bluuebunny/crossref_metadata_2025 huggingface dataset:
Relative Abundances and Recoil Energies of Fragment Ions Formed from the X-Ray Photoionization of N2, O2, CO, NO, CO2, and CF4
Authors: Carlson Thomas A. , Krause Manfred O.
Published: February 2004

A specially designed mass spectrometer has been used to study the fragment ions following x-ray photoionization of several simple gaseous molecules, containing only first-row elements. Two photon sources were used: CuLα (930 eV) and CKα (280 eV). The higher-energy x-ray source produced initial vacancies primarily in the K shells of the different elements, while the lower-energy radiation produced ionization entirely in the valence shells. Data were obtained as a function of collection voltage, and relative abundances of the fragment ions were evaluated under conditions of equal collection efficiency. From the shape of the collection efficiency curve the average recoil energies were estimated. The results are briefly discussed in terms of the effect of the KLL Auger processes and multiple ionization.
Ice I-Lattice Dynamics and Incoherent Neutron Scattering
Authors: Prask H. J. , Trevino S. F. , Gault J. D. , Logan K. W.
Published: February 2004

Incoherent neutron inelastic scattering (INIS) data for polycrystalline ice Ih are presented for E0= 240 cm−1, θ8=18°, T=82°K, E0=443 cm−1, θ8=25°, T=82°K, and E0=30 cm−1, θ8=65°, T=250°K. These data and other INIS measurements are analyzed in terms of a lattice-dynamical model for ice Ic with ordered hydrogens, the dynamics of which are very similar to ice Ih. Dispersion curves, the frequency distribution, rms amplitudes of displacement, and specific heat are calculated from the model; INIS cross sections, including two-phonon contributions-which are found to be significant in some cases-are also calculated. Calculated values and the various measurements are found to be in very good agreement.
Burnett Theory of Thermal Transpiration with Wall Accommodation
Authors: Cha C. Y. , McCoy B. J.
Published: February 2004

Thermal transpiration in a capillary is analyzed by use of second-order Chapman-Enskog constitutive relations for heat flux and stress. Constants of integration are determined by imposing the usual boundary conditions and results for low and for standard pressures. No arbitrary parameters appear, but energy and momentum accommodation coefficients enter in a way which significantly influences the interpretation of experimental data. The theory is worked for polyatomic gases with the result that the total thermal conductivity appears rather than the translational part. The results are compared with the dusty gas model, the Weber-Schmidt equation, and experimental data.
Atom-Molecule Reaction D+H2 → HD+H Studied by Molecular Beams
Authors: Geddes J. , Krause H. F. , Fite W. L.
Published: February 2004

Collisions between deuterium atoms and hydrogen molecules have been studied in a modulated crossed beam experiment. The relative signal intensity and the signal phase for product HD from reactive collisions allowed determination of both the angular distribution and HD mean velocity as a function of angle. From these a relative differential reactive scattering cross section in center-of-mass coordinates was deduced. The experiment indicates that reactively formed HD having little or no internal excitation, departs from the collision anistropically, with maximum amplitude 180° from the direction of the incident D beam in c.m. coordinates, which shows that the D–H–H reacting configuration is short-lived compared to its rotation time. Nonreactive scattering of D by H2 was used to assign absolute values to the differential reactive scattering cross sections.
Birefringence and Optical Rotatory Dispersion of a Compensated Cholesteric Liquid Crystal
Authors: Teucher I. , Ko K. , Labes M. M.
Published: February 2004

Utilizing deVries' model for the optical rotatory power of cholesteric mesophases, the temperature and wavelength dependence of the birefringence, Δn, of a compensated cholesteric liquid crystal was determined from optical rotatory dispersion measurements. Deviations from symmetry in the optical rotation around Tnematic of such a compensated system are due to the temperature dependence of Δn. Refractometer measurements of the ordinary refractive index were also performed and values of the three principal refractive indices were calculated. The ordinary refractive index of the focal conic texture was observed to be higher than that of the plane texture of the cholesteric phase.
Hydrogen Formation in the Photolysis of Propyne at 1236 Å
Authors: Payne W. A. , Stief L. J.
Published: February 2004

Hydrogen formation in the photolysis of propyne at 1236 Å occurs exclusively via a molecular process. The following reactions are energetically possible at 1236 Å: CH3C≡CH + hν → C3H2 + H2→ C3H+H+H2→ C3+2H2→ CH+C2H+H2.Photolysis of CH3C≡CH yields equal amounts of D2 and HD; addition of inert gas (C2F6) reduces HD relative to D2. These observations are taken as evidence for participation of the over-all Process (E) which may be rewritten as follows: CH3C≡CH + hν → C3HD* + D2,C3DH* → C3 + HD,C3DH* + M → C3DH + M.The dissociative lifetime of C3DH* is estimated to be 10−10 sec assuming unit efficiency for collisional deactivation.
Studies of Electron Impact Excitation, Negative Ion Formation, and Negative Ion-Molecule Reactions in Boron Trifluoride and Boron Trichloride
Authors: Stockdale J. A. , Nelson D. R. , Davis F. J. , Compton R. N.
Published: February 2004

