bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-Temperature Electron Injection and Space-Charge-Limited Transients in Anthracene Crystals
Authors: Pott Gerard T. , Williams Digby F.
Published: February 2004

A solid electrode has been prepared able to efficiently inject electrons into anthracene crystals at temperatures as low as 77°K. Currents as high as 10−2 A·cm−2 may be injected with no evidence of saturation. No activation energy is associated with the injection process, the current–temperature dependence is determined only by the trap distribution of the crystal. Using this electrode the temperature dependence of the electron mobility has been measured from 300 to 170°K. The results corroborate those from phototechniques, namely, as temperature decreases the electron mobility increases along the a and b crystal axes and decreases slightly along the c′ axes. At low temperatures, a second maximum in the current transient along the a and b directions is observed.
Phosphorescence Mechanism in CdF2:Eu3+
Authors: Banks E. , Schwartz R. W.
Published: February 2004

Single crystals of CdF2, doped with Eu3+, were grown by the Bridgman method, and shown by fluorescence and absorption spectra to contain no detectable Eu2+, irrespective of the presence of Na+ or Y3+ as added impurities. Ultraviolet irradiation at 77°K produced a new broad absorption band near 3400 Å, which disappeared on warming, with associated thermoluminescence. A complex ESR signal associated with this band is attributed to Eu2+, formed during the irradiation. A mechanism proposed for the luminescence processes involves formation of Eu2+ in the excitation and trapping steps, with the luminescence of Eu3+ resulting from a resonance transfer of excitation energy from the recombination site.
ESR Studies of Radical Fragments Produced in X-Irradiated Disodium Succinate
Authors: Bales Barney L. , Schwartz Robert N. , Hanna Melvin W.
Published: February 2004

The room-temperature stable paramagnetic species in x-irradiated disodium succinate has been shown to be −O2CĊHCH3, rather than −O2CCH2CĊCH2 proposed by earlier workers. This result is in marked contrast to that found in x-irradiated succinic acid, where HOOOCĊCHCH2COOH is the principal fragment. This difference in the radiation damage behavior in succinic acid and its disodium salt is attributed to differences in molecular configuration within the crystal lattice, which allows intermolecular hydrogen-atom transfer to occur in succinic acid, but not in disodium succinate. In the latter compound intramolecular transfer occurs instead. A mechanism for radiation damage in the dicarboxylic acids is proposed.
Deuteron NMR Study of Lithium Hydrazinium Sulfate
Authors: Howell Francis L. , Schmidt V. Hugo
Published: February 2004

The magnetic resonance spectrum of the deuterons in single crystals of LiN2D5SO4 has been studied between 78 and 458°K. The electric quadrupole coupling constants, the asymmetry parameters, and the orientations of the electric field gradient (efg) tensors relative to the crystal axes were obtained at 78°K, 193°K, 25°C, 65°C, and 95°C. The orientations of the efg tensors are considered in relation to nearby atomic sites. At 78°K the spectrum contains distinct lines due to the five deuterons of the N2D5+ ion. Between 110 and 150°K the lines due to the three deuterons of the ND3 group broaden and merge to a single pair which is still visible at 185°C. A small change with temperature in the coupling constant associated with this pair of lines is discussed. The lines due to the two deuterons of the ND2 group broaden and merge to a single pair between 0 and 50°C. These line mergers result from hindered rotation of the ND2 and ND3 groups. Estimates of the activation energies for these motions are made. These changes in the spectrum are related to the results of earlier NMR studies and to previous electrical conductivity and dielectric constant measurements.
Dynamic Jahn–Teller Effect in Trigonally Distorted Cubic Systems
Authors: Stephens P. J.
Published: February 2004

The Jahn–Teller problem in trigonally distorted cubic systems is discussed. Particular attention is devoted to the effects of a trigonal distortion on cubic systems in which a triply degenerate (T) electronic state interacts with a doubly degenerate (e) vibrational mode. The T state is split into singly and doubly degenerate (A and E) states and both “genuine” and “pseudo” Jahn–Teller interactions are present. It is shown how the Jahn–Teller problem can be formulated such that the relation of the trigonal and cubic problems is apparent. This leads to the definition of “isotropic” and “anisotropic” Jahn–Teller parameters, the latter arising from the trigonal distortion and vanishing in the cubic limit. When the trigonal distortion is small, it can be treated by perturbation theory. Explicit calculations are made of the trigonal splitting of the ground triplet state of the cubic vibronic system for several specific cases. It is shown that, to first order, this depends on both the A–E splitting in the trigonal geometry and the “anisotropic” first-order Jahn–Teller interaction. The inclusion of second-order Jahn–Teller terms and other electronic perturbations, such as spin–orbit coupling, is also discussed. The results are employed to complete the analysis of the zero-phonon levels of the T2g3 state of V3+ / Al2O3. The different quenching of the first-order electronic trigonal field and spin–orbit splittings, previously observed, is attributed to the “anisotropy” in the Jahn–Teller interaction. Values for both the “isotropic” and “anisotropic” first-order Jahn–Teller parameters are derived.
Interpretation of ESR Experiments on the Triplet Dianions of 1,3,5-Triphenylbenzene and Triphenylene
Authors: Sommerdijk J. L. , van Broekhoven J. A. M. , van Willigen H. , de Boer E.
Published: February 2004

