bluuebunny/crossref_metadata_2025 huggingface dataset:
Volume Generation and Hall Mobility of Holes in Anthracene
Authors: Dresner J.
Published: February 2004

The reversible chemisorption of iodine vapor on anthracene is shown to produce a space charge layer of trapped holes extending from 2 × 10−4 to 2 × 10−3 cm below the surface. The surface iodine level lies approximately 0.73 eV above the valence band. The Hall effect for free holes, generated in this layer by weakly absorbed light and moving parallel to the surface, was measured for currents along the b axis and magnetic field along the c′ axis. For eight samples, p-type signals were measured with mobilities 8–15 cm2 V−1·sec−1. The theoretically predicted sign reversal was not found. For both electrons and holes generated by strongly absorbed light in the singlet band, the measured mobilities exceed 20 cm2 V−1·sec−1. The order-of-magnitude difference between Hall mobilities in the region of generation and drift mobilities in the bulk (1 cm2 V−1·sec−1) is compatible with the existence of both narrow and broad bands for electrons and holes. The wide band mobility is calculated to be of the order of 200 cm2 V−1·sec−1. In a time-of-flight experiment, only the low-mobility component is observed. These results also eliminate the hypothesis that the Hall effect may be anomalous in the narrow bands only.
NMR Study of the Ferroelectric Transitions in Diglycine Nitrate and Trissarcosine Calcium Chloride
Authors: Blinc R. , Jamšek-Vilfan M. , Lahajnar G. , Hajduković G.
Published: February 2004

A proton magnetic resonance spin–lattice relaxation and second moment study of the ferroelectric transitions in diglycine nitrate (DGN), trissarcosine calcium chloride (TSCC), and its deuterated analog has been performed. The results support the order–disorder model of the ferroelectric transition in these compounds and demonstrate the freezing in of a “flipping” mode on going to the ferroelectric phase in analogy with triglycine sulphate. Deuteration results in a shift of the Curie temperature of TSCC from 127 to 149°K.
Relevance of the Concentration Dependence of Methyl Radical Formation from Methyl Bromide Solutions to the Distribution of Ion-Pair Separation Distances in Irradiated Hydrocarbons
Authors: Warman John M. , Rzad Stefan J.
Published: February 2004

A radiochromatographic technique for measuring the yields of methyl radicals formed on electron capture by methyl bromide in irradiated liquid hydrocarbons is described. G(CH3) has been measured in n-hexane, cyclohexane, and iso-octane from 3 × 10−6 to 0.3M CH3Br. Below 3 × 10−4M the yields are described by the equation G(CH3) = Gfi + K(CH3Br)1/2 in agreement with theory. The values of Gfi (free-ion yield) determined are 0.12, 0.13, and 0.36 for n-hexane, cyclohexane, and isooctane, respectively. Over the entire concentration range studied, G(CH3) can be described by the empirical expression G(CH3) = Gfi + (Ggi(αCH3Br[CH3Br])1 / 2 / {1 + (αCH3Br[CH3Br])1 / 2}). A value of approximately 4 is found for the yield of geminately recombining ion pairs for the three hydrocarbons. It is argued from the high-concentration data that the distribution of thermalization distances are closely similar for the different hydrocarbons, in contradiction to the predicted longer distances in iso-octane required to explain the high free-ion yield.
Nuclear Magnetic Resonance of Polymer Fibers
Authors: McBrierty V. J. , McCall D. W. , Douglass D. C. , Falcone D. R.
Published: February 2004

Nuclear magnetic relaxation times T2, T1, and T1ρ are reported for a tetrafluoroethylene–hexafluoropropylene copolymer (FEP), both in bulk and in drawn fiber form. The measurements cover a temperature range from − 200 to + 250°C. Relaxation times T2 and T1ρ exhibit marked anisotropy with fiber orientation in the magnetic field. The effect is less pronounced in the T1 case. Theoretical expressions for T2, T1, and T1ρ have been derived for the fiber case, and the results are presented. Lattice sums have been computed for rotation and rotation–translation modes of molecular activity and the values used to predict theoretical relaxation times. Quantitative agreement between theory and experiment is excellent for T2 and satisfactory for T1 and T1ρ, in support of the proposed molecular models. Relaxation effects previously labeled γ or Glass II are assigned to molecular reorientation about the helix axis, with the motion occurring in both the amorphous and crystalline regions of the polymer. The α or Glass I relaxation is assigned to longitudinal molecular translation. The possibility of a helix interconversion cannot be ruled out. The NMR data show that the perfluoromethyl–CF3 groups are locked in and do not rotate. Finally, no indication of a preferred orientation of chains in the amorphous regions is in evidence.
U Band in CsI
Authors: Tanton G. A. , Shatas R. A. , Singh R. S. , Mitra S. S.
Published: February 2004

The first electronic transition of substitutional H− and D− ions in CsI has been measured between 8 and 300°K. The absorption peak of the U band is located at 4.60 eV at 8°K. A small blue shift of 0.015 eV is observed upon replacing H− by D−. The method of moments has been employed to analyze the U-band shape. The temperature-induced broadening of the half-width and the shift of the peak is explained in terms of an effective phonon frequency representing the phonon spectrum of the host lattice.
Study of the Statistical Model for Molecular Collisions
Authors: Miller William H.
Published: February 2004