Electron attachment processes in BF3 and BCl3 have been studied with both electron swarm and electron beam techniques. Thermal electron attachment rates were determined by the drift-dwell-drift technique to be 〈5 × 105 sec−1 · torr−1 for BF3 and 9 × 107 sec−1 · torr−1 for BCl3. Beam studies showed that F−, F2−, and BF2− were produced from BF3 by electrons of energy near 11.5 eV while Cl− and Cl2− were produced in BCl3 near 1 eV. The SF6− threshold electron impact excitation spectrum of BF3 exhibited no structure, however, a number of peaks were seen in BCl3, the chief one being near 7.6 eV. A low energy peak was observed in BCl3 at ∼ 2.5 eV. The thermal energy SF6−* ion was found to react readily with both BF3 and BCl3, yielding BF4− and BCl3F−, respectively. BF4− was also produced through the reaction F2−/BF3+BF3→ BF4−+F. Thermal energy rate constants for these reactions determined by a pulsed source method were 1.8 × 10−9, 1.6 × 10−10, and 6.1 × 10−11 cm3 molecules−1 · sec−1 in the order above.
Intramolecular Nonradiative Transitions in the ``Non-Condon'' Scheme
Authors: Nitzan Abraham , Jortner Joshua
Published: February 2004

In this paper we consider the nonradiative intramolecular decay of a large molecule utilizing Born-Oppenheimer wavefunctions as a zero order basis, and bypassing the conventional Condon approximation for the calculation of the electronic coupling matrix matrix elements. The electronic adiabatic wavefunctions are expanded in terms of the Wigner-Brillouin perturbation series in the weak electronic-vibrational coupling limit. We have applied a generalized version of Feynman's operator calculus to derive general expressions for the nonradiative decay probability of a statistical harmonic molecule characterized by displaced potential surfaces. Numerical calculations were performed for the decay of the vibrationless excited electronic state in the ``non-Condon'' scheme. The numerical data for the decay rate in a two electronic level system in the weak electronic vibrational coupling limit exceed the results obtained invoking the Condon approximation by 2–3 orders of magnitude; the exact correction factor depends on the molecular parameters, and, in particular, is roughly proportional to the square of the (frequency normalized) electronic energy gap. Finally, the relevant off resonance coupling terms in the adiabatic representation are shown to be appreciably smaller than the near resonance coupling terms, demonstrating the superiority of the adiabatic basis over the crude adiabatic basis in describing electronic relaxation processes.
Excitation Energy and Deuteration Dependence of Fluorescence in Naphthalene Vapor
Authors: Lim E. C. , Uy J. O.
Published: February 2004

Combining fluorescence quantum yields with the lifetime data of Laor and Ludwig [J. Chem. Phys. 54, 1054 (1971)] radiative and nonradiative decay rates of dilute naphthalene vapor were determined as a function of excess vibrational energy in the lowest excited singlet state (S1). It is shown that the nonradiative decay probability increases, first approximately linearly then nearly exponentially, as the excess energy is varied over a range of 15 000 cm−1. The deuterium isotope effect on fluorescence yield is found to be large only in the region of high excess energy in which nonradiative decay rate exhibits an exponential energy dependence. These results are interpreted in terms of two different pathways of intersystem crossing which compete in the nonradiative deactivation of molecules in S1. In the low excess energy region, an intersystem crossing to a close-lying triplet state apparently dominates, so that the nonradiative decay rate is not sensitive to the deuteration of the molecule and it exhibits a linear variation with excess vibrational energy. At high excess energy, on the other hand, the dominant radiationless transition appears to be the S1→T1 (the lowest triplet) intersystem crossing, and a relatively large S1—T1 energy gap leads to the near exponential energy dependence of its decay rate and a rather large deuteration effect on fluorescence quantum yield. The near linear variation of the radiative decay rate with excess vibrational energy is accounted for on the basis of Herzberg-Teller intensity borrowing mechanism.
Photodissociation of OSCl2 in the Vacuum Ultraviolet; the Heat of Formation of SO
Authors: Okabe H.
Published: February 2004

The photodissociation of OSCl2 at the Kr (1165-, 1236-Å) lines has resulted in the production of SO A 3π, B 3Σ. The threshold energy of incident photons to produce the reaction, OSCl2→ lim hνSO A 3 Π0 + 2Cl2 P3/2,was 9.41 ± 0.03 eV from which Δ Hf0 ° (SO)=1.3 ± 0.7 kcal/mole is derived. The If/I0-vs-incident wavelength curve shows many diffuse bands found in the absorption spectrum of OSCl2 indicating that the process is predissociative. Various other methods to obtain Δ Hf0 ° (SO) are compared. It is concluded that the best value is 1.3 〈 Δ Hf0 ° (SO) 〈 0.8 kcal/mole.
Debye Model and the Primitive Model for Electrolyte Solutions
Authors: Stokes R. H.
Published: February 2004

It is shown that the free energy terms arising from charge-charge interactions in the Debye theory and in modern statistical-mechanical treatments of electrolyte solutions can be brought into close agreement when it is borne in mind that the ``ionic diameter'' is an adjustable parameter of both theories. By considering the case of an ideal nonelectrolyte solution, it is shown that the imperfect gas (co-volume) term in the modern theories is an artefact of the ``primitive model'' which cannot validly be included in comparisons with experimental data.
Absolute Total Scattering Cross Section of He–H2 over the Relative Velocity Range of 2000–3000 m/sec
Authors: Grace R. S. , Skofronick J. G.
Published: February 2004