A theoretical account is given for the ESR experiments on the triplet dianions of 1,3,5-triphenylbenzene (Tpb) and triphenylene (Tp). The calculated ZFS parameters of the unperturbed dianions agree nicely with the experimental values obtained in strong solvating solvents, e.g., diglyme. The triplet spectra obtained in less solvating solvents like MTHF are attributed to the dianions perturbed by two counterions (Tpb2−·2Me+ and Tp2−·2Me+). The ZFS parameters and the energy of the lowest triplet and singlet states have been calculated as function of the counterion positions. Comparing the results of our calculations with experiments, we propose that in Tp2−·2Me+ the counterions are above and below the center of the same noncentral ring. In Tpb2−·2Me+, on the other hand, all experimental and theoretical results suggest strongly that the counterions are on the threefold axis of Tpb. In both cases the cation–aromatic plane distance is estimated between 2 and 3 Å.
Theoretical Study of the Potential Surface for the H4 System by Double-Zeta Configuration-Interaction Calculations
Authors: Rubinstein M. , Shavitt I.
Published: February 2004

A number of shapes of the H4 system have been studied by ab initio full-CI calculations, using a scale-optimized, eight-orbital (1s, 1s′) basis set. The scale optimization consisted of the simultaneous variation of all the orbital exponents, keeping their relative values constant. Potential-energy curves were obtained for the uniform stretching of the square (1B1g, 3A2g, 1A1g, 1B2g), the rectangle with a 5:4 side ratio (1Ag, 3B1g, 1B1g), the tetrahedron (1E, 3T1, 1T2, 1A1), and the linear equidistant configuration (1Σg+, 1Σu+), while a more limited study was made of the rhombus with a 70° apex angle (1B1g, 3B1g, 1Ag). The order obtained for the energy levels of each shape is as listed above. The lowest energy for each shape, relative to the energy of two H2 molecules, was 43 kcal/mole for the linear case, 109 for the rectangle, 142 for the square, 151 for the rhombus, and 188 kcal/mole for the terahedron. It seems unlikely that the square configuration can be a transition state in a four-center exchange reaction H2 + D2→2HD, for the required energy is considerably above that needed to dissociate one hydrogen molecule and carry out a three-center exchange. Possible transition-state shapes which have not been studied include the isoceles trapezoid (suggested by the low energy of the linear form) and the kite.
COS Photoinitiated cis–trans Isomerization of 2-Butenes in the Gas Phase: A Chain Process
Authors: Schmidt Manfred W. , Lee Edward K. C.
Published: February 2004

It is found that 2-butenes cleanly undergo cis–trans isomerization by a chain process when the gaseous mixtures of COS and cis-2-butene (or trans-2-butene) is photolyzed at 2537 Å; no intramolecular doublebond migration in 2-butenes is observed and the process is largely homogeneous. The catalytic chain initiator is proposed to be the ground-electronic-state S atom (3P) which is produced by the photodecomposition of COS. The quantum yield of trans-2-butene resulting from the gaseous mixture of COS (1.0 torr) and cis-2-butene (2.5 torr) is (2.3 ± 0.5) × 102 at room temperature, and the chain length is estimated to be (1 − 3) × 103. The photostationary-state ratio of trans to cis, [(t-B) / (c-B)]Pss, and the initial isomerizationrate ratio, [Rc→t / Rt→c], have been determined to be 2.83 ± 0.06 and 2.8 ± 0.2, respectively. A competitive quenching study has been carried out to determine the efficiencies of various chain terminators. 1,3-Butadiene is found to be the most efficient terminator, more efficient than O2 or mono-olefins by 3 orders of magnitude.
Free-Energy Density near the Critical Point
Authors: Stell G.
Published: February 2004

We show how both the “classical” expression for the free-energy density and Widom's alternative expression can be modified so as to be consistent with known and expected critical behavior in all dimensions. In so doing, we postulate that an increase in the effective range of interaction due to an increase in dimensionality will tend to inhibit density fluctuations.
Photodissociation of NCN3 in the Vacuum-Ultraviolet Production of CN B2Σ and NCN A 3Π
Authors: Okabe H. , Mele A.
Published: February 2004

The photodissociation of NCN3 in the vacuum ultraviolet has yielded CN B2Σ and NCN A 3Π. No NCN 1Π was detected. The fluorescence intensity of the CN violet emission, If / I0, was measured as a function of incident wavelength, λ. The If / I0−vs−λ curve shows structure, indicating that the process NCN3 →  lim hvCN B2Σ + N3, is predissociative. The yield at 1216 Å is estimated to be approximately 2.5% The threshold wavelength of incident photons to yield CN B2Σ is 1685 ± 20 Å. The production of NCN A 3Π is attributed to the formation of N2 A 3Σ, NCN3→ lim hνNCN X3Σ + N2 A 3μ, with the threshold wavelength of 1915 ± 30 Å followed by the sensitized reaction, N2 A 3Σ + NCN3 → NCN A 3Π + 2N2. Bond dissociation energies obtained from threshold energies are D(NC–N3) = 4.2 ± 0.1 eV (= 96 ± 2 kcal mole−1) and D(NCN–N2) = 0.3 ± 0.1 eV (= 7 ± 2 kcal mole−1) where the error limit indicates an experimental uncertainty of the threshold energy measurements. Estimated heats of formation are ΔHf°(NCN3) = 4.7 ± 0.2 eV (= 108 ± 5 kcal mole−1 and ΔHf°(NCN) = 5.0 ± 0.2 eV (= 115 ± 5 kcal mole−1) from which D(N–CN) of 4.3 ± 0.2 eV (= 99 ± 5 kcal mole−1) is obtained. The absorption coefficient of NCN3 in the region 1200 to 2000 Å has been measured. A comparison is made of bond energies of several azide compounds.
Zeeman Populations of Optically Produced Triplet Excitons in Anthracene
Authors: Soos Zoltan G.
Published: February 2004

