bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation of NaI Vapor and the Energy Dependence of the Quenching of Na* (3p 2P) by Foreign Gases
Authors: Earl Boyd L. , Herm Ronald R. , Lin Shen-Maw , Mims Charles A.
Published: February 2004

Fluorescence of the Na* (3p 2P) D lines is observed upon photodissociation of NaI vapor by 1900–2500 Å radiation obtained through a monochromator from a high current, low pressure H2 arc continuum source; the properties of this source are briefly described. The D lines fluorescence efficiency exhibits a threshold at ∼ 2500 Å, a relative maximum at ∼ 2225 Å, and a relative minimum at ∼ 2050 Å. An expression is derived for the distribution in laboratory speeds of an atom produced by photodissociation of a diatomic molecule at thermal equilibrium. This expression is then employed to calculate the distributions in speeds of the photodissociatively generated Na* and these computed Na* speed distributions are employed to analyze the observed attenuations of the D-lines fluorescence upon addition of foreign gases. In this manner, the dependences on relative collision velocity g of the cross sections Qq for collisional quenching of Na* by CO2, C2H4, CH3CN, CF3Cl, C6H6, SO2, and I2 have been determined. Over the range in g studied in this work (∼ 1.0 - 2.5 km/sec), Qq varies approximately as g−4/8, with s in the range of 4–6, for all seven quenching gases. At a fixed value of g, Qq varies as: I2 > SO2> C6H6 >CH3CN> CF3Cl>C2H4 > CO2. These results on the quenching cross sections are discussed in terms of the long-range forces between the reactants, including a possible long-range electron transfer curve crossing for the collision of Na* with I2 or SO2. These measured quenching cross sections are also compared with previous literature values; good agreement is found in most cases, although some discrepancies are noted.
On the Mechanism of Low-Temperature Termolecular Atomic Recombination
Authors: Pack Russell T , Snow Richard L. , Smith Wesley D.
Published: February 2004

A simple theory of gas-phase termolecular atomic recombination kinetics is presented which treats the bound-complex (BC) mechanism in a fashion which parallels the Roberts, Bernstein, and Curtiss resonance theory of the energy-transfer (ET) mechanism. The ET rate constant has a low temperature maximum, but the BC rate constant does not. The contributions of the two mechanisms to the low-temperature recombination of hydrogen atoms, with He, Ar, and H2 as third bodies, are estimated and compared. The presence (absence) of a low-temperature maximum in the observable total rate constant is determined by absence (presence) of bound vibrational levels of the hydrogen-third body van der Waals dimer. Hence, experimental studies of these reactions could yield valuable information about the interatomic potentials.
On the Electronic States of Substitutionally Disordered Molecular Crystals
Authors: Hoshen Joseph , Jortner Joshua
Published: February 2004

In this paper we consider the optical properties of heavily doped molecular crystals where the constituents differ by isotopic substitution. Four different situations were considered determined by the perturbation strength relative to the second moment and to the width of the density of states in the pure crystal: (a) separated bands; (b) persistence case; (c) incipient band gap; (d) amalgamation limit. The properties of the effective Hamiltonian for a configurationally averaged mixed molecular crystal containing an arbitrary number of molecules per unit cell were explored. Information on the intensity distribution in optical absorption by a binary crystal can be obtained from the moments of the spatially averaged mixed crystal Hamiltonian. In the amalgamation limit the crystal exhibits the gross features of a virtual crystal and the number of the polarized intensity distributions is equal to that of the pure crystal, while in the separated band case and in the persistence case the number of polarized intensity distributions is double that in the pure crystal. The coherent potential approximation based on a local approximation for the self-energy and on neglecting multiple scattering effects was extended to handle the general case of a multiply branched exciton band. Concerning the general question of the persistence of the Davydov splitting in a mixed crystal, general arguments are provided that symmetry restrictions can be relaxed and that the Davydov splitting is exhibited by a substitutionally disordered system described by a Hamiltonian which is characterized by a random diagonal part and by a translationally invariant off-diagonal part. The number of these Davydov components is determined by the perturbation strength.
Orbitally Nondegenerate Iron(III) Dithiocarbamates. II. Intermolecular Exchange in a Ferromagnetic Homolog
Authors: Wickman H. H.
Published: February 2004

The chelate bis-(N,N-diethyldithiocarbamato)iron(III) chloride is known to exhibit a ferromagnetic transition at 2.43°K. The magnetism of this complex is of some interest since crystallographic data show the lattice to be composed of discrete molecules at normal van der Waal's radii, containing one iron atom per molecular unit. The magnetic exchange is primarily intermolecular in origin, since iron—iron separations rule out significant dipole—dipole contributions. Single crystal Mössbauer experiments are presented which may be analyzed to show the exchange to be of the form Fe–S··· S–Fe, the Fe–S units belonging to distinct molecules, and the actual geometry being nonlinear. The Mössbauer data are consistent with a spin polarization parallel to the major axis of the base of the rectangular pyramid formed by the immediate iron ligands.
19F Rotating Frame Nuclear Magnetic Relaxation in Liquid CHFCl2 and CFCl3
Authors: Sears R. E. J.
Published: February 2004

By a suitable choice of the forced precession frequency ω1, the rotating frame nuclear magnetic relaxation rate T1p−1 of 19F in liquid CHFCl2 and CFCl3 at room temperature can be dominated by the effect of the scalar coupling of 19F with 35Cl and 37Cl. From the measured variation of T1ρ with ω1, the isotropic scalar coupling constants were determined with the following results: JF3519Cl = 27.6 ± 0.5 Hz, JF3719Cl = 23.0 ± 0.5 Hz in CHFCl2; JF3519Cl = 13.7 ± 0.5 Hz, JF3719Cl = 11.4 ± 0.5 Hz in CFCl3.
Studies in Molecular Dynamics. XI. Correlation Functions of a Hard-Sphere Test Particle
Authors: Herman P. T. , Alder B. J.
Published: February 2004