Absolute helium-hydrogen molecule total cross sections for thermal energy collisions were measured using molecular beams derived from nozzle sources and crossed at right angles. Both nozzles were adjustable in temperature over limited ranges to provide variation in the relative velocities. The absolute density of the target gas (H2) was measured with a capacitance manometer. Over a relative velocity range of 2000–3000 m/sec the cross section, corrected for finite angular resolution, was found to be nearly constant in magnitude averaging 48 Å2. A spherical potential which best fits the experimental data [Lennard-Jones (12, 6), ε=0.7 meV, rm=3.57 Å] was obtained for atom-molecule separations of 2.7–3.3 Å. The range was determined from the response of the cross section to piecewise variation of the theoretical potential. The results are consistent with previously reported theoretical and experimental works.
Low Temperature Infrared Study of Association in Dissolved Dimethylamine
Authors: Bernard-Houplain Marie-Claude , Sandorfy C.
Published: February 2004

The fundamental, first, and second overtone of the NH stretching vibration of dimethylamine were measured at temperatures ranging from 22 to -190°C, in solvents forming rigid glasses at low temperatures. The self-association of dimethylamine was examined. At room temperature only monomers and dimers were found while at low temperatures a more highly associated species is also observed. The overtones of the dimer were observed at low temperatures and lead to normal anharmonicities.
Localized Excess Electrons in Water and Ice. An Ice Cavity Model for the Localized Electron and the Microscopic Relaxation Time
Authors: Nilsson G.
Published: February 2004

In this paper we have identified the structure of the trapping center for localized excess electrons in water and ice with the structure of the large cavities existing in ordinary hexagonal ice (ice I). The short-range attractive interactions with the 12 water molecules on the cavity boundary were treated as charge point dipole interactions, and the long-range interactions were represented by a Landau-type polaron potential. The cavities were permitted to expand isotropically with increasing temperature, and the cavity size was calculated from the OO distance which was taken from x-ray data for ice and identified with the position of the first peak of the radial distribution function for water. Without the use of adjustable parameters this ``ice cavity model'' gives a reasonable agreement between calculated and experimental values for the optical transition energy of excess electrons in both water and ice, the general behavior of the temperature dependence of the transition energy is well reproduced between -180 and +83°C, and the pressure shift of the absorption peak is accounted for. The relaxation of the nearest-neighbor water molecules in ice has also been considered and their relaxation time was shown to be much less than the dielectric relaxation time.
Simple Model for the Energy Accommodation Coefficient
Authors: Manson J. R.
Published: February 2004

Quantum mechanical expressions for the energy accommodation of a gas at a solid surface are developed using the general theory of inelastic surface scattering as developed by Manson and Celli. Calculations are presented for the accommodation of helium at a simplified model of a tungsten surface. The phonon spectrum at the surface is taken to be that of a semi-infinite isotropic continuum, and contributions to the accommodation are obtained from bulk, mixed, and Rayleigh phonon modes. For temperatures not too small the calculations agree qualitatively with experiment, and exhibit the property that the accommodation is nearly independent of surface temperature over a large range.
Kinetics of Reactions Leading to O2-(H2O)n in Moist Oxygen
Authors: Payzant J. D. , Kebarle P.
Published: February 2004

Most of the rate constants for the reactions by which O2− and O− are converted to the hydrates O2−(H2O)n and O3−(H2O)n in moist oxygen were determined with a high pressure mass spectrometer utilizing a pulsed electron beam ion source. The total pressure range was 0.6–7 torr and most reactions were investigated at room temperature.
Ab Initio Calculations on Large Molecules Using Molecular Fragments. Oxygen-Containing Molecules
Authors: Cheney B. Vernon , Christoffersen Ralph E.
Published: February 2004

An ab initio procedure for the investigation of large molecules is applied to a series of oxygen-containing molecules. Prototype molecules used to characterize the procedure include H2O, H2O2, CH3OH, CH3OCH3, furan, H2CO, CH3CHO, (CH3)2CO, HCOOH, CH3COOH, and HCOOCH3. The method is found to provide interesting and useful information concerning electronic structure and molecular geometry in spite of the restricted sets of basis orbitals employed. When possible, comparisions are made with other theoretical results as well as experimental values.
Effect of Pressure on the Electronic Structure of Twelve Ferric β-Diketone Complexes
Authors: Frank C. W. , Drickamer H. G.
Published: February 2004

The effect of pressure to 180 kbars on the electronic structure of 12 ferric β-diketone complexes has been measured by means of Mössbauer resonance and optical absorption. The systematic variation of the electron donor and acceptor properties of the ligands permits a more detailed interpretation of the results than in previous studies. These properties are correlated with the isomer shift of the Fe(III), and that of the Fe(II) produced under pressure, and with the change of the isomer shifts with pressure. The degree of conversion of Fe(III) to Fe(II) correlates well with the change of isomer shift with pressure, and thus with changes of the electronic properties of the ligands. Optical absorption studies as a function of pressure were made on both the charge transfer and intraligand π → π* transitions of several complexes. The decrease in area under the CT peaks correlates very well with the amount of reduction of Fe(III) as obtained from the Mössbauer resonance studies. The shifts of both the CT peaks and π → π* transitions are recorded and discussed briefly.
Near-Resonant Electronic Energy Transfer from Argon to Hydrogen
Authors: Fink Ewald H. , Wallach Daniel , Moore C. Bradley
Published: February 2004

