bluuebunny/crossref_metadata_2025 huggingface dataset:
Light Scattering as a Probe of Fast-Reaction Kinetics: The Depolarized Spectrum of Rayleigh Scattered Light from a Chemically Reacting Medium
Authors: Berne Bruce J. , Pecora R.
Published: February 2004

The spectral distribution of the depolarized component of light scattered from a dilute solution of molecules in dynamic chemical equilibrium between two states differing in optical anisotropy and dynamics is calculated. It is assumed that the molecules undergo isotropic translational and anisotropic rotational diffusion. It is furthermore assumed that the duration of a molecular transformation is much less than the characteristic times for rotational and translational diffusion. It is shown that in the most general case considered, the spectrum consists of 20 superposed lines with widths dependent on the translational diffusion coefficients and components of the rotational diffusion tensors of the two species and the backward and forward chemical rate constants. The relative strengths of the lines depend on the optical anisotropies and rotational diffusion tensor components of the two species as well as on their equilibrium concentrations. When the reaction is so fast that rotational and translational diffusion contributions to the linewidth may be ignored altogether, the spectrum reduces to a single Lorentzian line with half-width independent of scattering angle and proportional to the sum of the backward and forward rate constants for the chemical transformation.
Thermal Ion–Molecule Reaction Rate Constants at Pressures up to 10 torr with a Pulsed Mass Spectrometer. Reactions in Methane, Krypton, and Oxygen
Authors: Durden D. A. , Kebarle P. , Good A.
Published: February 2004

A mass spectrometer for high-pressure ion–molecule reaction studies was constructed using a pulsed 4-keV electron beam and pulsed “gate-open” mass-spectrometer detection. The ions drift out of the field-free ion source by diffusion and mass flow. Pressures up to 10 torr can be used and the ion reaction times can be as long as several hundred microseconds. The system is specially suited for study of reactions with very small rate constants, long reaction sequences, and reactions dependent on third-body deactivation. Also when reverse reactions are operative the establishment of the equilibrium can be directly followed. The thermal rate constants for the reactions of Kr+ with CH4, and CH4+ and CH3+ with CH4, were measured. The reaction of O2+ with O2 to form O4+ is found to be third order, proceeding with a rate constant K = 2.8 × 10−30cm6molecules−2·sec−1 at 298°K. The activation energy for the reaction is negative and equal to 1–2 kcal/mole. The equilibrium constants for the reaction O2+ + 202⇄O4 + +O2 were determined over the temperature range 295°–350°K. A Vant Hoff plot of the equilibrium constant gives D(O2+–O2≃10 kcal/mole. The equilibrium constants and bond dissociation energy are in very good agreement with earlier results by Yang and Conway. Interesting comparisons with the N4+ formation in nitrogen can be made.
Low-Lying Valence Band States and Intrinsic Photoconductivity in Crystalline Anthracene and Tetracene
Authors: Geacintov N. , Pope M.
Published: February 2004

The wavelength dependence of the intrinsic photoconductivity of anthracene and tetracene single crystals has been measured with photon energies up to 6.5 eV. Both materials exhibit three peaks or shoulders in the 3.0–6.5-eV range. The photoconductivity spectra are interpreted by comparing them to the kinetic energy distributions of external photoemitted electrons. The energy sequence of the photoconductivity peaks is in good agreement with characteristic energy losses suffered by the externally emitted photoelectrons. The decreases in the photocurrent, as the photon energy is increased, are shown to arise at energies at which transitions from lower-lying valence states set in. When transitions from lower-lying valence states are involved, the energy of the final state may be below the photoconductivity threshold energy and the current decreases. The energy sequences of the valence states are compared to theoretical HMO and extended-Hückel-method calculations. Finally, the absorption spectra of these crystals are related to the photoconductivity action spectra.
Final-State Interactions in Collisions of an Atom and a Diatomic Molecule
Authors: Miller William H.
Published: February 2004

