bluuebunny/crossref_metadata_2025 huggingface dataset:
Trajectory Studies of Hot-Atom Reactions. I. Tritium and Methane
Authors: Bunker Don L. , Pattengill Merle D.
Published: February 2004

We have studied the reactions of T+CH4 and T+CD4, treating these as six distinct particles, using a variety of potential energy surfaces subject to the restriction that only one methane hydrogen at a time is reactive. Our principal findings are: (1) This trial assumption about the potential is unjustified. Substitution (products CH3T+H and CD3T+D) involves strong interactions between at least four atoms. (2) There were no inertial isotope effects of any kind when CH4 was replaced by CD4. (3) From (2) and the details of the trajectories, there is suggestive but not conclusive evidence that substitution in CH4 proceeds by Walden inversion. (4) Abstraction (products CH3+HT and CD3+DT) is direct and concerted and occurs at relatively low energy. In our calculations it had a maximum cross section of 3.5 Å2 for a reactant translation energy of 65 kcal. At sufficiently high energy it is a stripping reaction. (5) About half the abstraction product energy is translational; the remainder appears as internal energy of both HT and CH3. When substitution occurs as a three-centered process, it deposits about 70 kcal in CH3T. Fragmentation is an important process above 100-kcal reactant energy.
Boundary Layer Effects on Chemical Kinetics Studies in a Shock Tube
Authors: Lifshitz Assa , Bar-Nun Akiva , de Boer P. C. T. , Resler E. L.
Published: February 2004

The effect of the cold boundary layer on kinetic parameters was studied behind reflected shocks in a single pulse shock tube. By inserting hollow cylinders into the test section, parallel to the tube walls, the surface relative to the volume was increased, and the lowering of the chemical conversion was determined as a function of the surface per unit volume. The test reaction studied was the unimolecular decomposition of perfluorocyclobutane (C4F8→2C2F4). It was shown that the lowering of the chemical conversion was proportional to the surface-to-volume ratio in the test section. A hypothetical cold layer δc in which no conversion occurred was found to be around 0.2 mm. It was concluded that in a shock tube of ∼ 2-in. i.d., at an initial pressure of p1 ≥ 100 torr the cold boundary layer has very little effect on kinetic parameters obtained in a single-pulse shock tube. At lower pressures and in smaller diameters shock tubes these effects should be considered when interpreting kinetics results.
Kinetics of Vaporization of Zinc Single Crystals
Authors: Mar Raymond W. , Searcy Alan W.
Published: February 2004

The rates of vaporization from (0001) and (101̄0) surfaces of zinc single crystals have been measured by the torsion–Langmuir method and have been compared to rates of vaporization of zinc from torsion effusion cells. A small concentration of impurities lowers the vaporization coefficient αυ for the (0001) surface to 0.7, but αυ for the (101̄0) surface of the same material and for the (0001) surface of high purity crystals are unity. The concentration of dislocations that were active in causing thermal etching of the (0001) surfaces was of the order of 105/cm3 for the samples tested. These experimental results are not in accord with the prediction of the Hirth and Pound model for metal vaporization that αυ = 13 for low index, low defect surfaces.
Effects of Ring Geometry on Triplet and Singlet Energies of the Metal Mesoporphyrins
Authors: Fischer Mark S. , Weiss Charles
Published: February 2004

Correlations were found between the hole size of 10 metalloporphyrins, determined by x-ray crystallography, and the ionic radius of the central metal atom, derived from a new compilation of ionic radii as a function of coordination number, spin state, and valence. These correlations were used to define representative porphyrin geometries for each of three different hole sizes. We recommend the use of these geometries for future molecular-orbital calculations. Molecular orbital calculations based on these porphyrin geometries predict a decrease in the phosphorescence energy as the hole size increases, a prediction in agreement with data in the literature on metal mesoporphyrins. Experimental excitation energy of the singlet Soret (but not the visible) band also decreases with increasing hole size, a result not given by calculations using either four-orbital or extensive configuration-interaction models. Electronegativity has a statistically significant effect on the lowest excited singlet and, to a lesser extent, the lowest triplet energy. Changes in hole size produce little change in calculated bond order, indicating that structural changes induced by increased hole size are due to strain induced in sigma bonds. There is no correlation between either singlet or triplet energy and the charge density of the central metal atom as determined by previous molecular-orbital calculations. Thus, the primary effect of changing metals on the lowest triplet state of metal porphyrins is due to effects on the pi electrons of the geometric distortion of the porphyrin ring imposed by the steric requirements of the central metal atom. The effect of the central metal on the singlet energies remains to be fully explained.
Photoelectron Emission by Solutions. II
Authors: Delahay Paul , Chartier Pierre , Nemec Ladislav
Published: February 2004

