bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Conductance from a Curved Surface through a Cylindrical Hole by Monte Carlo Methods
Authors: Sandry T. D. , Stevenson F. D.
Published: February 2004

The fraction of molecules that vaporize from a surface and escape through a cylindrical hole to a condenser (escape probability) has been calculated by Monte Carlo methods for the case of hemispherical (concave and convex) and flat surfaces. The results for vaporization from the flat surface are in agreement with results predicted by the Clausing factor. The effect of condensation coefficients has also been investigated. A semiempirical equation of adjustable parameters was developed which represents the escape probabilities as a function of the length-to-radius ratio (L / R) and the condensation coefficient (α).
Molecular Collisions. XIII. Nuclear Spin and Statistics Effects for Nearly Spherical Potentials
Authors: Biolsi Louis
Published: February 2004

The effects of nuclear spin on the expressions for the degeneracy averaged cross section for the collision between rigid diatomic molecules with rotational structure which interact through a soft, nearly spherical potential are considered. The development is based on a perturbation method which is essentially an extended distorted wave treatment. Explicit expressions for the moments of the cross section to second order are obtained. Collisions between identical molecules are also considered and the total inelastic cross section is calculated.
Positron Annihilation in Synthetic Zeolites 4A and 13X
Authors: Perkal M. B. , Walters W. B.
Published: February 2004

We have measured the rates of positron annihilation in synthetic zeolites 4A and 13X under a variety of conditions. For zeolites 4A and 13X, respectively, long lifetime values (τ2 in nanoseconds) and intensities I2 (listed in parentheses) were found as follows: hydrated, 2.26 ± 0.18(20.5%) and 1.95 ± 0.20(21.6%); evacuated, 3.76 ± 0.10(25.4%) and 4.42 ± 0.25(21.8%); filled with O2, 3.34 ± 0.25(4.2%) and 4.15 ± 0.35(4.6%); filled with He, 2.90 ± 0.11(23.8)% and 3.28 ± 0.12(20.0%); filled with N2, 2.97 ± 0.12(23.7%) and 3.39 ± 0.10(25.3%); filled with methane, 4.10 ± 0.06(28.2%) and 3.78 ± 0.11(24.8%). Very long components (τ3) with lifetimes of 42.2 ± 11.2 nsec and 41.4 ± 4.0 nsec for 4A and 13X, respectively, were observed in evacuated zeolites. Because the cavity sizes are well known from x-ray diffraction studies, the slightly larger lifetime observed in 13X relative to 4A for evacuated zeolites is thought to reflect the larger free volume present in the 13X configuration. The reduced lifetimes observed with He or N2 inclusion are postulated to result from reduced free volumes, whereas the reduced intensities observed with O2 inclusion may result from low O2 and positronium mobility coupled with nonuniform O2 distribution. Values for the short-lifetime component (τ1) were also observed and found to be relatively constant for each type of zeolite.
Trapping of Electrons in Low-Temperature Glasses. A Pulse Radiolysis Study
Authors: Richards J. T. , Thomas J. K.
Published: February 2004

The pulse radiolysis of 3-methylpentane, 3-methylhexane, 2-methyltetrahydrofuran, cumene, ethanol, and alkaline ice glasses have been studied at 77°K using 12-nsec pulses of 3-MeV electrons. Trapped electrons in varying yields were observed in all cases and in some instances short-lived species were observed. These short-lived transients have been correlated with different trap depths in the various media, and the trapping process and types of sites have been discussed. The effect of carbon tetrachloride and biphenyl on the transients observed in 3-methylhexane has been studied and the results interpreted in terms of electron capture by these solutes in accord with previous 60Co γ-radiolysis data. Addition of positive hole scavengers to the later system has also been investigated. Pulse radiolysis of 10−2-M naphthalene in 3-methylhexane glasses at 77°K produced small yields of solute ions, which recombined on warming to about 123°K giving the naphthalene triplet (G = 0.95) and naphthalene singlet.
Padé Approximants for Two- and Three-Body Dipole Dispersion Interactions
Authors: Langhoff P. W. , Karplus M.
Published: February 2004

A new method is presented for using theoretical or experimental optical dispersion data to construct bounded estimates to dispersion force coefficients. The bounds are obtained from the Casimir–Polder integral formula and an analytic continuation of the power series expansion of the dynamic multipole polarizabilities for the interacting species. Recognition of the dynamic polarizabilities at imaginary frequency as series of Stieltjes allows construction of the necessary bounded continuations with Padé approximants expressed directly in terms of Cauchy dispersion coefficients. The sequence of bounds obtained incorporates the London and Slater–Kirkwood values in lowest order, and provides improvements to these two well-known bounds in higher order. The method is applied to the two- and three-body dipole interactions of atomic and molecular hydrogen and of helium using theoretical Cauchy dispersion coefficients, and to the inert gases, alkali atoms, and molecular hydrogen, nitrogen, and oxygen using coefficients obtained from optical dispersion and absorption data. Comparison is made with dispersion force estimates obtained from ab initio calculation, semiempirical methods, and molecular beam and low-temperature gas kinetic measurements. In addition to supplying accurate numerical bounds for dispersion force coefficients, the Padé procedure provides the basis for a discussion of previously devised techniques for estimating dispersion force coefficients. The importance of utilizing a proper analytic continuation of the dynamic polarizability in the successful application of these procedures is emphasized. Finally, the accuracy of well-known dispersion force combination rules is discussed within the framework of the Padé procedure, and the relation of the latter to alternative bounding methods which have recently appeared is discussed.
Lattice Dynamics of Hexamine—A Rigid-Molecule Approximation
Authors: Rafizadeh Hamid A. , Yip Sidney
Published: February 2004