The distorted-wave Born approximation (DWBA) with a semiclassical (WKB) description of relative translational motion is used to characterize final-state interactions in the collision of an atom with a diatomic molecule. The resulting cross section is found to be the product of one factor that varies slowly with energy and another which characterizes the final-state interaction. This latter factor, containing the resonant-type of energy dependence, is determined solely by the distorting potential, being independent of the interaction producing the given transition. Simple physical interpretations and useful extensions are obtained by employing the concept of time delay.
Molecular SCF Calculations for SiH4 and H2S
Authors: Boer F. P. , Lipscomb W. N.
Published: February 2004

Wavefunctions and Hamiltonian matrices are obtained for SiH4 and H2S by the LCAO MO SCF method. Integrals were calculated accurately over minimum basis sets of Slater-type atomic orbitals, augmented by the addition of the 3d orbitals. Best atom exponents were used, except for the H1s, Si3d, and S3d orbitals, which were optimized to values SiH4: H1s, 1.26; Si3d, 1.30, and H2S: H1s, 1.22; S3d, 1.71. The calculated molecular energies are − 290.5187 a.u. for SiH4, and − 397.8415 a.u. for H2S, and net atomic charges are +0.127 for each hydrogen in SiH4 and + 0.176 for each hydrogen in H2S.
Diffusion Coefficients of Noble Gases in Water
Authors: Boerboom A. J. H. , Kleyn G.
Published: February 2004

The diffusion rate of noble gases in water has been measured by the mass-spectrometrical determination of the amount of noble gas diffused in a certain time through a water column of known size. For Xe the absolute value of the diffusion coefficient was measured; for the other noble gases the value relatively with respect to Xe. The diffusion coefficients are given for He, Ne, Ar, Kr, and Xe at 0°, 25°, and 50°C.
Light Scattering from Binary Solutions
Authors: Mountain Raymond D. , Deutch J. M.
Published: February 2004

The spectrum of the light scattered by a binary solution is calculated from thermodynamic fluctuation theory and the linearized hydrodynamic equations appropriate to a two-component fluid. The spectrum consists of three peaks. Expressions are obtained for the positions and widths of the two-side, Brillouin peaks. In general the central, unshifted Rayleigh peak is found to consist of a superposition of two Lorentzians that involve the combined dynamical effects of heat conduction and diffusion. The condition is stated under which it is possible to separate the central peak simply into two contributions, one arising from diffusion and one from thermal conduction. For many binary systems this separation is justified. In these cases measurement of the spectrum of the scattered light should prove to be an attractive alternative means of measuring the diffusion coefficient of binary solutions.
Singlet–Triplet Absorption of Crystalline Naphthalene by High-Resolution Photoexcitation Spectroscopy
Authors: Castro George , Robinson G. W.
Published: February 2004

The high-resolution singlet–triplet absorption spectrum of crystalline naphthalene at 4.2°K was obtained by a photoexcitation technique. The spectrum is presented and analyzed. The spectrum primarily consists of strong Ag modes that exhibit factor-group splitting and that are primarily polarized normal to the molecular plane. The total factor-group splitting of − 32 cm−1 is in agreement with that measured directly in absorption. The crystal polarization properties of the spectrum serve to identify the high-energy components of the Davydov pairs as belonging to the (+) or Au factor-group component. The spectrum clearly reveals the presence of certain antisymmetric modes, which appear with an intensity that is about two orders of magnitude less than that of the 0, 0.
Approximate Analytical Solutions of the Thomas–Fermi Equation for Positive Ions
Authors: Csavinszky P.
Published: February 2004

Approximate analytical solutions of the Thomas–Fermi (TF) differential equation for positive ions are obtained in two different forms. In the Sommerfeld-type approximation the solution is expressed as φS = φ0(1 + ηS), and in the Fermi-type approximation as φF = φ0 + ηF. The function φ0 is an approximate analytical solution of the TF equation for neutral atoms, obtained previously by making use of a variational principle, and the functions ηS and ηF are determined by an iterative solution of second-order ordinary linear differential equations with the boundary and subsidiary (normalization) conditions of the TF theory of positive ions. The approximate solutions φS and φF are tested by calculating the second ionization energies of Ne, Ar, Kr, Xe, and Rn. The calculated values are found in better agreement with the experimental values than those obtained from the exact solution of the TF equation for ions. The reason for this is explained.
Ground States of σ-Bonded Molecules. V. MINDO Treatment of Organic Compounds Containing Nitrogen or Oxygen
Authors: Baird N. Colin , Dewar Michael J. S. , Sustmann Reiner
Published: February 2004