The velocity autocorrelation function of a hard-sphere solute particle of the same size but different mass from the solvent particles has been studied at two different densities in order to quantitatively evaluate the mass dependence of the back scattering and vortex correlation contributions to the mutual diffusion coefficient. For an infinitely heavy solute particle, the time integral of the force autocorrelation function shows surprisingly little negative structure as a result of an unexpectedly large number of successive collisions with the same solvent particle.
Microwave Spectra of Iodogermane and Bromogermane
Authors: Wolf Stephen N. , Krisher Lawrence C.
Published: February 2004

Measurements have been made of the pure rotational spectra of four isotopic species of GeH3I and six species of GeH3Br, using a microwave spectrometer. The Ge—I bond length derived from the analysis is 2.5075±0.0006 Å. Hyperfine structure in the spectrum due to quadrupole coupling and centrifugal distortion gave an iodine quadrupole coupling constant eQq = −1381 ± 4 MHz and a centrifugal distortion constant D JK = 0.0077 ± 0.0002 MHz. The results obtained for GeH3Br are: Ge–Br bond length, 2.2970±0.0002 Å; eQq ( 79Br) = 384 ± 2 MHz; eQq ( 81Br) = 321 ± 3 MHz; and D JK = 0.013 ± 0.001 MHz.
Studies of Charge Exchange in a High-Pressure Pulsed Electron Impact Source
Authors: Sharma D. K. Sen , Hierl P. M. , Franklin J. L.
Published: February 2004

A high pressure pulsed ion source has been used in a time-of-flight mass spectrometer in order to study the charge exchange reactions in Ar–H2 and Ar–D2 systems using the ion source in the Čermák mode of operation. As the source was used in a pulsed mode, it was possible to identify the various secondary ions arising from the charge exchange reactions in these systems. Very good agreement has been shown to exist between the experimental results and simple theoretical deductions. Calculations have been made to determine the cross sections for the charge exchange reactions between the various species from the experimental data.
Theorem Useful for Interpretation of Magnetic Circular Dichroism Spectra
Authors: Seamans Lloyd , Moscowitz Albert
Published: February 2004

It is shown that the magneto-optical activity associated with a symmetry-forbidden transition is of second or higher order in any perturbation and that the contributions to the magnetic rotational strengths of perturbations belonging to different irreducible representations are additive. This theorem can be useful in analyzing the vibrational structure of a forbidden spectral band and in elucidating the conformations of molecular systems. The problem of the origin dependence of the magnetic rotational strengths is also discussed.
Large-ω Asymptotic Expansions and Padé Approximants for the Harmonic Oscillator
Authors: Osche G. R. , Iafrate G. J.
Published: February 2004

An approximate solution to the Schrödinger equation for a particle in various perturbed harmonic-oscillator potentials is obtained by utilizing large-ω (ω is the natural frequency of unperturbed oscillator) asymptotic expansion theory. Both the basic asymptotic and perturbation theory solutions are obtained and are shown to be related. Explicit high-order perturbation expansions for the energy and mean-square displacement of the symmetric anharmonic oscillator are obtained and are seen to be divergent but asymptotic. It is shown that by rearranging these expansions as Padé approximants, accurate numerical values are obtained for these observables, even for nonperturbative values of the coupling constant.
Pulsed Mass Spectrometric Study of Associative Ionization in Argon—Nitric Oxide and Argon—Acetylene Mixtures
Authors: Holcombe Norman T. , Lampe F. W.
Published: February 2004

Pulsed mass spectrometric studies of ArNO+ and ArC 2H 2 + ions formed by associative ionization show that even at very short times after excitation by 22−eV electrons, the predominant excited reactants are argon atoms in metastable states. Products of the specific reaction rates and the cross section for excitation into the triplet states of argon have been measured. From these results an estimate of 3 × 10 −17 cm 2 for the excitation cross section is made.
On a Comparison of the Electron Diffraction and Microwave Spectroscopic Structures for t-Butyl Chloride
Authors: Hilderbrandt R. L. , Wieser J. D.
Published: February 2004

An rz structure for t-butyl chloride has been determined from previously published microwave rotational constants. The resulting parameters: rz (C–Cl) = 1.831 ± 0.015 Ao, rz(C–C) = 1.525 ± 0.005 Ao, rz (C–H) = 1.081 ± 0.004 Ao and >CCCl=107.0± 0.7° were found to be in excellent agreement with our recent electron diffraction investigation. It was found that small skeletal differences in the rz distances resulting from isotopic substitution substantially influenced the structure obtained for the parent molecule. An rAv structure was also obtained for the molecule by combining the microwave and electron diffraction data. A modified set of Kraitchman's equations for symmetric-top molecules is also given which includes corrections for isotopic changes in bond lengths. Equilibrium bond lengths for C–Cl(1.821±0.006 Å) and C–C(1.521±0.004 Å) estimated from the rAv structure are found to be substantially different from the r8 parameters for the corresponding bonds, rs(C–Cl)=1.803± 0.002 Å and rs(C–C)=1.530± 0.002 Å.
Vibrational Relaxation in SO2/O2 Mixtures
Authors: Anderson B. , Shields F. D. , Bass H. E.
Published: February 2004

