related to the standard deviation of R; and, according to Fisher, δ = 2 / (1 − ε), where ε is defined by 〈R2〉 = b2N1+ε for polymers with step length b. We have calculated the angular dependence of light scattering, the translational frictional coefficient and intrinsic viscosity, and the dimensional statistics of cyclic chains. Comparison has been made with a previously used distribution function which took excluded volume effects into account in a less well-founded way. The present distribution function gives properties which differ measurably from those obtained with the previous one, reflecting the greater average expansion of the chain implied by the distribution proposed by Domb et al.
Electrical Forces between Uncharged Plates in Ionic Solutions
Authors: Bell G. M. , Levine S.
Published: February 2004

The nature of the electrical force between a pair of parallel uncharged plates of infinite thickness and low dielectric constant, immersed in an aqueous 1–1 electrolyte, is examined. The force is caused by the ion-image self-atmosphere potentials associated with the electrolyte ions in the neighborhood of the two plates. For anions and cations of the same size and polarizability in the electrolyte, these potentials of the two ion species are equal and consequently with uncharged plates there is no mean volume charge density. We have only considered point ions and neglected corrections due to ion size, except for introducing a distance of nearest approach of an ion to the plane of dielectric discontinuity. There are a number of contributions to the ion-image force between the two plates. The simplest is the ideal osmotic pressure term which is attractive because the repulsive image forces cause a depletion of ions in the region between the plates. Another term is the Debye–Hückel self-atmosphere contribution to the osmotic pressure, which is responsible for attraction. In addition, there are two direct image effects on the ions. One already occurs explicitly in the work of Buff and Stillinger and gives an attractive force which at large plate separations decays as the inverse cube of this separation. The second, which is new, may be described as a “cross effect” representing the direct image force of each plate on the diffuse layer of the second plate. It is repulsive and likewise decays as the inverse cube law at large separations, where it exceeds the first direct-image force. The net result is that the force between the two dielectric plates is an attraction at small separations but a repulsion at large separations. Except for sign this repulsion takes the same form as the Hamaker attraction, with an equivalent Hamaker constant equal to about 3 × 10−14 erg at room temperature. In Appendix C it is shown that the local free energy density and local pressure formalisms used in this paper yield the surface tension relations which Buff and Stillinger obtained by using molecular distribution functions in grand ensemble theory. It is also shown that the three different forms for the surface tension considered by these authors give the same values when the dependence of local ion density on distance from the interface is introduced consistently, even though superposition and linearization approximations are implicit in the expression for the potential of the mean force between two ions.
Vibrational Analysis of Thiophosphoryl Fluoride
Authors: Crowder G. A.
Published: February 2004

Vibrational calculations have been made for thiophosphoryl fluoride, using a valence force field with nine parameters. The calculations support assignment of the 983-cm−1 band to the P–F3 symmetric stretch and assignment of the 698-cm−1 band to the P=S stretch.
Low-Frequency Raman Spectrum of Imidazole Single Crystals
Authors: Colombo Lidija
Published: February 2004

The low-frequency Raman spectrum of imidazole single crystals is given. Of 12 frequencies active in the Raman spectrum, 11 have been recorded. Rotational oscillations have been assigned by the method of Kastler and Rousset. Owing to the specific structure of the imidazole crystal lattice, translational vibrations have been separated into two groups and thus a better assignment has been achieved.
Electrophoresis in Liquids of Low Permittivity and Field-Induced Polarization
Authors: Parreira H. C.
Published: February 2004

An equation is derived which correlates the electrophoretic velocity of a particle suspended in a liquid of low permittivity with the polarization of the particle material and that of the surface groups. Under certain conditions the equation reduces to the Hückel formula, thereby allowing the true particle mobility to be obtained.
Alkali–Alkali Glory Scattering in the Σ3 State
Authors: Rothe Erhard W. , Helbing Reinhard K. B.
Published: February 2004

The velocity dependence of the total cross section for the scattering of 7Li by Na, K, Rb, and Cs has been measured in a velocity range from about 1–4.5 km/sec, and that for Na–Cs scattering from 1–3 km/sec. The observed glory undulations are due to the Σ3 state of the diatom. A Lennard-Jones (8,6) potential is assumed. The product of the potential parameters εrm were obtained, and for the lithium scattering, the separated parameters ε and rm were estimated. Independent of potential model, the number of extrema indicates that the minimum number of bound states for the Li–Na, Li–K, Li–Rb, Li–Cs, and Na–Cs triplet states is 3, 4, 5, 6, and 7, respectively. The Na–Cs results are not consistent with previous results.
Photoemission of Some Organic Donor–Acceptor Complexes
Authors: Hibma Tjip , Vegter Johan G. , Kommandeur Jan
Published: February 2004

