bluuebunny/crossref_metadata_2025 huggingface dataset:
ESR Study of Electron Capture by Guest Molecules in Irradiated Single Crystals of Impurity-Doped Organic Compounds: Fumaric Acid-Doped Succinic Acid
Authors: Iwasaki Machio , Muto Hachizo , Toriyama Kazumi
Published: February 2004

The highly selective capture of an ejected electron by a small amount of guest molecules has been found by the ESR study of a single crystal of fumaric acid-doped succinic acid irradiated at 77°K. The four-line spectrum due to the molecular anion found in an irradiated single crystal of pure succinic acid was replaced by the three-line spectrum when about 0.005 mole fraction of fumaric acid was doped in a single crystal of succinic acid. The analyses of the hyperfine and g tensors of this spectrum lead to the conclusion that the radical responsible for this spectrum is the molecular anion of fumaric acid, which is believed to be a precursor of the stable radical formed by the addition of the acidic proton to fumaric acid in the room temperature irradiation. Our results suggest that the hydrogen addition radical is formed by the acidic proton transfer from the host molecule to the fumaric anion. The mechanisms for the formation of the stable radical and for the stereospecific addition of the acidic proton to the C=C bond are proposed based on the orientation of the guest molecule in the host crystal, which was determined by comparing the principal directions of the hyperfine and g tensors with the x-ray crystallographic data.
Ultrasonic Investigation of Acetic Acid Hydrogen Bond Formation. II Acetic Acid–Water Solutions
Authors: Corsaro Robert D. , Atkinson Gordon
Published: February 2004

Ultrasonic absorption has been measured in acetic acid–water mixtures in the range 3.6 molar acetic acid to 17.38 molar (glacial). The frequency range covered was 0.5–150 MHz and measurements were made at 18, 25, 35, and 50°C. Measurements were also made with added H+ and OAc− ions. The data for every solution measured could be fitted to a single relaxation curve. The simplest mechanism to explain the data is shown to be D⇆ lim krkfC, where D and C are believed to the “closed” and “open” forms of the acetic acid dimer. In aqueous solution this process is catalyzed by both H+ and OAc− ions. This mechanism gives a quantitative fit of the ultrasonic data and also provides complete kinetic and thermodynamic parameters. One finds that the rate constants show strong dependence on viscosity.
A Gas-Dynamic Model for Coupled Vibrational and Radiative Nonequilibrium in CO2—with Application to the Spectrophone
Authors: Tripodi Robert , Vincenti Walter G.
Published: February 2004

Macroscopic equations are formulated for the nonequilibrium interaction of vibrational and radiative rate processes in CO2. A model for CO2 is used that assumes that the energy levels of each vibrational mode are evenly spaced and that each mode can be assigned a vibrational temperature. The bending and symmetric-stretching modes are, however, taken to be in mutual equilibrium so that they have the same vibrational temperature. Collisional rate equations for the V—T and V—V processes are derived on a phenomenological basis. The resulting phenomenological coefficients are interpreted, with the help of basic knowledge of the microscopic physics, in terms of characteristic relaxation times and parameters measuring the relative amounts of energy exchanged by the various modes during V—V transitions. Radiative transfer equations are developed for the three strongest infrared bands, located in the spectrum at 15, 4.3, and 2.7 μ, by appropriately summing a previously derived microscopic transfer equation. It is found that the absorption coefficient is a function of the kinetic and the vibrational temperatures in a manner that depends on the band in question. The source function, which is also different for each band, depends strongly on the temperature of the vibrational modes involved in the transition and weakly on the kinetic temperature. The rate and transfer equations are then incorporated with the conservation equations of gas dynamics for application to linearized nonequilibrium flows of carbon dioxide gas. As a particularly simple example, a solution for the spectrophone is presented, including expressions for the phase lag in the pressure signal that results from radiative excitation via each of the bands individually. One of these expressions can be used in conjunction with experimental results from other sources to draw conclusions about predominant V—V transitions in CO2. The results of doing this using the 4.3-μ band have been reported previously. The utility of a spectrophone can also be extended by exploiting the possibility of selectively exciting CO2 through the different bands and making use of all the phase-lag expressions. All equations were derived without restriction to room temperature, allowing for possible use of spectrophones at higher temperatures. The approach used to derive the macroscopic rate equations can equally well be applied to other polyatomic molecules.
Ultrasonic and Relaxational Spectroscopy of Fast Reactions. III. Characteristics of Ultrasonic Spectrum
Authors: Tabuchi Daisaku
Published: February 2004

The complex velocity of sound due to multiple reactions in gas is derived in the first paper I and in liquid in the second paper II. The characteristics of the complex heat capacity are examined, and the complex velocity of sound is expressed in terms of the complex heat capacity. Velocity and absorption are calculated from the complex velocity of sound. There are some relations between the parameters of the ultrasonic spectrum and the heat capacity, and they are important when we compare the experiments with the theory. A numerical example about two reactions in gas is shown when the type of reactions, the relaxation times and the heat capacities at constant volume are given, and the frequency dependence of the heat capacities and the ultrasonic spectra of velocity and absorption are calculated and shown in the figures.
Liquid Water: Molecular Correlation Functions from X-Ray Diffraction
Authors: Narten A. H. , Levy H. A.
Published: February 2004