Measurements of sound absorption due to vibrational relaxation in SO2/O2 mixtures indicate a vibration—vibration (v—v) coupling between the asymmetric stretching mode of SO2 and the vibrational mode of O2. The theory of Tanczos, modified to allow for four quantum interchanges, was used to analyze the results. The acoustic absorption measurements of 5% SO2/95% O2 and 10% SO2/90% O2 mixtures enabled a distinction to be made between the series and parallel relaxation schemes in SO2. The measurements indicate that SO2 relaxes by way of the series process with energy passing through the stretching modes into the bending mode by vibration—vibration (v—v) energy transfer. SO2/SO2 collisions are of the order of 5 times as effective as SO2/O2 collisions in producing v—v exchanges within the SO2 molecule.
Hyperfine Interaction and Chemical Bonding in the BeH Molecule
Authors: Knight L. B. , Brom J. M. , Weltner W.
Published: February 2004

ESR spectra of the BeH and BeD molecules in their ground 2Σ+ states were measured while isolated in solid argon matrices at 4°K. g tensors and hyperfine structure (hfs) constants for interaction with 9Be, H, and D were determined. |Ψ (0)|2 values at Be and H nuclei were found to be 0.317(2) and 0.0434(2) a.u., respectively, as compared to 0.3223 and 0.0367 a.u. recently calculated by Bender and Davidson using a 1039 CI wavefunction. The HF SCF function of Cade and Huo gave poorer agreement with the measured |Ψ (0)|N2 values but gave satisfactory agreement with experimental values of 〈(3 cos2θN−1)/rN3〉. Approximate spin distributions and bonding among the alkaline-earth hydride molecules are briefly discussed.
Excitation Mechanisms of the Nitrogen Frist-Positive and First-Negative Radiation at High Temperature
Authors: Flagan Richard C. , Appleton John P.
Published: February 2004

The kinetic mechanisms responsible for the excitation of the first-positive and first-negative emission of nitrogen have been investigated in a re-examination of previously reported shock-tube measurements of the nonequilibrium radiation for these systems. The rate coefficients of the collisional quenching reactions, N2(A 3Σu+)+N(4S)⇒ lim k−2(N)N2(X 1Σg+)+N(4S)and N2+(B 2Σu+)+N2(X 1Σg+)⇒ lim kq(N2)N2+(X 2Σg+)   or   N2+(A 2Πu)+N2(X 1Σg+)were found to be given by the empirical expressions, k−2(N)=5.1× 10−3 T−2.23 cm3 sec−1 and kq(N2)=1.9 × 10−2 T−2.33 cm3 sec−1, respectively, over the approximate temperature range 6000–14 000°K.
Cluster Formation of Ion-Defect Pairs in Compensated Lattices
Authors: Naberhuis Steven L. , Fong Francis K.
Published: February 2004

Expressions for the dimer and trimer formation of aliovalent cation—lattice-defect pairs in crystalline compensated lattices have been derived from the minimization of free energy expressions which are given in terms of canonical partition functions. Numerical calculations have been made for the KCl:Sr2+ system by assuming the principle of linear superposition. It is shown that while extensive dimer formation occurs through wide ranges of temperatures and concentrations, trimer and higher cluster formations are of significance only at relatively high concentrations. Although the uncertainties of the interaction energies in many cases may weaken some quantitative aspects of the theoretical calculations, the general procedure given in the paper can be readily applied to other compensated lattices such as alkaline-earth halides containing trivalent cations. The over-all qualitative features of the theory serve to elucidate some of the more puzzling experimental observations reported recently in the literature.
Calculation of Transition Probabilities for Collinear Atom—Diatom Collisions with Nonpairwise Interactions
Authors: Shipsey Edward J.
Published: February 2004

Vibrational transition probabilities are calculated for a model collinear atom—diatom collision. The collision is that of an O atom with an N2 molecule. The interaction is 0.3 that of the corresponding triatomic, N2O. The calculation is done in the close coupling approximation which is briefly discussed. The results show many Feshbach resonances which influence the ground-state elastic probability, P11, less and less as the number of open channels increases.
Liquid Gallium: Comparison of X-Ray and Neutron-Diffraction Data
Authors: Narten A. H.
Published: February 2004

Structure functions for liquid gallium at 20°C from x-ray and neutron-diffraction data are compared. There is no significant difference between the x-ray and neutron data, and hence the equivalence of the two methods has been established, for the first time, with high precision.
Zeeman Fluorescence Studies of Neodymium in Calcium Tungstate
Authors: Kulpa Stanley M.
Published: February 2004

Polarized Zeeman fluorescence spectra of triply ionized neodymium in sodium compensated calcium tungstate have been taken at 2°K. Values of g‖ for the F 3/2 4Γ 7,  8 and several levels of the 4I ground term have been obtained. Accurate energy and polarization measurements were made, allowing assignment of S4 irreducible representations for all 11 levels of the 4I9/2 and 4I11/2 multiplets. Results are found to be consistent with recent predictions from crystal field calculations employing an effective spin—orbit Hamiltonian. Summation of the g∥'s for the three Γ 5,  6 levels of the 4I11/2 multiplet yields a value of −7.53± 0.19, whereas the partial g-sum rule predicts G 11/2,  6,  3/2(Γ 5,  6) = − 8.69 and the crystal field calculations yield −8.47.
Compton Scattering of X Rays from Ne, N2, and O2: A Comparison of Theory and Experiment
Authors: Eisenberger P. , Henneker W. H. , Cade P. E.
Published: February 2004