Formation of a collision complex is considered in terms of Feshbach's formulation of resonant scattering, and a generalized statistical approximation is obtained. It is seen how the usual statistical model follows as a special case; possible extensions are also discussed. A detailed application of the statistical model to rotationally inelastic collisions is presented, and general features of the total and differential cross sections are discussed. A simplified treatment of a chemical reaction which proceeds via complex formation is also presented.
Thermal Diffusion Measurements with a Modified Savart Interferometer
Authors: Wallin Lars-Erik
Published: February 2004

Measurements of the Soret effect have been made with the Savart interferometer, utilizing a double exposure technique for increased sensitivity. Data are reported on different concentrations of cadmium sulfate in water, the lowest concentration giving a difference of optical path between top and bottom of the order of one wavelength in the stationary state. The technique should be capable of giving data from solutions a few thousandths molal in cadmium sulfate.
Primary Processes of Hydrogen Formation in the Gas- and Liquid-Phase Radiolysis of Propane
Authors: Fujisaki Noboru , Shida Shoji , Hatano Yoshihiko
Published: February 2004

The yield of hydrogen formed in the gas- and liquid-phase radiolysis of propane has been measured in the presence of a “radical scavenger” (C2H4) and an electron scavenger (N2O or SF6). Further, the isotopic compositions of the hydrogen formed in the gas-phase radiolyses of C3H8–C3D8, C3H8–4 mole % SF6–C3D8, and C3H8–4 mole % C2H4–C3D8 mixtures and also those in the liquid-phase radiolyses of C3H8–C3D8, C3H8–0.4M SF6–C3D8, and C3H8–0.2M C2H4–C3D8 mixtures have been measured to determine the unimolecular fraction of hydrogen for each mixture. Combining these results, the yields of six primary processes by which hydrogen is formed have been evaluated. The results are that for the gas-phase radiolysis G (thermal hydrogen atom), G (hot hydrogen atom), and G (unimolecular hydrogen) are, respectively, 2.5, 0.9, and 1.4 by direct excitation and ionization, and that G (thermal hydrogen atom and hot hydrogen atom) and G (unimolecular hydrogen) are, respectively, 1.9 and 0.7 by neutralization. For the liquid-phase radiolysis G (thermal hydrogen atom), G (hot hydrogen atom), and G (unimolecular hydrogen) are, respectively, 0.9, 1.2, and 0.4 by direct excitation and ionization and 0.6, 0.7, and 1.0 by neutralization with probable errors of ± 0.2. The G values of the six primary processes for propane in the liquid-phase radiolysis are almost the same as those for n-butane previously reported. A possible role of hot hydrogen atoms has been pointed out in the gas-phase as well as in the liquid-phase radiolysis of propane. It is also suggested that ethylene scavenges with high efficiency not only thermal hydrogen atoms but also positive ions, expecially in the gas phase.
Nonexponential Relaxation of Rotating Three-Spin Systems in Molecules of a Liquid
Authors: Hubbard Paul S.
Published: February 2004

The longitudinal and transverse nuclear magnetic relaxation of a system of three identical spin-12 nuclei at the corners of an equilateral triangle which undergoes hindered rotation with respect to a molecule which undergoes rotational Brownian motion has been calculated on the assumption that the relaxation is due to dipole–dipole interactions between the three nuclei, including the effects of cross correlations. The longitudinal relaxation is in general the sum of three decaying exponentials. If the rotational Brownian motion is rapid compared to the Larmor frequency, the longitudinal and transverse relaxations are both proportional to A1exp(s1t) + A2exp(s2t). For a molecule of arbitrary shape, the expressions for the coefficients A1 and A2 and the negative exponents s1 and s2 are quite complicated. The expressions simplify somewhat for the case of a symmetric top molecule, and even more for the case of a spherical molecule. It is shown for a molecule of arbitrary shape that, if the hindered rotation is rapid compared to the rotational Brownian motion, which in turn is rapid compared to the Larmor frequency, then the predicted relaxation is distinctly nonexponential.
Positron Annihilation in Concentrated Cesium–Ammonia Solutions
Authors: Arias-Limonta Jose A. , Varlashkin Paul G.
Published: February 2004

The momentum distribution of photons from positrons annihilating in liquid and solid cesium–ammonia solutions was found to be concentration dependent. The narrow component was found to broaden with increasing concentration. The results are compared with those obtained for pure cesium, and one is led to believe that a bound species is formed in these solutions even at very high concentrations. No evidence of a Mott transition is found in cesium–ammonia solutions, and no abrupt change is found in the shape of the momentum distribution in going from 22 to 94 mole % cesium in ammonia. A conduction-electron-type analysis is presented for the density of states and the probability density. Methods of preparing relatively stable cesium–ammonia solutions are discussed in some detail.
Effect of Spin–Orbit Interactions on Radiative Triplet–Singlet Transitions in Aromatic Hydrocarbons
Authors: Adrian Frank J.
Published: February 2004

The 3(ππ*) → 1(ππ) phosphorescence spectra of many aromatic hydrocarbons are out-of-plane polarized. The electric-dipole transition moment due to the indicated mechanism of σπ singlet–triplet mixing by the spin–orbit interaction is calculated as the sum of contributions from σπ excitations of the individual localized σ bonds. The singlet 1(ππ) is mixed with triplets produced by σ → π* and π → σ* excitations of 1(ππ). The triplet 3(ππ*) is mixed with singlets produced by σ → π and π* → σ* excitations of 3(ππ*). A general expression is obtained for the radiative triplet–singlet transition probability in terms of the π-electron charge distribution in the ground and triplet states. Semiempirical estimates of the σπ transition energies and other parameters in the expression for the transition moment give order-of-magnitude agreement between the computed and observed triplet-state lifetimes.
Estimation of High-Polymer Excluded Volume from Numerical Studies on Short Chains
Authors: Bloomfield Victor A. , McKenzie Donald S.
Published: February 2004