For nearly spherical molecules the x-ray scattering from liquids yields structure and correlation functions for molecular scattering centers. The distribution of electron density in a water molecule is very nearly spherical, and orientational correlation between molecules in the liquid is not ``seen'' by x rays. Structure and correlation functions for molecular scattering centers are derived from x-ray data on water and tabulated. They provide a sensitive test for future work on a molecular theory of liquid water.
Total and Dissociative Charge Transfer of Positive Ions with Perfluoroalkanes Studied by Mass Spectrometry
Authors: Smith David L. , Kevan Larry
Published: February 2004

Total and dissociative charge-transfer cross sections of rare gas and molecular positive ions with CF4, C2F6, C3F8, n-C4F10, and cyclo-C4F8 are reported. The positive ions had an average energy of 50 eV so an electron jump mechanism occurs. When the total cross sections are plotted against the projectile ion recombination energy pseudoresonance effects are seen. It is suggested that the total cross section depends on the Franck—Condon factors for the transition of the neutral target molecule to a vibrationally excited ion. Thus higher vibrational states are less probable since they are generally associated with lower Franck—Condon factors. The highest cross sections occur with N2+. For a given projectile ion the total cross sections increase with the molecular weight of the fluorocarbons. The dissociative cross-section data indicates fragment ion appearance potentials in remarkably good agreement with recent photoionization measurements except for CF3+ from CF4.
Krypton Monofluoride and Its Positive Ion
Authors: Liu Bowen , Schaefer Henry F.
Published: February 2004

Nonempirical quantum mechanical calculations have been carried out on KrF and KrF+. A large basis set of Slater functions was used, and the self-consistent-field (SCF) total energies for both diatomics are estimated to lie no more than 0.005 hartree above the exact nonrelativistic Hartree—Fock energies. The KrF SCF potential curve is repulsive and dissociates smoothly to Kr+F atoms. The KrF+ potential curve displays a minimum at 1.68 Å and spectroscopic constants De=−0.02 eV, and ω e=810 cm−1. The importance of polarization functions (d and f orbitals) in the single configuration wavefunctions is discussed. Electron correlation in both molecules has been investigated using first-order wavefunctions, which have yielded dissociation energies in excellent agreement with experiment for several smaller diatomic molecules. A 158-configuration correlated wavefunction predicts a flatter but nevertheless repulsive potential curve for KrF. We conclude that, except for the van der Waals interaction, KrF is unlikely to exist in the gas phase. The relation between this work and the observation of the KrF ESR in crystals of KrF2 is discussed. First-order wavefunctions containing 210 configurations for KrF+ yield the following predictions: re=1.75 Å, D0=1.90 eV, ω e=621 cm−1. Chupka and Berkowitz report the experimental value D0≥1.58 eV for KrF+. Several molecular properties are predicted for KrF+.
Electron Spin Echoes and Spectral Diffusion in Organic Radical Solids
Authors: Brown I. M.
Published: February 2004

The echo decay envelopes of the two-pulse and three-pulse electron spin echoes have been investigated in radical cations, anions, and neutral radicals randomly oriented in rigid glass matrices. The shape of the two-pulse echo decay envelopes indicates that the analytical form of the spectral diffusion function is approximately Lorentzian, whereas the three-pulse echo decay envelopes imply that it is Gaussian in the wings. In most cases, at sufficiently low concentrations spin—spin flips and spin—lattice flips of the matrix nuclei are the dynamic processes responsible for the local field fluctuations which determine the phase memory times, whereas the local hyperfine fields resulting from the interactions with the radical nuclei are essentially static. Investigations of the spin—lattice relaxation using an echo technique show that the electron spin—lattice relaxation times of anions and cations with orbitally degenerate ground states are sufficiently smaller than those with nonorbitally degenerate ground states so that electron spin—lattice relaxation processes can in some cases shorten the phase memory times. Electron spin echo spectroscopy experiments are also described in which the echo height is monitored as the magnetic field is swept.
Fluorescence from the Second Excited Singlet State and Radiationless Processes in Pyrene Vapor
Authors: Baba Hiroaki , Nakajima Akira , Aoi Masakatsu , Chihara Kohji
Published: February 2004

The fluorescence spectrum originating from the second excited singlet state S2 of pyrene in the vapor phase at 170°C and 0.21 mm Hg has been studied with particular attention to changes of emission characteristics with the excitation energy. The fluorescence from the first excited singlet state S1 has a comparatively constant quantum yield Φ1F (between 0.14 and 0.08), regardless of the excitation energy. In contrast to this, the quantum yield Φ2F of the fluorescence from S2 increases rapidly with excitation into successively higher singlet states. The ratio Φ2F/Φ1F is of the order of 0.001 for excitation into S2, but it is larger than 0.1 for excitation into the fourth excited singlet state S4. Addition of cyclohexane as an inert foreign gas to the pyrene vapor results in a marked decrease in Φ2F/Φ1F, which seems to reach a constant value at high pressures of cyclohexane (〉60 mm Hg). From an analysis of the experimental data and a consideration of the kinetics of radiative and radiationless processes in a collision-free molecule, the rate constant of the internal conversion S2*→S1*, k21 (the asterisk refers to excited vibrational states), and that of the reverse process S2*←S1*, k12, are estimated to be 5.8×1011sec−1 and 3.2×108 sec−1, respectively, for excitation into the S4 state. The ratio k12/k21 is shown to be related to the ratio between the densities of vibrational states for the vibronic states S2* and S1*. Most of the anomalous S2-fluorescence emission occurs through the mechanism involving the reverse internal conversion process. This mechanism is expected to be particularly effective in the case of the pyrene molecule, because it possesses the following favorable properties: (1) the rate constant for the radiative transition S0←S2 from S2 to the ground state is much greater than that for the radiative transition S0←S1; (2) the S2 state is located close to the S1.
Stabilization of Metal Atoms in Solid Hydrocarbons
Authors: Graham W. R. M. , Duley W. W.
Published: February 2004