Experimental and theoretical Compton profiles are presented for gaseous Ne, N2, and O2. A comparison between experiment and theory indicates the ability of the Compton scattering technique to accurately probe many-electron systems if impulse approximation corrections are made. The need to achieve Hartree— Fock accuracy in molecular wavefunctions is emphasized.
Interaction Potential between Ground State Helium Atom and the B 1Σu+ State of the Hydrogen Molecule
Authors: Schaefer Henry F. , Wallach Daniel , Bender Charles F.
Published: February 2004

In order to aid in the interpretation of the energy transfer experiments of Moore and co-workers, ab initio quantum mechanical calculations have been carried out on the four electron system 1S He+ B 1Σu+ H2. Nine C2v geometries were considered, all involving a fixed H–H separation of 2.43 bohr. A contracted Gaussian basis of three s and two p functions on each atom was used, and calculations were performed including 1, 416, and 706 configurations. The 416 configuration calculation was designed to include only the electron correlation between He and H2 and should yield the most reliable results. The 416 configuration calculation predicts a large van der Waals interaction between He and the lowest 1Σu+ state of H2. The potential curve has a well depth of 401 cm−1 (0.05 eV) and this minimum occurs for a distance 3.72 bohr (1.97 Å) between He and the H2 center of mass. SCF calculations using the same basis and geometries were done on the He+ X 1Σg+ H2 and He+b 3Σu+ systems. The potential curves for these two states lie well below that of He+B1Σu+ H2 for thermal energies and it is suggested that curve crossing with the two lower surfaces is not responsible for the observed quenching of the B1Σu+ H2 fluorescence by He.
Effect of Slow Rotational Diffusion on Angular Correlations
Authors: Marshall Alan G. , Meares Claude F.
Published: February 2004

The theory for perturbed angular correlations of gamma radiation has been extended to include the possibility of adiabatic variation in the interaction Hamiltonian, K, for the intermediate state. The calculation begins from a polycrystalline model; it is then shown that adiabatic variation in K introduces a time dependence into the angles which express the orientation of the molecular frame. If this variation arises from slow rotational diffusion of the principal axis of K, then each term of the polycrystalline correlation is simply modified to include the factor, exp[−k(k+1)Dt], where D is the rotational diffusion constant for molecular reorientation in the intermediate state. The relevance of the adiabatic limit to the use of perturbed angular correlations of gamma radiation for study of the motion of radioactive species in viscous media is discussed.
Long-Range Many-Molecule Interactions in an Extended van der Waals Equation of State
Authors: Baer S. , Ben-Shaul A.
Published: February 2004

Distinction between the effects of pairwise and nonadditive interactions on thermodynamic functions can be made only in an approximate sense and then only for the long-range part of the interaction. Within such an approximation the entire contributions of all long-range many-body interactions to the free energy and pressure are obtained in closed form. These appear as simple extensions of the well known long-range terms of the van der Waals equations. Using a simple harmonic oscillator model for the molecule it is found that the relative contributions of the entire nonadditive to additive interactions to the free energy and pressure are functions only of a reduced polarizability of the molecule. Taking actual values of polarizabilities and densities of the noble gases it is found that the nonadditive effects contribute at most about 7% to the pressure and 4% to the free energy even at the liquid densities. Using van der Waals' expression for the short-range part of the pressure, the corrections to the critical constants are also estimated. As a rule inclusion of nonadditive effects improves the agreement between calculated and experimental values of all thermodynamic functions.
Mössbauer Study of Irradiated Iron Chelates and Chemical Consequences of 57Co Electron Capture in Complex Ligand Compounds
Authors: Baggio-Saitovitch E. , Friedt J. M. , Danon J.
Published: February 2004

The Mössbauer absorption spectra of Fe(III) acetylacetonate, Fe(III) citrate Fe(III) EDTA and Fe(III) trisdipyperidyl perchlorate irradiated with electrons at liquid nitrogen temperature are found to be similar to the emission spectra after electron capture of 57Co in analogous Co(III) compounds. This result suggests that an autoradiolysis mechanism is responsible for the stabilization of the anomalous iron charge states in these compounds. The main effect of the radiolysis is found to be the reduction of the trivalent iron complex to the corresponding bivalent one. A mechanism involving the formation and fast annihilation of free radicals is invoked for explaining the radiation reduction process.
Stochastic Model for a Fluid of Hard Cubes with Attractive Potential
Authors: Alexandrowicz Z. , Mostow M.
Published: February 2004

A Monte Carlo computer study of a fluid, consisting of hard cubes not limited to only lattice sites, and with a square well attractive potential, is undertaken with the help of the ``Stochastic models'' method. Thus two parameters, fixing the transition probabilities, are used to produce arrays of the molecules of desired density and degree of clustering, while the method's variation principle enables the choice of the ``best'' configurations for given N, P, and T. The results strongly suggest the existence of a gas—liquid phase transition, yielding what appears to be a correct estimate of the critical pressure and temperature.
Rotational Barrier and Electronic Structure of Monomethylphosphine from Ab Initio LCAO–MO–SCF Calculations
Authors: Absar Ilyas , Van Wazer John R.
Published: February 2004

Ab initio LCAO–MO–SCF calculations have been carried out on three conformations of monomethyl-phosphine, CH3PH2, using an uncontracted, moderate-sized Gaussian basis set, with and without d orbitals. The relative stability of the three forms (staggered, semieclipsed, and eclipsed) was studied. The effect of allowing d character to the phosphorus atom in this molecule was investigated in detail, with special reference to the carbon–phosphorus bond. Three-dimensional electron-density plots are presented and discussed in this context.
NMR Relaxation Studies of Solid Hexamethylethane and Hexamethyldisilane
Authors: Albert S. , Gutowsky H. S. , Ripmeester J. A.
Published: February 2004

