bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic Mechanism for Self-Diffusion and Relative Diffusion in Simple Liquids
Authors: Fehder P. L. , Emeis C. A. , Futrelle R. P.
Published: February 2004

Graphical displays of computer-generated data simulating the microscopic dynamics of a two-dimensional dense fluid of Lennard-Jones disks indicate that the microscopic mechanism for diffusion in simple liquids may be largely cooperative in nature. The cooperative effects are important for processes involving the relative motion of two particles, such as the relative diffusion of solute molecules separated by short distances in a liquid, or electromagnetic absorption or scattering in which the dipole moment or polarizability tensor varies with the interparticle separation. The statistical time-correlation functions describing self-diffusion in the model fluid are found to behave in a manner consistent with existing theoretical treatments of diffusion in simple liquids. The anomalous long-time behavior of the velocity autocorrelation function seen by Alder and Wainwright is discussed. It leads to a divergent diffusion coefficient in two-dimensional fluids. It appears not to have a strong effect on our results in the important kinetic regime (t ≤ 10−11sec). Relative diffusion of particles separated by distances of 1 to 3 diam is found to proceed more slowly than predicted by theories that neglect correlations in the motions of neighboring particles. The time-displaced distribution function G2,s[r12(τ), r12(0)] describing this relative motion is more complex than the ordinary Van Hove function Gd[r2(τ) − r1(0)]. The Smoluchowski equation describing G2,s is shown to be a consistent description only if the relative diffusion coefficient at close separations is depressed about 40% below the singlet value.
Interaction Energy Curves of LiHe and NaHe (X 2Σ+, A 2Π, B 2Σ+) and X 1Σ+ Ions
Authors: Krauss M. , Maldonado P. , Wahl Arnold C.
Published: February 2004

Hartree–Fock interaction energy curves have been calculated for the X 2Σ+, A 2Π, and B 2Σ+ states of neutral LiHe and NaHe as well as for the ground state X 1Σ+ ions over a range of distances from 3 to 10 a.u. Since it is intended to apply these results to scattering problems, the variation of the dipole and quadrupole moments and the electronic transition probabilities with internuclear distance were also obtained. Both Slater-type functions and Gaussian-type functions were used as variational trial functions with the intention of gauging the efficacy of the Gaussian basis. Except for situations involving small energy minima the Gaussian basis yielded results accurate relative to the Slater basis. The features of the Hartree–Fock interaction energy curves can be summarized as follows:

(1) The X 2Σ+ interaction energy is purely repulsive for both molecules to the accuracy of the present calculation.

(2) The A 2Π and X 1Σ+ curves are strikingly similar for both Li and Na confirming the penetration of the He for this interatomic orientation.

(3) The long-range repulsive behavior of the B 2Σ+ curve compared with estimates of the correlation energy shows that the internuclear distance dependence of the energy in the region of 10 a.u. is dominated by the Hartree–Fock repulsive curve.

Charge-density plots have been obtained for all states and distances. These are used to illustrate the physical basis of the energy curves.
Dielectric and Pressure Virial Coefficients of Imperfect Gases. III. CClF2H
Authors: Sutter H. , Cole R. H.
Published: February 2004

Second dielectric and pressure virial coefficients at 50°, 100°, and 150° are reported, together with low-density dielectric results which give a dipole moment μ = 1.48 D and polarizability 5.3 × 10−24 cm3. The second virial data can be reasonably well fitted by a pair potential including a Lennard-Jones radial function, polarizable dipole interactions, and an anisotropic shape factor proposed by Buckingham and Pople, but difficulties in consistently accounting for these and other results for polar molecules are pointed out.
Energy of Excess Electrons in Nonpolar Liquids by Photoelectric Work Function Measurements
Authors: Holroyd R. A. , Allen Mark
Published: February 2004

The energy of the excess electron state has been determined for six nonpolar liquids by a photoelectric injection method. The functional dependence of the photocurrents on wavelength is the same in the liquid as in the vacuum and work functions can be evaluated from Fowler plots. For several liquids the work functions are less than the vacuum value and these lowerings are characteristic for each liquid. For tetramethylsilane, neopentane, cyclopentane, and 2,2,4-trimethylpentane the work functions are lowered by 0.62, 0.43, 0.28, and 0.18 eV, respectively. For n-pentane and n-hexane the work functions are close to the vacuum value. The magnitude of the lowering is correlated with the energy of the first electronic absorption band. A variation of electron mobility with the work function changes is noted and discussed relative to the trap theory. The photocurrents measured in the liquids increase with voltage, and at the voltage employed the currents in neopentane and tetramethylsilane are comparable to the vacuum photocurrent. The magnitude of the observed photocurrent depends on the distance the electrons penetrate.
Study of Self-Avoiding Walks with Excluded First Nearest Neighbors
Authors: Kumbar Mahadevappa , Windwer Stanley
Published: February 2004

A study of the direct enumeration of self-avoiding walks on the tetrahedral and four choice cubic lattices with first nearest neighbors excluded is presented. Data is given for the mean square end-to-end distance, the mean square radius of gyration, the number of returns to the origin, the mean square radius of gyration of the rings, and probability calculations. The results are interpreted in light of the theories of excluded volume.
Ultraviolet Spectra of Alkali Halides in Inert Matrices
Authors: Oppenheimer Michael , Berry R. Stephen
Published: February 2004