Na, K, Ca, Ti, and Mn atoms have been stabilized in aromatic and paraffin matrices at 55°K and studied in absorption. Metal impurities in aromatic solids such as benzene and toluene, are stable to a temperature of 120°K. Bleaching of spectral features in some hydrocarbon solids doped with Ca and Mn species indicates photochemical reactions. Many systems are characterized by simple spectra consisting of perturbed resonance transitions in the metal atoms. Several metal impurity-solid hydrocarbon combinations have progressions of bands in absorption which may be interpreted as vibronic transitions.
van der Waals Forces Including Exchange in the Small Overlap Region
Authors: Boehm Richard , Yaris Robert
Published: February 2004

A linear response approach based on the many-body Green's function techniques of Martin and Schwinger is employed to obtain an intermediate range theory of interatomic forces between ground state atoms. Electron exchange is included to lowest order in a natural and physically transparent way. Our result for the interaction energy is expressible in terms of local and nonlocal response functions associated with the individual atoms. Moreover, the long-range limit of our expression for the interaction energy is explicitly demonstrated to reduce to the familiar van der Waals dispersion interaction. Finally the extension of the linear response approach to include exchange into the theory of nonadditive three-body interactions is briefly discussed.
More on Vibrational Relaxation in SO2/Ar Mixtures
Authors: Shields F. Douglas , Anderson Buford
Published: February 2004

A new system for making sound velocity and absorption measurements in gases has been constructed using capacitance transducers, resulting in a system having greater accuracy than that previously employed. Using this system and an improved method for correcting for the tube effect, the relaxation absorption has been measured in a mixture of 25% SO2/75% Ar to an accuracy sufficient to distinguish between the series and parallel relaxation schemes in SO2. The measurements favor the series process with energy passing through the bending mode into the stretching modes by vibration—vibration (v—v) energy transfer. The effectiveness of SO2/Ar collisions in producing v—v exchanges is of the order of 0.05 that of SO2/SO2 collisions.
Photochemical Recoil Spectroscopy: Chlorine Spectrum and Scattering Analysis
Authors: Diesen R. W. , Wahr J. C. , Adler S. E.
Published: February 2004

The measurement and analysis of the velocity distribution of the recoiling chlorine atoms from the laser induced photodissociation of Cl2 at 3471 Å using a time-of-flight technique is described. The expected velocity distribution is numerically calculated from the experimental geometry by appropriate center-of-mass (c.m.)-laboratory (LAB) coordinate transformations. By making the measurements at moderate densities in the interaction zone, the elastic scattering of Cl atoms by Cl2 molecules is observed. Since scattering angles in c.m. correspond to velocity changes in LAB coordinates, the direct recoil signal must be corrected for the effects of multiple elastic scattering. Careful comparison of expected and observed velocity profiles enables one to deduce directly the energy state of the products, the bond dissociation energy D0 ○, and the rainbow scattering angle. The analysis on Cl2 yields D0 ○ (Cl–Cl)=57.05±0.2 kcal/mole, which is in excellent agreement with the JANAF value of 57.04±0.06 kcal/mole and the precise value of 57.1743 kcal/mole [R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 52, 3869 (1970)]. Using a Lennard-Jones (12–6) potential, the rainbow scattering angle for Cl–Cl2 collisions corresponds to a Cl3 well depth (ε) of 2±0.5 kcal/mole.
Pi-Electron Theory of Acetylene. II. Unrestricted Hartree—Fock Theory
Authors: Dancz John , Harris Robert A.
Published: February 2004

Unrestricted Hartree—Fock (UHF) theory was applied to a Pariser—Parr—Pople pi-electron Hamiltonian for acetylene. The symmetries that must be broken in order to obtain the best UHF ground state were examined as a function of the Hamiltonian parameters. Improved ground-state wavefunctions were obtained for this two-band system by projecting out the undesired symmetries. Ground-state energies and correlation functions of the symmetrized UHF states were compared to the exact diagonalization of the Hamiltonian. For the model used, electron correlation is very well represented by the symmetrized UHF states.
Velocity Autocorrelation Function and Self-Diffusion in Liquids
Authors: O'Reilly Donald E.
Published: February 2004

Evidence is presented to show that density fluctuations that occur in van der Waals and polar liquids can be ascribed to voids approximately equal to a molecular volume. On the basis of a simple quasilattice model for the liquid a velocity autocorrelation function is derived and compared with the result of computer experiments on liquid argon. This comparison is made with the choice of three parameters: the mean time τc between ``hard'' collisions and the mean barrier heights for the creation of an interstitial molecule in the absence (ε¯0′) and presence (ε¯0) of a nearest-neighbor vacancy. A general expression for the self-diffusion coefficient of a liquid is derived in terms of τ c, ε¯0, and ε¯0′. Calculated pre-exponential factors (D∞) for self-diffusion are compared with experimental values for a variety of van der Waals and polar liquids. For simple liquids the agreement is good but for more complex liquids the observed values of D∞ are larger than the calculated values which are ascribed to an entropy of activation. The entropy of activation needed is approximately proportional to the energy of activation for diffusion. Barrier heights ε¯0 correlate well with the Lennard-Jones energy parameter for the free gaseous molecules. Volumes of activation for diffusion and relaxation are compared with experiment for argon, benzene, and chlorobenzene. An extension of the theory to solid-state diffusion is outlined.
One-Dimensional Cooperative Kinetic Model. Equilibrium Solution for Finite Chains
Authors: Simha Robert , Lacombe R. H.
Published: February 2004