Pulsed NMR studies in the static and rotating frames (T1 and T1ρ, respectively) of the proton spin-lattice relaxation time were carried out on solid hexamethylethane (CH3)3CC(CH3)3 and hexamethyldisilane (CH3)3SiSi(CH3)3 between 77°K and the melting point. The phase transition in each of the solids is accompanied by a discontinuity in the T1 and T1ρ plots. In the low-temperature phase of HMDS, the proton relaxation produced by reorientation of the CH3 group about its threefold axis (C3 rotation) is resolved from that produced by reorientation of the molecule about its threefold axis (C3′ rotation). The corresponding activation energies are 1.56±0.08 and 7.50±0.40 kal/mole. (Actually, the results for HMDS are more in keeping with the CH3 group motional narrowing and relaxation being caused by quantum mechanical torsional-state effects than by classical rotation.) In HME the effects of the two types of rotation are combined and have an apparent activation energy of 3.10±0.15 kcal/mole. The minimum T1 and T1ρ values were calculated for these motions by means of the BPP and extended BPP theories and are in good agreement with our experimental results. In the high-temperature phase of both solids, self-diffusion as well as over-all molecular reorientation or tumbling governs the proton relaxation. The activation energy for tumbling was found to be ·1.5 kcal/mole for HME and 2.20±0.10 kcal/mole for HMDS. Torrey's diffusion theory is employed to analyze the contribution to T1ρ caused by self-diffusion, and the activation energy for the process is estimated to be 18±2 and 8.7±0.4 kcal/mole for HME and HMDS, respectively. The properties of these two compounds are discussed briefly in relation to the NMR and thermodynamic studies of them.
Metamagnetism in Neptunium(V) Oxalate
Authors: Jones E. R. , Stone J. A.
Published: February 2004

Magnetic properties of neptunium(V) oxalate, (NpO2)2C2O4·4H2O, were studied from 2.4 to 37.5°K with magnetic-susceptibility measurements using a vibrating-sample magnetometer at applied fields of 50–10 000 Oe; Mössbauer spectra of the 59.54−keV γ ray in 237Np were taken from 4.2 to 80°K. A metamagnetic transition was found at 11.6°K. Below 11.6°K the magnetization isotherms had the characteristic S shape of metamagnets. Below 8°K a pronounced hysteresis was observed as the field was reduced from saturation. At 3.15°K the remanent magnetization at 100 Oe was 96% of the saturation magnetization at 10 000 Oe, and 125 times greater than the magnetization in the low-moment state. The Mössbauer spectra exhibited magnetic hyperfine splitting which was independent of temperature, with g0μnHeff = 10 cm/sec; splitting above the transition temperature is attributed to a long paramagnetic relaxation time.
Molecular Autoionization Lifetimes and Cross Sections for Penning Ionization: Numerical Results for He* (1s2s 3S) + H(1s 2S)
Authors: Miller W. H. , Slocomb C. A. , Schaefer Henry F.
Published: February 2004

The width Γ (or lifetime ℏ/Γ) for autoionization of He* (1s2s 3S) + H(1s2S) has been calculated as a function of internuclear distance, and cross sections for Penning and associative ionization (He*+H → He+H++e−, HeH++e−) have been determined for low collision energies. Associative ionization is 22% of the total ionization cross section in the limit of zero collision energy; this fraction decreases with increasing energy, being ·18% at a collision energy corresponding to 300°K. The distribution in energy of the ionized electron is also calculated, and it is seen that measurement of this quantity should lead to a good estimate of the well depth of the He*–H potential. Comparison of these results with those obtained by an orbiting model shows that the model (suitably scaled) is adequate in predicting the total ionization cross section, but is less accurate for the more detailed collision properties.
Electronic Spectroscopy of Maleimide and Its Isoelectronic Molecules. III. Quantum Chemical Calculations
Authors: Seliskar Carl J. , McGlynn Séan P.
Published: February 2004

The electronic transitions of energy <6.5 eV observed in maleic anhydride, maleimide, and 4-cyclopentene-3, 5-dione are assigned using all-valence-electron computational results and experimental data. In the one case where an experimental ground state dipole moment is available (i.e., maleic anhydride: 3.94 D), the calculated moment of 3.81 D provides excellent agreement. The solvent-polarity sensitivity and chemical substitution effects observed for several of the lower-energy transitions are discussed in terms of calculated ground-state and excited-state electron densities. The T1 states of all three isoelectronic molecules are tentatively assigned as 1 3B2(nπ*) states.
Two-Dimensional Anharmonic Oscillator. Application to 2,5-Dihydrofuran
Authors: Carreira L. A. , Mills I. M. , Person W. B.
Published: February 2004

A program has been developed to calculate the energy levels and corresponding wavefunctions for a two-dimensional anharmonic potential surface of at least C2v symmetry. This program has been employed to explain the high resolution splittings observed in the far infrared spectrum of 2,5-dihydrofuran. The magnitude of the cross term connecting the ring-twisting and ring-puckering modes of 2,5-dihydrofuran is sufficiently large to be significant. The potential surface determined also suggests that the ring-twisting mode may be slightly anharmonic.
Applications of Variational-Perturbation Theory to Molecules. II. Some 10-Electron Hydrides
Authors: Montgomery H. E. , Bruner Buddy L. , Knight Robert E.
Published: February 2004