The mean-square length 〈R2〉 of a polymer molecule composed of n segments is assumed to behave asymptotically as 〈Rn2〉 ∼ An1+ε for large n, where A is a constant and ε takes into account the excluded-volume effect. This paper compares calculations of 〈Rn2〉 for the model of a polymethylene chain without excluded volume studied by Flory and Jerigan with the model of a polymer as a self-avoiding walk on a crystal lattice which has been studied by the method of exact enumeration developed by Domb. For the first model ε = 0, whereas for the second, which includes the excluded-volume effect, ε = 15. It is shown using the extrapolation technique of Neville tables that the exact-enumeration method correctly predicts ε = 0 for the Flory–Jernigan model. However, the rate of convergence is very much slower than comparable calculations using the fcc lattice and taking account of excluded volume.
LCAO–MO–SCF Calculations Using Gaussian Basis Functions. VI. LiBeLi
Authors: Kaufman Joyce J. , Sachs Lester M.
Published: February 2004

A LCAO–MO–SCF investigation of LiBeLi utilizing Gaussian 985p basis functions established LiBeLi to be a stable species, linear in its ground state. The equilibrium bond distance was determined, and force constants and normal frequencies were obtained. Using a procedure for estimating the correlation energy similar to the one we employed successfully for BeH2, we determined the dissociation energy of LiBeLi. Our results imply that it may be possible to form an LiBeLi alloy which would be denser than either Li metal or Be metal themselves.
Equilibrium Properties of Ionic Solutions; The Primitive Model and its Modification for Aqueous Solutions of the Alkali Halides at 25°C
Authors: Rasaiah Jayendran C.
Published: February 2004

The hypernetted-chain (HNC) equation, which is accurate up to concentrations of 1M, is applied to test the validity of some models that have been used previously to describe ionic solutions. The osmotic coefficient (φ) calculated for the primitive model, with the distances of closest approach (aij) chosen to fit the experimental data at low concentrations (< 0.1M), is too large in comparison with experiment at 1M. This contradicts the basic assumption in several hydration theories that have appeared, in which the primitive model contributions to φ are assumed, on the basis of approximate theories, to be too small at high concentrations. When the aij parameters are the sums of the Pauling radii, the characteristic order of the experimental osmotic coefficients of the alkali halides is preserved even when the primitive model contributions have been subtracted out. It is shown that the residual short-range effects necessary to obtain qualitative agreement with the experimental osmotic coefficients and heats of dilution can be represented by a square well, and that the sign and magnitude of the well depth and its temperature coefficient seem to reflect the nature of the overlap contributions of the cospheres around each ion.
Infrared and Raman Spectra of Polycrystalline Glyoxal
Authors: Verderame F. D. , Catellucci E. , Califano S.
Published: February 2004

The infrared spectrum from 4000 to 70 cm−1 and the laser Raman spectrum of thin films of polycrystalline glyoxal at − 185°C are reported. A 400:1 N2–glyoxal matrix frozen at 5°K helped to identify exciton components. There appears to be little difference in symmetry between the “free” and the crystallized or matrix isolated molecule which retains a centrosymmetric s-trans structure in the crystal. The infrared bands originating from fundamental vibrations ν10, ν11, and ν12 are each exciton split into three components while two of the Raman bands, ν1 and ν4 exhibit four components. These observations lead to the conclusion that each molecule occupies a Ci site in the crystal, and the factor group or unit cell group of the crystal must be isomorphous with D2h.
Irradiation of Xenon–Propane and Argon–Propane Mixtures with γ Rays and 21.2-eV Photons
Authors: Lias S. G. , Rebbert R. E. , Ausloos P.
Published: February 2004

Mixtures of xenon or argon with propane (20:1) were irradiated in the presence of radical scavengers by γ rays and by 21.2-eV photons from a helium resonance lamp. The ion-pair yields of products orginating from ionic fragmentation reactions or ion–molecule reactions are derived from isotopic analysis of product molecules formed in C3D8–C3H8 mixtures or in CD3CH2CD3. At a given pressure, there are only small differences between the radiolysis and photolysis in the modes of dissociation of parent propane ions formed by charge transfer with a particular rare gas. Higher-energy ionic fragmentation processes are quenched when the pressure is increased; in the case of xenon–propane mixtures it is demonstrated that the parent ion yield increases as primary fragmentation processes are quenched. In the irradiated argon–propane–scavenger mixtures, isotope effects on the ionic fragmentation of the excited deuterated fragment ions are pronounced. The decomposition of neutral propane molecules excited to lower resonance states accounts for not more than 10%–12% of the product-forming processes in the xenon mixtures.
“Anomalies” in the Radial Distribution Functions for Simple Liquids
Authors: Fehder Paul L.
Published: February 2004

Subsidiary features appearing in the radial distribution functions obtained by x-ray or neutron diffraction measurements on a number of simple liquids have long been the object of speculation and some controversy. Distribution functions calculated from the configuration data for a computer-simulated model of a dense fluid of Lennard-Jones disks show similar additional structure. In the case of the model system, this additional structure may reflect an alternative configuration for local ordering within the fluid.
New Approach to Nonradiative Intramolecular Transitions
Authors: Berrondo Manuel , Weiss Karl
Published: February 2004