The argon sensitized fluorescence of molecular hydrogen has been observed. Relative cross sections have been determined for transitions of the type H2(X 1Σg +,  v=0, J) + Ar* → H2(B 1Σu +,  v′, J′) + Ar + Δ E,where Ar* is either Ar 4s[112]J=1 0 or Ar 4s′[12]J=1 0 and the hydrogen molecules, H2, D2, and HD are initially thermally distributed among J states at 300 ± 30°K. Calculations indicate that the largest cross sections, on the order of gas kinetic or larger, are due primarily to long range dipole-dipole interactions. These cross sections are for transitions for which |  Δ E  |  〈 100 cm−1 when Δ J ≡ J′ − J=± 1. The long range dipolar calculations establish an absolute magnitude scale for the cross sections. A curve crossing mechanism is possible for negative ΔE and provides a plausible explanation for the remaining levels excited. Generally, only one vibrational level of the hydrogen molecule is excited by each argon state. This contrasts sharply with many previous sensitized excitation experiments in which most of the energetically accessible energy levels are excited.
Triatomic Photofragment Spectra. III. NOCl Photodissociation
Authors: Busch George E. , Wilson Kent R.
Published: February 2004

We have obtained the photofragment spectrum of NOCl in the near ultraviolet at 28 810 cm−1 by crossing a molecular beam of NOCl with brief pulses of polarized laser light and then measuring the distributions of speed and direction of recoil of the photodissociation fragments. Approximately 70% of the energy in excess of that required to break the ON–Cl bond goes into translational energy of the recoiling fragments. Although vibrational excitation of the NO fragment with v=1, 2, or possibly 3 is energetically allowed, and up to ∼1000 cm−1 may be present in electronic excitation of the NO and Cl, the results are closely matched by a simple modified impulsive model which predicts that v=0 and almost all of the internal energy goes into rotational motion of the NO. The experimental angular distribution peaks along E, the electric vector of the polarized light, and is nearly zero in the plane perpendicular to E, indicating that the transition moment lies in the molecular plane, probably close to the N–Cl bond. This means that the predominant transition at 28 810 cm−1 is to a state of A′ symmetry. The very high angular anisotropy also shows that the average lifetime in the excited state before dissociation is short compared to the rotational period of the molecule, being at most ∼8 × 10−14 sec.
On a T1 and T1ρ Study of Molecular Motion and Phase Transitions in the Tetramethylammonium Halides
Authors: Albert S. , Gutowsky H. S. , Ripmeester J. A.
Published: February 2004

The temperature dependences of methyl group and (CH3)4N+ ion reorientations in the tetramethylammonium salts [(CH3)4N+X− where X is Cl, Br, I] were studied by means of the proton spin-lattice relaxation times in the static and rotating frame (T1 and T1ρ). The T1 values calculated for the motions are in good agreement with the double minima observed for each compound. Activation energies and frequency factors were obtained for both motions from the temperature dependence of T1 in the three salts. Rotating frame spin-lattice relaxation times were also measured over temperatures from 200 to 380°K for (CH3)4NCl, giving results in agreement with those for T1. A discontinuity in the T1 of (CH3)4NCl was found near 418°K. This led us to make a preliminary DTA investigation with results indicating that a first-order phase transition may occur near 434°K and another definitely at 528 ± 1°K. The appearance of the phase above 434°K is accompanied by a modulation on the free induction envelope which we ascribe to piezo-electric properties. The phase can be supercooled to room temperature and remains for a considerable length of time.
EPR Studies of Crystal Field Parameters in Fe3+: LiNbO3
Authors: Towner H. H. , Kim Y. M. , Story H. S.
Published: February 2004

ESR studies were performed on single crystals of LiNbO3 doped with a trace amount of iron. Fe3+(S=52) is in an axially symmetric site and is described by the spin Hamiltonian H=β (H· g· S)+B20O20+B40O40+B43O43. The crystal field parameters at room temperature were found to be B20=596 G, B40=−0.85 G, and B43=10 G.
Theory of Nuclear Overhauser Enhancement and C13–1H Dipolar Relaxation in Proton-Decoupled Carbon-13 NMR Spectra of Macromolecules
Authors: Doddrell David , Glushko Victor , Allerhand Adam
Published: February 2004

Expressions are presented for the nuclear Overhauser enhancement (NOE), the spin-lattice relaxation time (T1), and the spin-spin relaxation time (T2) of a 13C nucleus relaxing by a dipolar interaction with one proton under conditions of complete proton decoupling, and without assuming that the extreme narrowing limit applies. Specific equations are derived for a C–H group in a rigid molecule rotating isotropically and also for a C–H group with one degree of internal motion attached to a molecule undergoing isotropic rotational reorientation. Numerical results are presented for T1, T2, and the NOE of a C–H group in a rigid molecule (undergoing purely dipolar relaxation) as a function of the rotational correlation time (τR) and the resonance frequency (ωC). T1 goes through a minimum when τRωC ≈ 0.8. The NOE varies from the expected value of 2.988 in the extreme narrowing limit to 1.153 when 1/τR is much smaller than the resonance frequency. The numerical results indicate that the signal-to-noise ratio in proton-decoupled 13C Fourier transform spectra of macromolecules in solution may not improve significantly by going to very high magnetic field strengths (such as 51.7 kG), because the increase in basic sensitivity can be offset by a decrease in the NOE and the T2/T1 ratio. The magnitude of the latter two parameters is strongly dependent on τR and the magnetic field strength. Numerical results are also presented for a C–H group with one degree of internal motion. 1/T2 is a monotonically increasing function of τR and τG (the correlation time for internal rotation). The NOE and T1 behave in a more complex manner. The onset of internal rotation may make T1 larger or smaller, depending on the value of τRωC. In the extreme narrowing limit, T1 increases monotonically as τG decreases, reaching a limiting value (when τG≫τR) of 4(1−3 cos2θ)−2T1R, where θ is the angle between the C–H vector and the axis of internal rotation and T1R is the value of T1 in the absence of internal rotation. However, for very slow over-all reorientation (with respect to the resonance frequency), the onset of internal rotation produces a decrease in T1 until it reaches a minimum and increases again. When τG→ 0, T1 reaches the same limiting value as in the extreme narrowing case. As expected, in the extreme narrowing limit for τR we get the full NOE of 2.988 regardless of the value of τG. For slow over-all reorientation, the onset of internal rotation first increases the NOE. As τG gets smaller, the NOE goes through a maximum and then decreases again. As τG→ 0, the NOE reaches an asymptotic value equal to that in the absence of internal rotation.
Single Crystal Neutron Diffraction Study of Potassium Cyanide
Authors: Price D. L. , Rowe J. M. , Rush J. J. , Prince E. , Hinks D. G. , Susman S.
Published: February 2004