Experimental conditions were determined for study of photoelectron emission (PEE) by aqueous solutions of sodium and lithium ferrocyanide. PEE currents for emission in pure water vapor were corrected for an appreciable Schottky effect and for electron back scattering. The Thomson equation yielded emission currents of the correct order of magnitude, but average velocities of electrons at emission, deduced from this equation, were incorrect. This result is explained by failure to consider in the Thomson equation the cross section for electron–water-molecule collisions and the energy dependence of this quantity. The error arising from electron attachment to water molecules in the gas phase is evaluated. The PEE spectral response is a sigmoid curve in the 186–214-nm explored range. This shape is accounted for by fluctuations in solvation structure and their effect on the energy required for PEE. The spectral response curve yields the vertical work function of the emitter in solution, i.e., approximately 6.2 eV for ferrocyanide in aqueous solution (0.03–1.3M). This is believed to be the first direct determination of its kind for species in solution. The difference between the vertical work function and the energy for charge transfer to solvent (CTTS) is calculated from the Platzman–Franck polaron model. The energy for PEE is also calculated directly from this model. Agreement with experiment is satisfactory, but the assignment of the CTTS band for ferrocyanide is uncertain. Theory and previous work on CTTS spectra suggest PEE by other systems, e.g., the varified PEE by lithium iodide in acetonitrile.
De-excitation Rate Constants for Helium Metastable Atoms with Several Atoms and Molecules
Authors: Schmeltekopf A. L. , Fehsenfeld F. C.
Published: February 2004

The de-excitation rate constants for He(21S) and He(23S) metastables have been measured with the following gases: Ne, Ar, Kr, Xe, H2, D2, N2, CO, NO, O2, CO2, NO2, NH3, SF6, CH4, C2H6, C3H8, and C4H10. These rate constants were measured in a flowing afterglow system using the neon 5689.8 and 7032.4-Å emission lines as detectors for the He(21S) and He(23S), respectively. In all of the above cases the He(21S) rate constants were several times faster than those of the He(23S).
Vibrational Spectra of Gaseous 1,1,1-Trifluoro-2-butyne
Authors: Tuazon Ernesto C. , Fateley William G.
Published: February 2004

The infrared and Raman spectra of gaseous CH3C≡CCF3 have been recorded and all of the fundamental frequencies have been identified. A normal coordinate analysis indicates appreciable mixing among the symmetric CF3 vibrations and the C–C stretchings. The values of the Coriolis coefficients, ζ9 and ζ10, are consistent with those of a freely rotating methyl group. Strong Coriolis coupling has been suggested as the cause for the sharp doublet contour of the ν16 band in the Raman spectrum. The high intensities of some combination and overtone bands are explained in terms of Fermi resonance with fundamental levels. Thermodynamic functions at several temperatures have been determined in the rigid-rotor harmonic-oscillator approximation.
Correlation Function Approach to Radiationless Transitions
Authors: Fischer Sighart
Published: February 2004

The rate constant for radiationless transitions is expressed in terms of a correlation function similar to the Kubo formulas for transport coefficients. A strong coupling theory for the interaction of the electron with the molecular vibrations is adopted to analyze the time dependent correlation function. For a large molecular system, rapid convergence of this function is found due to strong interference effects. This fact is taken into account by applying the saddle point method for the time integration. The results directly show the dependence of the rate constant upon the energy difference between the relevant electronic states as well as the dependence upon the displacement and distortions of the adiabatic potentials.
Absorption, Fluorescence, and Energy Levels of Ho3+ in LaF3
Authors: Caspers H. H. , Rast H. E. , Fry J. L.
Published: February 2004

Optical absorption and fluorescence spectra of Ho3+ in LaF3 have been obtained between 2500 and 21 000 Å. From an analysis of the data, it was possible to assign 2J + 1 Stark components to the observed states below 26 000 cm−1. A least-squares fit of 22 assigned J levels, with the inclusion of configuration interaction, gave a rms deviation of 29 cm−1 between the calculated and experimental barycenters with the parameter values F2 = 423.9 cm−1, F4 = 63.37 cm−1, F6 = 6.817 cm−1, ζ4f = 2144 cm−1, α = 20.34 cm−1, β = − 578.5 cm−1, and γ = 1267 cm−1. The observed line groups above 30 000 cm−1 agree with the calculated free-ion levels.
Theory of Phenomenological Coefficients in Solid-State Diffusion. II. Cluster Expansion for Ionic Crystals
Authors: Allnatt A. R. , Rowley L. S.
Published: February 2004

Previously derived time correlation formulas for phenomenological coefficients of solid-state diffusion are studied. The model is that the system evolves by the master equation of a Markoff process and is characterized by known defect-configuration dependent jump probabilities. To study the effect of defect interactions on matter transport the cluster and diagram expansion methods of equilibrium theory are used to convert the expressions to series expansions in powers of defect concentrations, the migration mechanisms being vacancy and interstitial. The coefficients of the expansion are cluster summations dependent on concentration through equilibrium correlation functions. To obtain convergent summations for ionic crystals a further chain summation step like that of Mayer ionic solution theory, as well as an additional bond summation of a different kind, are necessary. Formal results are given for ionic conducting crystals whose defect composition is arbitrary except that there are no macroscopic amounts of vacancy-migrating impurities (although the results suffice for substitutional tracer diffusion in such crystals). A preliminary classification into cycle, two defect, three defect, ··· diagrams and future applications are briefly indicated.
Theory of Phenomenological Coefficients in Solid-State Diffusion. III. Limiting Law Conductance for Pure Ionic Crystals
Authors: Allnatt A. R. , Rowley L. A.
Published: February 2004