Photoemissive thresholds were determined of a number of donor–acceptor complexes (tetramethyl-p-phenylenediamine and p-phenylenediamine as donors with chloranil and bromanil as acceptors) by extrapolation of the wavelength-vs-photocurrent curves to zero current. These thresholds were compared with those of the constituent components. The thresholds of the complexes are found to be somewhat higher than those of the donors (by about 0.1–0.2 eV) and also the behavior of the wavelength-vs-photocurrent curves near the threshold appears to be different. The observed phenomena are consistent with a model in which the lattice of a donor–acceptor complex contains neutral molecules as well as ions.
Reaction and Deactivation of O(1D)
Authors: Young R. A. , Black Graham , Slanger T. G.
Published: February 2004

Relative rates for quenching of O(1D) to O(3P), and for removal of O(1D) by reaction, have been measured for various gases. O(1D) was produced by photolysis of O2 at 1470 Å. The rates were determined by measuring the variation of the production rate of O(3P) atoms as a function of the amounts of quenching and reactive gases added to the system. From limits set by the lack of detection of the 6300-Å line of the O(1D) → O(3P) transition, and the maximum rates of two-body reactions, the rate coefficients were put on an absolute basis. Except for Ar and He, all gases measured interact with O(1D) with rate coefficients ≥ 4 × 10−11 cm3 molecule−1·sec−1.
Viscosity and Diffusion Coefficient of Six Binary Mixtures
Authors: Kestin J. , Yata J.
Published: February 2004

The paper contains the results of measurements of the viscosity of six binary mixtures: CO2–Kr; H2–N2; He–O2; CH4–CO2; CH4–C4H10; H2–He. Measurements were made in an oscillating-disk viscometer at 20° and 30°C and at pressures from 1–25 atm, approximately; they were evaluated by the relative method. An analysis of the data shows that the interaction quantity μ12 which appears in the Chapman–Enskog theory for low densities is sensibly independent of composition. Although no direct check on the accepted mixing rules can be made owing to the narrowness of the temperature range covered, it is concluded that a knowledge of the viscosities μ1 and μ2 of the pure components together with that of μ12 at each temperature is sufficient to determine the dependence of the viscosity of the mixture on composition with the aid of the Chapman–Enskog theory. This dependence is reproduced well by the theory at virtually zero density and at a constant temperature. The experimental values of μ12 are used to calculate the binary diffusion coefficient D12 for all mixtures. The results agree well with the correlations provided by Mason and Marrero. The system H2–He does not exhibit the anomaly detected for thermal conductivity by Mukhopadhyay and Barua.
Improved Calculation of the nπ* Rotational Strength in Polypeptides
Authors: Woody Robert W.
Published: February 2004

Previous calculations of the nπ* rotational strength in the α helix using the Kirkwood theory as extended by Moffitt and Tinoco have led to results which are of the right order of magnitude and correct sign but too small by a factor of 6–8. It is possible that contributions neglected by the Kirkwood approach, e.g., charge-transfer transitions, are important in the α helix. A molecular-orbital treatment of an amide dimer indicates that mixing of the nπ* and charge-transfer states makes a negligible contribution to the nπ* rotational strength in the α helix. However, when two modifications are made in previous Kirkwood-type treatments, the calculated rotational strength is found to be about − 0.2 debye bohr magnetons (D·μB), in good agreement with experiment. In addition, the calculated exciton splitting for the ππ* transition agrees better with experiment than do previous theoretical values. The modifications are (a) a more consistent choice of permanent and transition monopole positions, and (b) solution of a secular determinant for the mixing of the nπ* and ππ* excited states, rather than using first-order perturbation with zero-order wavefunctions. Analyses of experimental circular-dichroism (CD) curves as sums of Gaussians have led to ππ* rotational strengths appreciably smaller than theoretically calculated ones. We show here that this discrepancy may be due to failure to include a non-Gaussian term predicted by theory. Using reasonable band positions and widths, theoretical CD curves agree reasonably well with experimental curves. It is also pointed out that the exact shape of the CD curve is very sensitive to the band frequencies and widths. Therefore, observed variations in the shape may result from rather subtle changes in these parameters.
Potential Curves and R Dependence of Electronic Transition Moments from Molecular Continuum–Continuum Emission Spectra. I. Theory
Authors: Smith Allan Laslett
Published: February 2004