Lattice dynamics of molecular solids is analyzed using a force model which treats the molecule as an extended point mass which can undergo translations and librations. The model is similar to the rigid-molecule formulation of Cochran and Pawley but contains fewer force constant parameters because definite relations now exist among translation, libration, and cross coupling force constants. These relations depend on the relative positions of an interacting pair of molecules and introduce an effective size parameter for each type of neighbors. The model has been used to discuss the lattice modes in hexamine. Recent neutron dispersion relation measurements on deuterated hexamine are successfully analyzed in terms of first- and second-neighbor interactions. Frequency distribution functions, including separate translation and libration components, are computed, and the results are used to obtain satisfactory correlations with specific heat data and inelastic neutron scattering spectra of normal hexamine. The ability of the model to predict dispersion curves using only elastic constant values and the triply degenerate optical mode frequency is discussed.
Comparison of Three Self-Consistent Ground States for the Linear Heisenberg Antiferromagnet
Authors: Silverstein A. J. , Soos Z. G.
Published: February 2004

The attractive interactions in the fermion representation of the linear Heisenberg antiferromagnet lead to three qualitatively different self-consistent ground states. In addition to the Hartree–Fock solution, Ruijgrok and Rodriguez have obtained an ordered state with lower energy. A new self-consistent solution, without long-range order but with lower energy than the Hartree–Fock, is discussed. The new solution agrees well with numerical estimates for the spin correlations among nearest, second, and third nearest neighbors. The correlations between the parallel and transverse spin components are also compared. This previously overlooked test indicates that all three self-consistent solutions are strongly anisotropic. They are axially symmetric and, in spite of excellent ground-state energies, poor representations of the spherically symmetric, singlet ground state. The possibility of three qualitatively different approximate ground states, each of too low symmetry, complicates the computation of finite-temperature properties.
Ab Initio Calculation of H2NO Geometry and Hyperfine Splittings
Authors: Salotto A. W. , Burnelle L.
Published: February 2004

The spin-unrestricted Hartree–Fock (UHF) method has been applied to the H2NO radical, using as basis functions Gaussians centered on the various atoms. The system is found to be nonplanar, with an out-of-plane angle of 26° and an HNH angle of 116°. Bond lengths of 1.34 and 0.99 Å are predicted for the NO bond and the NH bonds, respectively. The nonplanarity of the system is found to be due to the steep slope of the highest occupied orbital (of b1 symmetry and NO antibonding). This last point is in agreement with the prediction of Walsh. As for the other orbitals, little similarity is observed with the Walsh orbital energy diagram. Hyperfine coupling constants have been computed for both N and H atoms, using the UHF wavefunction as well as carrying out a single annihilation and a complete projection. The UHF method provides the most satisfactory results, namely, 11.2 G for N and − 8.8 G for H. These are to be compared with the experimental results (absolute values) of 11.9 G for both atoms. The importance of averaging over vibrational states in indicated.
Intermolecular Triplet Exciton Transfer in Anthracene Mixed Crystals
Authors: Baessler H. , Vaubel G. , Kallmann H.
Published: February 2004

Measurements of the time decay of delayed fluorescence in anthracene single crystals doped with perylene, tetracene, and pentacene are reported. It is found that the exciton drift velocity in the host lattice does not depend on the presence of guest molecules. The transfer time for triplet excitons between host and guest molecules depends strongly on the energy separation between host and guest triplet levels following the relationship τDA = τh exp (ΔE / E0) where τh is close to the exciton hopping time in crystalline anthracene and E0 is an empirical parameter of the magnitude E0 = 0.115 ± 0.01 eV. A close similarity between the mechanisms for host–guest triplet transfer and both radiationless decay of the triplet state (T1↝S0) and intersystem crossing (S1↝T1) is found. Earlier work concerning triplet energy transfer in fluid solution is re-examined.
Infrared Investigation of Structural and Ordering Changes in Ammonium Chloride and Bromide
Authors: Schumaker Norman E. , Garland Carl W.
Published: February 2004

Infrared spectra of NH4Cl, ND4Cl, NH4Br, and ND4Br have been recorded at numerous temperatures between 21 and 300°K using thin sublimed films, single-crystal sections, and pressed disks. At 21°K each spectrum contains a large number of sharp intense bands which can be assigned to combinations involving the internal vibrational modes of the ammonium ion, the librational (torsional oscillation) mode, and various lattice modes. Observation of the first and second librational overtone makes it possible to discuss the potential barrier hindering the rotation of the ammonium ion. Barrier heights of 1860 cm−1 (5.32 kcal mole−1) for NH4Cl and ND4Cl and 1520 cm−1 (4.35 kcal mole−1) for NH4Br and ND4Br were calculated from the Gutowsky–Pake–Bersohn model, but the experimental frequencies show that the libration is more anharmonic than this model would predict. The temperature dependence of an anomalous component of the ν4 bending fundamental was studied in detail. The intensity of this component can be directly correlated with the breakdown of translation symmetry due to disorder in the ammonium ion orientations. In addition, this feature of the spectrum indicates that there is appreciable distortion in the ordered cubic phase of NH4Br and ND4Br prior to the first-order transition into the ordered tetragonal phase.
Linear Antiferromagnetism in Spin-1 Systems; CsNiCl3
Authors: Smith John , Gerstein B. C. , Liu S. H. , Stucky Galen
Published: February 2004