A unified and completely time-independent approach based on Feshbach's formalism for nuclear reactions, in which radiative and nonradiative processes are treated on an equal basis, is used to describe the behavior of isolated molecules in a radiation field. The expressions developed are complete and link excited states of measurable lifetime to resonances arising from the action of Green functions. Approximations in the form of the assumption that the ground and excited states can be described as Born–Oppenheimer states are finally introduced only to show how matrix elements could be estimated and to relate our results to those of previous studies. An important feature of the present formulation is that, except for particular models as the ones used by Jortner and co-workers, it demonstrates that the radiative and nonradiative decay modes for large molecules are independent of each other to a high degree of approximation. Without substantial modification, the present model yields no information about transitions between excited states.
Molecular Structure of Xenon Tetroxide, XeO4
Authors: Gundersen Grete , Hedberg Kenneth , Huston John L.
Published: February 2004

The molecular structure of XeO4 has been investigated in the gas phase by electron diffraction. The data are completely compatible with the tetrahedral structure proposed from analysis of the infrared spectrum. Refinement of the structure by least squares based upon intensity functions, treating each distance and amplitude as independent parameters, yielded the results rXe–O = 1.736 Å (0.0023), rO...O = 2.832 Å (0.0086), lXe–O = 0.0468 Å (0.0025), and lO...O = 0.0866 Å (0.0076). Parenthesized values are 2σ and include estimates of systematic error.
Predissociation
Authors: Riess Ilan
Published: February 2004

The coupling of unstable states within an isolated system is discussed with special emphasis on intramolecular transitions. Using time-independent perturbation theory and normalization in a closed box, the coupling of a discrete state with a one-dimensional free-particle continuum is considered. The structure of the exact levels depends on the relative positions of the discrete state and the continuum. When the discrete state is degenerate with a state in the continuum, it is broadened to approximately an asymmetric Lorentzian shape, slightly shifted towards higher energy and of width given approximately by the “golden rule.” A new state is also found below the continuum. When the discrete state lies below the continuum, it is quasishifted towards lower energies. In this case the broadening is far from being Lorentzian, but is rather a delta function at the exact discrete state and much smeared throughout the continuum. The coupling of two discrete states with a continuum and the coupling of two continua is then discussed. The first case is quite similar to the single state–continuum coupling. For the latter case the manifold of exact states is found to be approximately equal to the union of both interacting continua, except for a new discrete state that is found below the two continua. The broadening of any state of either continua has a strong singularity at its original energy value E0 of the form 1 / (E0 − E)2. Hence the transition probability from one continuum to another is very small, and predissociation by optical excitation into one continuum is hardly possible.
Projected Hartree Product Wavefunctions. IV. Radial Correlation in Some Three- and Four-Electron Atoms
Authors: Heikes L. G. , Gallup G. A.
Published: February 2004

The energies of some three- and four-electron atoms including radial correlation have been determined by a spin-free version of the projected Hartree–Fock method. The wavefunction possesses the maximum flexibility allowable under the general restrictions of the method, and it is shown that this flexibility is necessary to adequately account for the differences between intra- and intershell correlation. Some comments on the AMO method are included.
Projected Hartree Product Wavefunctions. V. Density Matrices and Operators Containing Spin
Authors: Gallup G. A.
Published: February 2004

Formulas are given to allow the calculation of the various spin components of the first-order density and transition density matrices from one spatial component of a pure spin-state wavefunction and certain reduced operators. Similar expressions are given for the second-order density for singlet and doublet systems. These results are useful for the calculation of matrix elements of operators containing the spin.
Corrections to the Kinetic Theory of Fluid Diffusion. II. Binary Mixtures
Authors: Dymond J. H. , Alder B. J.
Published: February 2004

An analysis of high-temperature diffusion experiments in dense-fluid mixtures reaffirms that the hard-sphere theory is nearly quantitative in its predictions. It is necessary to apply the accurate hard-sphere theory which involves corrections to the Enskog theory due primarily to a long persisting vortex flow. This correction has the expected qualitative dependence on the mass of the diffusing species as determined from molecular dynamics and confirmed by the analysis of the experiments.
Microwave Spectrum, Dipole Moment, and Barrier to Internal Rotation of 1-Methylcyclobutene
Authors: Huang Y. S. , Beaudet Robert A.
Published: February 2004

The microwave spectrum of 1-methylcyclobutene was observed in the range 8–38 GHz. The potential barrier hindering the internal rotation of the methyl group was determined to be 1666 cal/mole and V6 to be −39 cal/mole. The dipole moment was found to be μa = 0.322 D, μb = 0.156 D, and μtotal = 0.357 D.
Thermodynamic Behavior of Dilute Solutions of Hydrogen in Niobium
Authors: McQuillan A. D. , Wallbank A. D.
Published: February 2004

In a recent paper on the thermodynamic properties of dilute solutions of hydrogen and deuterium in β-titanium and β-zirconium anomalous behavior was observed. It was concluded that a similar investigation on the niobium–hydrogen system might well provide a better insight into this phenomenon, should it prove to exist in the system, because the temperature range of dilute hydrogen solubility in the body-centered cubic structure of the niobium–hydrogen system is much more extensive than in the corresponding titanium and zirconium systems. It has now been shown that departures from Sievert's law are present in dilute hydrogen solutions in niobium. Departures from random solution are well marked and prove to have a strong temperature dependence. An analysis of the thermodynamic behavior of the niobium–hydrogen system together with some results on the niobium–deuterium system would seem to indicate that a small degree of clustering is present in such dilute alloys.
Metastable Ions and Isotope Effects in the Mass Spectra of Propane and Deuterated Propanes
Authors: Vestal Marvin , Futrell Jean H.
Published: February 2004