A procedure is developed for analyzing the oscillatory structure observed in the continuum–continuum emission of diatomic molecules when the upper state is attractive and the lower state repulsive. Given the potential curve of the lower state and the positions of the diffuse maxima and minima, the procedure uniquely determines the potential curve of the upper state. A simple semiclassical method is presented for deriving both the upper-state potential and the R dependence of the electronic transition moment from the measured relative intensity distribution. Criteria for the applicability of this method are presented and discussed.
Self-Consistent Perturbation Theory. II. Extension to Open Shells
Authors: McWeeny R. , Diercksen G.
Published: February 2004

The self-consistent perturbation theory developed in earlier papers is extended to the open-shell case. Density matrices for both shells are calculated iteratively until first-order self-consistency is achieved. A numerical application indicates the importance of considering both shells separately when discussing the effects of polarization on the charge density and the spin density.
Formation and Detection of Metastable Ions in a Double-Focusing Mass Spectrometer
Authors: Schulze Peter , Burlingame A. L.
Published: February 2004

By changing the acceleration voltage independently from the voltage applied to the electrostatic analyzer of a double-focusing mass spectrometer, it is possible to detect “metastable” ions which are formed in the field-free drift region in front of the electrostatic analyzer. The parent and daughter ions of the decomposition can be determined unambiguously. Applying this technique to various compounds it was found that it is also possible to detect ions whose decomposition originates during acceleration by the ion gun as well as during flight through the electrostatic analyzer. Theoretical calculations are presented which verify these assumptions and show the location of decomposition in the electrostatic analyzer. Using this technique, it was also possible to show that doubly charged ions may undergo a metastable transition which leads to another doubly charged ion.
Electron Spin Resonance Absorption Spectrum of CO2− Molecule Ions Associated with F− Ions in Single-Crystal Calcite
Authors: Marshall S. A. , McMillan J. A.
Published: February 2004

Single crystals of calcite subjected to γ irradiation exhibit a number of paramagnetic defect centers. One of these has been identified as the CO2− molecule ion which is in close association with an F− impurity ion. An analysis of the ESR superhyperfine structure due to the fluorine nucleus suggests that for 3.3% of the time, negative charge is transferred from the F− ion to the CO2− molecule ion. This transfer of charge creates the diamagnetic CO22− molecule ion and atomic fluorine whose electron configuration is approximately 90% (2s22p5) and 10% (2s2p6). The effective magnetic field at the fluorine nucleus due to core polarization and 2s contact is calculated to be 21 kOe.
Magnetic and Electric Field Spectra of Organic Crystals: Optical Measurements of Zero-Field Splittings
Authors: Hochstrasser Robin M. , Lin Tien-Sung
Published: February 2004

The theory of the Zeeman and Stark effects in molecular crystal exciton bands is presented. The development emphasizes the role of the intermolecular potential in determining the spin alignments in the crystal, and it is shown that the interactions V, and the projections of spins from one molecule onto those of another 〈Tα | Tβ〉, determine the crystal spin quantization in the absence of external fields through matrix elements of the type V 〈Tα | Tβ〉. A complete anisotropic Zeeman study in weak and strong fields is presented for the benzophenone crystal. It is shown that relatively low-resolution optical studies can be used to determine zero-field splitting parameters to about 5% accuracy. The higher resolution studies have led to the assignments, and relative ordering of the twelve k = 0 exciton components of the nπ*3 state of benzophenone crystals. These results are confirmed by Stark and Stark–Zeeman experiments which demonstrate the applicability of the theory outlined. The final best set of molecular spin-splitting parameters, and intermolecular matrix elements are X = − 0.03,Y = − 0.09,Z = + 0.12 cm−1V12 = − 0.23,V13 = + 0.03,V14 = + 0.07 cm−1.. The benzophenone crystal triplet exciton states, at k = 0, are close to those expected for strong spin coupling, but the similarity of magnitude of the V terms and the spin splittings causes the pure excitonlike spin states to be slightly mixed at zero field.
Fluorescence of Trivalent-Europium-Doped Yttrium Oxysulfide
Authors: Sovers O. J. , Yoshioka T.
Published: February 2004