The MINDO approximation, described in Paper IV, has been extended to compounds containing nitrogen and oxygen. With assumed molecular geometries, the method gives very satisfactory estimates of heats of formation and dipole moments.
Pressure-Induced Fundamental Infrared Absorption Band of Gaseous Chlorine
Authors: Winkel R. G. , Hunt J. L. , Clouter M. J.
Published: February 2004

The pressure-induced fundamental infrared absorption band of chlorine gas at 298° and 369°K has been investigated. The frequency of the maximum absorption of the band has been found to agree with that calculated for the free molecule when the effects of the three isotopic species 35Cl2, and 35Cl 37Cl, and 37Cl2 are considered. The binary absorption coefficient has been measured and using it with the theory of Van Kranendonk a value of 57a02 ± 25% has been derived for the rate of change of polarizability with internuclear distance. The temperature dependence of the binary absorption coefficient indicates the possible presence of bound states (Cl2)2 at room temperature.
Kinetics of the Nitrous Oxide–Hydrogen Reaction
Authors: Henrici Hans , Bauer S. H.
Published: February 2004

The reaction of hydrogen with nitrous oxide was studied behind incident shock waves over the temperature range 1700°–2600°K. Experimentally, the concentration profiles of OH and the concentration of NO after the decomposition of N2O were measured by uv absorption. The OH profiles have qualitatively similar features to those determined in the H2–O2 reaction. The concentration of NO after the disappearance of N2O remains constant for several hundred microseconds. Interpretation of the experiments was based on a mechanism which contains the currently accepted data on the H2–O2 reaction and four additional reactions. Computer calculations show that the rate constants for the reaction O+N2O→ lim k32NO as cited in the current literature are too low. The magnitude of k3 determined in this work is consistent with published data on the reverse reaction, and k3 = 6.3 × 1014exp(− 26 700 / RT) cm3mole−1·sec−1 is believed to be correct to within a factor of 3 between 1700° and 4000°K. The present investigation also permitted estimation of the constant for the reaction H+N2O→ lim k5OH+N2. If the literature rate constants for the H2–O2 system are assumed to be correct the best fit to our data is achieved with k5 = 4.0 × 1013exp(− 12 000 / RT). However, a number of inconsistencies remain in the lower temperature range which cannot be reconciled within the framework of the assumed mechanism. These are similar to difficulties which were reported by others on the H2–O2 reaction.
Gas Adsorption Studies on the (100) Plane of Vanadium by Low-Energy Electron Diffraction
Authors: Vijai Kamala Kant , Packman Paul F.
Published: February 2004

Gas adsorption studies were conducted on the (100) plane of a vanadium single crystal using low-energy electron diffraction (LEED). The crystal was exposed to hydrogen and to oxygen, in the pressure range from 10−6–10−9 torr, with the substrate held at temperatures between 25°–1400°C. A procedure for cleaning the surface of vanadium is described. Several important conclusions were reached: (1) the adsorption of hydrogen is random, i.e., amorphous; (2) the adsorption of oxygen is ordered; a (1 × 1) structure is formed and the intensity of the (00) beam is substantially higher than that from the clean surface; (3) when the temperature of the oxygen adsorbed crystal is raised to 1100°C, a (2 × 2) structure is formed; (4) on further heating of the oxygen-adsorbed crystal to 1400°C, all structures due to oxygen disappear; (5) with prolonged heating satellite spots appear on the (00) and the (10) beams, and cannot be removed by heating or argon ion bombardment. Thermal faceting takes place, and patches of a crystalline solid solution of tantalum (from the crystal support) and vanadium appear to form.
Proton and 19F Magnetic Resonance of Partially Oriented Fluoromethanes
Authors: Bernheim R. A. , Hoy D. J. , Krugh T. R. , Lavery B. J.
Published: February 2004