Ultraviolet absorption spectra have been observed for some of the alkali halides in several inert matrices at wavelengths between 2000 and 4500 Å. The matrix-isolated molecules exhibit simpler spectra than those in the gas phase, permitting vibrational analysis. The LiI spectrum contains a strong absorption showing at least nine peaks with a mean spacing of 363 cm−1 in argon and 367 cm−1 in krypton, at wavelengths below 2900 Å. In addition, LiI exhibits one broad structureless peak near 3500 Å in both matrices. The NaI spectrum shows a weak absorption with as many as five peaks just below 4000 Å. The mean spacings are 166 cm−1 in argon, 129 cm−1 in kyrpton, and 141 cm−1 in nitrogen. NaBr exhibits a strong absorption at wavelengths below 3100 Å showing as many as 16 peaks with mean spacings of 453 cm−1 in argon, 529 cm−1 in krypton, and 472 cm−1 in nitrogen. LiBr, KI, NaCl, and RbBr show no absorption in this region at similar concentrations. The observed absorption peaks are interpreted as arising from transitions between the ground states (1Σ+) and different vibronic levels of the first excited 0+ bound states of these molecules. The structureless peak near 3500 Å in LiI probably represents a transition to a purely repulsive state. The appearance of absorption in LiI in a region where none existed in the gas phase is attributed to a change in Franck–Condon factors induced by the matrix. The results are interpreted in terms of a charge transfer model of molecular absorption and support the interpretation of adiabatic behavior when the ionic and atomic potential curves cross at a relatively short internuclear separation; the smaller the internuclear distance at the crossing, the more likely the noncrossing rule is obeyed. The unexpected appearance of discrete structure in the NaBr spectrum is discussed in terms of matrix-induced charge transfer.
Phase Equilibria in Fluid Mixtures at High Pressures: The Neon–Argon System
Authors: Streett William B. , Hill James L. E.
Published: February 2004

Results of phase equilibria studies are reported for the neon–argon system at temperatures from 87.34 to 123.83°K and pressures to 3880 atm. These results, combined with those of earlier studies, provide a complete description, partly quantitative and partly qualitative, of the lines and surfaces comprising an important segment of the three-dimensional, pressure–temperature–composition, phase diagram for neon–argon. A three-dimensional sketch of the diagram is used to show the important phase boundary lines and the qualitative features in the region of transition from fluid to solid at high pressures. The relevance of these studies to problems of deep atmosphere structures in the outer planets is briefly discussed.
Shape Resonances and Rotationally Predissociating Levels: The Atomic Collision Time-Delay Functions and Quasibound Level Properties of H2(X 1Σg+)
Authors: Le Roy Robert J. , Bernstein Richard B.
Published: February 2004

The energy dependence of the collisional time-delay function has been computed for H(1S) atoms interacting via the ab initio H2(X 1Σg+) potential. Peaks in this function determine the scattering resonance energies Er and widths Γ, and the lifetimes for each of the corresponding quasibound vibrational–rotational levels. Small differences are found between these Er and Γ, and the values obtained by a “maximum internal amplitude” approach (intended to characterize the spectroscopically observable predissociating levels). Approximate procedures for rapid, accurate numerical evaluation of Er are appraised; a new outer-boundary-condition criterion for resonances leads to the best agreement with the exact results. Also, a primitive WKB procedure yields Γ's of usable accuracy. For ground-state H2, HD, and D2 the onset of line broadening due to centrifugal barrier penetration is found to occur at energies some hundreds of cm−1 below the locus of barrier maxima. The predissociation method of estimating long-range interatomic forces therefore cannot be expected to yield valid results for hydridic diatomics.
Optical and Microwave Spectra of Cr3+ in the Spinel ZnGa2O4
Authors: Kahan H. M. , Macfarlane R. M.
Published: February 2004

The EPR, optical absorption, and fluorescence spectra of Cr3+ in the spinel ZnGa2O4 have been measured and interpreted using a crystal-field model. The EPR experiments can be described by a spin Hamiltonian whose parameters are: 2D = + 1.048 ± 0.001 cm−1, g‖ = 1.9774 ± 0.0005, g⊥ = 1.9761 ± 0.0005. The broad band transitions to the T24 and lower T14 terms show a trigonal field splitting of < 500 and 1500 cm−1 respectively. The zero field splittings of the A24 level (1.048 cm−1) and E2 level (40.0 cm−1) are considerably different than in the two other spinel systems which have been reported, i.e., ZnAl2O4 and MgAl2O4. The model parameters used in the numerical calculations are: Δ = 18 000 cm−1, B = 670 cm−1, C = 3230 cm−1, ζ = 250 cm−1, υ = − 650 cm−1, υ′ = − 1100 cm−1, k = 0.7. The detailed structure of the phonon sidebands and fluorescence, which is tentatively assigned to exchange-coupled Cr3+ pairs, are significantly different from other spinels.
Absorption and Luminescence Spectra of Titanium Atoms Isolated in Noble Gas Matrices
Authors: Gruen Dieter M. , Carstens Dean H. W.
Published: February 2004