The powder susceptibility of CsNiCl3 has been measured from 1.3 to 130°K. The results, indicative of one-dimensional antiferromagnetic interactions, are used to test the one-dimensional Ising model for spin unity, and the Fisher model utilizing the classical Hamiltonian for infinite spin. The Fisher model with J = − 14.2k is found to give the better fit, but both models are found to deviate appreciably from the experimental results for kT less than J.
Transport Properties of Polymer Chains in Dilute Solution: Hydrodynamic Interaction
Authors: Yamakawa Hiromi
Published: February 2004

Several problems related to hydrodynamic interaction in flexible chain polymers without excluded volume are investigated. First, a correction to the Oseen hydrodynamic interaction tensor for a system of many spheres is derived, taking into account the finite volume of the spheres. The new hydrodynamic interaction tensor gives the positive definite diffusion tensor identical with that of Rotne and Prager. Second, the possible effects of the correction term in the hydrodynamic interaction tensor on the translational diffusion coefficient and intrinsic viscosity are examined when preaveraging the hydrodynamic interaction tensor is avoided following the procedure of Pyun and Fixman. If the Rouse free-draining normal coordinates together with the diagonal approximation are used, the effects are shown to be negligibly small in the case of flexible chains. Third, however, the intrinsic viscosity and diffusion coefficient are re-evaluated, taking the lower limit of | i − j | as unity in the evaluation of sums over the segment indices i and j. The results are similar to those derived by Hearst and Stockmayer for wormlike chains. That is, the present theory also predicts that the draining effect vanishes under certain conditions. It is emphasized that the Oseen tensor has still some practical value in the case of flexible chains when it is used carefully.
Surface Adsorption of Light Gas Atoms
Authors: Hollenbach David , Salpeter E. E.
Published: February 2004

The adsorption on solid surfaces of very light gas atoms is reconsidered. A variational wavefunction is constructed for the quantum-mechanical ground state of the adsorbed atom, and improved values of adsorption energies are calculated for a few cases. The surface mobility at low temperatures is determined quantum mechanically, by use of the variational ground-state wavefunction. Simple analytic approximations are derived for the sticking coefficient S and the accommodation coefficient α for gas atoms impinging on a surface. A few previous accurate calculations and some experiments are used to derive some general analytic semiempirical formulas for S and α as a function of the gas temperature. Surface recombination of diatomic molecules from two adsorbed atoms is considered briefly.
Rotational Strength of Molecules: Application of the Exchange Theorem
Authors: Chang Tai Yup
Published: February 2004

The calculation of the rotational strength of molecules through the first order of the badness of the molecular wavefunction is investigated by applying double perturbation theory and the exchange theorem for off-diagonal matrix elements. The method is applied to Eyring and co-workers' one-electron model of 3-methylcyclopentanone, and exact and variational solutions for the model are obtained. The result leads one to hope that for more practically interesting problems the variational approach can be employed with some confidence.
Photoionization of Mercury Atoms between 10.5 and 72 eV
Authors: Cairns R. B. , Harrison Halstead , Schoen Richard I.
Published: February 2004

The photoionization cross sections to produce Hg+ and Hg2+ have been measured from their respective thresholds at 10.434 and 29.186 eV to a high-energy limit of 72 eV. For single ionization the cross section rises to a maximum at more than 10 eV above the threshold associated with d-electron removal. The cross section for double ionization is a nonnegligible fraction of the total cross section.
NMR, Calorimetric, and Diffraction Study of Molecular Motion in Crystalline Carboranes
Authors: Baughman Ray H.
Published: February 2004

Molecular motion of two nearly icosahedral molecules, ortho- and meta-carborane, in the solid state has been studied by x-ray diffraction, calorimetric, and NMR techniques. At 25°C the powder diffraction patterns for both crystals are consistent with a fcc unit cell of dimension 9.86 Å containing four molecules. For both isomers the corresponding molecular site symmetry is higher than the molecular symmetry, implying molecular orientational disorder at this temperature. That the orientational disorder is of a kinetic nature is confirmed by NMR measurements. The proton resonance line second moment at 25°C is 0.711 ± 0.051 G2 for o-carborane and 0.82 ± 0.10 G2 for m-carborane. This can be compared with a line second moment of 0.630 G2 calculated assuming rapid isotropic molecular reorientation and a line second moment of 34 G2 calculated for the rigid lattice. For both isomers the observed second moment is much smaller than calculated assuming rotation about the molecular twofold axis, implying rapid reorientation between thermodynamically indistinguishable orientations. Measurements above and below room temperature were made only for the ortho isomer. A phase transition was observed at 4.0 ± 3.0°C both by calorimetry and x-ray diffraction. The associated transition enthalpy and entropy were respectively 0.84±0.05 kcal/mol and 3.0 e.u. The proton resonance line second moment observed at − 0.6°C is 1.13 ± 0.1 G2, implying a rather general molecular reorientation between indistinguishable molecular orientations at this temperature in the low temperature phase. With decreasing temperature the line second moment rapidly increases in the vicinity of − 80°C. At − 98°C, the lowest temperature measurement, molecular reorientation is still rapid enough to significantly narrow the resonance line. No high entropy phase transition was found by calorimetric measurements from below the 4°C transition to − 130°C, indicating either that an order–disorder transition lies below − 130°C or that for kinetic reasons the ordered phase is not attained, necessitating a nonzero entropy at 0°K. On increasing temperature above 25°C, no change was observed in line second moment until about 200°C. Above this temperature the linewidth and second moment decrease due to rapid molecular translational diffusion. Apparent activation parameters for molecular rotation and for translational diffusion have been obtained from linewidth measurements.
Anisotropy of the Singlet Transitions of Crystalline Anthracene
Authors: Clark Leigh B. , Philpott Michael R.
Published: February 2004