It is shown that the absolute sign of the indirect proton–fluorine nuclear spin–spin coupling constant JHF is positive in the fluoromethanes from their spectra in the nematic liquid crystal p,p′-di-n-hexyloxyaz-oxybenzene and the knowledge of the previously determined absolute signs of the 19F magnetic shielding anisotropies in CH3F and CHF3. Values of the 19F and proton magnetic shielding anisotropies are determined and the problems of shift referencing are discussed. If it is assumed that the contributions to magnetic shielding of the H and F nuclei are axially symmetric about the respective C–H and C–F bonds, the experimental values can be transformed into magnetic shielding anisotropies along bond directions.
Rotational Collision Number and Eucken Factors from Thermal Transpiration Measurements
Authors: Healy R. N. , Storvick T. S.
Published: February 2004

The formal kinetic theory of Wang Chang and Uhlenbeck as modified by Mason and Monchick provides an explicit expression for the heat conductivity of a polyatomic gas. The rotational collision number is an important parameter in this expression. The maximum differential pressure supported by a thermal transpiration measurement is related to the rotational collision number by the dusty gas model proposed by Mason. Thermal transpiration measurements were made that confirm the dusty gas model relationship and provide rotational collision numbers for nitrogen, oxygen, and carbon dioxide at 366°, 398°, and 444°K. Single values of Zrot were obtained for hydrogen, methane, and tetrafluoromethane. These values of Zrot agree with those obtained by ultrasonic measurements and the kinetic theory equations provide Eucken factors that agree within the experimental error with the factors obtained from direct heat conductivity measurements.
Melting Curves and Phase Transitions of the Ammonium Halides to 40 kbar
Authors: Pistorius Carl W. F. T.
Published: February 2004

The phase diagrams of the ammonium halides were studied to 40 kbar by means of differential thermal analysis. The present work confirms the recent piston-displacement study of the phase diagram of NH4F. The NH4F II–IV–liquid triple point is probably located at 16.8 kbar, 309°C. The melting curve of NH4F III rises to 529°C at 40 kbar. The present λ-transition curves of NH4Cl and ND4Cl are in good agreement with earlier hydrostatic work at low pressures. There are indications that the λ transition may change into a first-order transition above ∼ 30 kbar. A pure octopole–octopole interaction appears to be insufficient to predict the course of the transition line at higher pressures. NH4Cl, NH4Br, and NH4I melt at 520°, 523°, and 495°C, respectively, at ∼ 0.2 kbar. The melting curves rise slowly to the II–I–liquid triple points, and rapidly thereafter.
Optical Spectra of Cr3+ Impurity Ions in Ferroelectric LiNbO3 and LiTaO3
Authors: Glass A. M.
Published: February 2004

The optical absorption, luminescence, and excitation spectra of LiNbO3 and LiTaO3 doped with chromium impurities are examined as a function of temperature. Several interesting differences between these spectra and those of other chromium-doped oxides are noted. The most important differences are the large halfwidth of the Cr3+-ion “R” lines (∼50 cm−1 at 4.2°K) and the absence of any “R”-line luminescence down to 4.2°K. Instead, a broadband emission peaking at about 1 μ is observed. It is shown that these differences arise from the low value of the crystal-field parameter Dq at the Cr3+-ion site in LiNbO3 and LiTaO3, so that the zero-phonon T24 state of the Cr3+ ion lies at a lower energy than the E2 state. Point-charge calculations of Dq for several oxides, including LiNbO3 and LiTaO3, and other considerations suggest that the most probable sites of the Cr3+-ion impurities are the Nb/Ta sites and not the Li sites.
Electronic Consequences of Vibrational Deficiency in Polyatomic Molecules
Authors: Gardner Phillip J. , Kasha Michael
Published: February 2004