Neutron diffraction measurements were made on single crystals of KCN to obtain information about the disorder of the CN− ions in the cubic phase. The crystals were obtained by pulling from the melt by a modified Czochralski method. Data were collected on two four-circle diffractometers at room temperature and at 180°K, just above the cubic-orthorhombic phase transition. Structure factors were obtained for 24 inequivalent reflections. Free rotation of the CN− ions is ruled out by the different structure factors obtained for reflections with equal values of (h2+k2+l2) but different (h, k, l). However, models fitted to the data assuming orientations along a particular symmetry direction did not establish a preferred direction. The fits indicated that both vibrational and librational thermal amplitudes are large and relatively independent of temperature. These amplitudes could not be reproduced by a dynamical model assuming harmonic forces even when rotational-translational coupling was included. A tentative explanation of these results is given in terms of rapid CN− reorientations caused by the low activation energy for rotation.
Lattice Vibrations, Crystal Structure, Dielectric Properties, and Elastic Constants of AgF
Authors: Bottger G. L. , Geddes A. L.
Published: February 2004

The far-infrared transmission spectrum of AgF has been measured at room temperature and at 143°K in the region of the lattice vibrations. The zone-center transverse and longitudinal optic modes were found and their frequencies determined. The room-temperature crystal structure of AgF was restudied using x-ray diffraction, and an NaCl-type structure (Oh5) was confirmed with a lattice constant of 4.936±0.001 Å. Ellipsometry was utilized to determine the high-frequency refractive index of AgF; n=1.73±0.02. The static dielectric constant at room temperature (ε0=10.6±0.7) and at 143°K (ε0=10.3±0.3) was calculated using the Lyddane-Sachs-Teller relation. The Szigeti estimates of effective ionic charge and of compressibility were calculated, and their significance is discussed. Values were also computed for the elastic constants and the Debye characteristic temperature.
Model Calculations of Transport Coefficients for a Gas of Tetrahedral Molecules in a Magnetic Field
Authors: Verlin J. D. , Cooper E. R. , Hoffman D. K.
Published: February 2004

Methods by which the classical collision integrals for nonlinear polyatomic molecules can be computed are developed for rigid models having tetrahedral symmetry. Using these models the thermal conductivity and shear viscosity coefficients for CH4, CD4, and CF4 in field-free space and in a static magnetic field are calculated. The agreement with experiment, whenever comparison is possible, is moderately satisfactory for the thermal conductivity but not for the shear viscosity. These results are comparable to those found using rigid models for linear molecules.
Low-Temperature Magnetic Characteristics of Tetrahedral CoCl4 2−. II. Nature of the Phase Transition in [(C2H5)4N]2CoCl4
Authors: McElearney J. N. , Shankle G. E. , Schwartz R. W. , Carlin R. L.
Published: February 2004

Magnetic susceptibility measurements have been made between 1.5 and 20°K on [(C2H5)4N]2CoCl4. The results show that the CoCl4 2− tetrahedra are oriented such that their unique axes lie in or near the (001) plane while maintaining the tetragonal symmetry exhibited by the crystal at room temperature. Values for the parameters D and E have been obtained, as well as the principal g values. The results indicate that the CoCl4 2− anions in this compound behave similarly to those doped into the isostructural material [(C2H5)4N]2ZnCl4.
Application of the Eikonal Approximation to Electron Scattering. I. Elastic Scattering from Spherically Symmetric Potentials
Authors: Duncan D. P. , Tuley F. H. , Kohl D. A.
Published: February 2004

A form of the eikonal approximation is presented which is slightly different from the familiar one found in the literature, but which reduces to it in the appropriate limit. The object of the paper is to investigate the possibility of using the eikonal theory to study processes which cannot be easily nor accurately studied by methods currently available. It is found that the eikonal theory is quite successful in reproducing partial wave results for elastic scattering on spherically symmetric potentials, and it is then proposed that it be used to study inelastic scattering processes for atoms and elastic processes for molecules.
Application of the Eikonal Approximation to Electron Scattering. II. Inelastic Scattering from Helium
Authors: Kohl D. A. , Duncan D. P. , Tuley F. H. , Fink M.
Published: February 2004

The eikonal approximation is applied to the electron impact excitation of the 2 1 S and 2 1 P states of helium for incident energies of 50 eV–500 eV. The differential cross sections for both excitations deviate significantly from the first Born approximation calculation. The results are compared with experimental cross sections.
Variational Wavefunctions for H2+
Authors: Weinhold Frank , Chinen Allan B.
Published: February 2004