The fluorescence spectrum of trivalent europium in the site of C3υ symmetry of polycrystalline Y2O2S:Eu3+ is investigated in the region 4000–9000 Å. Of the total of 363 transitions between the crystal components of the J = 0 to J = 3 states of D5 and those of the J = 0 to J = 6 states of 7F, almost two thirds are observed. Transitions originating in the D35 state of Eu3+ are particularly intense at 77°K. Some lines originating in unidentified levels, higher than D35 in energy, are also present. Thirty-nine of the 44 crystal components of D0–35, 7F0–6 are identified. Y2O2S:Eu3+ is perhaps an optimal case of identification of degeneracies of the crystal components without benefit of the additional information afforded by polarization studies or by the Zeeman effect. From a consideration of all possible assignments of symmetry species for the crystal levels of F1–47 in a systematic fashion, a small number of self-consistent sets of assignments and crystal-field parameters is obtained. Intensity data reduce the number of possibilities to four. For the most likely one of these, the deviation between observed and calculated crystal levels is 9 cm−1. The crystal-field parameters are B02 = 124, B04 = 1344, B34 = 823, B06 = 286, B36 = − 362, and B66 = 554 cm−1. The effect of J mixing is considerable. A fit to the barycenters of F0–47 and D0–25 yields values of the spin–orbit and electrostatic parameters η = 1326 cm−1 and F2 = 395.8 cm−1. A mechanism involving lattice vibrations is proposed which may explain the dramatic increase in intensities of transitions originating in D35 when the temperature is lowered from 300° to 77°K.
High-Temperature Phase Transition in KH2PO4
Authors: Blinc R. , Dimic V. , Kolar D. , Lahajnar G. , Stepišnik J. , Žumer S. , Vene N. , Hadži D.
Published: February 2004

The high-temperature phase transition in KH2PO4 has been investigated by differential thermal analysis, thermogravimetric analysis, proton magnetic resonance and relaxation, infrared spectroscopy, and x-ray and quasielastic cold-neutron scattering. The transition is not connected with a static breaking up of the hydrogen-bond network but rather with the onset of disordered hindered rotation of the H2PO4 groups around all three axes. It seems that rotation of H2PO4 groups is as well rate determining for the proton conductivity of this crystal.
Kinematic Model for Atom–Diatom Reactions
Authors: Suplinskas Raymond J.
Published: February 2004

A simple model for atom–diatom reactions is completely analyzed. The model represents the molecule by two hard spheres nearly in contact and the atom by a third hard sphere. Energy and momentum transfers among the three particles are calculated for all values of molecular orientation and collision parameters. On the basis of the kinematic analysis and in terms of an activation parameter, probabilities are assigned to the various reactions which may occur. Using as parameters the bond length of H2 and the activation energy for H+H2, the model predicts (a) the energy dependence of the H+H2 cross section, in agreement with the complete classical analysis of Karplus et al.; (b) rate constants for hydrogen isotopic exchange reactions which agree with experiment when threshold effects do not dominate; and (c) the hot-atom behavior of the T+H2 and T+D2 systems in substantial agreement with both experiment and the complete classical analysis. Methods for extending the model are discussed.
Nuclear Magnetic Resonance Study of Ethylene Adsorbed on a Zeolite. Chemical-Shift Measurements
Authors: Muha G. M. , Yates D. J. C.
Published: February 2004

The measurement is reported of the relative NMR chemical shifts of ethylene adsorbed on a zeolite in which the usual sodium cation has been exchanged with lithium, potassium, silver, cadmium, or barium ions. When measured relative to the proton signal from gas-phase ethylene, the proton shifts in the adsorbed phase are to high field. The sample bulk susceptibility correction cannot explain the magnitude nor direction of the shift, nor can an analysis based on a simple consideration of the electric field of the cation. A linear correlation is observed between these high-field shifts and certain frequency shifts in the infrared spectra of the adsorbed molecule. A discussion of this correlation is based on an analogy with the mechanism of the temperature-dependent chemical shifts observed in gases. The hypothesis is offered that these chemical shifts result from deformations in the electronic and geometric structure of the adsorbed ethylene. A brief discussion of the low-temperature wide-line NMR spectra and the linewidth dependence on the spectrometer operating frequency is included.
Vibrational Spectra of Single Crystals of Group I Nitrates
Authors: James David W. , Leong W. H.
Published: February 2004