Vibrationally deficient molecules have been defined as those molecules whose symmetry species for the normal vibrational modes of a molecule fail to span all of the irreducible representations of the corresponding point group. A comprehensive group-theoretical study of all of the finite point groups and of the infinite point groups C∞ν and D∞h has been undertaken in order to determine the extent of vibrational deficiency for various molecular geometries. An important electronic consequence of vibrational deficiency is the identification of special examples of electronic transitions which are forbidden even when considering perturbation by a single antisymmetric normal vibrational mode. Thus, in the second part of this group-theoretical study, electric-dipole selection rules have been determined in order to ascertain possible examples of these forbidden transitions in vibrationally deficient molecules. First-order forbidden vibronic transitions are found to be physically possible in certain polyatomic molecules in D3h, D4h Td, Oh, C∞ν, and D∞h point groups. An analysis of electronic transitions of vibrationally deficient molecules in the above point groups was made in order to establish and to predict examples of this specially forbidden transition which will be manifest as an anomalously weak absorption. A second electronic consequence of vibrational deficiency is predicted in small polyatomic molecules where missing antisymmetric vibrations may have a critical influence on the pathway of intramolecular radiationless processes. The probability of a radiationless transition in an isolated polyatomic molecule may be considered to depend upon a vibrational factor and an electronic factor. This present work indicates that in vibrationally deficient molecules, the electronic factor may have a significant influence on the nonradiative transitions. The fluorescence of SO2 from the second excited singlet state to the ground state is considered as a possible example of the influence of the electronic factor on radiationless processes in vibrationally deficient molecules.
Vibrational Energy Transfer in Gases: Atom–Triatomic-Molecule and Diatomic–Diatomic-Molecule Collisions
Authors: Dubrow Robert , Wilson David J.
Published: February 2004

The interconversion of molecular vibrational energy and translational energy is investigated by a Monte Carlo method and numerical integration of the equations of motion. Colinear collisions of a linear triatomic molecule (having Morse bond potentials) with an atom via a Lennard-Jones 6–12 potential were investigated, and histograms characterizing the probability of transfer of various amounts of energy were constructed. Colinear collisions of two diatomic molecules (Morse bond potentials) interacting via a 6–12 potential were studied, and information on vibration–vibration transfer was obtained. In both models, total energies were in the range 1–20 × 10−12 erg.
Radial Distribution Functions and Equation of State of the Hard-Disk Fluid
Authors: Chae Dong Ghie , Ree Francis H. , Ree Taikyue
Published: February 2004

We evaluated by a Monte Carlo method the hard-disk radial distribution function at densities: ρ / ρ0 = 0.4, 0.5, and 0.6 (ρ0 is equivalent to the close-packed density of hard disks). The results were used to find out the relative merits of solutions of the four integral equations (the Born–Green–Yvon, the modified Born–Green–Yvon, the Percus–Yevick, and the convolution–hypernetted-chain equations) describing approximately the behavior of the radial distribution function. We also evaluated the first several density coefficients of the pressure and the radial distribution functions corresponding to these approximate equations. The Padé approximants formed by the calculated coefficients for the approximate equations were found to satisfactorily describe pressures for these equations over the range of low to medium density of the hard-disk fluids. A simple analytic expression, Eq. (13), for the pressure which describes fairly well the dense hard-disk fluid was also derived by using the modified Born–Green–Yvon equation. This expression yields better or comparable results for virial coefficients and pressures at low and medium fluid densities compared with the results obtained from the Percus–Yevick equation in combination with the virial theorem.
Polarization of Electronic Transitions of Aromatic Hydrocarbons
Authors: Gallivan J. B. , Brinen J. S.
Published: February 2004

The polarizations of the fluorescence, phosphorescence, and triplet–triplet absorption of a series of aromatic hydrocarbons were determined by the method of “photoselection.” The compounds studied; phenanthrene (h10and d10), chrysene (h12and d12), and picene exhibit relatively intense and well-characterized luminescence as well as clearly defined singlet–singlet and triplet–triplet absorption spectra. All spectra including the singlet–singlet absorption spectra were measured in dilute solutions of organic glasses at 77°K. For all five compounds the polarization of the fluorescence was primarily in-plane short axis, the phosphorescence principally out of plane, and the T–T absorption mainly in-plane long axis. Variations in the polarization of vibronic bands within a given transition were noted, as were differences in detail between light and heavy compounds. The polarization results permit the assignment of the states involved in these transitions in addition to confirming the π, π* nature of the upper triplet state.
Lingering Time of the Proton in the Wells of the Double-Minimum Potential of Hydrogen Bonds
Authors: Brickmann Jürgen , Zimmermann Herbert
Published: February 2004