The usual approximations to the molecular orbitals of H2+ are shown to have an improper exponential dependence on the radial elliptic coordinate λ. However, the exact λ dependence is easily incorporated into simple trial functions and leads to substantially improved numerical accuracy for both the 1sσg and 2pσu states.
Exact Solutions to the Coupled Hartree-Fock Perturbation Equations
Authors: Alexander Millard H. , Gordon Roy G.
Published: February 2004

A new method is presented for obtaining exact solutions to the time-dependent coupled Hartree-Fock perturbation equations. We choose an iterative approach, relying on the Aitken δ2 transformation to insure and accelerate convergence. At each iteration the resulting uncoupled inhomogeneous differential equations are solved using the technique previously presented by Alexander and Gordon, based on piecewise polynomial approximation of both the potential and the inhomogeneity. As a numerical application we calculate the frequency-dependent dipole polarizability of the helium atom within the coupled Hartree-Fock approximation. Comparison is made with the results of previous variational and numerical calculations. The method can be extended to the solution of integro-differential equations arising in other areas of chemical physics.
Quantum-Mechanical Hamiltonian for Chemical Reactions. II. Practical Considerations
Authors: Shipsey Edward J.
Published: February 2004

The work of a previous paper is extended. The most vexing practical problems are solved. The present work, given a suitable basis set, will allow the complete three-dimensional chemical reaction problem to be expressed in terms of ordinary coupled differential equations for the class of reactions considered. A useful treatment of collinear reactions is also given.
Calculation of the Spin-Spin and Spin-Orbit Contribution to the Zero-Field Splitting in Hemin
Authors: Han P. S. , Das T. P. , Rettig M. F.
Published: February 2004

Earlier theoretical studies of the electronic structure of hemin [iron (III) protoporphyrin (IX) chloride] have been extended to include calculation of both the spin-spin and spin-orbit contributions to the observed zero-field splitting in this molecule. The wavefunctions used for the calculations were obtained by the self-consistent-charge extended Hückel method. The iron-based one-center spin-spin result for D is 0.0407 cm-1. The two-center spin-spin contribution to D is 0.0345 cm-1, and is dominated by chlorine-3p-iron-3d interactions. The total spin-spin result (0.0752 cm-1) is much smaller than the observed zero-field splitting of 6.95 cm-1, and the major contribution is shown to arise from spin-orbit effects. The spin-orbit contribution is large and strongly dominant because of the low-lying 4A2 electronic state in hemin. The accuracy of the spin-orbit calculation is limited by difficulties in calculating the energy of excitation to the 4A2 state, but, with reasonable choices of parameters in terms of which this excitation energy may be expressed, the experimental zero-field splitting is well accounted for. We conclude that in most hemin-type molecules, the zero-field splitting would be strongly dominated by spin-orbit effects.
Quantum Calculations of Planar Reactive H+H2. II. Application
Authors: Saxon Roberta P. , Light John C.
Published: February 2004

The theoretical approach of the previous paper [J. Chem. Phys. 56, 3874 (1972)] is applied using the potential surface of Shavitt et al. [J. Chem. Phys. 48, 2700 (1968)]. Total reactive cross sections were found to be monotonically increasing functions of translational energy virtually independent of initial rotational state. Reactive angular distributions were forward and back peaked with peaks shifting with energy and product rotation. Nonreactive differential cross sections were back peaked. The rate constants agreed in order of magnitude with classical and transition state theory calculations and with experiment, but showed evidence of large amounts of tunneling. At low temperatures, they were dramatically influenced by the threshold form of the cross section assumed.
Investigation of the cw Saturation Technique for Measurement of Electron Spin-Lattice Relaxation: Application to the Benzene Anion Radical
Authors: Rataiczak R. D. , Jones M. T.
Published: February 2004

A detailed consideration of the cw saturation technique for measuring spin-lattice relaxation times (T1) in liquid solutions is given. An improved method for more accurate measurement of cavity Q is described. The saturation technique was used to measure the T1 of the benzene anion radical over the temperature range 150–250°K. The measurements were made at five different concentrations (0.01), 0.03, 0.1, 0.3, and 1.0M). For benzene concentrations ≦0.1 M, the linewidths and hence T2 are independent of concentration. However, the linewidth displays a minimum and T2 a maximum at 173°K for all benzene concentrations. Our measurements of T1 show that although there is no discernable dependence upon benzene concentration, a maximum in T1 is observed at approximately 188°K. Below 188°K, T1 decreases to the solvent freezing point. In addition, analysis of the line shape of the benzene anion radical spectrum yields strong evidence that the benzene anion radical prepared from Na–K alloy in 2:1 tetrahydrofuran-1,2-dimethoxyethane is ion paired.
Role of Electron Correlation in a Priori Predictions of the Electronic Ground State of BeO
Authors: Pearson Peter K. , O'Neil Stephen V. , Schaefer Henry F.
Published: February 2004