In connection with a cooperative kinetics on linear lattices, the well-known equilibrium solution for the infinite Ising model is readily derived by the use of detailed balancing. The natural variables in this approach are conditional probabilities. For nearest-neighbor interactions and a two-state model, the independent variables are two conditional probabilities for doublets. The simplicity of the procedure suggests various generalizations. We consider a finite chain where the above probabilities become functions of location within the chain. They are obtained as the solution of a set of elementary difference equations. A second such set then yields the singlet frequencies as a function of position and thus determines the frequency of any sequence as a function of location, length, and composition. These quantities are also required in connection with solutions of the coupled kinetic equations. Our results lead to expressions derived by others by explicit use of a partition function, evaluated by matrix methods.
On the Quantum Mechanical Basis for Bond Energy Schemes
Authors: Nelander Bengt
Published: February 2004

The possibility to use a partitioning of the first-order density matrix to obtain a quantum mechanical basis for bond energy schemes is pointed out. Methods for obtaining such partitionings are discussed. The problems arising, when a bond energy scheme is based on the kinetic energy of the electrons is discussed in the light of the quantum mechanical virial theorem. A small numerical example is given.
Orbital Angular Momentum in the Theory of Paramagnetic Clusters
Authors: Lines M. E.
Published: February 2004

The statistical theory of paramagnetic clusters described in the literature to date is largely restricted to ``spin-only'' magnetism for which orbital angular momentum is quenched, or enters the calculations only as a trivially modified g factor. This paper discusses the extension of the theory to magnetic ions for which large unquenched orbital angular momentum is present, rendering the conventional approach completely inoperative. A theory is developed in detail for cobaltous cluster complexes, the cobaltous ion generally being one of the more difficult of the 3d transition metal ions to describe statistically. A preamble into relevant crystal-field theory is followed by the development of a statistical theory valid, to a good approximation, over the entire temperature range. Numerical results are presented to indicate the separate effects of spin—orbital reduction, and inter- as well as intracluster exchange. The full theory is also compared numerically with the results of simpler, but less adequate, approximations to demonstrate the penalty in accuracy paid for algebraically less onerous calculations. The theoretical principles involved are readily extendable for use with other magnetic ions (e.g., ferrous) for which unquenched orbital angular momentum plays an essential role.
Electronic Absorption Spectrum of LiMnO4·3H2O in LiClO4·3H2O
Authors: Johnson L. W. , McGlynn S. P.
Published: February 2004

Polarized absorption spectra of single crystals of LiClO4·3H2O/LiMnO4·3H2O have been measured at 4.2°K. The 5200-Å band of MnO4− has been identified as a T12←A11 transition; the 3600-Å band has been identified as T11←A11 or, just possibly, E1←A11; and the 3000-Å band has been identified as T12←A11. A brief discussion of two higher energy transitions is also provided. The site splitting of the 1A1 and 1E states, which are produced from the T2 state upon reduction of symmetry from Td to C3v, is large ( ∼500 cm−1). Vibronic activity is observed in the 5200 - Å1T2←1A1 transition and is discussed.
Collisional Deactivation of CO (a 3Π)
Authors: Young Robert A. , Van Volkenburgh Gregg
Published: February 2004

Using a weak, pulsed, Tesla-type discharge, the (a 3Π) v′= 0, 1 states of CO were generated and their rate coefficients for deactivation by CO and NO were measured by observing the time decay of the (0, 0) and (1, 0) CO Cameron bands. The rate constants for quenching of the v′= 0, 1 levels of CO (a 3Π) were found to be k0=1.2×10−10 and k1=2.8×10−10 cm3/ (sec·molecule). Quenching by NO gave k0′= 3.1×10−10 and k1′= 7.0×10−10 cm3/ (sec·molecule), with an error limit of ±25 % on all four numbers. The efficiencies of energy transfer between CO (a 3Π) v′= 0, 1 and NO to give NO (A 2Σ) v′= 0, 1, respectively, were determined by measuring the decay rates and intensities of the (0, 2) and (1, 3) NO gamma bands and the (0, 0) and (1, 0) Cameron bands for a fixed concentration of CO and NO. The efficiencies were found to be g0=15 % and g1=25 %, with an error limit of ±10 %.
EPR Study of O17- on Magnesium Oxide
Authors: Wong Ning-Bew , Lunsford Jack H.
Published: February 2004

The EPR spectrum of O17- on MgO has been obtained by the reaction of N2O containing 71.9% oxygen-17 isotope with trapped electrons on an MgO surface. The spectrum of the ion is characterized by g ⊥=2.042 and g ∥=2.0013 with A⊥=19.5±0.1G and A ∥=103.2±0.1G. A quadrupole interaction of Q⊥=1.3±0.2G was also determined. From the hyperfine coupling tensor the coefficients of the wavefunction of the spin state were estimated and were found to be consistent with those obtained from the g tensor. The unpaired electron is localized mainly in one 2p orbital on the O− species. These results suggest that O− is trapped at an oxide ion vacancy on the surface.
Matrix Infrared Spectrum of OF and Detection of LiOF
Authors: Andrews Lester , Raymond James I.
Published: February 2004