The movement of a proton which moves in a one-dimensional effective double-minimum potential V(x) and can pass by means of the tunnel effect from one well into the other was investigated. The lingering times of the proton in the potential wells (a) and (b) were calculated for more than 150 potential models as functions of the height Vmax of the potential barrier separating the two wells, the energy difference ΔV, and separation d of the minima. In the determination of the lingering times, it was assumed that the proton at time t = 0 is to be found with certainty in one of the wells and can be represented by means of a statistical operator related to the eigenstates of a one-well Hamiltonian H(a)orH(b), respectively. The times determined on this basis were compared with those which result with a quasiclassical method from the classical frequency of oscillation of a particle in a well and from the penetration coefficient of the potential barrier. The deviation between the values determined with both methods is largest if an eigenvalue of H(a) becomes approximately equal to an eigenvalue of H(b). For the potential model V(x) a polynomial of fourth degree in the position coordinate x was chosen. The eigenfunctions of H(a)andH(b) as well as those of the effective Hamiltonian H corresponding to V(x) were represented as linear combinations of eigenfunctions of the harmonic oscillator with angular frequency ω. ω was chosen so that the mean square deviations of the approximated eigenvalues of H became as small as possible for the states of lowest energy.
Relativistic Theory of H2+
Authors: Luke S. K. , Hunter G. , McEachran R. P. , Cohen M.
Published: February 2004

The Dirac equation for H2+ is reduced to a form similar to that of the Schrödinger equation with some perturbation terms. A first-order perturbation calculation for the 1sσg state then reveals that these relativistic effects lower the electronic energy by a small amount of order 0.2α2 for most internuclear separations and thereby make the molecule slightly more stable. The formalism is readily generalized to the isoelectronic sequence of H2+, and the ratio of the relativistic correction to the nonrelativistic energy is found to be of order Z2α2. Consequently, the relativistic correction becomes progressively more important as Z increases.
Transfer of Electronic Excitation Energy in Solid Solutions of Perylene in N-Isopropylcarbazole
Authors: Klöpffer W.
Published: February 2004

The electronic excitation energy transfer from the host material to guest molecules has been studied in the system perylene/N-isopropyl carbazole by measuring the fluorescence intensity ratio guest/host over a great range of guest concentrations. Exciton diffusion and single-step transfer are discussed as possible transfer mechanisms; it is shown that the former process prevails in this system. According to the hopping model, the number of steps during the lifetime of the exciton is calculated to be 4 × 104, and the average displacement from the starting point amounts to 1350 Å.
Asymptotic Coriolis Interactions in Slow Atomic Collisions
Authors: Thorson Walter R.
Published: February 2004

In slow atomic collisions there appear formal long-range interactions between axis-coupled (BO) states due to asymptotic Coriolis interactions, which behave as R−2. These interactions have been considered by a number of workers. In this paper is it shown that these long-range interactions are usually of trivial significance and can be “ignored.” An error in the asymptotic correlation proposed earlier by us is corrected.
Role of Work Function in Electron Ejection by Metastable Atoms: Helium and Argon on (111) and (110) Tungsten
Authors: MacLennan D. A. , Delchar T. A.
Published: February 2004