The absorption spectra of titanium atoms, isolated in argon or xenon matrices, have been measured at low temperatures. The spectra consist of a large number of bands in the visible and near-uv regions, which are correlated with transitions of gaseous neutral titanium atoms. Oscillator strengths are obtained for the absorption bands of isolated titanium, and these values are compared to intensity data on the gaseous spectrum. In an argon, but not in a xenon matrix, upon excitation with appropriate uv light, the metal atoms yield a relatively intense luminescence spectrum consisting of two bands. One, occurring in the 16–16 600 cm−1 region, is assigned to the z5G0 → a3F transition. The origin of the second band system, occurring at 14–15 000 cm−1, is uncertain. The matrix shifts are found to depend on the transition energy, increasing in argon but decreasing with energy in xenon matrices. These results provide new data for testing current theories of matrix shifts.
Vibrational Spectra and Structure of Diboron Tetrachloride in the Crystalline and Fluid States
Authors: Durig J. R. , Saunders J. E. , Odom J. D.
Published: February 2004

The infrared spectra of B2Cl4 have been studied from 4000 to 33 cm−1 in the gaseous and crystalline states. The Raman spectra have been studied to 45 cm−1 in the liquid phase at several temperatures between − 30 and − 190°C, the sample being in a supercooled liquid or glassy state in the latter cases, and in the crystalline state after annealing. An assignment of the normal vibrations of the molecule in the planar form is given, and the frequency of the expected torsional mode has been calculated. Splitting of the modes resulting from the natural isotopic ratios of boron have been observed, especially in the higher-frequency modes. Evidence is presented that the conversion from the nonplanar D2d form in the liquid to the planar D2h form in the solid results from a greater lattice stabilization for the D2h form rather than a greater inherent thermodynamic stability of the planar form.
Nuclear Magnetic Resonance Study of β-Rhombohedral Boron and Boron Carbide
Authors: Hynes Thomas V. , Alexander Michael N.
Published: February 2004

The 11B nuclear magnetic resonance (NMR) in β-rhombohedral boron and in boron carbide has been investigated at room temperature. Continuous wave (cw) measurements were performed at NMR frequencies 2–22 MHz, and pulse measurements at 29.5 MHz. Because of the crystal symmetry and the large number (105) of atoms per unit cell, the NMR in single-crystal β-rhombohedral boron is characterized by a very complex first-order quadrupole spectrum which is extremely sensitive to crystal orientation in the magnetic field. Measurements were made on powders to observe the orientation average of the spectrum, and first- and second-order quadrupole interactions were observed. The cw spectrum is a superposition of spectra corresponding to quadrupole coupling constants νQ1 = 680 ± 70 kHz (this apparently being the central value for a distribution of electric field gradients whose spectra cannot be resolved) and νQ2 in the range 90 ≲ νQ2 ≲ 170 kHz. There may also be a few sites for which νQ3 ≅ 0 (νQ = eqQ / 2). The intensities of the spectra corresponding to the various quadrupole couplings were measured by a cw technique, and also by a pulse technique which utilizes the fictitious spin-12 formalism. 80% ± 5% of 11B nuclei in β-rhombohedral boron were found to experience the quadrupole coupling νQ1 = 680 kHz, corresponding to the fact that 80% of the atoms in the crystal are six coordinated with icosahedral nearest-neighbor linkages. The spin–lattice relaxation mechanism in β-rhombohedral boron was found to be quadrupolar in nature. The spin–lattice recovery following a saturation pulse sequence can be fitted by the sum of two exponentials having time constants 5 and 52 msec (the central transition was observed). When the time constants are “normalized” so that only phonon contribution to the relaxation is considered, the strength of the spin–phonon interaction is found to be extremely large (more than an order of magnitude greater than in any alkali halides). The cw spectrum in a powder of boron carbide was found to be similar to that of β-rhombohedral boron in that it is characterized by quadrupole couplings νQa = 520 ± 30 and νQb < 30 kHz. At higher NMR frequencies an additional powder pattern was observed which has divergences corresponding to νQc = 2.74 ± 0.10 MHz. Intensity measurements showed that 4 ± 2 times as many 11B nuclei experience the coupling νQa as experience νQb; the powder pattern which has divergences corresponding to νQc arises from 17%–58% of all 11B nuclei in the boron carbide crystal. These results lend support to a model of the boron carbide crystal which assigns a boron atom to the central position of the three-atom chain and a carbon atom to the icosahedron (which, in the usual simple models of boron carbide, is assumed to be occupied only by boron atoms).
On the Electronic Structure of Some Nitrogen-Containing Molecules as Studied by Ab Initio SCF MO Calculations and High-Energy Electron Spectroscopy
Authors: Wyatt J. F. , Hillier I. H. , Saunders V. R. , Connor J. A. , Barber M.
Published: February 2004

Nitrogen 1s binding energies in HN3 and the potassium salts of NO2−, NO3−, and N3− are measured and interpreted in terms of ab initio SCF MO calculations. The valence orbitals must be represented by a double-zeta basis of STO's to adequately interpret the chemical shifts. Inclusion of the lattice potential in the case of the ionic compounds allows a good correlation between calculated and experimental 1s binding energies to be obtained for both ionic and nonionic compounds.
Monte Carlo of Chains with Excluded Volume: Distribution of Intersegmental Distances
Authors: Alexandrowicz Z. , Accad Y.
Published: February 2004