The Raman and infrared spectra of single crystals of the nitrates of the alkali metals and silver have been studied at room temperature. The internal vibrations of the nitrate ion and the external vibrations of the crystal lattice are assigned on the basis of a factor-group analysis of the crystal. For the rhombohedral crystals the frequencies were little changed from those expected for a free ion with the exception that infrared and Raman frequencies were distinct. For crystals of lower symmetry the vibrations of the free ion give rise to many components which can, however, be adequately described on the basis of the factor-group analysis. Strong combination bands between internal and external vibrations are observed and assigned in the infrared spectra.
Dissociative Photoionization of O2
Authors: Doolittle P. H. , Schoen R. I. , Schubert K. E.
Published: February 2004

Energy distributions of the ions produced in the dissociative photoionization of O2 have been measured with photons in the spectral interval 584–358 Å. Tentative identifications of several ion groups with particular states of the O2+ molecular ion have been made. Mass spectrometric measurements were carried out with a pulsed, slitless mass spectrometer to identify the ions produced with 584 radiation and to confirm the production of energetic ions. It is concluded that mass spectrometer discrimination against energetic ions may have caused an understimate of the amount of dissociative photoionization produced by wavelengths shorter than 584 Å.
Work Function of Metals and the Potential at Which They Have Zero Charge in Contact with Solutions
Authors: Bockris J. O'M. , Argade S. D.
Published: February 2004

A deduction is made of a relation between the work function of a metal and the potential of the metal (functioning as an electrode) in solution, at zero charge, is derived. The deduction is consistent with the value of the surface potential of water attached to mercury of 0.20 ± 0.11 V. The absolute potential difference across a hypothetical Na–Na+ electrode is calculated to be − 1.24 V.
Raman Spectrum of Neptunium Hexafluoride
Authors: Gasner Earl L. , Frlec Boris
Published: February 2004

The three Raman active fundamentals of NpF6 have been observed at 654 (ν1), 535 (ν2), and 208 cm−1 (ν5) for the gas and at 651 (ν1) p, 524 (ν2) dp, and 218 cm−1 (ν5) dp, for the liquid. The values observed for the gaseous sample were within 7 cm−1 of the values calculated by Malm, Weinstock, and Claassen on the basis of infrared combination bands. Spectra were obtained through the use of a 6328-Å He–Ne laser-excited, photoelectric recording Raman spectrophotometer.
Positron Annihilation in Condensed Ammonia
Authors: Holt W. H. , Chuang S. Y. , Cooper A. M. , Hogg B. G.
Published: February 2004

Two-photon angular correlation and lifetime studies have been performed for positron–electron pairs annihilating in liquid (− 65°C) and solid (− 105°C) ammonia. The angular distributions were the same for both the liquid and solid samples. A low-momentum component indicated an amount of positronium formation that is consistent with the lifetime results. The momentum distribution determined from the data is compared with a calculated distribution based on a cell model, using SCF molecular orbitals for ammonia and realistic positron wavefunctions. The lifetime results are discussed in the light of the free-volume model.
Stochastic Model for Gel-Permeation Chromatography at High Flow Rates
Authors: Carmichael Jack B.
Published: February 2004

Calculations are presented for the elution curve of monodisperse polymer obtained during gel-permeation chromatography at high flow rates. The derivation parallels an earlier stochastic treatment by McQuarrie valid at low flow rates. Under those conditions the result of the present model reduces to that obtained by McQuarrie.
Symmetric H3: A Semiempirical and Ab Initio Study of a Simple Jahn–Teller System
Authors: Porter R. N. , Stevens R. M. , Karplus M.
Published: February 2004

Semiempirical and multiconfiguration ab initio results for the potential energy of H3 are used to obtain a detailed description of the potential-energy surface in the region of interest for the Jahn–Teller problem. Topographical properties of truncated perturbation expansions of the potential-energy function about the D3h minimum are compared with those of the complete semiempirical contour map; six symmetric dissociation “troughs” are found in the complete semiempirical surface, in contrast to three stable basins in the second-order surface and three dissociative troughs in the third-order surface. The relationships among the various parts of the complete three-dimensional H3 potential-energy hypersurface are discussed and illustrated by means of a single compact diagram. A convenient matrix-projection method for identifying normal-mode coordinate and electronic wavefunction partners associated with degenerate representations is discussed in an appendix.
Pressure-Induced Microwave Absorption of Nonpolar Gases
Authors: Gersten J. , Foley H. M.
Published: February 2004