The reflection spectra of three faces of crystalline anthracene in the region 5000–1820 Å have been observed at room temperature, and the corresponding absorption spectra have been derived by a Kramers–Kronig analysis. The results for face (001) agree with previous direct absorption work. For faces (201̄) and (1̄10) the Davydov splittings of the 3800-Å transition are found to be substantially different from those observed for the (001) face. The strong 2500-Å transition gives rise to reflection bands which are 1.5–2.0 eV wide. These bands are examples of metallic reflectivity which had previously been observed only for some organic dye crystals. For radiation incident upon the (201̄) and (1̄10) faces the splittings of the strong transition are smaller by nearly an order of magnitude relative to that observed in the case of the (001) face. These results are interpreted as strong evidence for the dependence of dipole sums on the relative orientation of the wave vector k to the transition moment d. The experimental observations are compared with the dipole theory of exciton states in molecular crystals. Qualitative agreement between dipole calculations and experiment is good, and quantitative differences are attributed to the presence of higher multipole and electron exchange effects. It is estimated that these nondipolar interactions contribute approximately − 1600 cm−1 to the total Davydov splitting of the intense 2500-Å transition.
Constrained-Variation Method Applied to Helium-Atom Wavefunctions
Authors: Whitman Donald R. , Carpenter Richard N.
Published: February 2004

The constrained-variation method is applied to helium-atom wavefunctions, using precise Pekeris values of electron moments both as constraints and as tests of the effectiveness of the method. The conclusions support the basic premise that constrained-variation wavefunctions may better represent the true electron densities in a system than do the corresponding free-variation functions.
High-Resolution Electronic Absorption Spectra of Diazanaphthalenes in the Vapor Phase
Authors: Glass R. W. , Robertson L. C. , Merritt J. A.
Published: February 2004

The absorption spectrum of quinoxaline vapor for λ > 2000 Å is presented. Two diffuse π* ← π systems and a single sharp π* ← n system are observed. A vibrational analysis of observed broad and sharp bands is given for the allowed B11(nπ*) ← 1A1 electronic system, and a 0–0 band is assigned at 27 071 cm−1. Characteristic band contours and correlation with low-temperature polarization data allow the assignment of nine fundamental vibrations in the electronic excited state. Six ground electronic state fundamental vibrations are also assigned. The observed band contours and prominent frequencies of quinoxaline are compared with other diazanaphthalenes, and new spectral data for quinazoline and cinnoline are presented.
Perturbation Theory of the Hooke's Law Model for the Two-Electron Atom
Authors: White Ronald J. , Brown W. Byers
Published: February 2004

The Hooke model for the two-electron atom replaces the electron–nuclear interaction by a harmonic oscillator potential, but retains the Coulomb repulsion of the electrons. The first-order perturbation equation for the electron repulsion is solved analytically, and the exact first-, second-, and third-order perturbation energies are obtained. A similar Z−1 perturbation treatment is carried out for the Hartree–Fock equation and other variational approximations. The Z−1 expansion of the correlation energy is compared with that for heliumlike atoms and found to be similar.
External Heavy Atom Perturbation of Vibronic Transitions in Singlet–Triplet Spectra
Authors: Giachino Gary G. , Kearns David R.
Published: February 2004

The high-resolution phosphorescence spectra of a number of compounds were obtained at 4°K both in the presence and absence of external heavy atom perturbers. A comparison of the two cases demonstrates that only totally symmetric modes of the molecule are enhanced by the external heavy atom effect. This result implies that the enhancement mechanism is purely electronic in nature and involves no vibronic coupling. The results also suggest that the external heavy atom effect can be useful in aiding the assignment of complicated singlet–triplet spectra.
Molecular Zeeman Effect of Cyclobutanone
Authors: Tigelaar H. L. , Flygare W. H.
Published: February 2004

The molecular Zeeman effect has been observed in cyclobutanone at magnetic fields near 21 000 G. The measured molecular g values are gaa = − 0.074 ± 0.002, gbb = − 0.0325 ± 0.0004, gcc = − 0.0279 ± 0.0004, and the two independent components of the magnetic susceptibility anisotropy are 2χaa − χbb − χcc = 14.8 ± 0.9 and 2χbb − χaa − χcc = − 10.6 ± 1.0 in units of 10−6 erg/G2·mole. The a axis is along the C=O bond and c is the out-of-plane axis. The molecular quadrupole moments are Qaa = − 9.4 ± 1.2, Qbb = 4.5 ± 1.1, and Qcc = 4.9 ± 1.7 all in units of 10−26. The individual diagonal elements in the total, paramagnetic, and diamagnetic susceptibility tensors are also evaluated. The results on cyclobutanone are compared to other small-ring compounds.
Energy Levels of Pr3+ in Various Crystal Hosts
Authors: Wensky D. A. , Moulton W. G.
Published: February 2004