A Monte Carlo study of non-self-intersecting chains on a four-choice cubic lattice is undertaken with the help of a recently described “dimerization” method. This method permits to avoid the difficulty of “sample attrition,” enabling the construction of relatively long chains. Certain improvements of the method are described and its validity is substantiated. The distribution and average values of the end to end, as well as of various intrachain distances, are determined for chain lengths varying from N = 32 to 8192. The results indicate that the basic assumptions of several theories on excluded volume need to be revised.
Kinetic Theory of Moderately Dense Gases
Authors: Henline William D. , Condiff Duane W.
Published: February 2004

A kinetic equation approach to the kinetic theory of moderately dense gases is developed by asymptotic expansions from a viewpoint independent of Bogoliubov chaos assumptions. The theory is applied to the rigid sphere model where the contributions which determine the Enskog dense gas theory are identified. The existence of divergences in the time integrations for triple collision events in the case of rigid discs and for quadruple collision events in the case of rigid spheres is confirmed by means of variable transformations. A scheme for renormalizing the divergences into an asymptotic density expansion containing nonanalytic logarithmic terms is proposed, illustrated for the case of rigid discs, and employed to deduce the fact that the first density corrections for rigid spheres, being convergent, remain intact as the lead term of the asymptotic density expansion. Numerical studies of the first density corrections to the thermal conductivity and shear viscosity of rigid spheres are performed in order to assess the relative importance of Enskog dense gas contributions and dynamical correlation effects. The results are in substantial agreement with preliminary Monte Carlo calculations of Sengers based upon Bogoliubov theory.
Spin Resonance of Gd:Y(OH)3 and Gd:La(OH)3
Authors: Scott P. D.
Published: February 2004

Spin Hamiltonian parameters for Gd:Y(OH)3 and Gd:La(OH)3 have been determined from EPR spectra. The nature of the effective crystal field is discussed. The observed angular variation of the Gd:Y(OH)3 spectrum is compared with calculated values.
Classical S Matrix for Rotational Excitation; Quenching of Quantum Effects in Molecular Collisions
Authors: Miller William H.
Published: February 2004

A previously developed theory in which exact solutions of the classical equations of motion for a complex collision system (i.e., numerically computed trajectories) can be used to generate the classical limit of the quantum mechanical S matrix (the “classical S matrix”) for the scattering process is applied to rigid rotor–atom collisions (rotational excitation). Comparison with essentially exact quantum results shows that transition probabilities (the square modulus of an S-matrix element) between individual quantum states are given reasonably accurately by classical dynamics provided the interference terms are properly accounted for; a strictly classical approach (neglect of interference) gives poor agreement with the quantum values. For averaged collision properties, however, it is found that interference and tunneling effects are rapidly quenched. The linear atom–diatom system (vibrational excitation) and the rigid rotor–atom system are both investigated with regard to this question, namely, how much averaging is necessary to quench these quantum effects. Results indicate that even summation over a few quantum states is often sufficient to make a completely classical treatment appropriate.
Electron Density Distribution in Lithium Hydride
Authors: Banyard K. E. , Dixon M. , Hayns M. R.
Published: February 2004

The electron density distribution ρ(r̄) has been examined for the ground state of LiH at its equilibrium bond length. In addition, by means of density difference maps and profile diagrams, the molecular density was compared with two hypothetical situations represented by the combined electron densities of (a) the free atoms and (b) the Li+ and H− ions. In each case, the nuclei of the component systems were separated by a distance equal to the bond length and the respective charge clouds were assumed to be mutually undistorted. All electron densities were derived from well-correlated wavefunctions; this was particularly essential for the present analysis of LiH, since a noncorrelated treatment for H− is known to give a distinctly poor wave function. In contrast with earlier work, the density difference contour maps δρ(r̄)SA (molecule—separated atoms) and δρ(r̄)SI (molecule—separated ions) are quite different in shape. The δρ(r̄)SA map indicates that considerable charge has been moved from around the Li nucleus into an extensive region around the proton. The contour map and profile diagram for δρ(r̄)SI show that, in the molecule, the outer region behind Li has a density distribution which is virtually identical with that for Li+. Further, the charge movement depicted by δρ(r̄)SI is also in accord with the change in density which would arise from the mutual polarization between Li+ and H−. Consequently, inspection of the density difference maps obtained here suggest that the chemical bonding in LiH, at its equilibrium bond length, could be classified as ionic in character—being composed essentially of two oppositely charged ions, each polarized by the electric field of the other.
EPR of Gd3+ in Some Rare Earth Trifluoride Single Crystals
Authors: Sharma V. K.
Published: February 2004

The EPR of Gd3+ ion has been observed in LaF3 and the paramagnetic CeF3, PrF3, and NdF3 single crystals at room temperature. The spectra exhibit, on the whole, similar over-all features in these homologous isostructure rare earth trifluorides, REF3. The crystal field parameters b20 and b22 increase progressively with decrease in the difference in ionic radii of Gd3+ and the RE3+ ion of the host crystal, with variation of b20 being linear. On the basis of an explanation which is offered for variation of the parameters bnm in a series of similar crystal, an order-of-magnitude calculation of the changes in b20 is found to be in reasonably good agreement with experiment. The EPR spectra are consistent with C2υ symmetry of the local crystalline field for Gd3+ in all the REF3, and the orientation of the y axis of the spectra is such that it convincingly supports the C6υ3 structure for these crystals but not the D3d4 structure. The Gd3+ line-widths are widely different in these crystals and show an unusual variation in the paramagnetic REF3. The widths of the transitions M ↔ M − 1 increase progressively as M takes the values 12, (32or − 12), (52or − 32), and (72or − 52). This variation in the paramagnetic REF3 is presumably due to the unequal influence of exchange interaction upon various transitions.
Microwave Spectra of the First-Excited Torsional States and Barrier to Internal Rotation of Dimethylamine
Authors: Wollrab James E. , Laurie Victor W.
Published: February 2004