Simultaneous deposition of OF2 or 18OF2 diluted in argon with a beam of Li, Na, K, or Mg atoms on a salt window at 15°K produces new infrared absorptions which are due to the appropriate metal fluorides. An intense absorption at 1028.6 cm−1 is independent of the metal used to generate the reaction, and this feature shows the appropriate oxygen-18 shift for assignment to the OF free radical. Additional bands show a metal dependence and are attributed to weakly bonded MF···OF dimers. Use of isotopic mixtures shows that bands at 714.0 and 417.5 cm−1 using 6Li are due to a new species containing single lithium and oxygen atoms which concentration studies suggest to be the molecule LiOF.
Survival Probability of Products of Hot Reactions of (n, γ)-Produced 38Cl with CH3Cl in the Gas Phase
Authors: Alfassi Z. B. , Baer M. , Amiel S.
Published: February 2004

The hot reactions of (n, γ)-produced 38Cl with CH3Cl have been studied in the presence of three noblegas moderators: He, Ar, and Kr, using Br2 as a scavenger. The main purpose of this work was to study the extent of decomposition of the newly formed products upon collision with Ar and Kr. Applying the method of Baer and Amiel, it was established that the average survival probability of the various hot products in Ar and Kr is 100%, within the experimental error. Kinetic parameters for the three hot products, CH338Cl, CH2Cl38Cl, and CH238ClBr were determined as well.
Velocity Corrections for Total Scattering Cross Sections
Authors: Lang Neil C. , Lilenfeld H. V. , Kinsey James L.
Published: February 2004

Velocity correction factors are given for a quantity κeff≡viQeff, where Qeff is an effective total scattering cross section as measured in a molecular-beam experiment, and vi is the beam velocity. If κeff is regarded as being measured at the average relative velocity, the correction factors are all near unity and only weakly dependent upon the true velocity dependence of the cross section.
Emission Cross Sections of CO2 by Electron Impact in the Interval 1260–4500 Å. II
Authors: Ajello Joseph M.
Published: February 2004

A survey of the most prominent emission features produced by electron impact excitation of CO2 over the wavelength range of 1260–4500 Å has been completed. Emission cross sections were measured from threshold to 300 eV for the Ã2Hu→X̃2Πg and B̃2Σu+→ X̃2Πg electronic transitions of CO2+ produced by ionization excitation of CO2. Emission cross sections were also measured for the A1Π→X1Σ+ electronic transition of CO and the B2Σ+→ X2Σ+ electronic transition of CO+ produced by dissociative excitation of CO2. The vibronic intensities differed from those for electron excitation of CO. In addition, emission cross sections are presented for the Sa0→3P, 1304-Å multiplet of OI, the D2→2P0, 1335-Å multiplet of CII, and several atomic multiplets of CI produced by dissociative excitation of CO2. The relative energy variation of the emission cross section of the a3Π→X1Σ+ electronic transition of CO and of the S50→3P, 1356-Å multiplet of OI were also measured. The processes leading to these emissions are discussed.
Collision of an Atom and a Diatomic. The Adiabatic Approximation
Authors: Thiele Everett , Katz Ronnie
Published: February 2004

The adiabatic approximation is used to compute vibrational transition probabilities for the collision of an atom and a diatomic molecule in one dimension. The accuracy of the results is evaluated in terms of a parameter β, which is proportional to the period of the oscillator divided by the length of the collision. It is seen that for small β, the error in adiabatic transition probabilities approaches zero, and that the approximation is accurate over a physically interesting range of β.
Reaction Matrix Method for Computing Probabilities of Vibration—Translation Energy Transfer; Range of Applicability for the Collinear Collision of an Atom and a Diatomic
Authors: Clarke James H. , Weare John H. , Thiele Everett
Published: February 2004

Transition probabilities for the case of a collinear collision of an atom and a harmonic oscillator are computed using both the distorted wave and K-matrix approximations. Results are shown as a function of a new parameter β which incorporates the parameters E, m, and α of Secrest and Johnson and which is proportional to the ratio of the oscillator period divided by the collision time. Ranges of applicability for the above approximate methods are discussed in terms of β for one- and two-quanta transitions. The K-matrix approach is seen to extend the range of β for which the distorted wave approximation maintains a given degree of accuracy for one-quantum transitions. Also the K-matrix method yields vastly improved results for two-quanta transitions which exhibit the same type of dependence on β as the one-quantum-transition probabilities.
Proton Magnetic Resonance and Relaxation in AlCl3·6H2O
Authors: Saffar Z. M. El , Mulcahy William , Rochau Gary
Published: February 2004

The proton magnetic resonance spectrum of AlCl3·6H2O has been studied in the single-crystal and polycrystal forms at room temperature; the second moment and the proton spin—lattice relaxation time have been studied in the temperature range 300–438°K. The H–H distance of the water molecule obtained by fitting the observed polycrystalline line shape with least squares to a theoretical function is found to be 1.59±0.01 Å, in disagreement with the value 1.70 Å reported by Buchanan and Harris using neutron diffraction. The H–H direction obtained from the single-crystal study is in agreement (within 4°) with the neutron-diffraction results. The temperature dependence of the second moment and relaxation time reveals that the water molecules begin to execute 180° flip motion just above room temperature. The barrier height hindering the motion is found to be 11.0±0.8 kcal/mole.
Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. II. Photoaquation
Authors: Shirom Miriam , Stein Gabriel
Published: February 2004