Metastable transitions occurring in the mass spectrum of C3H8, CH3CD2CH3, CD3CH2CD3, and C3D8 have been investigated by means of the defocusing technique using a double-focusing mass spectrometer. In addition, detailed calculations of the decomposition of propane ions have been carried out, using the quasiequilibrium theory of mass spectra. Very strong isotope effects are observed and are accounted for quantitatively, and it is concluded that the quasiequilibrium theory provides an adequate framework for the description of both metastable ions and the general fragmentation processes responsible for the mass spectra of the deuterated propanes.
Scattering of Low-Energy Electrons from Hydrogen-Covered Mo(100) Surfaces
Authors: Dooley George J. , Haas T. W.
Published: February 2004

Hydrogen has been found to adsorb onto the Mo(100) surface at room temperatures in two states. The first gives rise to LEED patterns with extra-order beams. The second structure arises from full monolayer coverage and gives only integral order spots. The full coverage structure is easily electron-beam desorbed to give the lower coverage structure.
Distribution Function for Self-Avoiding Walks
Authors: Chay T. Ree
Published: February 2004

In this paper the asymptotic behavior of the distribution function for self-avoiding walks is derived from the generating function Gn(θ), where n is the number of steps and θ is a variable of the function. The asymptotic form of Gn(θ) for large n is determined as follows: First, from the subadditive property of lnGn(θ) we prove rigorously the existence of κ(θ) = limn→∞(1 / n) lnGn(θ) and determine its upper and lower bounds. Second, from the enumeration data for small n we determine the form of Gn(θ) exp[− nκ(θ)] for large n. The relation between δ and ν, defined, respectively, by Fn(x) ∼ exp[− (x / xn)δ] and xn ∼ nν, are derived from the distribution function thus obtained.
Microwave Spectra, Barriers to Internal Rotation of the Methyl Group, and Molecular Electrical Dipole Moments in trans- and cis-1,3-Pentadiene
Authors: Hsu S. L. , Flygare W. H.
Published: February 2004

The microwave spectra of trans- and cis-1,3-pentadiene have been assigned. The barriers to internal rotation of the methyl group and the molecular electric dipole moments are also measured. The barriers to internal rotation are discussed in light of recent theories and the experimental work on other substituted propenes. The specific results obtained here are listed below. trans-1,3-pentadiene: The rotation constants are A = 28.2 ± 0.7 GHz, B = 2160.61 ± 0.01 MHz, and C = 2033.21 ± 0.01 MHz. The barrier to internal rotation of the methyl group is V3 = 1805 ± 60 cal/mole. The electric dipole moments in the molecular plane are | μa | = (0.561 ± 0.002)D, | μb | = (0.16 ± 0.03)D, and | μ | = (0.585 ± 0.010)D. The total dipole moment is at an angle of 16.5° ± 3.0° from the a axis (see Fig. 1). cis-1,3-pentadiene: The rotational constants are A = 15.7 ± 0.1 GHz, B = 2659.17 ± 0.02 MHz, and C = 2306.12 ± 0.01 MHz. The barrier to internal rotation is 741 ± 50 cal/mole. The electric dipole moments in the molecular plane are | μa | = (0.465 ± 0.010)D, | μb | = (0.185 ± 0.015) D, and | μ | = (0.500 ± 0.015) D. The total dipole moment is at an angle of 21.6° ± 1.5° from the a axis.
Some Topics Concerning the Radius of Gyration of Linear Polymer Molecules in Solution
Authors: Fujita H. , Norisuye T.
Published: February 2004

Fixman's integral for the distribution function P(S2) obeyed by S2 of the unperturbed polymer chain was evaluated analytically, and its asymptotic forms valid for t ≪ 1 and t ≫ 1 were derived. Here S is the radius of gyration of the chain and t is the ratio of S2 to its statistical average 〈S2〉. The values of P(S2) calculated from the analytic solution agreed with Koyama's computer values over the entire range of t. The leading term of the asymptotic form for t ≪ 1 confirmed the previous result of Fixman. The asymptotic form for t ≫ 1 agreed not with Fixman's but with that of Forsman. The Flory–Fisk theory for the excluded volume effect in polymer chains was reformulated with the exact form being substituted for P(S2) in their basic equation for αs2. Here αs is the linear expansion factor of a polymer molecule defined in terms of 〈S2〉. The calculated curve for αs3 as a function of the interaction parameter z differed markedly from that derived by Flory and Fisk who had employed an approximate expression for P(S2). Our curve followed quite closely the one recently deduced by Yamakawa and Tanaka from an entirely different treatment. However, both are different in the limiting behavior at indefinitely large z, our curve tending to the fifth power type as represented by αs5 = 1.668z, whereas Yamakawa–Tanaka's approaches αs4.35 = 1.11z.
Scattering of High-Velocity Neutral Particles. XVII. Ar–O2, Ar–N2, Ar–CO
Authors: Jordan J. E. , Colgate S. O. , Amdur I. , Mason E. A.
Published: February 2004