The cycle diagrams of the previously derived cluster expansions are studied in detail for pure ionic crystals containing Schottky defects. The chain diagram approximation for the equilibrium pair correlation function is employed throughout. Some diagrams are then zero and the remainder yield, in the continuum limit, the Onsager limiting law for the relaxation effect for the conductance at zero frequency. The coefficient of the correction of lowest order in κa to this limiting law which arises when the cycle diagrams are evaluated allowing for the discreteness of the lattice is determined for the NaCl structure. The predictions of this correction are compared with exact numerical evaluations of the cycle diagrams. The corrections from the time-dependent correlation terms are well approximated by the analytic result and are very close to those predicted by the Pitts ion-size correction for electrolyte solutions. However, the remaining corrections from the time-independent correlation terms are but poorly represented by the analytic result and tend to cancel the other corrections. It is concluded that the major approximation in the use of the Onsager limiting law is the use of the chain diagram approximation for the pair correlation function (equivalent to the Debye–Hückel linearized result in the continuum limit) and this is probably relatively more important than effects arising from the discreteness of the lattice or corrections arising from the dynamics of vacancy pairs, triplets, etc. Further numerical study of this point is required. The generalization of the results to Frenkel defects is considered briefly but without detailed results.
Molecular Motion in Liquids: On the Prevalence of Large-Size Rotational and Translational Diffusion Steps
Authors: Rothschild Walter G.
Published: February 2004

It is shown experimentally (Fourier inversion of vibrational band contours) that the molecular orientational motion in some representative common liquids [methylene chloride (CH2Cl2), methyl iodide (CH3I), chloroform (CHCl3), cyclohexane (C6H12)] appears to consist of angular jumps of the order of 20° to 60°. During a jump, which is completed in times of the order of 0.4 × 10−12 sec, the molecules are observed to rotate as if they were in their vapor phase, regardless of whether or not the orientational motion involves the tilting of a permanent dipole moment or of the size of the molecule. The infrared data are compared with results from nuclear magnetic relaxation studies. It is seen that the rotational motion in liquids, as it is observed by these techniques, is not sensitive to molecular association, weak hydrogen bonding, etc. On the basis of the extended Torrey model of intermolecular spin–lattice relaxation, it appears that molecular translational diffusion in a large variety of liquids, some of them strongly associated, occurs by a jump of the order of a molecular diameter after a trapping time of the order of 10−11–10−10 sec. As far as can be predicted, liquids which obey a classical diffusion model of very small diffusion steps would be the exception rather than the rule. The possibility is discussed that a dimer is the diffusing entity in acetic acid.
Energy-Transfer Processes in Monochromatically Excited Iodine Molecules. IV. Collisional Release in υ′ ≃ 50
Authors: Steinfeld J. I. , Schweid A. N.
Published: February 2004

Quenching and vibrational energy-transfer cross sections are reported for the B state of I2, excited into its 50th and 53rd vibrational levels, colliding with the monatomic gases. Vibrational energy transfer is less efficient than in the 43rd level of this same state (see preceding paper). This is attributed to a competition by electronic quenching, which proceeds via the collisional-release mechanism suggested by Callear.
Theory of the Hall Effect in Aromatic Hydrocarbon Crystals
Authors: Munn R. W. , Siebrand W.
Published: February 2004

The Hall effect is studied for a triangular model of a molecular crystal, with the quadratic electron–phonon interaction previously applied to triplet exciton diffusion coefficients and carrier drift mobilities. General expressions are derived for the Hall mobilities in three different limits of perturbation theory, corresponding to slow-electron hopping, slow-phonon hopping, and slow-phonon coherent transport. The results are generalized to anisotropic lattices and it is shown that the Hall mobility may be anomalous in sign in each of these limits, depending on the signs of the intermolecular exchange intergrals. The results of model calculations are compared with experimental Hall mobilities for anthracene. Of the conflicting sets of experimental data, one is chosen to analyze the contributions of various lattice triangles to the observed Hall mobilities. The analysis yields results similar to those of the previous analysis of drift mobilities and shows the necessity of considering more than one transport mechanism.
Spin–Orbit and Crystal-Field Parameters for the Ground Term of Nd3+ in CaWO4
Authors: Karayianis N. , Farrar R. T.
Published: February 2004

A theoretical analysis is made of the I4 ground term of Nd3+ in CaWO4 to determine the spin–orbit and (S4 point-group symmetry) crystal-field parameters for this term. The analysis is carried out by diagonalyzing the Hamiltonian HE + HX in the ground term where HE is the parametrized effective spin–orbit interaction HE = Σλn(L·S)n developed by one of the authors and HX is the usual crystal-field interaction HX = ΣBkm†Ckm. Frequency and polarization measurements are taken of the absorptions to the I15/24 and I13/24 multiplets and are compared with data found in the literature in order to establish the symmetry and energy of the ground term levels. By varying the spin–orbit and crystal-field parameters, λn and Bkm, an identification of all 26 levels for the ground term is obtained with a standard deviation between theory and experiment of 4.33 cm−1. The empirical spin–orbit parameters λ1 and λ2 agree to within 2.5% of those predicted previously, and g‖ and g⊥ for the ground state agree to within 0.2% and 1.9% of those reported, thus further establishing the meaningfulness of the crystal-field parameters obtained. For the I4 ground term of Nd3+ in Na-compensated CaWO4 we find λ1 = 294.651, λ2 = − 2.4186, λ3 = 0.04172, B20 = 548.81, B40 = − 942.41, B44 = 1004.66, B60 = − 17.12, B64 = 946.84 + i1.34 cm−1, and obtain theoretical values of g‖ = 2.029 and g⊥ = 2.587 for the ground state. Theoretical g values for all the ground term levels are also calculated, and ζ and F2 values for the 4f3, Nd3+ configuration of 876.9 and 327.4 cm−1, respectively, are obtained.
Positive Ion Scavenging in the Radiolysis of Liquid Chloroform
Authors: Baxter J. N. , Bibler N. E.
Published: February 2004