The steady-state Zeeman populations of optically produced triplet excitons in crystalli e anthracene are obtained for a localized diffusional model. The transition rates among the Zeeman sublevels induced by exciton motion between the two anthracenes, with differently oriented dipolar principal axes, in a unit cell are calculated for an arbitrarily oriented static magnetic field. Triplet exciton motion maintains uniform high-field sublevel populations even in the limit T → 0, thus explaining qualitatively several low-temperature experiments on the delayed fluorescence from collisions of free and trapped excitons. Triplet exciton motion is also shown to account for spin–lattice relaxation at room temperature.
Relativistic and Correlation Effects for Optical Levels of Large Atomic Systems: Application to Tl ii
Authors: Beck Donald R.
Published: February 2004

The effects of relativity and correlation on bound levels of large atomic systems are investigated by using the low-Z Pauli approximation for the Hamiltonian. A linear combination of Slater determinants is used to approximate the wavefunctions. The radials are obtained from the Hartree–Fock–Slater equation with six potential forms, and the coefficients are found by application of the variational principle. Results are obtained for a matrix element involving any two configurations composed of a closed core and two valence spin–orbitals. Application is then made to some of the bound levels of Tl ii. The average error in the energy levels (the ground state is the reference level) is found to be 5% as compared to the average error of 25% in the nonrelativistic results of Beck. Both correlation and relativistic effects are important.
Kinetics of Nucleation: The Steady State
Authors: Gerlach Robert L.
Published: February 2004

The kinetics of steady-state nucleation are treated using a generalized Becker–Döring formulation. The concepts of critical nucleus, dynamic equilibrium, and steady-state nucleation are examined in detail. Two classes of nucleation are defined: (1) rapidly increasing stability nucleation in which the decay probability decreases very rapidly with increasing cluster size in the size region of the critical nucleus and (2) slowly increasing stability nucleation in which the decay probability decreases very slowly with increasing cluster size in the size region of the critical nucleus. It is shown that in the steady state the critical nuclei may be considered to be in dynamic equilibrium with the smaller nuclei only for the first class of nucleation. In addition, the equation for the rate of nucleation, generally accepted as being the rate at which critical nuclei grow to the next larger size, is shown to be valid only for the first class of nucleation in the steady state. The rapidly (slowly) increasing stability nucleation is shown to be characteristic of small (large) cluster nucleation at low (high) temperatures.
Out-of-Plane Force Constants of Benzene
Authors: O'Reilly E. J.
Published: February 2004

It is argued that the out-of-plane force constants associated with the null symmetry coordinates of benzene are zero. On this basis 11 valence force constants associated with the internal coordinates are calculated. These constants do not give rise to inconsistencies when considering redundant coordinates. They reproduce the spectra of the deuterobenzenes within 1.2 cm−1
Collisional Quenching of Competitive Unimolecular Reactions. Vibrational Energy Transfer
Authors: Larson C. W. , Rabinovitch B. S.
Published: February 2004

Two competitive–consecutive unimolecular reaction systems were investigated. An H+olefin chemical activation technique was used to produce highly vibrationally excited polyatomic alkyl radicals (Rp*) in a narrow, nonequilibrium distribution of energy states centered near 45 kcal mole−1. The radicals studied, 3,3-dimethylhexyl-2·* and 3-methylhexyl-2·*, each decompose unimolecularly by several competitive C–C rupture reaction channels to stable olefins and smaller radical fragments; also, isomerization by H-atom transfer to secondary radicals, 4,4-dimethylhexyl-2·* and 4-methylhexyl-2·*, respectively, competes with C–C rupture. The isomerized species retain the vibrational excitation and undergo further consecutive unimolecular reactions including C–C rupture and H-atom isomerization. The spectrum of competitive reactions of Rp* via various paths, each characterized by a different energy threshold, provides a type of competitive chemical reaction “spectroscopy” wherein the amounts of the various decomposition events associated with each threshold act as transducers of the steady-state energy level populations of the reactive species. The competition between collisional stabilization and reaction via the several reaction paths of Rp* was measured for H2 and CF4 heat-bath molecules. The experimental results are compared with calculations made on a stochastic basis. Various collisional transition probability models characterized by different distributions of vibrational energy jump sizes were examined. Based on a step ladder model of transition probabilities, H2 transfers ∼ 400 cm−1 per collision and CF4 >̃ 1600 cm−1 per collision; for an exponential distribution of transition probabilities, a larger amount of vibration energy, e.g., ∼ 500 cm−1 for H2, is appropriate. The conclusions obtained by this technique are independent of information on the magnitudes of the collision cross sections. Some of the more important aspects of the radical chemistry are considered and several disproportionation–recombination ratios were obtained.
EPR Study of the Hole Centers in Phenacite
Authors: Lozykowski H. , Wilson R. G. , Holuj F.
Published: February 2004

The centers created by x and uv irradiation of the single crystals of phenacite (Be2SiO4) at room temperature were studied by the EPR method. Well-documented proofs were offered that the centers involved ·SiO4−3 and ·PO3= radicals. The components of the g factor of the ·SiO4−3 radical are presented in the text. They fit this model well. Also supporting this model are the HFS splittings which were attributed to two Be atoms. The components of the g factor and the HFS tensors of the ·PO3= radical were measured. The HFS tensor fits this description of the center well. The paper following this one attempts to correlate these measurements with luminescence of phenacite.
Luminescence in Phenacite
Authors: Lozykowski H. , Holuj F.
Published: February 2004

Luminescent behavior and optical absorption of phenacite BeSiO4 are compared with the results obtained for x- or uv-irradiated naturally occurring and synthetic phenacite crystals using the EPR method. It is thought that these crystals contain ·PO3= and ·SiO4−3 radicals, and our results indicate that they both act as luminescence centers. Emission bands, which might be assigned to both, have been identified. Thermoluminescence, thermoconductivity, and uv re-excitation were also investigated and compared to EPR results.
Determination of Diffusion Coefficients of Thermal Electrons with a Time-of-Flight Swarm Experiment
Authors: Nelson D. R. , Davis F. J.
Published: February 2004