Ab initio wavefunctions including electron correlation have been calculated for the 3II state of BeO. A (4s2p1d) basis set of Slater functions was centered on each atom. The iterative natural orbital method was used to optimize the set of molecular orbitals employed in each 591 configuration first-order wave-function. The 3II energy calculated here is 0.73 eV above the 1Σ+ energy obtained in a comparable calculation. Since near Hartree-Fock calculations result in a 3II energy below the 1Σ+ energy, it seems clear that electron correlation plays a crucial role in the ordering of these states. Predicted spectroscopic constants for the 3II state are: Re=1.463 Å, ωe= 1270 cm−1, and Be= 1.365 cm−1. Natural orbital occupation numbers and coefficients of important configurations in the CI wavefunctions are presented to describe the electronic structure of 3II BeO. First-order calculations (519 configurations) were also carried out for the lowest 3Σ− state of BeO. These calculations confirm our previous SCF prediction that the 3Σ− state is repulsive.
Influence of an External Electric Field on the Charge Transfer State of the TCNE-trans-Stilbene Molecular Complex
Authors: Eckhardt Craig J.
Published: February 2004

An experimental investigation of the effect of an external electric field on the absorption spectrum of the tetracyanoethylene-trans-stilbene molecular complex in cyclohexane solution yielded values of 1.1±0.1 D for the ground state electric dipole moment and 10.5±0.9 D for the change of the electric dipole moment upon transition to the charge transfer excited state. The ground state electric dipole moment determined by the electro-optic effect is in agreement with the value of 1.4±0.5 D obtained from the dielectric constant measurements made on solutions of the complex. The directions of the transition moment, the ground state electric dipole moment, and the excited state electric dipole moments were found to be parallel. These results give a 5% ionic character for the ground state of the complex and 76% ionic character of the excited state. These results are consistent with the theory of Mulliken.
Formaldehyde: Comprehensive Spectral Investigation as a Function of Solvent and Temperature
Authors: Bercovici Tuvia , King Jimmie , Becker Ralph S.
Published: February 2004

Absorption, emission, and NMR studies indicate that: (1) Formaldehyde exists as a monomer (form A) in nonpolar solvents to a temperature of approximately −95°C, (2) at −95°C in nonpolar solvents, formation of a formaldehyde polymer (form C) begins which is thermally reversible at temperatures > −90°C. The form C also exists at room temperature in water, dimethyl sulfoxide, dimethylformamide, and tetrahydrofuran. The formaldehyde-formaldehyde interaction is principally of a dipole-dipole type. (3) Formaldehyde can act as a hydrogen-bonding donor or acceptor in a complex with solvent molecules or silica gel (form B). In aromatic hydrocarbons, formaldehyde acts as a donor whereas in 2-methyltetrahydrofuran, chloroform, and silica gel it acts as an acceptor. Also, in 2-methyltetrahydrofuran, forms A and B exist in addition to C. Monomer formaldehyde exhibits a fluorescence from the lowest Sn,π* state in nonpolar solvents to −95°C. Both the forms B and C exhibit emission. In addition to the classification of the forms of formaldehyde, it is possible to classify the solvents into four main groups based on the rate of formation of paraformaldehyde or the fact that formaldehyde essentially totally reacts (strong hydrogen-bond donors or molecules with active α-hydrogen atoms).
Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Studies of the Chemical Reactions of Diphenyldiazomethane and of Diphenylmethylene in Single 1,1-Diphenylethylene Crystals
Authors: Doetschman David C. , Hutchison Clyde A.
Published: February 2004

Electron paramagnetic resonance and electron nuclear double resonance methods were used to investigate the rates and mechanisms of the chemical reactionsdiphenyldiazomethane→ lim hνdiphenylmethylene+N2,diphenylmethylene+1,1-diphenylethylene→ 1,1,2,2-tetraphenylcyclopropane, and diphenylmethylene+diphenyldiazomethane→ benzophenone azine. These reactions occurred in single crystal solutions of diphenyldiazomethane in 1,1-diphenylethylene during irradiation with visible and ultraviolet light or after termination of irradiation, without any general disruption of the structure. The crystal structure of these chemically reacting solutions was investigated by electron paramagnetic resonance spectroscopy, by electron nuclear double resonance spectroscopy, by x-ray diffraction, and by optical absorption spectroscopy. All of these methods and techniques nicely supplemented each other to give a very detailed picture of this single crystal chemically reacting system and of the mechanisms of the processes which were involved.
Testing an Approximation Method for Lattice Statistics
Authors: Clayton David B. , Woodbury George W.
Published: February 2004

Calculated results based on an approximation method developed by Woodbury are compared to exactly known results for the lattice gas model and the Ising model of ferromagnetism. Results based on the Bethe-Guggenheim and Bragg-Williams approximations are included in several cases for comparison. These calculations show the Woodbury approximation to be at least equivalent to the above approximations even in its lowest order forms. The convergence (on increasing the order of the approximation) of the approximate results to exact results is shown to be very rapid. Also, a new low temperature form of the approximation is developed and used.
Electrocapillary Studies on Partially Immersed Mercury-Plated Electrode in DMF-Water Solutions
Authors: Morcos Ikram
Published: February 2004

Meniscus rise vs potential studies were carried out on a mercury-plated gold electrode in DMF-water media. The magnitude as well as the variation of the meniscus minimum with the nature and electrolyte concentration correspond to the electrocapillary maximum of mercury in the same solutions. The interfacial tension and surface charge calculated from the meniscus rise data agree with their corresponding values obtained from directly measured electrocapillary curves on liquid mercury. Possible explanations for the lower interfacial tensions values calculated from the meniscus rise data in aqueous solutions are given.
Production of Excited States in the Collision of He+ with HCl and HBr; Selective Production of the v=0 Level in the A2Σ State of the Molecule-Ion
Authors: Haugh M. J.
Published: February 2004