The results of a least squares fit of the energy levels of Pr3+ in various crystal hosts are reported. By using an expanded eight-variable semiempirical free-ion Hamiltonian, the average deviation between the calculated and observed levels was reduced by an order of magnitude from the average deviation found using the usual four-variable free-ion Hamiltonian. The best fit parameters F2′, F4′, ζ4f′, α′, β′, γ′, and M0′ determined for various crystal hosts are reported. In addition the magnitudes of the configuration interaction screening factors of the 4f interelectric Coulombic F2, F4, and F6 integrals are presented. The effects of the crystalline hosts on the empirical parameters are examined. It is suggested that changes in the various screening factors involved in the expanded free-ion Hamiltonian may be responsible for sizable contributions to the over-all host effects on the empirical parameters.
Solute–Solute Interactions of Matrix Isolated Cu, Ag, and Au Atoms
Authors: Brewer Leo , King Baldwin
Published: February 2004

We examined the matrix spectra of Cu, Ag, and Au separately in matrices of Ar, Kr, Xe, and SF6 and compared them with the spectra of the pairs Ag and Au and Cu and Au in Kr. We observed that the spectra of the metal atoms were identical whether deposited separately or in pairs, and we can exclude solute–solute interaction as a source of the shifts, splitting, and broadening of atomic lines compared to the gaseous spectra. Similar results were obtained for combinations of Cu, Au, and Hg. We also studied the spectrum of Au atoms in mixtures of Kr and SF6.
Vibrational Relaxation of Nitric Oxide in Argon
Authors: Kamimoto Goro , Matsui Hiroyuki
Published: February 2004

Vibrational relaxation times of NO diluted in Ar were measured by monitoring infrared emission of NO at 5.3 μ. Measurements were made in shock waves over the temperature range of 900–2700°K. NO was found to relax exponentially and the relaxation times of NO in NO–NO and NO–Ar collisions were estimated. The results were expressed by logpτNO–NO = 6.1T−1/3 − 1.1 atm·μsec and logpτNO–Ar = 14.7T−1/3 + 0.15 atm·μsec.
Further Development of Scaled Particle Theory of Rigid Sphere Fluids
Authors: Tully-Smith D. M. , Reiss H.
Published: February 2004

A new approach to the scaled particle theory of fluids is developed which is based on detailed consideration of the reversible work required to create a fixed spherical cavity of radius r in a rigid sphere fluid. The new results depend only on the mechanical concept of work and the classical virial theorem. A cycle greatly facilitates the necessary detailed analysis. A new integral equation for G(r), the primary function in scaled particle theory, culminates the new results. Approximation schemes may be employed relatively easily in this integral equation. The series approximation G(r) =  ∑ n = 0∞ Gn(r−1)n, which is asymptotically correct as r → ∞, is discussed. A new set of exact conditions on the coefficients Gn in this asymptotic expansion, the most striking of which is G3 ≡ 0 in three dimensions, is derived. These exact conditions relate to surface properties. The infinity and integral exact conditions on G(r) are shown to be truly independent. The jump condition on the second derivative of G(r) is incorporated successfully for the first time with other exact conditions in the derivation of two new expressions for G(a), both of which give rise to equations of state. One of these expressions yields the best virial coefficients thus far obtained for a rigid sphere system from any analytic equation of state not based on cluster expansions. The new approach and results are easily generalized to arbitrary dimension. Future work will center on attempts to extend the new integral equation for G(r), on possible extensions of the present method to real fluids, and on studies of the nature of the asymptotic series approximation scheme.
Pseudopotential Approaches to Localized Orbitals for Polyatomic Systems
Authors: Schlosser Herbert
Published: February 2004

We present a method for constructing optimized pseudopotentials for general polyatomic systems which yield the “smoothest” orbitals localized in particular regions of space. (“Smoothest” orbitals are those for which the expectation value of the kinetic energy operator is a minimum.) We specifically consider the general case in which one can have several occupied orbitals localized in a particular region, rather than a single orbital. Recently, Anderson presented a pseudopotential formalism for constructing localized orbitals for a single isolated band in a polyatomic system. We have extended his formalism to the general multiorbital case, and show that for a specific choice of pseudopotential the Anderson equation yields optimal orbitals. We also show that the Hückel form of the Anderson pseudopotential equation yields identical eigenvalues (neglecting terms of order S2) to the optimal form, and orbitals differing from optimal orbitals by terms linear in S. The correction terms to the Hückel orbitals are obtained by first-order perturbation theory. Finally we examine the Adams–Gilbert equation, which is valid only for systems representable by a single Slater determinant, and indicate how one may solve this equation for the localized orbitals using perturbation theory with Anderson orbitals as the basis set.
Polarized Spectroscopy of Excited States of Substituted Anthracenes on a Nanosecond Time Scale
Authors: Kliger David S. , Albrecht A. C.
Published: February 2004