The effects of the positive ion scavengers NH3 and n-C4H9OH, on the organic product yields in the radiolysis of liquid CHCl3 at 25°C were investigated. As the solute concentrations increase, the values for G(CH2Cl2), G(C2HCl5), and G(C2Cl6) increase, G(sym-C2H2Cl4) and G(C2Cl4) remain unchanged, and G(CCl4) decreases. These results are consistent with a free radical mechanism for all the radiolysis products except CCl4. The decrease in G(CCl4) indicates that CCl4 is formed by a secondary positive ion reaction. The concentration dependence of this decrease can be described by the expression G(CCl4)O−G(CCl4)S=G(CCl4)O((α[S])1/2/{1+(α[S])1/2}), where G(CCl4)O is the 100-eV yield in the absence of a scavenger, and α is an empirical parameter proportional to the reactivity of the solutes, and [S] is the concentration of the scavenger in moles per liter. Values of α were determined for each solute and compared to published values determined in cyclohexane. In chloroform, values for α were 4–19 times larger than in cyclohexane suggesting that the solutes are more efficient in the alkyl halide than in the hydrocarbon. One mechanism accounting for this efficiency increase is the formation of anions by the free electrons in the alkyl halide. This anion formation increases the average lifetimes of the ions in the alkyl halide and hence the efficiencies of the scavengers. The increase in the 100-eV yields of three of the products formed by the free radical mechanism indicates that the presence of a cation scavenger increases the radical yields. Thus, some of the charge neutralizations in pure CHCl3 do not produce radicals. This observation is in accord with the existence of the collisionally stabilized CHCl3− anion in the irradiated solutions. It is suggested that this anion may be an intermediate in the formation of CCl4.
Vibrational Energy Distribution in the Dichloroethylene Photoelimination Chemical Lasers
Authors: Berry Michael J. , Pimentel George C.
Published: February 2004

The HCl laser emission caused by photolysis of the three dichloroethylenes has been examined in detail. All three emit in 3 → 2, 2 → 1, and 1 → 0 vibrational transitions, and 1,1-dichloroethylene also emits in the 4 → 3 transition. The highest-gain transition at 15°C is P3→2(5) for 1,1-dichloroethylene, P2→1(5) for cis-1,2-dichloroethylene, and P2→1(6) for trans-1,2-dichloroethylene (100 torr total pressure with 200 to 1 ratio of argon to dichloroethylene). Equal-gain temperature measurements give population ratios N3 / N2 = 0.87 ± 0.02 for the 1,1 isomer, N2 / N1 = 0.81 ± 0.02 for the cis-1,2 isomer, and N2 / N1 = 0.70 ± 0.02 for the trans-1,2 isomer. These ratios are all below unity, as was found in the chemically activated trifluoroethane HF elimination laser. Further, the data show that the HCl vibrational mode is decreasingly significant in the sequence 1,1 > cis-1,2 > trans-1,2. The results might be attributable, in part, to competition between α, α and α, β elimination. Even so, the differing HCl excitation may indicate, by microscopic reversibility, a stereospecific influence of HCl vibration in the reverse, addition reaction between HCl and chloroacetylene.
Orientation Dependence in Triplet–Triplet Energy Transfer: Analysis of Magnetophotoselection Experiments
Authors: Judeikis Henry S. , Siegel Seymour
Published: February 2004

We have examined theoretically and experimentally triplet–triplet energy transfer for several different donor–acceptor pairs dissolved in low-temperature glasses in order to determine if there is some observable preferred mutual orientation for energy transfer, especially at large intermolecular distances. In particular, we have analyzed experiments that used magnetophotoselection techniques. Our results substantially support earlier findings that indicated the absence of observable orientation effects. In analyzing our experiments, we find that for any given system a large fraction of the donor molecules will transfer efficiently and will exhibit little in the way of orientation effects unless delta-function-type orientation dependences exist. Even at low acceptor concentrations, these molecules represent a significant portion of all molecules that transfer and their presence will tend to minimize experimentally observable effects.
Proton Spin Relaxation in the Chloroethanes
Authors: Miller Charles R. , Gordon Sidney L.
Published: February 2004

T 1 and T2 proton relaxation times were measured for the chloroethanes (CH3CH2Cl, CH3CHCl2, CH2ClCH2Cl, CH3CCl3, CH2ClCHCl2, CH2ClCCl3, CHCl2CHCl2, and CHCl2CCl3) at room temperature as a function of concentration in CS2. The data was extrapolated to infinite dilution. The extrapolated relaxation times were interpreted in terms of two mechanisms: internal dipolar interactions, and scalar coupling between proton and chlorine nuclei. The extrapolated relaxation times indicated that the dipolar correlation times increased as the number of chlorines increased, while the chlorine relaxation times decreased as the number of chlorines increased. Internal dipolar correlation times were computed from the extrapolated T1 relaxation times. The relative trends of the reorientational correlation times were explained equally well by the BPP viscosity model and an inertial model due to Steele. The inertial model predicted correlation times which were too low by a factor of 2, whereas the BPP model predicted correlation times too high by an order of magnitude. This was interpreted to mean that the reorientational motions of the chloroethanes at infinite dilution in CS2 are controlled more by the moments of inertia of the chloroethanes than by the frictional forces of the solvent.
93Nb and 23Na NMR in Polycrystalline Sodium Niobate
Authors: Wolf F. , Kline D. , Story H. S.
Published: February 2004