A theory is developed which describes the microwave absorption of nonpolar gases. The contributions arising from collisionally induced absorption and from dimers are evaluated separately. It is found that at low enough temperatures the absorptive properties of such gases are dominated by the dimeric states. This accounts for the observed anomalous temperature dependence of the absorption coefficient. A comparison of the theoretical predictions with experimental data on carbon dioxide and nitrogen yields good agreement.
Spin–Lattice Relaxation and the Anisotropic Part of the He–H2 Intermolecular Potential
Authors: Riehl John W. , Kinsey James L. , Waugh John S. , Rugheimer John H.
Published: February 2004

The spin–lattice time T1 of gaseous mixtures of H2 and He has been measured as a function of composition and of temperature in the range 77°–300°K at pressures such that T1 is proportional to the density. The relationship between the experimental T1 and cross sections for molecular reorientation of H2 is given. From comparison of computed cross sections to the data, experimental values are obtained for parameters in an approximate form for the He–H2 anisotropic intermolecular potential.
Charge Separation and Similarities of Ionic Processes in the Radiolysis of Alkanes and of Water
Authors: Leone James A. , Hamill William H.
Published: February 2004

The charge-pair separation distribution function, d lnn / d lnR = − 2.7 is consistent with results from the rate of correlated ion-pair recombination, as well as the temperature and field dependence of the electrical conductivity of n-hexane under x irradiation for data reported by Allen et al. The T dependence derives from (1) and Rescape = e2 / εkT. The combined T and E dependence fits the description Rescape = e2 / ε(kT + gλ′Ee) where g is a geometric factor and gλ′5 × 10−7 cm (which implies λ′∼102 Å for energy gained from the field), while (1) and (2) together account for the E dependence. The integral distribution function is P(R) = (R / Rm)−2.7 for separation ≥R, with P = 1 at Rm≅100 Å. It is shown that many electron reactions in organic liquids and glasses fit a common concentration dependence, which is predictable from (3). It is also shown that in the region of ∼ 10−4 − 10−1 M solute in water, the yield of chemical reduction obeys the same concentration dependence as liquid alkanes. This effect is attributed to a population of electrons which are not trapped and do not become hydrated. Consequently their electrical behavior depends upon the high-frequency dielectric constant, accounting for similar behavior in cyclohexane and in water.
Effect of the Internal Coulomb Field upon the Viscosity of a Fused Salt
Authors: Morrison Graham , Lind John E.
Published: February 2004

An experimental determination is made of the effect of the internal Coulomb field in a fused salt upon the viscosity of the fluid. Two fluids of isoelectronic molecules are compared: the salt tetrabutylammonium tetrabutylboride and the nonelectrolyte 5,5-dibutylnonane. The molecules of these fluids are identical except for the charge on the nucleus of the central atom and their coefficients of viscosity are compared at the same temperature and particle number density between 120° and 180°C. The salt viscosity varies from 10 times that of the nonelectrolyte to less than 5 times as the temperature and volume increase. The Brownian model of Kirkwood and Rice yields a ratio of 2 for a similar model system and suggests that the deviation of this ratio from unity is not necessarily caused by the dissipation arising directly from the Coulomb field. Rather it is caused by the repulsive forces which are more effective in the salt because of the shorter intermolecular distances demanded by the Coulomb field of the salt at any given density.
Positron Annihilation in the Neon Isoelectronic Series, Diamond and Solid Iodine
Authors: Klotz A. S. , Hafemeister D. W.
Published: February 2004