A typical time-of-flight electron-swarm experiment was modified by replacing the electric-field–ring assembly with a conducting paper cylinder in order to (1) increase the lateral dimension of the swarm region, and (2) remove nonconducting surfaces which become charged with electrons and thereby provide the zero-field condition necessary for thermal electron diffusion and capture studies in gases. A new drift–dwell–drift technique was devised whereby electron swarms were (1) drifted to the center of the swarm region, (2) allowed to dwell there for varying periods of time under field-free conditions, and then (3) drifted to the anode where proper sampling with a differentially pumped electron-multiplier detection system allowed the reconstruction of the swarm time-of-arrival distribution. Diffusion coefficients were determined for thermal electron swarms in nine gases. The mean thermal diffusion-pressure (DtP) values were 0.214, 2.01, 0.885, 0.133, 0.294, 0.178, 0.0151, 0.253, and 0.281 cm2 μsec−1 torr for He, Ne, Ar, H2, N2, CO, CO2, CH4, and C2H4, respectively.
Thermal Expansions of Some Alkali Halides
Authors: Morley G. L.
Published: February 2004

A simple model is presented for the calculation of the equilibrium nearest-neighbor separation R0 of the alkali halides as a function of temperature at atmospheric pressure. Theoretical values of R0 are compared with the experimental data for NaCl, KCl, KBr, KI, CsCl, CsBr, and CsI for the temperature range from room temperature to the melting point. Excellent agreement with experiment is obtained for the potassium halides by using a central-force two-particle potential composed of the Coulomb interaction and a nearest-neighbor exponential repulsive potential.
Threshold Electron-Impact Excitation and Negative-Ion Formation in XeF6 and XeF4
Authors: Begun G. M. , Compton R. N.
Published: February 2004

Electronic excitation and dissociative electron attachment in XeF6 and XeF4 were investigated by studying the products of low-energy electron–molecule collisions in the gas phase. The relative abundances of the major negative ions produced were recorded as a function of the electron-beam energy. Both molecules attached electrons at ∼0 and ∼5 eV and dissociated into a number of negative-ion products. The similar energy dependence of the various ion currents suggested that the fragment ions were competing for the electron attached to XeF6 or XeF4. The threshold electron-impact excitation spectra were determined by means of the “SF6-electron-scavenger” technique. No evidence for low-lying electronic states was found.
Energy Migration in Lanthanide Chelates
Authors: Kleinerman Marcos
Published: February 2004

Intramolecular energy transfer in lanthanide chelates can occur through different paths depending on the nature of the chelate. Contrary to prevailing notions, it is shown that energy transfer does not require the participation of the lowest triplet level of the chelate. The experimental results from a study of solutions of more than 600 chelate systems are consistent with a transfer mechanism via the ligand-excited singlet state, with rate constants ket of about 1011 sec−1 and higher. Transfer via the lowest triplet state may predominate, however, when the rate of energy transfer from the singlet excited level is smaller than the rate of intersystem crossing to the triplet level. Whatever the mechanism, intramolecular energy transfer is generally a very efficient process, regardless of the fluorescence quantum yield of the chelated lanthanide ion. Low quantum yields of sensitized fluorescence are usually the result of radiationless decay processes following intramolecular energy transfer. This quenching is most pronounced in chelates in which the lowest triplet level lies below the emissive level of the lanthanide ion.
Guest-Impurity Heterogeneous Triplet–Triplet Annihilation in Organic Mixed Crystals: Benz[f]indan Guest in Biphenyl and in Fluorene Hosts
Authors: Misra T. N.
Published: February 2004

Delayed fluorescence has been observed in mixed crystals with benz[f]indan guest in biphenyl host and also in fluorene host. At 77°K, the delayed fluorescence intensity shows a second-order dependence on the phosphorescence intensity, indicating the biexcitonic origin of the delayed fluorescence observed. Temperature dependence of intensity and of decay and growth behavior of delayed fluorescence and phosphorescence is similar to that generally observed in mixed-crystal systems where T1G < T1H < S1G < S1H. The thermal energy gap obtained from various Arrhenius plots, however, do not give the spectroscopic energy gap in either case; instead a lower activation energy has been obtained. Evidence is presented that an unknown impurity causes the delayed fluorescence in these cases. Kinetics for such guest-impurity heterogeneous triplet–triplet annihilation is discussed.
Nuclear Spin–Lattice Relaxation in Some Pure Liquids
Authors: Sandhu H. S.
Published: February 2004

The measurements of proton spin–lattice relaxation time T1 have been carried out in pure samples of CH2Br2 and CH2I2 between 0–100°C. The results are compared with Bloembergen, Purcell, and Pound (BPP) theory modified to include the effect of the interaction between nonidentical spins. The results show that intramolecular interaction is predominant in causing relaxation in both liquids. The experimental results are in good agreement with the modified BPP theory for CH2Br2, but not for CH2I2. This discrepancy for CH2I2 could be due to the failure of the simplified model of molecular motion.
Equilibrium Spatial Distribution Function of Reaction Coordinates
Authors: Kostin M. D.
Published: February 2004