Nuclear magnetic resonance (NMR) spectra of 23Na (I = 32) and 93Nb (I = 92) from a powdered sample of NaNbO3 have been studied. At room temperature the 23Na spectrum shows the presence of two distinct sites, one having axial symmetry with a coupling constant of 21.5 ± 0.2 MHz and the other having an asymmetry parameter lying between 0.80 and 1.0 with a coupling constant of 1.0 ± 0.1 MHz. The niobium spectrum arises from a single site with a coupling constant of 19.7 ± 0.5 MHz and an asymmetry parameter of 0.82 ± 0.02. The (12 ↔ − 12) transition in the 93Nb spectrum exhibits some unusual features which were confirmed by computer-simulated powder patterns. The temperature dependence of one 23Na site indicated a rather linear decrease in the coupling constant, from 2.6–1.6 MHz, and the asymmetry parameter was approximately zero as the temperature increased from − 170–280°C. The 93Nb quadrupole interaction showed a linear decrease not only in the coupling constant, from 19.7–10 MHz, but also in the asymmetry parameter, from 0.82–0.64, as the temperature rose from room temperature to 270°C. Electric-field-gradient calculations for 93Nb produced coupling constants lower than the measured value by about an order of magnitude while yielding values of the asymmetry parameter comparable to that observed. Similar calculations for 23Na gave relatively high values of the asymmetry parameter, and the coupling constants were of the same order of magnitude as those determined experimentally. Third-order corrections to the NMR frequencies perturbed by the quadrupole interactions for the case of the asymmetry parameter unequal to zero were calculated. These proved to be negligible except for the case of niobium at low frequencies.
Three-Dimensional Model of Collision-Induced Vibrational Transitions in Homonuclear Diatomic Molecules
Authors: Hansen C. Frederick , Pearson Walter E.
Published: February 2004

The one-dimensional, semiclassical theory of vibrational transitions in homonuclear diatomic molecules is extended to three dimensions. Small perturbation methods are applied to harmonic-oscillator, rigidrotator wavefunctions, and the interaction potential is assumed to be a linear superposition of exponential repulsions between atomic centers. This potential is expanded to terms of first-order influence on the transition probabilities, and the spherically averaged potential is used to determine the collision trajectory and the time-dependent perturbation. Vibrational transitions (predominantly Δυ = ± 1) are found to be accompanied by simultaneous rotational transitions (predominantly Δl = 0, ± 2). The effect of coupled rotations increases the vibrational transition rates by 50% or more. Analytic approximations are derived for cross sections, rate coefficients, and relaxation rates. The three-dimensional model permits one to fit both the gradient and the magnitude of an effective exponential repulsion to observed vibrational relaxation data, whereas only the gradient can be determined using one-dimensional theories. This “effective” interaction potential for vibrational excitation is steeper and of shorter range than potentials normally responsible for pure scattering. We surmise that a multiplicity of interaction potentials exist for molecules, just as for atoms, and that the steeper inner potentials resulting from paired electron spin configurations are responsible for most vibrational transitions, whereas the outer potentials resulting from unpaired spins are responsible for most of the scattering.
Polyhedral Clathrate Hydrates. XVI. Structure of Isopropylamine Octahydrate
Authors: McMullan R. K. , Jeffrey G. A. , Panke D.
Published: February 2004

The crystal structure of isopropylamine octahydrate, 10(CH3)2CHNH2·80H2O, mp − 4°C, has been determined from three-dimensional Weissenberg film data obtained at − 160°C with CuKα radiation. The crystals are hexagonal, P63 / mmc, with a = 12.30 Å, c = 24.85 Å. The structure was determined from the Patterson function and refined anisotropically to an R of 0.09 for 1041 observed reflections. The water structure is a clathrate-type hydrogen-bonded framework, which can be related to that of the cubic gas hydrates. It consists of layers of face-sharing 14-hedra (42 × 58 × 64), separated by pentagonal dodecahedra (512) and 16-hedra (512 × 64). The amine molecules occupy positions with sixfold orientational disorder in the six 14 hedra and four 16 hedra per unit cell. They are hydrogen bonded to the water structure, singly within the 16-hedra and doubly within the 14-hedra. Because of the distortions caused by the inclusion of the amines in the 14-hedra, there are additional voids in the arrangement of the 12-, 14-, and 16-hedra, which do not occur in the gas hydrates. These are 8-hedra with six squares and two hexagons (46 × 62). They are vacant, as are the pentagonal dodecahedra.
Scattering of Atomic Helium and Molecular Deuterium Beams from Platinum (111) Surfaces Characterized by LEED
Authors: Smith D. L. , Merrill R. P.
Published: February 2004

The scattering of helium and deuterium from platinum (111) surfaces whose crystallographic order and structure has been characterized by in situ LEED measurements in an ultrahigh vacuum system (≈ 10−10 torr) is described. The effect of surface temperature, gas temperature, incidence angle, and lattice stiffness are in qualitative agreement with the cube theories and thermal roughening of the surface. Indications of the excitation of internal degrees of freedom are discussed. The use of a dc detection system allows mass balances between the incident and scattered beam which agree within 3%.
Laser-Induced 16-μ Fluorescence in SF6: Acoustic Effects
Authors: Bates Richard D. , Flynn George W. , Knudtson J. Thomas , Ronn A. M.
Published: February 2004