Fast Ar beams with energies between about 200–1500 eV have been scattered by room-temperature O2, N2, and CO to obtain the following potentials (where the energy is given in electron volts and the interaction distance in angstroms): Ar–O2 V(r) = 1.36 × 103 / r8.34 2.01 < r < 2.50, Ar–N2 V(r) = 567 / r7.06 2.04 < r < 2.53, Ar–CO V(r) = 551 / r6.99 2.09 < r < 2.68. Results for these systems in the indicated ranges have not been previously reported from this laboratory. They are, however, in quite satisfactory agreement with independent measurements of Leonas and coworkers in Moscow, and show that even at the small separation distances involved in these high energy collisions, the average force fields on N2 and CO are essentially identical. The peripheral-force approximation, which assumes that the centers of force in a molecule are located at the nuclei of each atom, has been used to construct the individual atom–atom interactions which combine to make up the experimental atom–molecule potentials. In the one case where direct scattering measurements are available, the Ar−O system, agreement between the derived and experimental atom–atom potentials is excellent. Potentials have also been derived for the following atom-molecule and molecule-molecule systems by continuing the peripheral-force approximation with established semiempirical combination rules: N–N2; N–O2; N–NO; O–O2; O–N2; O–NO; N2–N2; O2–N2; O2–O2; N2–NO; O2–NO; and NO–NO. With the exception of molecule–molecule systems involving NO there is good agreement between the derived potentials and those obtained directly from scattering experiments. There is also reasonable agreement, considering the limitations of the models, between the derived potentials and theoretical calculations for systems where it is possible to make comparisons.
Pure Rotational Spectrum of Water from 108 to 40 μm
Authors: Hall Richard T. , Dowling Jerome M.
Published: February 2004

The pure rotational spectrum of water vapor has been obtained in the frequency range 92–250 cm−1 at a resolution of ≈0.14 cm−1. The frequencies of the 73 lines assigned to single transitions in the region ν > 122 cm−1 are compared with those calculated from energy levels determined from previously reported far-and near-infrared spectral data. In general, the agreement is within ±0.014 and ±0.011 cm−1 (rms deviations) for the energy levels determined from the far-infrared and near-infrared spectra data, respectively.
Electron Correlation and Separated Pair Approximation in Diatomic Molecules. III. Imidogen
Authors: Silver David M. , Ruedenberg Klaus , Mehler Ernest L.
Published: February 2004

The geminals of a many-electron separated pair wavefunction are expanded in terms of their natural orbitals, and a variational procedure is developed for their determination. Within this framework, natural orbitals of the separated pair wavefunction for the 3Σ− ground state of the imidogen, NH, molecule and the S4 ground state of the nitrogen atom are calculated, and in order to give a pictorial appreciation of their structures, contour maps have been drawn for them. Several physical properties of the NH molecular system are calculated, namely the total energy, the dissociation energy, the equilibrium internuclear distance, the molecular potential-energy curve, the spectroscopic constants, the molecular dipole and quadrupole moments, and various other one- and two-electron expectation values. Of greatest interest is the correlation energy recovered by the separated pair wavefunction and, for this reason, a detailed analysis of the correlation is given in terms of the geminals and the natural orbitals. The study reveals that the separated pair approximation provides a suitable description of part of the electronic correlation in NH but, on the other hand, is too restrictive to yield a complete description of all the various electronic correlations in this molecular system. Nevertheless, the analysis does shed light on the nature of the omitted part of the correlation and suggests directions for future improvements.
Nuclear Quadrupole Resonance Study of the Inductive Effect in Chemical Bonding in Chloroalkenes
Authors: Koltenbah David E. , Silvidi A. A.
Published: February 2004

An investigation was made of the inductive effect upon the C–Cl bonds in some straight-chain chloroalkenes at 77°K by means of the shifts of the 35Cl NQR frequencies. Attempts to crystallize the monochloro-1-butenes were unsuccessful, but the monochloropropenes were successfully crystallized, and their NQR frequencies were measured: 1-chloropropene (cis, trans mixture), 33.424 ± 0.030 MHz; 2-chloropropene, 32.619 ± 0.030 MHz; and 3-chloropropene (allyl chloride), 33.445 ± 0.006 MHz, the last of which had been previously measured by Hooper and Bray. Tabulation and discussion includes the other straight-chain mono- and dichloroalkenes for which NQR data have been previously reported. The relation of the NQR frequency to the bond parameters was interpreted according to the theory of Townes and Dailey. The interpretation of the NQR data for the chloroalkenes was made by analogy with that for the chloroalkanes and by comparison with microwave data for the chloropropenes and vinyl chloride. A double-bond character of Π = 0.05 was assumed for each of the nonconjugated chloroalkenes by analogy to both Bersohn's calculations and the microwave data for vinyl chloride. The hydridization parameters were estimated by the Townes-Dailey rules, and the asymmetry parameters were obtained from available microwave data. The ionicity, I, calculated from the NQR frequencies with these assumptions yielded values in the range 0.18–0.21 for the monochloroalkenes, which is closely that for the analogous monochloroalkanes. For the monochloropropenes, the ratio r between the principal quadrupole coupling constant in the gas phase and in the solid phase was found to range from 1.050 to 1.072, which is somewhat less than r = 1.083 reported by Dailey as the average for alkyl halides. It was concluded that the magnitude of the inductive effect on the NQR frequency of the nonconjugated chloroalkenes is not significantly different from that in the analogous saturated chlorine compounds. The NQR data for the chloroalkenes were consistent with the assumption that both the effect of hybridization and double-bond character are virtually constant in an homologous group of chlorine compounds and that shifts in NQR frequencies arise almost entirely from the inductive effects in the σ-bond distribution.
Anisotropies of the 19F Chemical Shifts in Fluorobenzene Compounds from NMR in Liquid Crystals
Authors: Nehring Jürgen , Saupe Alfred
Published: February 2004