Ground state energies are calculated for 12 different 10-electron hydrides and hydride ions using Rayleigh—Schrödinger variational-perturbation theory. The zero-order wavefunctions were products of screened single-center hydrogenic orbitals. The energies are calculated through the second order for three different selections of the screening and compared with previously determined SCF values. The potential of the procedure is discussed in light of these results.
Radio Frequency Spectrum of the X 2π State of 7Li16O
Authors: Freund S. M. , Herbst E. , Mariella R. P. , Klemperer W.
Published: February 2004

The molecular beam electric resonance spectrum of the high temperature free radical 7Li16O is measured in areas of the region 5–12 350 MHz. ΔJ=0, ΔF=0, ±1 electric dipole transitions are observed in the Π3/22 and Π1/22 spin—orbit components of the ground electronic state. Transitions of the two lowest vibrational states are analyzed. The fine structure parameters are: v=0v=1AΠ−112.0(1) cm−1−108(1) cm−1〈 Π | (A/2+B)L+ | Σ 〉 〈 Π | BL+ | Σ 〉−139.3(2)cm−1−139.5(2.0) cm−1ωe800(100) cm−1μ6.84(3) D.The Frosch and Foley hyperfine parameters are: aΠ + 1/2 (bΠ + cΠ)3.98(1) MHz3.83(3) MHzaΠ − 1/2 (bΠ + cΠ)8.25(4) MHz6.76(8) MHzbΠ−19.67(14) MHz−20.2(5) MHzdΠ1.94(10) MHz1.65(3) MHz〈 Π | aL+ | Σ 〉−11.0(1.4) MHzeQq00.444(8) MHzeQq20.109(18) MHz.The observed fine structure constants are in excellent agreement with Van Vleck's model of pure precession and with detailed calculations of Wahl.
Enhancement of the Reaction Cross Section of He+H2+→HeH++H by Vibrational Excitation of H2+ According to the Statistical Phase-Space Theory
Authors: Truhlar Donald G.
Published: February 2004

The quantum-mechanical statistical phase-space theory of Peuchukas, Light, and Nikitin is used to calculate the cross sections for the endothermic reaction He+H2+→HeH++H as a function of initial vibrational state and relative translational energy. The theory predicts enhancement of the reaction cross section by vibrational energy of the reagent, in qualitative agreement with the experimental results of Chupka. However, the experimental enhancement is much greater than that predicted by this theory. The energy dependence of the reaction cross sections for each initial vibrational state is also in only qualitative agreement with experiment.
Dielectric Properties of Some Nematic Liquid Crystals with Strong Positive Dielectric Anisotropy
Authors: Schadt M.
Published: February 2004

Measurements of the temperature dependence of the dielectric constants ε‖, ε⊥, and εis of some nematic liquid crystals with strong positive dielectric anisotropy (ε‖ ≃ 3 ε ⊥) are reported. The low frequency dispersion regions of the dielectric constants parallel to the unique axes were measured to be in the range from 10 kHz to 10 MHz. A correlation between the lengths of similar molecules and their relaxation frequency in the nematic state was found. The height of the potential barrier which hinders the reversal of the rodlike molecules in the nematic order was determined by measuring the temperature dependence of the low frequency relaxation and comparison with the temperature dependence of viscosity.
Lifetime of the a 3II Metastable State of Carbon Monoxide
Authors: Johnson Charles E. , Van Dyck Robert S.
Published: February 2004

The rotational dependence of the lifetime of the a 3II metastable state of CO has been investigated using a time-of-flight technique. The velocity distribution of metastable molecules is sampled and detected at two positions, 1.9 and 6.7 m from the pulsed electron gun used to excite the ground state molecules effusing from a cooled source slit. The initial population distribution among rotational levels is changed by varying the temperature of the source. A comparison of the number of metastables within given velocity intervals at the two detectors determines the number which decay in flight and yields an experimental plot of the number which decay versus time of flight. A theoretical decay plot is also obtained using the a 3II metastable state lifetimes calculated by James, who predicts rotational level lifetimes ranging from 3 msec to several hundred msec. For two different source temperatures, the experimental and theoretical decay plots are in good agreement.
Intermolecular Potentials from Crossed-Beam Differential Elastic Scattering Measurements. IV. Ar+Ar
Authors: Parson J. M. , Siska P. E. , Lee Y. T.
Published: February 2004

High-resolution low-energy elastic differential cross sections have been measured for Ar–Ar over a wide angular range. Rainbow structure as well as symmetry oscillations at wide angels were resolved. These results are used to evaluate 16 different Ar–Ar potentials proposed previously and also to construct a new potential. Vibrational energy levels and the second virial coefficient calculated for the new potential are compared with experimental results.
Polarized Raman Spectra of β-Quartz
Authors: Bates John B. , Quist Arvin S.
Published: February 2004

The Raman spectra of single crystal quartz were measured over the temperature range from a few degrees below the α-β transition temperature (573°C) to 700°C. The polarization characteristics of the fundamental optical modes of the β phase were obtained, and the temperature dependent behavior of the bands in the region below 200 cm−1 was examined. A complete assignment of the Raman active optical modes of β-quartz was determined from the polarization measurements.
Convergence of a Product Representation of the Exponential Hamiltonian Operator. An Example from Statistical Theory
Authors: Handler George S. , Wang Pearl S. C.
Published: February 2004

Using an iterated product representation of an exponential Hamiltonian, the density matrix for the potential V (x)=ax, a>0, is derived in nth-order approximation. The density matrix so found is shown to converge to the exact quantum mechanical result in the limit of n→ ∞.
Nuclear Magnetic Resonance Study of Transition Metal Monoborides. I. 11B Nuclear Electric Quadrupole Interaction Parameters
Authors: Creel R. B. , Barnes R. G.
Published: February 2004