Damped oscillations in the laser-excited 16- and 10-μ infrared fluorescence decay from SF6 and SF6–rare-gas mixtures have been observed. This fluorescence modulation is shown to be due to a laser-induced sound wave propagating in the sample cell. The generation and propagation of this sound wave is analyzed by solving the coupled mass, momentum, and energy transport equation for the gas following laser excitation. Similar effects have been observed in BCl3, and the phenomenon is expected to occur in general for molecular systems which strongly absorb pulsed laser radiation and also have very rapid vibration–vibration and vibration–translation energy transfer rates. The risetime of the 16-μ fluorescence from SF6 following excitation by a Q-switch CO2 laser oscillating at 10.6 μ gives a lower limit of 5 × 104 sec−1 at 1 torr for the vibration–vibration energy transfer rate in SF6 between states fluorescing at 10 μ and states fluorescing at 16 μ.
Observation of Laser-Induced Acoustic Waves in SF6
Authors: Burak I. , Houston P. , Sutton D. G. , Steinfeld J. I.
Published: February 2004

The acoustic waves reported by Bates et al. are directly observed by means of a solid-dielectric capacitance microphone. The initial time development of the pressure wave indicates that the vibration–vibration energy transfer rate in SF6 is not less than 2 × 106 sec−1·Torr−1.
Generalized Susceptibility Theory. III. Spectral Properties of Extended Molecular Systems: Optical Rotation of Helical Polymers
Authors: Rhodes William
Published: February 2004

The spectral properties of an extended molecular system, such as a linear polymer, are considered in terms of the linear response tensor (LRT) which determines the part of the average current density that is proportional to the transverse electric field. In order to formulate such phenomena as optical rotatory dispersion (ORD) and circular dichroism (CD) use is made of Maxwell's equation for the transverse field components. Periodic boundary conditions are used for both the molecular system and the radiation field. The symmetry properties of the LRT with respect to time translation, time reversal, and reversal of propagation direction of the photon are given. Discussion is given of the relation between the components of the LRT and various optical properties such as ORD, CD, and linear birefringence. For an extended molecular system, having discrete symmetry operations (e.g., translations or screw axis), it is shown that a multicenter expansion is useful for formulating the LRT in terms of unit-cell spaces. This approach is applied to a helical polymer, and it is found that (in addition to Umklapp processes) components of the field having wave vector q are coupled to those having wave vector q ± 2s, where s corresponds to a wavelength equal to the pitch of the helix. These interactions produce second-order corrections to the refractive indices of right and left circularly polarized light. For the purpose of clarifying recent theoretical work by several groups and in order to present a unified development of the theory, a precise formulation is made of all terms (to first order in q) contributing to the ORD–CD of a helical polymer. It is found, in agreement with the work of others, that the leading term is an electric dipole strength term which is not included in the conventional Rosenfeld formula whenever periodic boundary conditions are used. The physical nature of the various terms are considered in detail and approximation methods for evaluating polymer properties in terms of a monomer basis are briefly reviewed.
Radiation-Induced Isomerization of [2,2,1] Bicycloheptadiene (Norbornadiene) to [2,2,1,02,6,03,5] Quadricycloheptane (Quadricyclene) in Cyclohexane Solutions
Authors: van Ingen J. W. F. , van Tieghem C. H. C. , Cramer W. A.
Published: February 2004

Radiation-induced isomerization of norbornadiene (NBD) into quadricyclene (Q) was studied in cyclohexane solutions. From the effects of charge and radical scavengers it could be concluded that at low NBD concentrations isomerization is initiated by a reaction of NBD with electrons and probably proceeds via singlet excited NBD. Low yields of Q show that this is not an efficient process. At higher NBD concentrations the isomerization becomes more efficient, indicating that other reactions may begin to contribute. Selfquenching of NBD* also becomes important at higher concentrations. In the presence of SF6, electron transfer from NBD− to this second solute occurs. Enhanced isomerization yields are observed in the presence of benzene, toluene, and acetophenone. Scavenger studies indicate that triplet-excited states of these solutes contribute to this increase in G(isom). They are formed by charge transfer from cyclohexane positive ions to the aromatic additives followed by neutralization. Complex formation between triplet-excited aromatic molecules and NBD is suggested to be an essential step in the isomerization reaction. Relative reactivities towards cyclohexane positive ions, estimated for methanol, toluene, and benzene, are, respectively, 1.0, 0.32, and 0.27. Calculated lifetimes of triplet-excited acetophenone, toluene, and benzene are between 10−6 and 10−7 sec. The excited states all react with NBD with rate constants between 5 × 108 and 5 × 109 M−1·sec−1.
Ionic Conductivity and Association in CdF2
Authors: Tan Y. T. , Kramp D.
Published: February 2004

The ionic conductivity of pure, yttrium-doped, and sodium-doped cadmium fluoride has been determined from room temperature up to a maximum temperature of 500°C. Results show the mobile defect to be fluorine vacancies with an activation energy of motion of 0.44 eV. A mechanism is developed for the conversion of yttrium-doped CdF2 to a semiconductor based on vacancy motion. An anomalous conductivity curve is observed in crystals containing sodium impurity. Possible explanations for this are discussed in terms of association effects.
General Proof of Lassettre's Theorem
Authors: Bonham R. A.
Published: February 2004