Rotational spectra in the first-excited torsional states of a number of isotopic species of dimethylamine have been assigned. Internal rotation splittings were observed and have been used to calculate a barrier of 3.22 ± 0.02 kcal mole−1. Inversion splittings differ significantly from those observed in the ground state spectra while 14N quadrupole effects are unchanged within experimental error.
Quantum Vibrational Transition Probabilities in Atom–Diatomic Molecule Collisions. V Effects of Mass and Well Depth
Authors: Wilson David J.
Published: February 2004

The method of Cheung and Wilson is used to investigate the dependence of vibrational transition probabilities on atomic masses and on the well parameters of the intermolecular potential. The presence of a well in the intermolecular potential causes quite large changes in the transition probabilities, as do changes in the masses of the colliding particles. Difficulties in using this method on systems having certain unfavorable mass ratios had been noted earlier; a procedure is presented in which these difficulties are overcome.
γ
Radiolysis of Liquids at High Pressures. IX. Cyclopentane and Its Solutions with N2O, ND3, and C2H5OD
Authors: Hentz Robert R. , Brazier David W.
Published: February 2004

The γ radiolysis of cyclopentane and its solutions with N2O, ND3, and C2H5OD was studied at room temperature and at pressures in the range 0–6.2 kbar. For pure cyclopentane at 1 atm (0 kbar), G(H2) = 5.2, G(bicyclopentyl) = 1.3, G(cyclopentene) = 3.0, and G(pentene-1) = 0.70. The yields of hydrogen and bicyclopentyl are independent of pressure. The yields of cyclopentene and pentene-1 decrease with increase in pressure, and at 6.2 kbar both yields are ∼ 10% smaller than the values at 1 atm. Yields of several unidentified minor products (G < 0.1 at 1 atm) show the largest pressure dependence. Such results are discussed in terms of possible effects of pressure on competitive decay modes of cyclopentane excited states, and ΔV‡ (disproportionation) − ΔV‡ (combination) for cyclopentyl radicals is estimated to lie in the range 0–1 ml mole−1. For 0.06M N2O solutions, the observed pressure independence of G(N2) = 3.0 and G(H2) = 3.7 indicates no effect of pressure on the yield of scavenged electrons. For both 0.06M ND3 and 0.32M C2H5OD solutions, G(H2+HD+D2) = 5.2 at all pressures. However, for both solutions, G(HD) increases with increase in pressure and at 6.2 kbar is ∼ 35% larger than the value at 1 atm. The increase in G(HD) with pressure is attributed to an increase in the yield of cations scavenged from geminate cation–electron pairs. Such an effect of increased pressure can be understood in terms of a viscosity-independent decrease in the electron diffusion coefficient (via an effect on the depth and concentration of electron traps) or an increase in the positive-charge diffusion coefficient (via an enhancement of positive-hole conduction).
Monte Carlo Calculations on Polypeptide Chains. I. A Three-State Hard-Sphere Model for Randomly Coiling Poly-l-Alanine
Authors: Knaell Kenneth K. , Scott Roy A.
Published: February 2004

A Monte Carlo study of randomly coiling poly-l-alanine was carried out using a model in which the bond lengths and bond angles were held fixed, the peptide bond was fixed in the planar trans configuration, free rotation was assumed about the backbone N–Cα and Cα–C′ single bonds, the individual atoms or groups of atoms interacted pairwise by hard-sphere potentials, and each peptide unit was restricted to three sterically allowed states, the right-handed α-helical state, the left-handed α-helical state, and a third state in the vicinity of the so-called “ribbon structure.” Non-self-intersecting chains of various chain lengths up to 154 amino acid residues were generated using the sample enrichment technique of Wall and Erpenbeck. The attrition parameter λ was found to be an increasing function of chain length and not to have reached its limiting value at N = 154. The data were fitted to an empirical equation of the form λ = λ∞[(N − c) / (N + d)]e. The mean square end-to-end distance was found to approximately obey the usual law 〈r2〉 = aNb and the mean square radius of gyration the usual law 〈s2〉 = a′Nb′ for n ≥ 28. Monte Carlo calculations were also carried out on this model omitting the hard-sphere nonbonded interactions to obtain the unperturbed values of the mean square end-to-end distance 〈r2〉0 and the mean square radius of gyration 〈s2〉0 for comparison with the calculations including the excluded volume effect. A brief discussion is presented for the use of the attrition data to calculate the entropy per residue for both the unperturbed and non-self-intersecting chains and of the interpretation and reliability of the parameters b and b′ and the ratio 〈r2〉 / 〈s2〉 for both of these models.
Absorption Spectra of Na Isolated in Solid C6H6
Authors: Graham W. R. M. , Duley W. W.
Published: February 2004