Fluorine-19 shift anisotropies, which have been determined from NMR spectra of nematic solutions, are reported for 1,2-, 1,3-, 1,4-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and hexafluorobenzene. Combination of the anisotropies of different molecules leads to the following results for the principal values of the anisotropic part σ̂a = σ̂ − 13Trσ̂ of the screening tensor σ̂ for 19F nuclei without fluorine neighbors in ortho or para position: σ11a = −94 ± 12, σ22a = 27 ± 9, σ33a = 67 ± 3 in parts per million (2 axis in direction of C–F bond, 3 axis perpendicular to molecular plane). Fluorine substitution in ortho position changes the shielding tensor by Δσ11 = (+6 ± 3) × 10−5, Δσ22 = (−3 ± 3) × 10−5, Δσ33 = (+4 ± 1) × 10−5. There are difficulties in explaining the experimental results by the theory of Karplus and Das since a very high ionicity in the C–F bond has to be assumed that increases by ortho fluorine substitution.
Molecular Spin–Orbit Coupling Constants. The Role of Core Polarization
Authors: Walker T. E. H. , Richards W. G.
Published: February 2004

The ab initio computation of molecular spin–orbit coupling constants from Hartree–Fock wavefunctions is extended to second-row nonhydride diatomic molecules, with only the smallest integrals treated approximately. Agreement with experiment is good (∼5%), providing core polarization effects are included. The extension to polyatomic molecules seems possible since reasonable values are obtained even from minimal basis set wavefunctions, and the contributions from two-center integrals are self-canceling.
Molecular Beam Kinetics: Reactions of K Atoms with Alkyl Iodides
Authors: Kwei G. H. , Norris J. A. , Herschbach D. R.
Published: February 2004

Angular distributions of reactively scattered KI have been measured for several K+RI reactions, with R=CH3, C2H5, n- and i-C3H7, n-, i-, s-, and t-C4H9, n-C5H11, and n-C7H15. The reaction cross sections are roughly ∼30 Å2 and the activation energies are undetectably small, <̃0.5 kcal/mole, for the whole series of reactions. The laboratory angular distributions are strongly anisotropic. A qualitative analysis which requires only the conservation laws for energy and momentum shows that these reactions proceed predominantly via a “rebound” mechanism: An observer stationed at the center of mass would see most of the KI recoil into the backward hemisphere (and the R group forward) with respect to the incoming K beam. The kinematic analysis also indicates that for K + CH3I roughly half (60% ± 20%) of the energy of reaction (∼15 ± 5 out of ∼25 kcal/mole) appears as internal excitation of the products, probably mainly in vibrational excitation. As the size of the R group is increased, the backward peaking of the KI appears to become less pronounced; also, the energy released into relative translational motion of the products decreases and the internal excitation increases. The angular distributions of the K atoms scattered without reaction resemble those for scattering from unreactive molecules of similar size, except for some falloff in intensity at wide angles. The customary “rainbow” analysis (only rigorous for atom-atom scattering) yields 0.5 ± 0.15 kcal/mole for the depth of the van der Waals potential well, with no noticeable variation with the size of the R group.
Irreversible Electronic Relaxation in Polyatomic Molecules
Authors: Freed Karl F.
Published: February 2004

The general phenomena which are classified as involving radiationless transitions, e.g., intersystem crossing, internal conversion, are discussed in terms of a unified Green's function formalism for the cases of the large, intermediate, and small molecule limits for both isolated molecules and molecules with external perturbers. Because of the existence of a natural time limit on any experiment, e.g., an isolated molecule description is only valid for a limited time, irreversibility can “effectively” occur on these relevant time scales in molecules which only have discrete energy levels. The introduction of this natural time limit allows the derivation of completely general criteria for the occurrence of irreversible electronic relaxation in isolated and perturbed polyatomic molecules. The dependence of these criteria on the type of radiationless process is elucidated in a general manner. For completeness, the excitation process is treated as well as the decay. Because we wish to examine the general phenomena encompressed in “radiationless transitions,” only a simplified model for the excitation is used. However, this simple model is sufficient to examine those general aspects of “radiationless transitions” which are independent of the “fine details” of the exciting light.
X-Ray Photoelectron Spectroscopy of Simple Hydrocarbons
Authors: Thomas T. Darrah
Published: February 2004

Carbon 1s ionization potentials for methane (290.8 eV), ethane (290.6), ethylene (290.7), acetylene (291.2), cyclohexane (290.3), benzene (290.4), 2,2-dimethylpropane (290.4), and fluoroform (299.1), together with the fluorine 1s ionization potential for fluoroform (694.1) and ionization potentials for the 2ag and 2au orbitals of ethylene (24.5 ± 1 and 19.5 ± 1) and for the 2σg and 2σu orbitals of acetylene (23.5 and 18.5), have been measured. These values, together with those from other measurements, are compared with orbital energies obtained by use of Koopmans' theorem. The comparison indicates that this method of calculation gives orbital energies that are about 5% larger than the experimental ionization potentials for carbon 1s electrons and 10%–15% greater for the outer electrons. These differences are consistent with calculations of the ionization potentials of carbon atoms. The carbon 1s binding energy decreases with hydrogenation and when a hydrogen is replaced by an alkyl group. The first of these results is consistent with theoretical calculations and with other chemical evidence. There is an apparent discrepancy between the second result and the result of theory or other chemical evidence. A possible source of this discrepancy is discussed.
Asymptotic Evaluation of WKB Matrix Elements. II. Use of Langer's Uniform Asymptotic Wavefunctions
Authors: Smith Wesley D. , Pack Russell T
Published: February 2004