Seven-nsec laser pulses at 337 nm from an N2 gas laser and continuous illumination at 254, 313, and 365 nm were used to investigate the formation of (Fe(CN)5H2O)3− from photoexcited states of Fe(CN)64− and its protonated form in aqueous solution. The same pentacyano-aquo complex is formed at all wave-lengths. It is formed in ≤ 1 nsec as shown by the laser results. The quantum yield is constant at 337 nm between pH 3.8–10.5. The spectrum of the product changes and the quantum yield appears to decrease at lower pH. The quantum yield at pH 9 is the same at 365 and 313 nm and decreases to half at 254 nm. Photoaquation occurs via the lowest singlet 1T1g state. Excitation to higher singlet states results in competition between decay to the lowest singlet and transition from the short-lived higher singlets to a CTTS state yielding the hydrated electron.
Theoretical Studies of Solvent Effects on Nuclear Spin—Spin Coupling Constants. II A Cubic Closest Packed Cluster Model
Authors: Johnston Milton D. , Barfield Michael
Published: February 2004

A theoretical study of the solvent dependence of nuclear spin—spin coupling constants is presented in terms of a model which assumes a cubic closest packed cluster in which six solvent molecules are arranged around the solute molecule. This model is described theoretically by means of a group function modification of self-consistent perturbation theory. Calculations for individual molecular groups were performed by the semiempirical INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. Intermolecular integrals associated with solute and solvent molecules were evaluated theoretically. This model provides some improvement over the previous theoretical study in this series, which was based on the reaction field model. The improvement can be attributed to the inclusion of electrostatic effects arising from the real charge distributions of the solvent molecules, especially those which are perpendicular to the solute dipole moment axis. A most important result is the good agreement found for the solvent dependence of the geminal H–H coupling constant in formaldehyde, as the reaction field results for this case were of the wrong sign. The calculated solvent dependence of the coupling constants of 1, 1-difluoroethylene in solvents of almost identical dielectric constant exhibits a greater range of values than either the experimental values or the reaction field results. On this basis, it is concluded that the cluster model is too specific and that improvement in the theoretical description of this phenomenon will incorporate rotational effects associated with the solvent molecules.
Some Superconducting Intercalation Complexes of TaS2 and Substituted Pyridines
Authors: Gamble F. R. , Osiecki J. H. , DiSalvo F. J.
Published: February 2004

Following the discovery of superconductivity in TaS2 (pyridine)1/2, we have prepared a number of similar complexes using substituted pyridines in order to learn how the structure, the stoichiometry, the stability, and the superconducting critical temperature of the complex are interrelated. In these complexes atomically thin, metallic layers of TaS2 and layers of substituted pyridines alternate. X-ray diffraction studies and chemical analyses show that they are usually highly crystalline and stoichiometric. The critical temperature varies between 1.5 and 4.5°K and generally increases as the number of molecules per TaS2 is increased.
Dissociation of CO2 by Electron Impact with the Formation of Metastable CO(a 3Π) and O(5S)
Authors: Freund Robert S.
Published: February 2004

Molecular beam time-of-flight measurements show that CO2 is dissociated by electron impact to give two metastable fragments, CO(a 3Π) and O(5S). The experimental dissociation limit for production of CO(a 3Π) lies at 11.65±0.30 eV which agrees with the 11.45-eV energy of CO2 → lim eCO(a3Π)+O(3P). The excitation function for production of CO(a 3Π) shows structure which can be correlated with dissociation processes involving higher triplet states of CO and excited states of O. The cross section for formation of CO(a 3Π) has a maximum at 27±1 eV, a magnitude which is 0.4± 0.2 times the maximum cross section for CO→ lim eCO(a3Π) and probably has a value not far from 4×10−17cm2.
Surface Lifetimes of Alkali Metals on Molybdenum
Authors: Scheer Milton D. , Klein Ralph , McKinley John D.
Published: February 2004

The surface lifetimes (τ¯) of all the alkali metals on a polycrystalline molybdenum surface in the temperature region 900–1350°K were measured. The temperature dependence followed an Arrhenius expression of the form τ¯° exp(l¯/kT). Desorption energies (l¯) were found to be 3.14, 2.60, 2.53, 2.31, and 2.10 eV for Li, Na, K, Rb, and Cs. The corresponding pre-exponential factors (τ¯°) were 3× 10−16, 3×10−15, 2× 10−14, 1×10−13 and 3×10−13sec. From these data, ion and neutral desorption energies were calculated using a model employing a partial surface charge for the adsorbed particle and the Schottky relation. The validity of the Schottky relation for an adsorbed alkali was supported by analysis of recently available theory of partial charges. An explanation of the variation of the pre-exponential factors is presented on the basis of relative mobility of the adsorbed species. A pre-exponential factor was calculated using a partition function for an adsorbate with mixied localized and nonlocalized character. The extremes of completely localized and completely nonlocalized adsorption led to pre-exponentials that spanned the experimental values.
Quadrupole Coupling and Anisotropic Chemical Shift in Dicobalt Octacarbonyl at Liquid Nitrogen Temperature
Authors: Mooberry E. S. , Pupp M. , Slater J. L. , Sheline R. K.
Published: February 2004