The absorption spectra of Na trapped in solid C6H6 matrices have been studied over a wide range of concentrations and temperatures. Dilute Na–C6H6 samples exhibit a spectrum which can be interpreted in terms of vibronic transitions. The active vibrational frequencies indicate that the symmetry of the complex cannot be higher than C6υ. Samples show relatively high chemical and thermal stability. In heavily doped Na–C6H6 dispersions absorption features attributable to Na2 are observed.
Potential for the Interaction of Atoms and Heavy Polar Molecules in Thermal Scattering
Authors: van de Ree J. , Okel J. G. R.
Published: February 2004

The anisotropy of a molecule interacting with an atom in thermal collisions is given a more fundamental basis by separating the effects of the motion of the electrons through mutual induction and through the rotation of the nuclei to which they are connected. For heavy polar molecules with an asymmetric mass distribution the positions of the centers of dispersive and electrostatic interaction on the internuclear axis are different from the center of mass. Most of the anisotropy is accounted for by the dynamical effect of these displacements. Two models are proposed which are applicable in classical mechanics. The total scattering cross sections have been evaluated for the partners TIF and Ar. The anisotropy parameter thus derived is in agreement with experiment.
Temperature Effects on the Phosphorescence of Benzene and Toluene
Authors: Kilmer Nelson G. , Spangler John D.
Published: February 2004

Phosphorescence lifetimes have been measured as a function of temperature in the region 2–160°K for the following polycrystalline mixtures: C6H6 and C6D6 in cyclohexane and in cyclopentane; toluene-h8, toluene-α-d3, and toluene-d8 in methylcyclohexane; and toluene-h8 in perdeuteromethylcyclohexane. In each case the results can be reasonably fit by a formula of the form 1 / τP = K0 + K1exp(− ΔE / kT), with the values of K0, ΔE, and K1 depending on both the solute and the solvent. An unexpected increase in lifetime at very low temperatures has been observed for benzene. For C6H6 and C6H6 in cyclohexane the ratio ΦP / ΦF = kpkISCτP / kF has been determined by numerical integration of the corrected emission spectra. Plots of ΦP / ΦF vs τP yield straight lines.
Iodine Mössbauer Studies of Chemical Bonding in Iodobenzene and Several Polyvalent Iodine Derivatives
Authors: Ehrlich Benjamin S. , Kaplan Morton
Published: February 2004

Mössbauer resonance experiments have been carried out at 4.2°K to investigate the iodine bonding in iodobenzene, iodobenzene dichloride, iodosobenzene, iodoxybenzene, and an iodosodilactone. The quadrupole splitting, asymmetry parameter, and isomer shift data have been analyzed to demonstrate the existence of pure p bonding in all of the compounds except PhIO2, where a small amount of s hybridization in the iodine p orbitals was found. A localized hybridization model cannot provide an adequate description of the iodine bonding in PhICl2, PhIO, and the iodosodilactone, whereas a delocalized molecular orbital approach is in good agreement with the experimental results. The C–I–O bond angle in PhIO is shown to be near 90°, rather than linear, and the iodosodilactone has an approximately planar configuration with a linear IO2 group. PhIO2 has iodine orbital populations significantly different from the other compounds, which can be associated with an O–I–O bond angle of about 95° in this molecule.
Relaxation Processes in Water. The Spin–Lattice Relaxation of the Deuteron in D2O and Oxygen-17 in H217O
Authors: Hindman J. C. , Zielen A. J. , Svirmickas A. , Wood M.
Published: February 2004

An experimental study of the deuteron relaxation time T1 has been made over a temperature range − 18–178°C. The equation for the temperature dependence of the relaxation time is of the form − lnT1 = ln(aeb/T + ced/T). The data are interpreted in terms of an equilibrium leading to a species which relaxes by an isotropic rotational diffusion process. Using transition rate theory and a quadrupole coupling constant derived for the relaxing species from dielectric data, heats and entropies are calculated for both the equilibrium and rate processes. For the equilibrium ΔH = 6.8 ± 0.2 kcal mole−1 and ΔS = 24.8 ± 0.9 e.u. mole−1. For the rate, ΔH* = 2.50 ± 0.06 kcal mole−1 and ΔS* = 3.6 ± 0.1 e.u. mole−1. Similar measurements for the oxygen-17 relaxation time T1 over the temperature interval − 14–180°C yield for the equilibrium ΔH = 5.6 ± 0.3 kcal mole−1 and ΔS = 20.7 ± 1.4 e.u. mole−1. For the rate, ΔH* = 2.43 ± 0.08 kcal mole−1 and ΔS* = 3.9 ± 0.2 e.u. mole−1. The results are discussed in terms of models for the water structure, the species present in the liquid, the relaxation process and molecular motion in the liquid.
Vibrational and Rotational Relaxation in Sulfur Dioxide
Authors: Bass H. E. , Winter T. G. , Evans L. B.
Published: February 2004

Ultrasonic absorption measurements have been used to study the vibrational and rotational relaxation in SO2 from 290 to 1090°K. This work extends the temperature range of earlier investigations by 500°K and is in agreement with the other studies in the low-temperature region. The vibrational data has been analyzed by assuming that the 518-cm−1 node exhibits a relaxation time τ1, that the 1151- and the 1361-cm−1 modes relax together with a relaxation time τ2,3, and that the two relaxations occur in parallel. It is found that τ1 increases with increasing temperature, while τ2,3 decreases with increasing temperature in the normal manner. The two vibrational collision numbers are a factor of 10 apart at room temperature but approach each other at high temperature. This is the first ultrasonic measurement of rotational relaxation in SO2, and the results confirm those of a previous thermal transpiration measurement but conflict with an earlier thermal conductivity measurement.
Quasilattice Features of Concentrated Aqueous LiNO3 Solutions
Authors: Irish D. E. , Nelson D. L. , Brooker M. H.
Published: February 2004