Measurements are reported of the nuclear electric quadrupole interaction parameters (coupling constant and asymmetry parameter) of the 11B nuclear magnetic resonance spectra of the transition metal monoborides, MB (M=Ti, V, Cr, Co, Ni, Mo, and W). A detailed account is given of the method of extracting these parameters from the experimental spectra with the aid of computer-synthesized spectra. The measured 11B quadrupole couplings and asymmetry parameters are quite small and are inconsistent with the assumption that the boron atoms in these borides are linked by strong covalent sp hybrid bonds.
Comparison of 31P Spin—Rotation Interaction Constants Derived from Chemical Shift Data and from NMR Relaxation and Viscosity Data
Authors: Gillen Kenneth T.
Published: February 2004

The complete rotational diffusion tensors of the three symmetric top molecules PCl3, PBr3, and POCl3 are obtained from viscosity and halogen NMR relaxation time data. These tensors allow us to obtain estimates of the average correlation times for each molecule, which in turn are used in Hubbard's isotropic spin—rotation equation to derive NMR values of their spin—rotation interaction constants Ceff. These and similarly derived values of Ceff for a number of other 31P compounds are compared to values of the spin—rotation interaction constant Cσ deduced from an absolute shielding scale based on a recent molecular beam determination of the 31P spin—rotation interaction tensor in PH3. In contrast to previous work the values of Cσ are in reasonable agreement with the values of Ceff, therefore offering evidence for the validity of Hubbard's relationship. Earlier discrepancies between Cσ and Ceff appear to be mainly due to poor estimates of the average correlation times, although effects of anisotropy appear to contribute slightly. The results of this paper offer evidence that the so-called microviscosity approach should be the favored method for estimating average reorientational correlation times in complex rigid molecules and correlation times for reorientation perpendicular to the symmetry axis in symmetric top molecules whenever direct quadrupolar or dielectric relaxation times are not available. The derived rotational diffusion tensors for PCl3, PBr3, and POCl3 are used to discuss a recent extension to Hubbard's isotropic spin—rotation equation which takes account of the anisotropic rotational motions in symmetric top molecules. In favorable circumstances this approach can be used to calculate the entire spin—rotation interaction tensor for a nucleus in a symmetric top molecule.
CO2 Extinction Coefficient 1700–3000 Å
Authors: Shemansky D. E.
Published: February 2004

The CO2 extinction coefficient in the 1700–3000 Å region has been measured using a 2-m spectrometer in conjunction with a multiple-path absorption cell. The results show excellent agreement with other recent observations to a long-wavelength limit of about 1950 Å. At longer wavelengths the present results indicate considerably lower cross sections; the extinction cross section is dominated by Rayleigh scattering in the region longward of 2035 Å, with values determined reasonably accurately by calculations using the measured refractive index and depolarization factor.
Formal Kinetic Theory of the Senftleben Effects
Authors: Moraal H. , McCourt F. R.
Published: February 2004

A formal theory for the magnetic field changes induced in the transport properties of paramagnetic gases (the Senftleben effects) is developed. In order to illustrate the formal theory, it is applied to a calculation of the low-pressure Senftleben effects for molecular oxygen. The calculated results are compared with experiment and quite good agreement is shown to exist. All kinetic collision cross sections relevant to these effects have been extracted and listed. One conclusion arising from this analysis is that O2 has a slightly more nonspherical intermolecular interaction potential than N2.
Ionization Cross Sections in Flames
Authors: Fowler G. N. , Preist T. W.
Published: February 2004

A model is described based on the Chew—Low approximation which accounts quantitatively for recent measurements of ionization rates of alkali metals in CO/O2/N2 or H2O2N2 flames. The basic physics of the model is the same as that proposed by Hollander; that is to say the ionization of the alkali metal takes place from a state close to the ionization continuum by collision with an excited gas atom—here taken to be nitrogen. The well-known divergence of the sum over states of hydrogenlike atoms is avoided by assuming Debye screening and using observational results on the series limit of the Balmer spectrum in the sun's chromosphere.
Ion Solvent Interaction. VIII. Thermodynamic Properties of Ions in Methanol Solution
Authors: Padova J.
Published: February 2004

The thermodynamic treatment of a fluid in an electrostatic field is extended to the calculation of partial molal volumes of electrolytes at infinite dilution in methanol solutions. The electrostatic part of the free energy of the electrolyte may be interpreted in terms of the same intrinsic radii that are used in the volume computations. An expression for the ``primary medium effect'' is obtained giving good agreement between calculated values and literature data.
Measurements of Anisotropic Light Scattering and Thermal Transition in Optical Anisotropy of Poly-α-methylstyrene
Authors: Utiyama Hiroyasu , Tsunashima Yoshisuke
Published: February 2004

The anisotropic optical scattering from solutions of flexible macromolecules is discussed, and a simple experimental method of obtaining information concerning the optical anisotropy of a segment is presented. Intensities of anisotropic and isotropic optical scattering have been measured on poly-α-methylstyrene samples in the ranges of molecular weight from 400 thousand to 7 million, in trans-decalin at 9.6° (theta temperature), 20.2°, 29.5°, 49.7°, and 91.6°C. Analysis of the experimental data by the present method shows that the optical anisotropy of poly-α-methylstyrene exhibits a transitionlike decrease with temperature with the midpoint of about 30°C. The origin of the slight cooperativity of the transition is discussed and concluded as indicative of the correlation between the skeletal bond rotation and the rotational motion of the phenyl group. Some change of the unperturbed mean-square radius of gyration with temperature is suggested as a possible consequence of the transition.
Classical Evaluation of Averaged Collision Rates: Ehrenfest Theorem in Near Classical Collision Theory
Authors: Levine, R. D.
Published: February 2004