A newly derived partial differential equation for the temperature-dependent spatial distribution function of quantum-statistical mechanics is applied to the case of calculating the spatial distribution function along reaction coordinates for a system at equilibrium. In the limit where Planck's constant tends to zero, it is shown that the partial differential equation yields the classical formula for the probability that the chemical complex is at a specified region of reaction coordinates. Explicit formulas are given for the quantum correction to the probability density as a function of temperature for stable chemical complexes, whose total energy is negative, and for unstable chemical complexes, whose total energy is positive.
Spin–Orbit Coupling and Radiationless Transitions in Aromatic Ketones
Authors: Dym S. , Hochstrasser Robin M.
Published: February 2004

The spin–orbital coupling mechanism for nπ*←13A was investigated for several substituted benzophenones by use of polarized absorption spectra and high-field Zeeman studies and was found to be consistent with the scheme already determined for benzophenone itself [S. Dym, R. M. Hochstrasser, and M. Schafer, J. Chem. Phys. 48, 646 (1968)]. Spin-polarization experiments on anthrone in n-heptane at 4.2 and 2°K and benzophenone in pyridine at 2°K showed that excitation into the singlet resulted in the population of the predominantly active spin state, |Tz〉. Examination of low-temperature absorption spectra of a variety of acetophenones and benzophenones revealed band broadening within the triplet manifold, suggesting the presence of higher triplets in many cases. No such evidence for a higher triplet state below the singlet origin could be detected in the spectra of benzophenone and perdeuterobenzophenone. The difference in linewidths for the triplet and singlet origins of benzophenone was measured and interpreted as uncertainty broadening due to strong intersystem mixing; the rate of this process was calculated to be about 1011 sec−1. Additional broadening of the nπ*1, 0–1 carbonyl stretching band was observed and attributed to vibrational relaxation and an increased rate of intersystem crossing from this level. The use of C2υ symmetry labels as an appropriate basis for discussion of aromatic carbonyl transitions is discussed. Finally a mechanism for intersystem crossing in aromatic ketones is given involving direct coupling of modified nπ*1 and nπ*3 states.
On Polytypism and Internal Rotation
Authors: Weltner William
Published: February 2004

The ordered stacking of eclipsed (E) and staggered (S) layers in SiC or ZnS crystals leads to formation of polytype lattices. The short-range forces acting between layers are considered to be analogous to those determining the barriers to internal rotation in molecules. This is applied to an understanding of eclipsing in covalent lattices (SiC, diamond) and in the more ionic lattices such as ZnS. The Madelung energies are calculated for the prevalent polytype structures, and there is a small effect favoring a cooperative formation of the completely eclipsed lattice, supporting Schneer's proposal that the zincblende–wurtzite transition is second-order. Configurational and vibrational entropy effects are not found to contribute to the stabilization of polytype lattices. The progressive distortions in these lattices with increasing numbers of E layers are attributed to “internal-rotation” forces and may be the explanation of polytype formation. A simple model, successfully applied to ethane, is used to calculate possible distortions in the recently prepared hexagonal diamond lattice. An undistorted lattice, not significantly more stable than the cubic structure, was obtained. Electron–nuclear interaction terms dominate over nuclear–nuclear terms, just the opposite of ethane.
Low-Speed Transport of Gas Mixtures in Long Cylindrical Tubes According to the BGK Model
Authors: Shendalman Lawrence H.
Published: February 2004

The interior flow of a binary gas mixture at all densities is analyzed by means of the BGK kinetic theory. A single long cylinder which is often considered part of an ensemble employed to model flow and dispersion of gases in porous media is considered. A linearization that is appropriate for describing low-speed isothermal flow and is necessary to yield linear constitutive relationships provides a pair of integro-differential equations. The formal integration of this set results in two simultaneous nonsymmetric linear integral equations. Two trial functions are chosen to illustrate the utility of the Bubnov–Galerkin method to obtain expressions for the axial flux of each species. Uniform-pressure counter diffusion and the Kramers–Kistemaker effect are considered as specific examples. High- and low-density limiting cases are evaluated analytically using trial functions with optimally chosen coefficients. The flat velocity profiles predict the ratio of the average axial fluxes to be inversely proportional to the ratio of the square roots of the molecular weights for constant pressure diffusion at low and high pressures. The parabolic velocity profile describes the Kramers–Kistemaker effect exactly at low pressures and agrees well with the “dusty-gas” model at the high-pressure limit.
Theoretical Study of the Chemical Shift in Co3+ (d6) Compounds
Authors: Betteridge G. P. , Golding R. M.
Published: February 2004

The NMR chemical shifts of a wide range of diamagnetic cobalt compounds have been interpreted by extending the basic theory of Griffith and Orgel to include calculations of spin–orbit coupling interaction, configurational mixing, bonding effects, and the distortion interaction. The important terms are the bonding effects and configurational mixing. The results tend to indicate a large quenching of the orbital momentum, resulting from a relatively large transfer of spin to the ligands. The value of the quenching within each of the three series of ligands appears to be constant.
Search for Anomalies in the Temperature Dependence of Ultrasonic Velocity in Water
Authors: Senghaphan W. , Zimmerman George O. , Chase C. E.
Published: February 2004

High-resolution measurements of the velocity of 9.9-MHz ultrasonic waves in pure distilled water have been made at about 2300 temperatures between 6° and 81°C. The results can be fitted to an 8th-order polynomial in temperature with scatter of approximately ± 30 ppm and standard deviation 10 ppm. A plot of the deviations of the experimental points from such a polynomial discloses no significant discontinuities or other anomalous behavior. A comparison with previous measurements is made.
Nuclear Quadrupole Coupling Constants and Charge Distributions in Ionic Crystals of the α-NaFeO2 Type
Authors: Peterson G. E. , Bridenbaugh P. M.
Published: February 2004