A general proof of Lassettre's theorem—the limit as K0n approaches zero of K0n times the exact scattering amplitude is equal to the same limit of K0n times the first Born amplitude including exchange where ℏK0n is the inelastic momentum transfer for excitation of the target system between the states 0 and n—is given.
Magnetothermodynamics of Single Crystal CuSO4·5H2O. V. Fields Along the β Axis. Thermodynamic Temperature without Heat Introduction below 0.5°K. A Reference at 0.035°K
Authors: Giauque W. F. , Fisher R. A. , Hornung E. W. , Brodale G. E.
Published: February 2004

The magnetic and thermodynamic properties of a 3.700-cm-diam spherical single crystal of CuSO4·5H2O have been investigated down to 0.02°K with fields along the β magnetic axis. Thirty-eight magnetizations from unknown low temperatures to reference conditions were performed under adiabatic conditions, and 35 were essentially isentropes. These have been used to determine the thermodynamic temperature and heat capacity, both in and out of magnetic fields, without heat introduction. A temperature reference at 0.035°K has been suggested. It has been pointed out that proving a process to be isentropic is no guarantee that “frozen-in” entropy and energy situations have not developed during the isentropic process, thus conserving nonequilibrium entropy. Entropy conserved in this way cannot be used as an equilibrium variable in determining temperature. It is evident that detailed methods of investigation designed to detect such nonequilibrium magnetic structural situations will be desirable. When “frozen-in” entropy situations develop on proven isentropes, as is inevitable in many cases, the isentropic transfer of the entropy values to unknown low temperatures will not suffice to determine the thermodynamic temperature by use of heat introduction and the equation T = dq / dS. Earlier magnetothermodynamic data have been supplemented by measurements between 0.4 and 4.2°K with a field of 33 kG.
Gas-Phase Reaction Rates of Some Positive Ions with Water at 296°K
Authors: Howard Carleton J. , Rundle Howard W. , Kaufman Frederick
Published: February 2004

Measurements of the rate constants for the reactions of He+, Ne+, Ar+, Kr+, O+, N+, and N2+ with H2O by the flowing afterglow technique at 296°K gave values of 0.56, 0.74, 1.43, 1.19, 2.33, 2.57, and 2.19 × 10−9 cm3/sec with standard deviation of 7%–15%. They are in good agreement with extrapolated beam results for Ar+, O+, N+, and N2+. Close agreement is also found for all but He+ and Ne+ with estimates based on classical trajectory calculations for similar ion plus dipole systems.
Transfer of Protons through “Pure” Ice Ih Single Crystals. II. Molecular Models for Polarization and Conduction
Authors: von Hippel A.
Published: February 2004

Neither the polarization of pure ice single crystals can be understood on the basis of rotating H2O molecules nor the conduction through such crystals by assuming migrating H3O+ and OH− ions. Bjerrum's concepts of L-, D-defect and ionic-defect formation are a useful starting point, but the subsequent development of double-well models leads into serious difficulties. A new interpretation of the intrinsic polarization of ice Ih visualizes L, D pair formation as a one-step process: a proton shifted by near-infrared phonon excitation to an empty corner of its H2O tetrahedron. Subsequent intramolecular proton transfers, slightly field directed, inscribe a dipole moment into the disordered proton system of the ice crystal; the defects die off by recombination. Thus the polarization builds up from a statistically triggered pulse spectrum; its correlation period is the waiting time, until the same H2O molecule forms again a defect pair. This waiting period, the relaxation time of the Debye spectrum, connects the low-frequency response of the ice to events in the near infrared. A simple quantitative formulation is given. Next the role of the ionic defects is investigated. These defects cannot follow the field in the ideal crystal: If they move downstream, the antipolarization created by this intermolecular proton transfer pumps the corresponding charge up-stream. Thus only the L, D defects carry charge through the volume of an ideal ice crystal. Interconversion of L, D defects and ionic defects can occur at the electrodes and may create low-dielectric-constant layers in front of them. In addition to these intrinsic effects, extrinsic phenomena are expected and observed. Lattice defects, dislocations, and doping can cause complexities akin to “color centers” in alkali halides. “Catalytic defect mills” may arise when extrinsic effects couple into the proton-ordering mechanisms of the ice crystal. Part III resumes the experimental account.
Mass Spectrometric Investigation of Surface Ionization. VI. Desorption of Th+, Np+, and Pu+ from Tungsten Surfaces
Authors: Smith David H. , Cameron A. E. , Dean John A.
Published: February 2004

The mean residence times of Th+, Np+, and Pu+ on polycrystalline tungsten surfaces have been determined as functions of temperature using the modulated beam technique. The desorption parameters, determined from the relationship 1 / τ = ω0exp(− Ei / kT), are Ionω0(sec−1)Ei(eV)Th+(1.7 ± 1.5) × 10147.83 ± 0.25Np+(6.8 ± 3.8) × 10136.32 ± 0.12Pu+(1.6 ± 0.9) × 10124.99 ± 0.15.
Mössbauer Electric Field Gradient Parameters in FeSO4·4H2O Single Crystals
Authors: Garg V. K. , Puri S. P.
Published: February 2004