The change in an observable due to collisions is written, using near classical collision theory, as a simple average over the initial internal state. For most observables of physical interest this average assumes the usual form of a classical phase average. The most valid means of characterizing the distribution of final states is in terms of its moments.
Matrix Isolation Studies of Group IV Oxides. IV. Infrared Spectra and Structures of PbO, Pb2O2, and Pb4O4
Authors: Ogden J. S. , Ricks M. J.
Published: February 2004

The matrix isolation of lead oxide vapors has led to the characterization by infrared spectroscopy of three prominent vapor species, PbO, Pb2O2, and Pb4O4. In a nitrogen matrix these molecules have absorption bands at 718.4 cm−1 (PbO), 557.4 and 467.7 cm−1 (Pb2O2), and 474.4 and 374.4 cm−1 (Pb4O4). With the aid of 18O substitution, it is shown that the spectra of Pb2O2 and Pb4O4 are consistent with molecular symmetries Vh and Td, respectively, and approximate molecular dimensions are obtained: Pb2O2, rPb – O=2.17 Å, 〈 OPbO=79° Pb4O4, rPb–O=2.27 Å, 〈 OPbO=81°. Several weaker features were also observed, and these are assigned to additional polymers.
Light Scattering in Aqueous LiCl Solutions; Evidence for a Low Temperature Immiscibility
Authors: Hsich S.-Y. , Gammon R. W. , Macedo P. B. , Montrose C. J.
Published: February 2004

Rayleigh and Brillouin light scattering spectra have been obtained in a number of concentrated aqueous LiCl solutions. The scattering from density fluctuations shows the dispersion effects typical of viscoelastic relaxation as the temperature is decreased. In the same temperature region the Rayleigh scattering associated with fluctuations in concentration becomes quite intense. This is interpreted as portending the presence of a miscibility gap in the LiCl–H2O system. The upper consolute temperature associated with this immiscibility is quite near the glass transition region so that no large-scale phase separation can be seen. In addition it appears there is yet another mechanism producing Rayleigh scattering in the solutions. We have tentatively identified this as arising from fluctuations in the order parameter(s) describing the progress of ionic hydration reaction(s).
Influence of Interband Transitions on the Optics of Anisotropic Crystals
Authors: Fulton Robert L.
Published: February 2004

The contribution of the interband transitions, single exciton→single exciton, to the susceptibility Cr(12) of an anisotropic crystal is evaluated. It is shown that this contribution is not of the form which may be inferred from the Maxwell equations using the usual constitutive relations. A general discussion of the optics of anisotropic crystals based on a previously derived equation of motion for the vector potential ensues. The modifications of classical optics necessitated by the interband transitions are found. The optics of uniaxial crystals is discussed in detail. It is shown that in regions of negligible absorption the presence of interband transitions modifies the angular dependence of the index of refraction of the extraordinary wave. The possible existence of a cone of optic axes is predicted for suitable values of the interband contributions. In regions of absorption it is shown that the interband transitions give rise to a characteristic dependence of the imaginary part of the index of refraction on propagation directions. This differs from that of classical optics and allows for a simple experimental determination of the contribution of interband transitions to Cr(12). A discussion of the relation between the directions of the optic axes and optical parameters in orthorhombic crystals is given. It is shown that crystals of this class have at least one pair of optic axes, and possibly three pairs of optic axes, depending on the strength of the interband transitions.
On a Reinvestigation of the Structure of Dinitrogen Tetrafluoride, N2F4, by Gaseous Electron Diffraction
Authors: Gilbert Michael M. , Gundersen Grete , Hedberg Kenneth
Published: February 2004

Analysis of electron-diffraction patterns from gaseous N2F4 at 25°C has verified conclusions from earlier work that the material contains both trans and gauche conformers. Assuming that these conformers have the same parameter values except for the torsion angle, the following principal results were obtained: 71.2% (8.1) trans, rNN=1.492 Å (0.007), rNF=1.372 Å (0.002), 〈 FNF=103.1° (0.6), 〈 NNF=101.4° (0.4), θ (the dihedral angle N2N1F4, N1N2F1)=64.2° (3.7), lNN=0.048 Å (0.005), and lNF=0.044 Å (0.002). These distances and root-mean-square amplitudes are ra and la values; the errors are 2σ and include estimates of systematic error. The diffraction results indicate that the energy difference Eg—Et is roughly less than 1.0 kcal but that the rotational barrier may be several kilocalories.
Matrix Isolation of Pb and Pb2
Authors: Brewer Leo , Chang Chin-An
Published: February 2004

Lead vapor trapped at 20°K in Ar, Kr, Xe, SF6, and in mixtures of these matrix gases showed an atomic absorption line with no multiplet structure and molecular bands of Pb2. Warming the matrices above 20°K shifted the atomic line reversibly to the red while the molecular features did not shift. Bulk diffusion of isolated Pb atoms to produce Pb2 did not take place in Xe even after 2 h at 50°K. However, surface diffusion of Pb atoms to form Pb2 can take place during the condensation process on a 20°K window under conditions of high heat load or poor thermal contact that cause high surface temperature.
Cotton—Mouton Effect in Atoms and Molecules
Authors: Chang Tai Yup
Published: February 2004

A more general description of the Cotton—Mouton effects in atoms and molecules is presented. By use of a time-dependent double perturbation approach, closed and spectral expansion expressions of required molecular parameters are derived. The method is applied to the hydrogen atom to generate accurate Cotton—Mouton dispersion curves for a wide range of wavelengths, including the first two lines of the Lyman series.