A tungsten single crystal, cut to expose the (111) and (110) planes, was employed to examine the influence of the work function on electron ejection by beams of helium or argon metastable atoms. The (111) and (110) planes have work functions which differ by approximately 1.5 eV. The electron yield of the (111) surface, with incident helium metastable atoms, is about 1.5% higher than that of the (110) surface; with argon this difference is about 4%. The ejected-electron energy distributions are essentially the same for both crystal planes, but different distributions are obtained with helium and argon. The work-function values inferred from these measurements are the same for both crystal planes, but different values are obtained depending on which metastable-atom probe is used. These results differ from what has been previously predicted, and they are qualitatively accounted for by recognizing that the resonance ionized metastable atom strongly perturbs the surface electron cloud, causing the local vacuum potential to be shifted. The shift in vacuum potential is calculated for a one-dimensional model using a pseudopotential approximation for the resonance ionized metastable atom.
Peptides from Non-Amino Acid Sources. II. A Quantum-Chemical Study of the Isomers of Formamide
Authors: Robb M. A. , Csizmadia I. G.
Published: February 2004

Ab initio molecular-orbital calculations have been carried out on nine isomers corresponding to the empirical formula of CH3NO. The near-Hartree–Fock wavefunctions were used to calculate the ground-state energies as well as dipole moments. The total energy values and transition moments of the first 12 singlet and triplet excited states have been calculated by the virtual orbital technique. These latter results were relevant to some questions in photochemistry and also gave the basis of the theoretical electronic spectra. The computed total energies for the electronic ground states of the nine isomers studied showed that formamide, the model compound for the peptide bond, was much more stable than the other eight isomers.
Kinetic Analysis of the Ion–Molecule Reactions in Nitrogen–Hydrogen Mixtures Using Ion Cyclotron Resonance
Authors: Bowers M. T. , Elleman D. D. , King J.
Published: February 2004

The thermal energy rate constants for the reactions N2++D2→ lim k1N2D++D, N2++H2→ lim k3N2H++H, N2++HD→ lim k4aN2H++D, → lim k4bN2D++H, have been measured using ion cyclotron resonance (icr) giving k1 = 1.15, k3 = 1.41, k4a = 0.65, and k4b = 0.69 × 10−9cm3molecule−1·sec−1. These results agree very well with the values calculated from the change-induced dipole model of Gioumousis and Stevenson. The absence of a significant isotope effect in k4a and k4b indicates these reactions proceed via a stripping mechanism as indicated by Wolfgang and co-workers and Mahan and co-workers. Energy dependence studies of k2 and the corresponding charge exchange rate constant k5 D2++N2→ lim k2N2D++D D2++N2→ lim k5D2+N2+ indicate that the decrease in k2 with increasing reactant ion energy is approximately offset by the increase in k5 as predicted by the phase-space theory of Light.
Effect of Localized Lattice Vibrations on the Motional States of Polar Impurities in Alkali Halide Matrices
Authors: Pandey Gopal Krishna , Agrawal Vinai Krishna
Published: February 2004

The effect of localized lattice vibrations on the librational and tunneling frequencies of diatomic polar impurities trapped in alkali halide matrices has been worked out. It is shown that Devonshire energy-level scheme corrected for the lattice-vibrational effects gives a good account of the librational and tunneling levels of the impurity.
Far-Infrared Spectrum and the Barrier to Pseudorotation of Silacyclopentane
Authors: Laane Jaan
Published: February 2004

The far-infrared spectrum arising from the transitions between the pseudorotational levels in silacyclopentane has been observed. Thirteen absorption maxima were found for the υ = 0 (radial ground) state and eight for the υ = 1 state. For each series a potential of the form V = (V2 / 2) (1 + cos2φ) predicts frequencies which agree very closely with the observed values. The values of V2, which represent the barriers to pseudorotation, were found to be 1362 ± 25 cm−1 for the radial ground state and 1301 ± 50 cm−1 for the first excited state. The pseudorotation constants for the two radial states were found to be B0 = 1.966 and B1 = 2.033 cm−1. The observed pseudorotation barrier of 3.89 kcal/mole represents the energy required to go from the more stable C2 half-chair conformation to the Cs envelope form and is higher than expected from previously derived formulas.
Reflection Method for Obtaining the Infrared Spectrum of a Thin Layer on a Metal Surface
Authors: Greenler Robert G.
Published: February 2004