Mössbauer absorption of 57Fe in single crystals of FeSO4·4H2O has been studied as a function of the orientation of the crystal axes with respect to the direction of the unpolarized γ-ray beam. The analysis of the peak intensity area ratios of the quadrupole split doublet spectra yields the principal-axis values, the asymmetry parameter, and the direction cosines of the efg axes with respect to the crystal axes. Furthermore, it exhibits the anisotropy of the recoilless absorption probability. The value of eq = 2.26 a.u. has been utilized to find the electronic population density in dε levels.
Cartesian Tensor Perturbation Scheme for the Senftleben Effect
Authors: Cooper E. R. , Hoffman D. K.
Published: February 2004

A perturbation expansion scheme using a Cartesian tensor expansion is developed for computing transport coefficients of diatomic gases in a static magnetic field. The collision operator is uniquely separated into an unperturbed part plus a “nonspherical” perturbation. Special attention is given to the saturation limit. Using a truncated expansion set, explicit expressions for the thermal conductivity and viscosity are obtained to second order in the “nonsphericity.” These solutions in the second-order approximation are identical with the approximate solutions of Levi and McCourt. Also, the fourth-order contributions from terms odd in the angular momentum are given in the saturation limit, since for certain molecular models the second-order contributions are zero.
High-Pressure Phase Studies on Sodium–Potassium Alloys
Authors: Anderson D. Ray , Ott J. Bevan , Goates J. Rex , Hall H. Tracy
Published: February 2004

The solid–liquid phase diagrams were investigated to 60 kbar for elemental sodium, potassium, and various sodium–potassium alloys. The data for Na and K are in general agreement with those of earlier workers. In the alloy system the temperatures of both the peritectic and eutectic halts increase as the pressure increases, but with different slopes, so that the two invariant points merge at approximately 35 kbar. Above this pressure, two halts are again observed. One increases with increasing pressure, and the other decreases with increasing pressure up to 55 kbar, where it abruptly starts increasing again. Melting points for samples in the 0.6–0.8 mole fraction potassium range indicate that an NaK2 intermetallic compound may form at high pressures.
Ab Initio Calculations on Large Molecules Using Molecular Fragments. Hydrocarbon Characterizations
Authors: Christoffersen Ralph E. , Genson Donald W. , Maggiora Gerald M.
Published: February 2004

An ab initio procedure for investigation of large molecules is described. The procedure involves two steps, the first of which is the determination of a set of orbitals by the examination of molecular fragments. These molecular fragments and associated orbitals are then combined to form large molecules of interest, using an ordinary LCAO MO SCF-type procedure. The molecular fragment investigations employed are designed to obtain nonlinear parameters for use in large molecules, and are not used in this study to obtain “small molecules” from which “large molecules” can be formed. Several descriptions of molecular fragments are explored, in which floating spherical Gaussian orbitals are employed, and applications of the method to ethane, propane, ethylene, and benzene are given. Comparisons are made with other methods, and several general characteristics of the method are also discussed.
Cross Sections for Electron Attachment Resonances Peaking at Subthermal Energies
Authors: Christophorou L. G. , McCorkle D. L. , Carter J. G.
Published: February 2004

A simple procedure is described whereby both the magnitude and the energy dependence of the cross section for subthermal peaking electron attachment resonances are determined from electron swarm data alone. For an initial energy region it is shown that the attachment cross section can be approximated by σa(ε) = Aγ / εγ, and a determination of the power γ of the electron energy ε as well as of the corresponding constant Aγ yields both the magnitude and the energy dependence of the cross section at thermal and epithermal energies. The values of Aγ [in (ergs)γ square centimeters], γ, and σa(0.05 eV) (in square centimeters) are 2.5 × 10−29, 1.12, 1.17 × 10−14; 7 × 10−34, 1.412, 2.17 × 10−15; 9.1 × 10−32, 1.148, 1 × 10−16; and 1.66 × 10−29, 1.031, 5.2 × 10−16 for sulfur hexafluoride, 1,4-naphthoquinone, anthracene, and 1,2-benzanthracene, respectively. The thermal (T = 298°K) value of the attachment rate for SF6 in the carrier gases N2 and C2H4 is found to be 8.7 × 109 sec−1·torr−1 and 9 × 109 sec−1·torr−1, respectively. These values are in excellent agreement with the most reliable data available which are summarized. Cross sections are reported also for SF5− at 298°K. This work demonstrates further the potential of the electron swarm method in electron attachment studies.
Mode Expansion in Equilibrium Statistical Mechanics. II. A Rapidly Convergent Theory of Ionic Solutions
Authors: Chandler David , Andersen Hans C.
Published: February 2004

The mode expansion theory, which was derived in a previous paper, is applied to the calculation of thermodynamic properties of an ionic solution. The specific system investigated is the 1–1 restricted primitive model aqueous electrolyte. In this model, the effective interactions (potentials of mean force at infinite dilution) between ions are a sum of pair potentials, and the pair potentials are hard-core repulsions (all with diameter 4.6 Å) plus a positive or negative Coulomb potential reduced by the dielectric constant of water. For this system, the mode expansion converges rapidly. At salt concentrations between 0.05 and 1.0M, only the first three terms in the expansion are needed to obtain osmotic coefficients and activity coefficients which agree very well with the accurate results obtained by integral equation methods. Due to the computational simplicity of the mode expansion, it is argued that the theory provides a convenient method for studying more realistic models for electrolytes.