Transitions originating from excited electronic states of anthracene, 2-methylanthracene, and 9-methylanthracene are investigated. Using kinetic spectroscopic techniques with nanosecond time resolution, spectra are measured in both ethanol and lucite solutions. Polarizations of the absorptions in lucite samples are also measured. Anthracene shows its well-known long-lived transient absorption of the triplet state at 425 nm which is assigned to a B1g3 ← B2u3 transition. In addition, a transient absorption having a very strong oscillator strength is observed at 427 nm which decays in a few nanoseconds. This is assigned to a Ag1 ← B2u1 transition out of the first excited singlet state of anthracene in excellent agreement with theoretical results. The methylanthracenes studied show triplet spectra almost identical to that observed in anthracene. However, the short-lived transient spectrum of anthracene changes dramatically upon methylation. In addition to the anthracene transition, now shifted to 405 nm, new strong transitions are observed at 420 and 435 nm. Their polarization tends to favor the direction of the methyl substituent. Isotope studies show that these new transitions do not originate from the first excited singlet state of the derivative. The evidence suggests that one may be seeing a transition out of an upper triplet state. The most appealing candidate is a low-lying σ–σ* triplet state with a natural lifetime on the order of fractions of nanoseconds.
Electric Polarizability of Molecular Hydrogen as Calculated by Use of the Nonsymmetrical Wavefunction
Authors: Saika A. , Musher Jeremy I. , Ando Toshihiko
Published: February 2004

The nonsymmetrical (NS) wavefunction has been employed to calculate the static electric dipole polarizability of the hydrogen molecule. It has been found that the double perturbation calculations carried through first order in electron correlation give results much better than those by zeroth-order Hartree–Fock (HF) calculations and comparable to first-order HF results with much less labor. Thus it appears better to obtain corrections to a starting wavefunction, even if it is exceedingly crude, rather than to find the best possible zeroth-order approximation and stop there.
Nematic Ordering of Hard Rods Derived from a Scaled Particle Treatment
Authors: Lasher Gordon
Published: February 2004

A fluid of hard rods is considered as a model for nematic liquid crystals. A direct application of the scaled particle approach results in an inconsistent thermodynamics. A simplified version which is consistent gives an integral equation for the orientational distribution which has the same form as that used by Onsager in the second virial approximation to the same problem. The coefficients, however, give finite density corrections which change his predicted order and density ratio at the transition in the direction of the experimental values.
ATR Infrared Spectra of Uniaxial Nitrate Crystals
Authors: Devlin J. P. , Pollard Gary , Frech Roger
Published: February 2004

The experimental ATR bands for the ν3 nitrate ion mode, in particular as measured for trigonal alkalimetal nitrate crystals with their trigonal axes perpendicular to the ATR element–sample interface, display interesting shapes which are radically different for TM and TE polarizations. Further, there is only a marginal amount of overlap of the intense bands which appear for the two polarizations. It is shown that the band structures are in complete accord with expectations based on a damped oriented oscillator model of the nitrates combined with the complex form of Snell's law and Fresnel's reflection equations as developed for a uniaxial crystal. The theoretical development indicates that the TM band structure closely reflects both the transverse and longitudinal modal frequencies since the band peaks sharply near ν3(t) and has an inflection point corresponding to ν3(l). This insight is used to deduce ν3(t) and ν3(l) from ATR data for the high-temperature trigonal forms of sodium and potassium nitrate.
Analysis of the Rotatory Strength for Some Model Aldehydes
Authors: Howell James M.
Published: February 2004

The rotational strength characterizes the magnitude and the sign with which a particular transition will contribute to the rotatory dispersion curve. Direct calculations of the rotatory strength, employing extended Hückel wavefunctions, are made on model aldehydes. The chirality is introduced through rotation of the terminal methyl group. The rotatory strength is decomposed through symmetry into contributions from perturbation of both the n and the π* orbital. When there is a cis relationship about the C1–C2 bond the contributions augment each other. A trans relationship produces near cancellation. This behavior is associated with the projective properties of the contribution arising from perturbation of the π* orbital. A perturbation analysis indicates that interaction between the perturbing atoms and the hydrocarbon chain (as opposed to the carbonyl group) is quite important.
Rydberg Series in the NO Spectrum: An Interpretation of Quantum Defects and Intensities in the s and d Series
Authors: Jungen Christian
Published: February 2004

The observed quantum defects in the s and d Rydberg series of NO are interpreted. It has been shown previously that the nf Rydberg states of NO conform to a long-range force model. This model assumes that the motion of the Rydberg electron is determined only by the long-range potential of the NO+ core. The same model, when applied to the nd states, fails to reproduce the observations. The differences between predicted and observed energies result from: (i) exchange effects and increased nuclear attraction inside the core, (ii) the required orthogonality of the wavefunction of the Rydberg electron with respect to core electrons, (iii) mixing of the dσ with the sσ Rydberg series. In order to describe these effects (called penetration effects) quantitatively, the long-range force model is supplemented by additional parameters. These parameters are determined independently from the observed electronic and from the observed rotational energy level patterns, and good agreement is found. The mixing of the dσ with the sσ Rydberg series is shown to influence strongly the intensities of numerous transitions in the near infrared, in the visible and in the ultraviolet spectral regions. Numerical calculations for the core orbitals of NO support this interpretation of the penetration effects. The possible assignments of “atomlike” quantum numbers to a molecular Rydberg orbital are discussed.
Infrared Spectra and Vibrational Potential Function of the Perpendicular Vibrations of CHF3 and CDF3
Authors: Hoskins L. C.
Published: February 2004