The single-crystal broadline 59Co NMR of dicobalt octacarbonyl was studied at liquid nitrogen temperature. Using these data, the complete orientation of the electric-field gradient tensor system was determined for both crystallographically independent sites. The z axis of the electric-field gradient tensor was within 6° of the terminal carbonyl opposite the missing bridging carbonyl. This direction agrees well with the value predicted by considering Co2(CO)8 to have both threefold and fourfold distortions at the cobalt site, but disagrees with a bent bond interpretation for Co2(CO)8. An anisotropic chemical shift was found at liquid nitrogen temperature. The anisotropy was small (0.5‰) and the z component was more paramagnetically shifted than the x and y components, as expected from the threefold and fourfold distortions at the cobalt site. A two-circle goniometer for determination of the NMR parameters at liquid nitrogen temperature is described.
Concentration Dependent Relaxation Time Studies of N2− in X-Irradiated Sodium Azide
Authors: Ford Bryce , Gelerinter Edward
Published: February 2004

The concentration dependence of the relaxation time of N2− in sodium azide was studied using paramagnetic resonance at 4.2°K. The relaxation rate was found to be independent of defect concentration within the experimental error of the saturation technique employed. The relaxation time product, T1T2, displayed the same strong anisotropy at K band (23 GHz) as was previously reported at X band. With the crystal platelet parallel to the static magnetic field, T1T2=0.5× 10−12sec2, and with the crystal platelet perpendicular to the static field, T1T2=1.6× 10−12sec2. The concentration was shown to be linear with irradiation time over the temporal range of interest by employing optical studies. These experiments indicate that the dominant relaxation mechanism is intramolecular in nature.
Molecular Collisions. XV. Classical Limit of the Generalized Phase Shift Treatment of Rotational Excitation: Atom—Rigid Rotor
Authors: Pattengill M. D. , Curtiss C. F. , Bernstein R. B.
Published: February 2004

The generalized phase shift (GPS) approach to the problem of rotationally inelastic molecular collisions is extended from the level of the first-order (semiclassical) approximation of Paper XIV to the essentially infinite order level, but specialized to the full classical limit. The limitations and assumptions are that (i) the de Boer reduced wavelength parameter be small (i.e., Λ*≪ 1), (ii) the relative translational motion takes place under the influence of the orientation-averaged (spherical) part of the anisotropic interaction potential (i.e., curved but planar trajectories), and (iii) the rotational energy, Erot, may be well approximated by its classical expression (i.e., rotational quantum numbers ≫ 1). The procedure is applied numerically to a model problem involving an anisotropic L-J (12,6) potential (as in XIV), taking advantage of the previously computed generalized action integrals. The program yields directly an arbitrarily chosen specified number of moments of the inelasticity probability density function P(Δ Erot) at various impact parameters b. Inversion of the set of moments leads to P(Δ Erot). For the examples chosen, the lowest eight moments suffice to obtain practical accuracy of convergence on the inversion. For large b, i.e., in the weak-coupling regime, the first moment vanishes and the second moment (which can be well-approximated using the first-order results of XIV) dominates, governing the breadth of the inelasticity density function.
Vibrational Frequency Spectrum for Polymers
Authors: Broadhurst M. G. , Mopsik F. I.
Published: February 2004

A method is given for calculating the vibrational frequency spectrum of a model linear polymer. The model is a chain of N masses having bending and stretching force constants. Each mass is quasiharmonically coupled to a Debye lattice which has a cutoff frequency ωL. Each of the 3N free chain eigenfrequencies ωj becomes a band with a low frequency cutoff ωj min2= ωj2, a high frequency cutoff ωj max2= ωj2+ωL2, and a pseudo-n-dimensional Debye distribution gj(ω)=nωn−1/(ωj maxn−ωj minn) for ωj min〈ω〈ωj max. The total frequency distribution agrees closely with the results by Genensky and Newell for the Stockmayer and Hecht lattice using their force constants and compares reasonably well with results of GF matrix calculations for polyethylene.
Molecular-Orbital Theory for Infinite Systems: Self-Consistent Field Perturbation Treatment of Hydrogen-Bonded Molecules
Authors: Bacon J. , Santry F. P.
Published: February 2004

A self-consistent field perturbation theory of intermolecular interactions is introduced with a view to its application to the study of hydrogen-bonded molecular crystals. A numerical perturbation scheme is also introduced that permits a very detailed numerical check to be made on both the perturbation equations and the corresponding computer program. From a numerical study of the (HF)2 and (HCN)2 systems it is shown that, if taken to the third order, the perturbation method yields dimerization energies and changes in charge densities to within 7% of the values obtained by direct calculations. Structural data calculated from perturbation theory are still more accurate.
Emission Spectra of Alkali-Metal Molecules Observed with a Heat-Pipe Discharge Tube
Authors: Sorokin P. P. , Lankard J. R.
Published: February 2004

Intense molecular emission spectra from vapors of each of the alkali metals with the exception of lithium have been observed using a new device, the heat-pipe discharge tube. For Na2, K2, and Br2 the total emission appears to be almost entirely comprised of transitions Σu+1→1Σg+ from the first excited singlet state. For Cs2, emission from a low-lying Πu3 state appears to contribute as well.
Emission of the Werner Band System and Lyman-α Radiation for 0.05–6-keV Electrons in H2
Authors: De Heer F. J. , Carrière J. D.
Published: February 2004