The Raman Spectra of concentrated aqueous lithium nitrate solutions contain lines at 737, 1389, and 1458–1464 cm−1, characteristic of a nitrate ion in the first coordination sphere of lithium and lines at 718, 1352, and 1429 cm−1 typical of more dilute aqueous solutions. These spectra are compared with those of the hydrates LiNO3·3H2O and LiNO3·2.4H2O, the anhydrous crystal, and the molten salt. Regions with the water-governing structure and with the latticelike structure simultaneously exist over a range of concentrations. This interpretation is consistent with a quasilattice model. A place-exchange equilibrium constant is estimated for a nitrate ion entering the first coordination sphere of a lithium ion.
Improved Error Bounds for the Long-Range Forces between Atoms
Authors: Starkschall George , Gordon Roy G.
Published: February 2004

Error bounds for the long-range attractive potential between pairs of atoms and molecules have been calculated by constructing oscillator strength distributions which reproduce known dynamic polarizabilities, oscillator strengths, and photoionization cross sections. Results from the theory of moments are applied to construct these distributions so that they provide rigorous error bounds on the force constant. Approximate solutions to the moment equations which result are found by means of linear programming. Results obtained for rare-gas interactions are found to be consistent with known values, with error bounds significantly tighter than those obtained from previous calculations. These same oscillator strength distributions are used to calculate error bounds on the dynamic polarizabilities for frequencies between those of the existing measurements and the first dipole-allowed transition.
Quantitative Comparison of Gas–Surface Theory with Molecular Beam Data
Authors: Lorenzen J. , Raff L. M.
Published: February 2004

The predictions of a detailed lattice model for an (Ar/W) system are quantitatively compared with molecular beam data reported by Yamamoto and Stickney. The quantities calculated include (a) phase shifts and average energy transfer as a function of scattering angle, (b) spatial density patterns, and (c) scattered velocity distribution patterns as a function of scattering angle. The results for surface temperatures of 2100 and 1090°K and incidence angles of 40° and 60° are reported. The extent of agreement between lattice model predictions and the beam data is fair to good. The average error is about 15%. The heat of adsorption for Ar on W predicted from the empirically determined potential surface is in quantitative agreement with experiment while the predicted Debye temperature of W surface atoms is significantly less than that of the bulk. This is in qualitative agreement with the surface data for Pt systems reported by Lyon and Somorjai. The general accuracy obtained in the computations indicates that such lattice models can provide a powerful tool for the interpretation, prediction, and correlation of molecular beam results.
Optical Pumping of the Lowest Triplet State and Multiple Resonance Optical Techniques in Zero Field
Authors: El-Sayed M. A.
Published: February 2004

The principles involved in the optical detection of zero-field transitions, electron spin–electron spin double resonance (EEDOR) transitions, and electron spin–nuclear spin double resonance (ENDOR) transitions in zero-field of the lowest triplet state are briefly summarized. Equations are then derived which relate the changes in the phosphorescence intensity originating from any of the zero-field levels to the radiative and nonradiative intramolecular rate constants of processes involving the triplet state upon:

(a) saturation of any of the two zero-field transitions involving the emitting level with microwave radiation;

(b) simultaneous saturation of two zero-field transitions, only one of which involves the emitting level optical detection of electron–electron double resonance (EEDOR);

(c) simultaneous saturation of a zero-field transition involving the emitting level and a hyperfine transition involving either of the two electron spin levels being saturated with microwave (optical detection of electron–nuclear double resonance, ENDOR).

These equations are derived for three different systems, in all of which the spin-lattice relaxation processes are assumed negligible (i.e., systems at very low temperatures). In System I, the triplet state is pumped by singlet–singlet absorption followed by the intersystem-crossing processes in the molecule studied (guest). In System II, pumping of the lowest triplet state is carried out by singlet→triplet absorption of the guest. In System III, pumping of the lowest triplet state of the guest is carried out by exciting either the singlet or the triplet states of the host, which is followed by triplet–triplet energy transfer. In System I, the changes in intensity are related to the rate constants of the intersystem-crossing processes to and from the individual zf levels of the molecule being studied. In System II, the radiative rate constants of the optical transition used for pumping, as well as the decay constants of the zf levels of the lowest triplet state of the guest molecule, determine the changes in the phosphorescence intensity upon microwave or radiowave saturation. In System III, in addition to the decay constants of the emitting triplet state, the intersystem-crossing (S1 ↝ T1) rate constant or the singlet→triplet absorption cross section of the host molecule determines the predicted changes in the phosphorescence intensity in the multiple resonance experiment. The relative orientation of the magnetic principal axes of the host to those of the guest is also important in System III. The conservation of spin direction in triplet–triplet energy transfer is briefly discussed. From the equations derived and the observed intensity changes upon microwave saturation, the ratios of important unimolecular rate constants for radiative and nonradiative singlet–triplet processes involving the individual zero-field levels can be determined. Comparison between theoretical predictions and observed effects is made whenever results are available.
Algebraic Symmetry and Quantum Secular Equations
Authors: Clark Michael G.
Published: February 2004