The emission of light over the 2000–6000-Å wavelength region resulting from the collision of the He+ ions on HBr and HCl has been investigated at ion energies from 0.3 to 4.0 keV. Emission is observed from the A 2Σ → X 2Πi transition of HBr+ and HCl+, from atomic hydrogen, and from Br+ and Cl+ ions. Relative cross sections were determined for the formation of individual H-atom quantum levels and vibrational levels of the molecular ion. Vibrational populations were proportional to the Franck-Condon factors for a transition between the neutral ground state and the A 2Σ state. An unusual dependence upon target gas pressure was observed for the intensity of the (0, 0) band of both HCl+ and HBr+. The intensity increases quadratically with target gas pressure in the 10−4-torr region. Similar studies of other ions impinging on HCl and HBr do not show this effect. The most plausible mechanism satisfying all of the experimental observations is that two molecules of target gas are involved in the excitation process. In the first step He+ reacts with HCl or HBr to form a reactive intermediate that subsequently reacts with a second molecule to produce the A 2Σ state of the molecule-ion in the v=0 level only.
Zero-Field Splitting of the Triplet State in Zinc Etioporphyrin
Authors: Gouterman Martin , Yamanashi B. S. , Kwiram Alvin L.
Published: February 2004

The zero-field splitting (ZFS) of the 3Eu state of Zn porphyrin is treated theoretically. Three types of interaction are discussed: (i) spin-orbit coupling Hso, (ii) spin-spin coupling Hss, and (iii) crystal-field coupling Hcef. Four coupling cases are defined depending on the relative sizes of these three terms. Experimental data on ZFS by optically detected magnetic resonance (ODMR) are given for Zn etioporphyrin. The data can be explained assuming either Hso ≪ Hss or Hcef ≫ Hso. In either case we find D ≈ 0.035 cm−1, a value obtained earlier by EPR studies on the triplet. For a 3Eu case, in addition to the usual E parameter there is a parameter E′. The ODMR experiments show that | E | + | E′ | ≲ 0.005 cm−1. The ZF ODMR spectrum is unusually broad. This can be explained by the effect of Hcef in the case Hso ≪ Hss or by the effect of Hso in the case Hcef ≫ Hso.
Application of Diffusion Model for Recombination of Isolated Pairs in Condensed Media
Authors: Abell G. C. , Mozumder A.
Published: February 2004

A diffusion model is presented for the recombination of isolated pairs of reactants. A general solution has been derived for neutrals and an approximate procedure, with specified limitations, is indicated for ionic recombination. Expressions (exact or approximate as specifically applicable to cases detailed in the text) are provided for the ultimate recombination probability, rate of the recombination process and the half-time of the reaction.
Viscosity of the Binary Gaseous Mixture Neon-Krypton
Authors: Kestin J. , Ro S. T. , Wakeham W. A.
Published: February 2004

The paper presents new relative measurements of the viscosity of binary mixtures of neon and krypton, and repeated determinations of the viscosity of the pure gases neon, krypton, and argon. The experiments were performed using an oscillating disk viscometer; the measurements were made at atmospheric pressure and in the nominal temperature range 25–700°C and with a precision of ±0.1%. Tables of consistent values of the thermal conductivity of the mixture as well as of the pure gases are given. The binary diffusion coefficient for the mixture has also been computed.
Proton NMR Studies of Bihalide Ion Equilibria in Solution
Authors: Fujiwara Fred Y. , Martin John S.
Published: February 2004

In aprotic solvents, bihalide ions may be formed by adding the tetra-n-butylammonium halide to a solution of the hydrogen halide. The shieldings of the bihalides are obtained by analysis of the salt concentration dependence of the proton shieldings, which are averaged by rapid exchange. The deduced shieldings are insensitive to temperature and the nature of the solvent, and, relative to internal TMS, are: ClHCl−, −13.9 ppm; BrHBr−, −10.2 ppm; IHI−, −0.9 ppm. These represent deshieldings of about 13 ppm in each case, relative to the corresponding solvated hydrogen halide. The complete and rapid equilibration of all protonated species precludes characterization of the heterobihalide ions: addition of Cl− or Br− to another hydrogen halide results in quantitative formation, in excess salt, of the homobihalide ion ClHCl− or BrHBr−. However, ClHI− and BrHI− may be formed in solutions with excess iodide. The equilibrium constants are in the order consistent with a simple electrostatic model of ion-molecule association.
Proton Shielding in the Bihalide Ions
Authors: Martin John S. , Fujiwara Fred Y.
Published: February 2004

The absolute proton shieldings of the bihalide ions, FHF−, ClHCl−, BrHBr−, and IHI−, are consistently about 14 ppm less than those in the corresponding gas-phase hydrogen halide molecules. It can be shown that in both species, the dominant contributions to the shielding are all proportional to the hydrogen charge density. Thus, the charge density in each ion must be less than that in the corresponding molecule, consistent with SCF calculations on the bifluoride ion. This is the charge shift predicted by a simple electrostatic model, but not by the independent-electron MO model.
Viscosity of Carbon Dioxide in the Temperature Range 25–700°C
Authors: Kestin J. , Ro S. T. , Wakeham W. A.
Published: February 2004

The paper presents new relative measurements of the viscosity of carbon dioxide at a pressure P=1 atm and in the temperature range 25–700°C. The accuracy of the reported viscosity data is estimated as ±0.1% at 25°C and ±0.3% at 700°C. The rotational collision number for carbon dioxide is computed with the aid of the kinetic theory for polyatomic gases and reliable thermal conductivity data.