We have measured relative cross sections as a function of impact energy for the emission of Werner bands C (2pπ 1Πu)-(1sσ 1Σg+) and Lyman-α radiation for 0.05–6-keV electrons on molecular hydrogen. The cross sections for the Werner bands are normalized on theoretical Born or Bethe cross sections, evaluated by means of the theoretical generalized oscillator strengths of Miller and Krauss and the theoretical optical oscillator strengths and transition probabilities of Allison and Dalgarno. By comparing the cross sections thus obtained and the relative intensity measurements, the sensitivity of our optical detection system can be determined between about 1030 and 1250 Å. It is then also possible to bring the relative cross sections for Lyman-α radiation at 1215.7 Å on an absolute scale. The latter cross sections appear to be 1.31 times smaller than those reported by Vroom and De Heer, which data were normalized on the so-called countable ultraviolet cross sections of Fite and Brackmann. In the latter normalization, the contribution of the Werner bands to the countable ultraviolet cross sections has been neglected.
Analysis of Symmetry in Chemical Reactions
Authors: George Thomas F. , Ross John
Published: February 2004

We investigate the effect of the symmetry of electronic and nuclear motion in chemical reactions. The analysis focuses on concerted reactions which are defined to be describable by a single transition matrix. The transition matrix for rearrangement collisions is derived in a quasiadiabatic representation of electronic motion such that the electronic degrees of freedom of reactants and products can be defined separately. Three alternative forms are obtained for the transition matrix from which symmetry rules are derived. Five approximations are applied: (1) separate conservation of total electronic spin; (2) neglect of virtual electronic transitions; (3) Born—Oppenheimer approximation; (4) fixed nuclear configuration (Franck—Condon-type approximation); (5) neglect of remaining effects of dynamics. The first approximation is the Wigner—Witmer spin angular momentum correlation rule. The first four approximations lead to an expression for the transition matrix from which Shuler's rules and the Woodward—Hoffmann rules follow as selection rules by application of the Wigner—Eckart theorem. The fifth approximation is necessary for a discussion of the Woodward—Hoffmann rules in connection with orbital and state correlation diagrams and the principle of maximum bonding. The inapplicability of one or more of these approximations are discussed; we derive corrections and cite examples of violations of various symmetry rules. Physical reasons are presented for the wide applicability of the Woodward—Hoffmann rules. It is suggested that the ``principle of maximum bonding'' proposed by Woodward and Hoffmann is a useful rule, but not supported by theory and experiment as a principle.
Interface Structure of the Two-Dimensional, Square Ising Model
Authors: Clayton David B. , Woodbury George W.
Published: February 2004

Numerical calculations are carried out on the two-dimensional square Ising model for the structure of the interface between two phases. The method consists of stabilizing the interface in a finite system, calculating interface properties (magnetization profile and surface tension), and then extrapolating to the limit of infinite sized systems where the stabilizing forces may be removed. From the numerical work we make the following tentative conclusions: (1) For temperatures below the critical temperature, the limit of infinite size for fixed magnetic field H (H 〉0) followed by the limit H→O+ does not give rise to an asymptotic degeneracy of eigen values of the transfer matrix. (2) The structure of the interface is sensitive to ``infinitesimal'' forces stabilizing the interface. (3) The surface tension is independent of these same ``infinitesimal'' forces, always being given by the Onsager expression. The first conclusion above implies that an Ising model of infinite size stabilized by an arbitrarily small magnetic field lacks long-range order.
Optical Constants of Simple Molecular Crystals. I. Results for CO and O2
Authors: Scharber S. R. , Webber S. E.
Published: February 2004

The reflection and transmission spectra of unannealed films of α-CO and α-O2 at 20.4°K have been obtained, and values for the optical constants (n and k) and the real and imaginary part of the dielectric constant (ε1 and ε2) determined. The oscillator strength of the A 1Π←X 1Σ+ transition (fourth-positive group) of α-CO has been estimated to be in the range 0.035 and 0.068 (compared to the gas-phase value of 0.094). The oscillator strength for the B 3Σu−←X 3Σg− (Schumann—Runge) transition in α-O2 is estimated to be between 0.22 and 0.35 (compared to the gas-phase value of approximately 0.156). The broad absorption of α-O2 is thought to be due to the mixing of the B 3Σu− and A 3Σu+ states of O2 in the crystal.
Theory of Static Dielectric Behavior of Mixtures of Polar Liquids near a Critical Point
Authors: Snider Neil S.
Published: February 2004

The theory of critical fluctuations was employed in an attempt to elucidate the possibly singular behavior of the static dielectric constant which has been observed for mixtures of polar liquids near a critical point. The treatment is similar to existing theoretical treatments of the specific heat singularity. A contribution to the dielectric constant which depends on the correlation length and on the second derivative of the logarithm of the osmotic compressibility with respect to electric field was derived. Some predictions about the existence of and the nature of a singularity in the dielectric constant could be made. These predictions are, however, ambiguous because the precise functional behavior of the osmotic compressibility with electric field in the critical region is still uncertain.
Lattice Dynamics and Thermodynamic Properties of Platinum
Authors: Konti A.
Published: February 2004

An angular force model has been applied to investigate the lattice dynamics of platinum metal. The calculated results show satisfactory agreement with the observed phonon dispersion curves and with Debye—Waller factors. The calculated calorimetric Debye temperatures, however, show a substantial disagreement with the experimental results; possible causes of this disagreement are discussed.