An improvement in a previous procedure for the analytic asymptotic evaluation of integrals arising in the quantum-mechanical theory of inelastic molecular collisions is presented. The integrals are evaluated using Langer's uniform asymptotic wavefunctions and the higher-order saddle-point or steepest-descents method. It is found that the Langer functions give no better results than WKB functions, but the higher-order steepest-descents corrections produce a marked improvement. The result is a simple method for evaluating a large class of integrals with an error of 0.1%–2.1%.
Diffuse Double-Layer Interactions in One-, Two-, and Three-Dimensional Particle Swarms
Authors: Philip J. R.
Published: February 2004

The theory of diffuse double-layer interactions in homogeneous isotropic swarms of particles is developed by means of a cell model. Energies of interaction are conveniently treated in terms of the partial volumetric Gibbs free energy. Calculations use (in part) a simple “matched Debye–Hückel approximation” to exact solutions of the nonlinear Poisson–Boltzmann equation. Geometrical effects exert a dominant influence on the variation of partial Gibbs free energy with particle area per unit volume of solution. Evaluation of the relative importance of double-layer interactions and particle Brownian motion provides a check on the cell model. Results for the three-dimensional swarm are found to be consistent with available estimates of the repulsive force between two spherical particles. Application of the approach to heterogeneous swarms is discussed.
Vaporization Kinetics and Thermodynamics of Antimony and the Vaporization Coefficient of Antimony Single Crystals
Authors: Rosenblatt Gerd M. , Lee Pang-Kai
Published: February 2004

The vaporization rate of Sb4 from Sb (111) single-crystal cleavage faces has been measured over the temperature range 606–697°K using a recording vacuum microbalance. The results, expressed as Langmuir vapor pressures PL, yield R lnPL(atm) = −[(49 740 ± 2200)) / T] + (35.67 ± 3.4). At 650°K PL = 1.2 × 10−9atm. Comparison with equilibrium vapor pressures PE yields a vaporization coefficient αυ = PL / PE = 0.17 ± 0.06 at 650°K which increases with temperature αυ = exp(−2310 / RT). The data are corrected for evaporated molecules which return to the sample. Vaporization rates of antimony powder and Kundsen effusion experiments in the same apparatus confirm the value of αυ. The apparent vaporization coefficient from the powder, 0.54, correlates well with the single-crystal results. Kundsen-effusion experiments at 683–819°K using a 0.4-mm-diam-orifice aluminum cell give R lnPE(atm) = − 223[(47600 ± 1330) / T] + (35.65 ± 1.79) and a third-law enthalpy of sublimation ΔH°298 = 49.67 ± 0.32 kcal/mole in good agreement with other determinations. Averaging these results with those from other investigations leads to ΔH°298 = 49.55 ± 0.31 kcal/mole. The vaporization behavior of arsenic and antimony is compared and the differences in vaporization coefficient are rationalized as reflecting differences in atomization energies of the gaseous tetramers and in equilibrium enthalpies of vaporization.
Surface van der Waals Forces
Authors: Lando D. , Slutsky L. J.
Published: February 2004

Adsorption isotherms at very high coverage of (CH3)4C and (CH3)4Si on flat gold surfaces are found to imply a potential energy of interaction between the molecule and metal which varies at the inverse square of the distance between the molecule and the metal surface.
High-Resolution NMR Study of Relaxation Mechanisms in a Two-Spin System
Authors: Freeman R. , Wittekoek S. , Ernst R. R.
Published: February 2004

A description of nuclear spin–spin and spin–lattice relaxation is presented for a system of two coupled nonequivalent spin−12 nuclei subject to mutual dipole–dipole interaction and interactions with external magnetic species. Expressions are derived for the six transition probabilities linking the four energy levels, in terms of an internal dipole–dipole relaxation parameter TD, external relaxation parameters TXA for nucleus A and TXB for nucleus B, and the degree of correlation (C) between the interactions of A and B with external magnetic species. These molecular relaxation parameters are related to the initial rates of recovery of the NMR signal intensities after a selective pulse has rotated the magnetization vector of a chosen line through π radians in the rotating frame of reference. A small general-purpose computer has been used to control these pulse experiments and to program a synthesizer to provide the required stable audiofrequency signals. Measurements have been made on the high-resolution proton spectra of dilute degassed solutions of 2,3-dibromothiophene and 3-bromothiophene-2-aldehyde in carbon disulphide. Assuming an interproton distance equal to that in thiophene, the direct dipole–dipole contribution to relaxation has been used to estimate the rotational correlation times τC = 4.4 × 10−12 sec for dibromothiophene and τC = 5.2 × 10−12 sec for bromothiophenealdehyde. Evidence is found for a small but significant difference between the external relaxation parameters TXA and TXB, and the degree of correlation between the interactions of A and B with external spins is found to be 0.67 in both molecules. This can be related to the nominal distance of closest approach of the species responsible for external relaxation.