The algebraic symmetry of a Hamiltonian of “spin–Hamiltonian type” having the form H = A·J + J·B·J is examined. The form of the coefficients in the characteristic equation is deduced, and symmetry-adapted parameters and coupling coefficients are introduced. Any polynomial function of the matrix elements of the Hamiltonian which is invariant under all transformations of the frame of reference can be written as a polynomial in the symmetrized parameters. By use of coupling coefficients direct calculation of all but a limited number of matrix elements can often be avoided. The study of nuclear hyperfine interactions in solids by spectroscopic techniques (NMR, NQR, Mössbauer) is taken as an illustrative example.
Ising Chain with Competing Interactions in a Staggered Field
Authors: Nagle J. F. , Bonner J. C.
Published: February 2004

Work on the Ising chain with competing interactions is extended to the case where a staggered field is included as well as the usual direct field. A simple transformation enables one to interpret the behavior of this model as a metamagnet. Tricritical points are found and analyzed, and the observability of the tricritical exponents, as distinguished from the usual critical exponents, is discussed.
ESR Studies of BO32− Ions in Potassium Borate Ceramics
Authors: Taylor P. C. , Griscom D. L. , Bray P. J.
Published: February 2004

BO32− ions have been observed and identified by ESR and diffuse reflectivity measurements in a series of potassium borate ceramics. The spin-Hamiltonian parameters for these ions in the ceramic were investigated as a function of temperature from 4.2 to 300°K. At 77°K, the parameters are found to be: A1 = 7.3(2) G, A2 = 7.6(2) G, A3 = 12.2(2) G, g1 = 2.0143(2), g2 = 2.0128(2), g3 = 2.0091(2). Useful trends in the isoelectronic series of paramagnetic ions BO32−, CO3−, NO3 are examined. Using the observed hyperfine coupling constants and general molecular orbitals for the unpaired spin consistent with a 23-electron, AB3-type molecule (a2′, e′, or e″ orbitals), the s density on the central atom is found to be small and negative for all three species. The p density is also found to be small and negative when a2′ or e″ orbitals are assumed, but may be positive when an e′ orbital is assumed. If the orbital for the unpaired spin is approximately a2′, then the g tensors for all three isoelectronic ions are in agreement with planar distortions of the AB3-type molecules with these distortions increasing as one goes down the series from NO3 to CO3− to BO32−.
Floating Spherical Gaussian Orbital Model of Molecular Structure. VIII Second-Row Atom Hydrides
Authors: Chu San Y. , Frost Arthur A.
Published: February 2004

The FSGO model is applied to the second-row hydrides: NaH, MgH+, AlH, SiH4, PH3, H2S, and HCl. The trends in the bond lengths and electronegativities are correctly predicted. The bond lengths are predicted quantitatively with an average deviation of 3%. The calculated bond angle in PH3 is 90.9° compared with the experimental value of 93.2°, while for H2S the values are 87.7° compared with 92.2°.
Optical Saturation of a Single Vibration–Rotation Transition in the ν3 Fundamental of SF6
Authors: Djeu N. , Wolga G. J.
Published: February 2004

A well-resolved vibration–rotation transition in the ν3 infrared fundamental of SF6 has been revealed at 200°K by using the tuned laser spectroscopy technique. The frequency-dependent saturation behavior of that transition has been studied at low pressures, and the results fitted to theoretically predicted curves. From the pressure dependence of the saturation parameter, the quantity (γ′ / γ)Aab, which is the product of the Einstein Aab coefficient and the ratio of the dephasing collision rate and the rotational relaxation rate, was determined to be 53 sec−1. This is considered as a good upper bound for the Einstein Aab coefficient itself.
Studies in the Single-Center Method
Authors: Anderson Alfred B.
Published: February 2004

The Coulson–Barnett expansion and a Taylor series in powers of cosθ for the off-center hydrogen 1s function are examined. Energy converges as L−4 in the former case and L−9/4 in the latter, where L is the expansion length. Other rates are found. The single-center integral Hellmann–Feynman method is briefly discussed. Radial-angular separation using a function of correct asymptotic behavior is also considered.
Experimental Determination of the P–T Melting Curve of Argon
Authors: Hardy W. H. , Crawford R. K. , Daniels W. B.
Published: February 2004

Techniques for the precise measurement of the P–T melting curves of gases at pressures up to about 12 kbar have been developed. In the method used, a pressure vessel is maintained at a temperature constant to within ± 0.002°K. The pressure in the system is adjusted by manipulation of the pressure generating intensifier until solid–fluid coexistence is obtained within the vessel. The melting temperature is then measured with a platinum resistance thermometer mounted on the vessel, while the corresponding melting pressure is measured with a high-precision manganin resistance pressure gauge, which is connected to the pressure system. This method has been employed in making extensive P–T melting curve measurements in argon at pressures ranging from 0.58–11.41 kbar and temperatures from 97.8–272.9°K. The estimated precision in the results is ± 0.4 bar in pressure and ± 0.002°K in temperature. The absolute accuracy of the measurements, especially pressure, may be somewhat less due to uncertainty in the defining scales. The data show systematic deviations of as much as 10 bar from a Simon melting equation of the form Pm = ATmc + B. These deviations can be substantially reduced by use of a melting equation of the form Pm = A(Tm + D)c + B.