The 23Na nuclear quadrupole coupling constant is determined for 10 compounds of the α-NaFeO2 type, namely, NaInO2, NaInS2, NaInSe2, NaCrO2, NaCrS2, NaCrSe2, NaTiO2, NaTlO2, NaScO2, and NaAlO2. The electric field gradient is calculated on the basis of a point-charge model, and charge distributions are obtained for the complex ions. The charges on the metal ions appear to correlate with the average electronegativity difference between the metal and chalcogen.
Fluctuations in Open Systems
Authors: Nicolis G. , Babloyantz Agnessa
Published: February 2004

The probability of occurrence of fluctuations around nonequilibrium steady states or states slowly varying in time is discussed from a kinetic viewpoint. The limitations of the thermodynamic theory of fluctuations are investigated. It is shown that in a large class of far-from-equilibrium systems, described by a discrete set of variables, it is possible to extend the classical Einstein fluctuation formula, provided one uses suitable steady-state parameters rather than equilibrium quantities. This conclusion is shown to be in agreement with the results of recent work by Prigogine and Glansdorff.
Self-Consistent Molecular-Orbital Methods. I. Use of Gaussian Expansions of Slater-Type Atomic Orbitals
Authors: Hehre W. J. , Stewart R. F. , Pople J. A.
Published: February 2004

Least-squares representations of Slater-type atomic orbitals as a sum of Gaussian-type orbitals are presented. These have the special feature that common Gaussian exponents are shared between Slater-type 2s and 2p functions. Use of these atomic orbitals in self-consistent molecular-orbital calculations is shown to lead to values of atomization energies, atomic populations, and electric dipole moments which converge rapidly (with increasing size of Gaussian expansion) to the values appropriate for pure Slater-type orbitals. The ζ exponents (or scale factors) for the atomic orbitals which are optimized for a number of molecules are also shown to be nearly independent of the number of Gaussian functions. A standard set of ζ values for use in molecular calculations is suggested on the basis of this study and is shown to be adequate for the calculation of total and atomization energies, but less appropriate for studies of charge distribution.
Apparent Molar Volumes and Osmotic Coefficients in Dilute Electrolyte Solutions
Authors: Indelli A. , De Santis R.
Published: February 2004

The apparent molar volumes and the osmotic coefficients of different types of electrolytes in dilute aqueous solutions are calculated on the basis of the Mayer theory. The sums which appear in the equations are calculated for different values of the ionic strength and suitable functions of the sums are represented graphically. The results show that the Mayer theory accounts in a satisfactory way for the molar volumes and the osmotic coefficients in very dilute solutions, with the distance of closest approach used as the only adjustable parameter. The limits of this agreement of the theory with the experiments are discussed.
Effect of Temperature on the Reactions of Electrons during the γ Radiolysis of Liquid n-Propanol
Authors: Jha K. N. , Freeman G. R.
Published: February 2004

Purified n-propanol was irradiated at temperatures from − 120° to + 264° and G(H2) increased from 4.1 to 5.9 over this range. Solutions of electron scavengers (acid and nitrous oxide) were irradiated at − 120°, + 25°, and 140°. The values of G(H2) for absolutely pure n-propanol were estimated to be 4.3 at − 120°, 4.9 at 25°, and 6.0 at 140°. The value of G(total ionization) ≈ 4.3 estimated at − 120° was assumed to be independent of temperature. The free-ion yield was G(esolv−)fi = 1.1 at − 120°, 1.2 at + 25°, and 0.8 at 140°. A reaction mechanism was proposed and the measured hydrogen and nitrogen yields were subjected to kinetic analysis; homogeneous kinetics were used for the reactions of the free ions and nonhomogeneous kinetics were used for the reactions in spurs. The activation energy of dielectric relaxation (6.1 kcal/mole) is greater than that of diffusion (∼ 4.5 kcal/mole); it was necessary to use a time-averaged value of the dielectric constant for the smaller ion–electron separation distances at − 120°, but the static value was satisfactory elsewhere. The reaction esolv−→→n-C3H7Osolv− + H has a rate constant of about 5 × 105 sec−1 at 25° and has an activation energy of 4.5 kcal/mole and an entropy of activation of − 19 cal/deg·mole. Only a fraction of the electrons that undergo geminate neutralization lead to hydrogen formation; the value of the fraction is 0.29 at − 120°, 0.52 at + 25°, and 0.63 at 140°.
Thermodynamics of Rare-Earth–Carbon Systems. II. The Holmium–Carbon and Dysprosium–Carbon Systems
Authors: Balducci G. , Capalbi A. , De Maria G. , Guido M.
Published: February 2004

The vaporization of the systems holmium–carbon and dysprosium–carbon has been studied by means of the Knudsen-effusion–mass-spectrometric technique. In addition to the gaseous atoms, dicarbide and tetracarbide molecules were identified, and their bond and atomization energies were obtained. The values found are practically coincident for the two systems. A determination of the heats of formation for HoC2(s) and DyC2(s) has also been made.
Thermodynamics of Rare-Earth–Carbon Systems. III. The Erbium–Carbon System
Authors: Balducci G. , De Maria G. , Guido M.
Published: February 2004

A sample of erbium dicarbide has been prepared by direct reaction of the elements in a graphite lined molybdenum crucible. The vaporization of the dicarbide has been studied by means of the Knudsen-effusion–mass-spectrometric technique over the temperature range 1750–2500°K. The vapor species above ErC2(s) were found to be Er(g) and, as minor constituent, ErC2(g). From the vapor-pressure equation the enthalpy of vaporization of ErC2(s) to Er(g) was found to be ΔH°298 = 101.1 ± 0.4 kcal/mole. From this value the standard heat of formation of C-rich ErC2(s) was calculated to be − 18.5 ± 0.4 kcal/mole. The stability of ErC2(g) molecule has also been determined.
Ab Initio Studies on BeH+ (X1Σ+)
Authors: Brown Richard Edwin
Published: February 2004