Infrared spectra of the perpendicular bands of CHF3 and CDF3 have been examined at high resolution. The ν4 band of CHF3 and the ν5 band of CDF3 were resolved and Coriolis constants and band centers determined. The measured band shapes of the ν6 bands permitted Coriolis constants to be calculated. These data were then used to calculate the E-species force constants of the general harmonic potential function. Normal coordinates were also calculated.
Nature of the Rigid-Rod Mesophase
Authors: Runnels L. K. , Colvin Carolyn
Published: February 2004

The thermodynamic phases of the rigid-rod gas are re-examined. Introduced in its present form by Zwanzig, the model postulates elongated molecules of parallelepiped shape with only three allowed orientations (mutually perpendicular). We have added to the virial series the cluster integrals represented by eight-point bicolored graphs. It is demonstrated that the Padé approximants generated by the truncated cluster expansion provide a much more stable sequence of parameters characterizing the isotropic–anisotropic phase transition. It is similarly shown that there are no stable phases predicted for which the orientational distribution is not axially symmetric. The relative volume change accompanying the transition is still predicted to be larger than that observed in the isotropic–nematic liquid crystal transition.
Intersystem Crossing Rate and Triplet State Lifetime for a Lasing Dye
Authors: Webb J. P. , McColgin W. C. , Peterson O. G. , Stockman D. L. , Eberly J. H.
Published: February 2004

Triplet state kinetics for rhodamine 6G in alcohol have been studied. The singlet-to-triplet intersystem crossing rate kST and triplet lifetime τT have been determined in air-saturated alcohol, where the spin-forbidden decay from the triplet manifold into the singlet ground state is enhanced by collision with oxygen molecules dissolved in the solution. Values of the kinetic parameters for air-saturated ethanol solutions at room temperature were determined to be τT = 250 ± 40nsec and kST = (3.4 ± 1.3) × 106sec−1.
Correlated Wavefunctions for the Ground State of H−, He, and Li+
Authors: Carlton Terry S.
Published: February 2004

Energies are calculated variationally using a closed-shell orbital product times the correlation function ga = [1 + λr12 + μ(r1 − r2)2 + νr122(r1 + r2)−n]1/2 (n = 1 or 2). Energy errors for H−, He, and Li+, respectively, are 0.00193, 0.00019, and 0.00021 a.u. when n = 1 and 0.00229, 0.00025, and 0.00064 a.u. when n = 2 or ν = 0. An explanation is proposed for the fact that ga is much more accurate for He and Li+ than for H−, whereas the opposite is true for the correlation function gb = 1 + λr12 + μ(r1 − r2)2. For He, expectation values other than the energy are typically accurate to four figures.
Compressibility of Inorganic Azides
Authors: Weir C. E. , Block S. , Piermarini G. J.
Published: February 2004

The compressibility of α lead azide, β lead azide, barium azide, potassium azide, sodium azide, and thallium azide have been measured by single-crystal x-ray diffraction techniques for the first time in a new application of the diamond anvil pressure cell. Both the anisotropic and volume compressibilities are reported. The pressures were determined by measurements at the known freezing points of n-hexane and ethanol. A phase transition occurs in thallium azide at a pressure between the freezing points of chloroform (5390 bar) and n-decane (2990 bar). Pressure–temperature observations in the diamond cell of lead azide were carried out to 300°C and approximately 30 kbar. No phase transitions were observed. Radiation damage to azide crystals under high pressures is reduced significantly.
Interpolation Formula for Radial Distribution Functions
Authors: Carley David D.
Published: February 2004

A Padé approximant is used as an interpolation formula to obtain the density dependence of radial distribution functions. The equation is of the form gexp(u / kT) = (1 + a1ρ + a2ρ2) / (1 + b1ρ + b2ρ2), where g is the radial distribution function, u is the pair potential energy, k is Boltzmann's constant, T is the absolute temperature, and ρ is the number density. Padé constants are determined for the Gaussian and hard-sphere models of a fluid.
Water Cluster Ions: Rates of Formation and Decomposition of Hydrates of the Hydronium Ion
Authors: Young C. E. , Edelson D. , Falconer W. E.
Published: February 2004

The rates of formation of hydrates of H3O+ in the reactions H3O+(H2O)n−1+H2O+M⇆H3O+(H2O)n+M, n = 1, 2, 3, were studied using drift tube techniques. Experiments were performed at 337°K with mixtures of 0.5% to 1.8% partial pressure of water in argon at total pressures from 0.6 to 3.0 torr. Rate constants for the forward reactions decreased slowly with increasing E / P0 in the range 4–30 V cm−1·torr−1. Values for k1, k2, and k3 at 337°K were 6 × 10−28, 6 × 10−28, and 2 × 10−28 cm6/sec. Limits were estimated for the reverse reactions. Mobilities of the cluster sequence from H3O+ to H9O4+ were almost independent of E / P in the range studied, with zero-field values of 3.0, 2.5, 2.2, and 2.0 cm2V−1·sec−1 in close agreement with the polarization-limit Langevin theory.
Calculation of the Effects of Divalent Metal Ions on Nucleotide Base Pairs. II. Single G–C and A–T and Poly(G–C) and (A–T)
Authors: Rozsnyai B. F. , Ladik J.
Published: February 2004