The angular correlation of the annihilation quanta has been measured in the neon isoelectronic series (neon–neon configuration). Although the curves had different shapes, they were analyzed in terms of a single parameter Γ, the full width at half-maximum with the result: NaF[100], 9.7 ± 0.2 mrad; MgO[100], 12.1 ± 0.2 mrad; AlN(polycrystalline), 11.1 ± 0.2 mrad; and SiC (polycrystalline), 12.0 ± 0.2 mrad. The increase between NaF and MgO is attributed to the increase in the ionic charge, and the decrease for AlN and SiC is attributed to the onset of covalency. A single [100] crystal of diamond was investigated and compared with data on Si[100] and Ge[100]. There appears to be a linear relationship between Γ and r0−1, the inverse of the lattice parameter. The SiC also agreed with this trend. Finally, Γ was obtained for KI(7.1 ± 0.2 mrad) and solid iodine (9.2 ± 0.2 mrad). The relative broadening in Γ for these two compounds is consistent with Mössbauer isomer-shift data and with the known atomic configurations Wigner–Seitz cellular-model calculations which were performed for the positron wavefunction for the NaF, MgO, KI, and I2 lattices roughly describe several aspects of the data.
Proton Magnetic Resonance Study of Hydrated Potassium Trioxalatoaluminate: Free Rotation of Proton Pairs
Authors: Rangarajan G. , Ramakrishna J.
Published: February 2004

A proton magnetic resonance study of hydrated potassium trioxalatoaluminate revealed that the lines are extremely narrow and intense at room temperature and much broader at 90°K, indicating the presence of free rotation of the proton pairs. Detailed investigations were carried out on both single-crystal and powder samples, and it was found that the p–p vectors rotate about the c-axis generating a cone having a semi-vertical angle of about 33°. The temperature variation of the second moment showed a transition around 155°K. This is in agreement with Saha's susceptibility measurements on the isomorphous chromium salt. Similar rotational narrowing was observed in the chromium and ferric salts, also. The Pake method was used to determine the length and orientation of the p–p vectors in the rigid lattice (at 90°K). The hydrogen-bond schemes suggested by the x-ray workers did not satisfy the criterion of parallelism between the p–p line and the line joining the acceptor oxygen atoms. Alternate schemes could not be found. An order-of-magnitude calculation gave a very low value for the barrier hindering the rotation (≈ 2kcal/mole). These results suggest that hydrogen bonds are either absent or, if present, extremely weak and are consistent with the presence of free rotation. A preliminary infrared study gave an insignificant shift of the O–H stretching frequency thus supporting our conclusions.
Temperature Dependence of Kinetic Isotope Effects in Gas-Phase Reactions. II. The Hydrogen-Atom–Formaldehyde Reaction
Authors: Macdonald Digby D. , Tschuikow-Roux E.
Published: February 2004

Kinetic isotope effects of atom-transfer reactions and their temperature dependences at high temperatures have been calculated for various hydrogen-atom–formaldehyde reactions. In the case of the “abstracting atom” kinetic isotope effect, a negative temperature coefficient is predicted in the high-temperature limit. The theoretical calculations are compared with experimental results in the literature and satisfactory agreement is obtained.
Molecular Acoustic and Spectroscopic Studies on the Internal Rotation in 1-Fluoro-1,1,2,2-tetrachloroethane
Authors: Pethrick R. A. , Wyn-Jones E.
Published: February 2004

An ultrasonic relaxation observed in liquid 1-fluoro-1,1,2,2-tetrachloroethane has been attributed to a perturbation of the equilibrium between the trans and gauche isomers. From the temperature dependence of the relaxation parameters, the rates of isomerization have been derived. The temperature dependence of the rate constants were found to obey the Arrhenius and Eyring rate equations giving the following parameters: A = 2.2 ± 1.5 × 1012sec−1, E = 9.0 ± 0.4 kcal/mole,ΔHgt+ = 8.3 ± 0.3 kcal/mole. The vibrational frequency of the torsional oscillations of the trans isomer is 86 cm−1, which is equivalent to a torsional barrier (ΔHgt+) of 8.89 kcal/mole. The acoustic and spectroscopic barrier are compared to the potential barrier found from a recent NMR study, (ΔHgt+ = 7.25 kcal/mole).
Gaussian-Orbital Approximation for the Hydrogen Atom by Minimization of Variance
Authors: Rendell Marc , Arents John
Published: February 2004

The determination of an approximate wavefunction in the form ∑ i=1n Aiexp(− bir2), 1 ≤ n ≤ 10, by minimization of the variance, ∫ |(H − E)ψ |2dτ, is tested on the H atom. Obstacles to using other definitions of variance are discussed. Results are less favorable than those obtained by minimization of 〈H〉, but improve rapidly as n increases.