A 38-configuration wavefunction was calculated for BeH+(X 1Σ+) at R = 2.48 bohr, the experimental equilibrium R distance, to give an energy of − 14.92196 hartree and an electric moment of 1.578 a.u. This function gave 79% of the binding energy and 73% of the correlation energy. Comparative studies were made with the important zeroth-order configurations, Be+(2S)H(2S), Be+(2P)H(2S), Be2+(1S)H−(1S), and Be(1S)H+, and with previous work on the LiH(X 1Σ+) molecule. The polar nature of BeH+ appears to be best described in terms of a valence-bond picture, where (2s–2pgu) hybridization amply explains the charge transfer.
Reaction of Carbon Dioxide with Atomic Oxygen and the Dissociation of Carbon Dioxide in Shock Waves
Authors: Clark T. C. , Garnett S. H. , Kistiakowsky G. B.
Published: February 2004

The reaction of oxygen atoms with carbon dioxide has been investigated from 2800–3200°K using the decomposition of N2O as a source of oxygen atoms: O+CO2→CO+O2 (1). Over the temperature range, the value of k1 was found to be logk1 = − 10.59(± 0.79) − (32 500 ± 10 900) / 4.58 T cm3/particle sec, which is much larger than the recent literature value. The addition of molecular oxygen decreased the observed rate of CO2 removal in Reaction (1). The decomposition of CO2 in the second-order pressure region in neon diluent was investigated over the temperature range 2900–4000°K, and it was shown that the contribution of Reaction (1) to the gross decomposition rate is significant, but does not account for the anomalously low activation energy of CO2 decomposition. The corrected first-order rate constant for the reaction CO2+Ne→CO+O+Ne (2) at a density of 1.35 × 1018particle/cm3, is logk2 = 7.71 (± 0.36) − (74 100 ± 6300) / 4.58T sec−1. The rate constant for the reaction O3+NO→N+O2 (3) is estimated as k3 = 1.5 × 10−14cm3/particle·sec at 3000°K, and a suggested upper limit for the rate constant of the reaction NO+CO2→NO2+CO (6), is k6 ≤ 5 × 10−15cm3/particle·sec at the same temperature.
Ternary Hydrides Possessing the Cubic Perovskite Structure. II. Vibration Spectra by the Inelastic Scattering of Cold Neutrons
Authors: Maeland Arnulf J.
Published: February 2004

Inelastic neutron scattering techniques in recent years have been employed in the study of lattice dynamics in solids. These techniques are particularly suited for the study of hydrogen-containing compounds because of the large incoherent scattering cross section of hydrogen and the absence of selection rules. In the present experiments we have obtained the vibration spectra of BaLiH3 and SrLiH3 by the energy-gain scattering of cold neutrons. Both these compounds have previously been shown to have the perovskite structure. The observed neutron time-of-flight spectra have been used to calculate approximate frequency distributions for each compound. Peak frequencies in cm−1 are: SrLiH3, 1130, 950, 660, and 340; BaLiH3, 920, 780, 650, and 320. These frequencies have been assigned in order of decreasing values to the Li–H stretch, the Li–H F2u bending mode, the Li–H F1u bending mode, and Sr–(LiH3) or Ba–(LiH3) stretch. Infrared data have also been obtained for both compounds as well as for the isostructural EuLiH3.
Infrared Lasers Resulting from Photodissociation of Cs2 and Rb2
Authors: Sorokin P. P. , Lankard J. R.
Published: February 2004

Infrared laser beams at 3.095 μ in helium-buffered cesium vapor and at 2.254 and 2.293 μ in helium-buffered rubidium vapor have been produced by irradiating these gases with beams from various giant pulse lasers. The mechanism of excitation is shown to involve two-step photodissociation of Cs2 and Rb2 molecules.
Laser–Raman Spectra of PH4I, PH4Br, and PH4Cl
Authors: Rush J. J. , Melveger A. J. , Lippincott E. R.
Published: February 2004

Laser–Raman spectra have been measured at 23°C for PH4I and PH4Br and at ∼5°C for PH4Cl. Peaks observed in the spectra of all the halides are assigned to “internal” PH4+ modes, ν1(A1g), ν2(A1g, B1g), ν3(Eg), and ν4(Eg) and to torsional [L2(Eg)] and translational [T1(A1g)] lattice modes. The torsional frequencies are 326, 343 and 363 cm−1, respectively, for PH4I, PH4Br, and PH4Cl, while the translational mode is observed at 55.5, 75, and 112 cm−1, respectively. Barriers to rotation are calculated from the torsional frequencies on the simple but unrealistic assumption of a fourfold cosine-type potential for the PH4+ ions. The barriers obtained are 6.7, 7.4, and 8.2 kcal/mole, respectively, for PH4I, PH4Br, and PH4Cl. The spectral results and calculated barriers are compared in some detail with previous optical, NMR, and neutron scattering results for both the phosphonium and ammonium halides. Suggested differences in force fields between the ammonium and phosphonium salts are discussed.
Absolute Chemiluminescent Reaction Rates for Emission of the O2 Herzberg Bands in Oxygen and Oxygen–Inert-Gas Afterglows
Authors: McNeal R. J. , Durana Stephen C.
Published: February 2004