SCF calculations have been carried out for the electronic energy levels and wavefunctions of single G–C and A–T and poly(G–C) and (A–T) in the presence of Mg2+ ion attached to the electronegative sites in various ways. The P–P–P approximation was used for the single base pairs, and its equivalent, the semiempirical SCF–LCAO–crystal-orbital (CO) approximation for the polymer. Energy levels and energy bands are presented for all cases that have been investigated. The results supplement those of an earlier paper.
Estimates of Error Bounds of Gas Transport Properties
Authors: Monchick L.
Published: February 2004

Error bounds are calculated for transport properties that do not have the form − (f, L−1f), where L is the linearized semiclassical Boltzmann collision operator. The error bound generated depends on error bounds estimated for two other gas properties that do have this form. The method is illustrated for the thermal diffusion ratio of a gas mixture of mechanically similar rigid spheres and for the perturbation of the rate of change of temperature in a gas of reacting rigid sphere molecules. In the Appendix a rederivation is given of some general relations between diffusive and thermal fluxes.
Mass Spectrometric Studies of Atom–Molecule Reactions Using High-Intensity Crossed Molecular Beams
Authors: Foner S. N. , Hudson R. L.
Published: February 2004

Elementary gas phase reactions have been studied with high-intensity crossed molecular beams. A detailed analysis is presented on the sensitivity factors involved in mass spectrometric detection of free radicals formed in ordinary chemical reactions as contrasted to surface ionization detection of the products of alkali atom reactions. Free radicals have been observed in a number of atom–molecule reactions, including: (1) Cl atoms with n-butane and isobutane, (2) H atoms with NO2 and hydrazine, and (3) O atoms with hydrazine, monomethylhydrazine, and 1,1-dimethylhydrazine. The O atom reactions with the hydrazines are very interesting because of the marked proclivity for an O atom to abstract two H atoms from opposite ends of the molecule, forming diimide (HNNH) in the case of hydrazine and methyldiazene (HNNCH3) in the case of monomethylhydrazine. Ionization potentials of the free radicals and unstable molecules produced in the reactions have been measured and used in conjunction with appearance potentials of the corresponding ions to calculate relevant molecular and radical bond dissociation energies and heats of formation.
Numerical Solutions for the Energy Distributions of Reactive Atoms in Gases. V
Authors: Phillips T. T. , Kostin M. D.
Published: February 2004

From the equation of continuity, an integrodifferential form of the time-dependent Boltzmann equation is derived which can be used to obtain numerical solutions for the energy distributions of reactive atoms for the case of realistic scattering collisions. Application of the integrodifferential equation is made to the problem of computing the energy distributions as a function of time of tritium atoms reacting with molecular hydrogen and molecular iodine and moderated by helium. A numerical procedure for efficiently obtaining the energy distribution at large times is reported. For those systems where the deviation from equilibrium is small a differential equation is derived which gives accurate results even though large changes in kinetic energy during scattering collisions can occur.
Molecular Zeeman Effect of Cyclobutene, Methylene Cyclopropane, and Methylene Cyclobutane
Authors: Benson R. C. , Flygare W. H.
Published: February 2004

The rotational Zeeman effect has been observed in cyclobutene, methylene cyclopropane, and methylene cyclobutane. The molecular g values, magnetic susceptibility anisotropies, and molecular quadrupole moments were obtained and they are listed below (the a and b axes are in the molecular plane; the a axis is perpendicular to the carbon–carbon double bond in cyclobutene, and it is along the carbon–carbon double bond in methylene cyclopropane and methylene cyclobutane). The units of χ are 10−6 erg/G2·mol and the units of Qxx are 10−26 esu·cm2: CyclobuteneMethylene cyclopropaneMethylene cyclobutanegaa− 0.0516 ± 0.0007− 0.0672 ± 0.0007− 0.0320 ± 0.0008gbb− 0.0663 ± 0.0007− 0.0231 ± 0.0004− 0.0218 ± 0.0010gcc− 0.0219 ± 0.0006+ 0.0244 ± 0.0004− 0.0184 ± 0.00112χaa − χbb − χcc− 0.9 ± 0.5+ 18.3 ± 0.5− 6.4 ± 0.5− χaa + 2χbb − χcc+ 5.0 ± 0.7+ 14.0 ± 0.6+ 4.3 ± 1.7Qaa− 0.3 ± 0.6− 0.7 ± 0.5− 1.2 ± 1.1Qbb+ 1.6 ± 0.7+ 0.9 ± 0.6− 1.1 ± 2.0Qcc− 1.3 ± 1.0− 0.2 ± 0.9+ 2.3 ± 2.4 The above results, in particular the magnetic susceptibility anisotropies, are compared with similar ring molecules. Local and nonlocal (ring current) contributions to the magnetic susceptibility anisotropies are discussed and an additivity scheme is proposed for local group anisotropies in hydrocarbons.
Theoretical Analysis of the Electronic Structure and Molecular Properties of the Alkali Halides. VI. Rubidium Fluoride and Sodium Bromide
Authors: Matcha Robert L.
Published: February 2004

The results of Hartree–Fock calculations in the SCF LCAO MO approximation on sodium bromide and rubidium fluoride are presented giving emphasis to a discussion of modecular properties including quadrupole coupling constants, dissociation energies, molecular force constants, and electronic dipole moments.