Chemiluminescent reaction rates for emission of the O2 Herzberg bands in the O2 and O2 + He or Ar afterglows have been measured in a discharge-flow system. Absolute intensities were measured between 260.0 and 420.0 nm at 0.5-nm resolution with a scanning spectrometer, and oxygen-atom densities were determined by NO2 titration. Intensities were obtained as a function of atom density, O2 pressure, and partial pressures of added nonreactive gases. The integrated intensity of the Herzberg band system is proportional to [O]2 and independent of O2 pressure between 1 and 10 torr. The A 3Σu+ state of O2 is quenched by ground-state O2, and the Herzberg bands are stronger in mixtures of O2 with He or Ar than in pure O2. The chemiluminescent reaction rate coefficient is 2.5 × 10−21 cm3/sec in O2 and is larger by about a factor of 10 in He or Ar. The vibrational population distribution is shifted to lower vibrational levels of O2(A 3Σu+) in the O2–inert-gas mixtures than in pure O2.
Second-Order Calculation of Vibrational Energy Transfer
Authors: Yao H. Y. Sun , Yao S. J.
Published: February 2004

A second-order calculation of the vibrational energy transfer is performed within the framework of classical mechanics. The ratios of the second-order to the first-order energy transfer values are expressed as functions of two parameters ̃M and 2Lω / V0. The dependence of these ratios on ̃M is linear, while the dependence of these ratios on 2Lω / V0 does not appear to be very sensitive. We conclude that the magnitude of the mass factor ̃M is the decisive factor over the validity of Rapp's first-order calculation.
Molecular Structure of Allene from its Proton NMR in a Nematic Solvent
Authors: Sackmann E.
Published: February 2004

The proton NMR spectrum of allene dissolved in the nematic mesophase of p,p′-di-n-hexyloxyazoxy-benzene has been obtained at 75°C. Computer analysis of this spectrum gives the parameters of the spin Hamiltonian. The structure of allene has been determined from the elements of the spin Hamiltonian assuming that the two CH2 planes are perpendicular to each other. It is found that the ratio of the geminal H–H bond length to the C–H bond length is 1.7133 and is in good agreement with the electron diffraction value of 1.7180. Again in comparison with electron diffraction, the C–C bond length is too short relative to the C–H bond length by 4%. This shortness could not be completely explained on the basis of molecular vibrations.
Molecular Quadrupole Moment, Molecular Magnetic Susceptibilities, and Molecular g Values in Benzene
Authors: Shoemaker R. L. , Flygare W. H.
Published: February 2004

The molecular quadrupole moment in benzene is derived from the values in fluorobenzene. The resultant moment in benzene perpendicular to the molecular plane is Q‖ = − (5.6 ± 2.8) × 10−26esu·cm2. The second moment of the electronic charge distribution in benzene is also given as 〈a2〉 = 〈b2〉 = (60.1 ± 1.5) ± 10−16cm2 and 〈c2〉 = (7.7 ± 1.6) × 10−16cm2. The molecular g values are gaa = gbb = − (0.068 ± 0.025) and gcc = + (0.053 ± 0.017). The diamagnetic and paramagnetic susceptibilities are χaad = χbbd = − (286 ± 10) × 10−6, χccd = − (508 ± 20) × 10−6, χaap = χbbp = (251 ± 10) × 10−6, and χccp = (413 ± 15) × 10−6, all in units of erg/G2·mole. The molecular quadrupole moment and the second moments of the electronic charge distributions are compared to similar molecules.
SCF MO CI Studies of Contact Nuclear Spin–Spin Coupling. I Pi-Electron Coupling between Protons
Authors: Barfield Michael , Reed Joann J.
Published: February 2004

A theoretical study of π-electron coupling between protons is presented in terms of the nuclear spin–spin coupling formalism which uses the generalized product approximation with intergroup configuration interaction. Calculations are performed for several unsaturated and aromatic hydrocarbons in which the π-electron systems are described by self-consistent-field molecular-orbital (SCF MO) wavefunctions in the variable-β modification of the Pariser–Parr–Pople method. Coupling constant results are found to be quite sensitive to the inclusion of configuration interaction (CI) in the triplet manifold of the π-electron system. Calculated values are compared with the experimental results.
Field Emission through Hydrogen and Helium Layers
Authors: Halpern B. , Gomer R.
Published: February 2004

Field emission through multilayers of H2, D2, HD, and He adsorbed on tungsten field emitters has been studied at 4.2–1.6°K and pressures of 10−5–10−7 torr of these gases under ultrahigh-vacuum conditions. In the case of H2, HD, or D2, small work function increases relative to the chemisorbed layer were obtained, indicating a barrier to the solution of free electrons in these layers. In the case of He an apparent decrease in work function was observed for vacuum work functions of 5–3.5 eV (obtained by Ba or O2 adsorption). The effect is explained in terms of a second-order resonance transition in which a He electron makes a virtual transition from the ground state to the 2p level, and then tunnels; an electron from the metal then fills the He ground state. In this way horizontal tunneling occurs, but avoids the strong repulsive He potential which would be seen by a metal electron tunneling normally. The mechanism fails when the work function is too low, or when the broadened He 2p level is raised above the Fermi level of the metal. The effect of chemisorbed layers on the physisorption of He on W is shown to be considerable; He physisorption is strongest on oxygen covered, and least on Ba-covered surfaces.
Optical Spectra of IrCl62− in Single Crystals of Cs2ZrCl6, Cs2HfCl6, and K2SnCl6 at Low Temperatures
Authors: Douglas I. N.
Published: February 2004

The optical spectra of IrCl62− in single crystals of Cs2ZrCl6, Cs2HfCl6, and K2SnCl6 have been studied at helium temperature. Both molecular-orbital theory and ligand-field theory are used to interpret the results. The rich vibrational structure observed is analyzed in terms of the vibrational modes of the IrCl62− complex. Several long progressions involving up to five quanta of the completely symmetric vibration, γ1g, are observed throughout the spectrum, indicating that the transitions are to states in which the Ir–Cl bond length has been changed.