Previous calculations have shown that a thin, moderately absorbing layer on a metal surface has an absorption factor in the infrared which varies greatly with the angle of incidence and the state of polarization of the incident radiation. The requirements on a spectroscopic system, which uses the insights of these calculations, are discussed under the realistic conditions of multiple reflections with a low f/number beam. A method is described for obtaining many reflections between closely spaced mirrors. Radiation emerges from a narrow aperture between the mirrors, the aperture serving as an effective source for the infrared monochromator. A system has been constructed and its use is illustrated by the spectrum of a cellulose acetate layer on a silver mirror.
Microwave Spectra of Substituted Propenes. V. The Microwave Spectra, Barrier to Internal Rotation, and Nuclear Quadrupole Coupling Tensor of trans-1-Bromopropene
Authors: Beaudet Robert A.
Published: February 2004

The microwave spectra of trans CH3CH = CH79Br and CH3CH = CH81Br were studied between 8–40 GHz. The height of the barrier hindering the internal rotation of the methyl group was found to be 2120 cal/mole. The electron nuclear quadrupole moment tensor was also determined for both isotopic species.
Pair Polarizability of Hydrogen Atoms. Relation to Collision-Induced Light Scattering and Dielectric Phenomena in Rare Gases
Authors: DuPré Donald B. , McTague J. P.
Published: February 2004

The polarizability tensor as a function of internuclear separation of hydrogen atoms is calculated, using the approximate variational wavefunction for ground- and first-excited-state H2 of Hirschfelder and Linnett. Two interacting hydrogen atoms in the nonbonding triplet state have certain qualitative features in common with colliding pairs of rare-gas atoms. In particular, the system exhibits the long-range dispersion attraction and short-range overlap repulsion of interacting closed shell atoms. Results of the calculations here on 3ΣH2 are capable, in analogy, of explaining observed trends in collision-induced light scattering intensities and the second dielectric virial coefficient of rare gases, which cannot be accounted for on the basis of long-range dispersion force models or perturbation theory alone. The major source of the effect is found to be the distortion of electronic charge density at intermediate and small internuclear separations.
Study of the Pressure Dependence of Dielectric Polarization
Authors: Chen Tunli , Dannhauser Walter , Johari Gyan P.
Published: February 2004

The pressure dependence of the dielectric polarization of polar fluids depends principally on three factors: (1) the fluid density, which is easily measured; (2) pressure-induced distortion of molecules, expressed as dμ0 / dP; and (3) pressure-induced changes in the liquid structure, expressed as dg / dP, where g is Kirkwood's dipole correlation factor. d(gμ02) / dP has been determined over a wide range of temperature to 4 kbar for n-propanol, sec-butanol, and 6-methyl-3-heptanol and is found to be either positive or negative depending on the temperature and the specific nature of the alcohol. At temperatures and pressures where g is near unity, increasing pressure enhances gμ02 because of pressure-induced chain association; when g is already large, increasing pressure decreases gμ02. Determination of the dipole moment in dilute hexane solution as a function of pressure shows that for diethyl ether and n-butyl chloride, μ0 is virtually constant to 4 kbar, μ0 of sec-butanol probably increases slightly, and μ0 of 1,2-dichloroethane increases by about 6%. The decrease of gμ02 of alcohols with increasing pressure is attributed to interchain correlations as the packing in the liquid approaches that of the crystal.
Concentration Dependency of the Sedimentation Constant
Authors: Imai Satoshi
Published: February 2004

Recently we have calculated the Huggins constant in terms of the characteristic relaxation spectrum. Here we apply the same method to the coefficient ks, which is defined by s / s0 = 1 − ksc···, where s is the sedimentation constant at the polymer weight concentration c, and s0 is that at infinite solution. It is found that ks is related to the expansion coefficient αξ (determined by viscosity measurements) and to the molecular weight of the polymer in the following manner: ks = C0M1/2(αξ − αξ−1), where C0 is a constant dependent on the unperturbed dimensions of the polymer coil. This equation is compared with recent experiments by Nagasawa and by Cowie and good agreement is obtained. Thus, plots of ks against M1/2(αgx−αξ−1 give excellent linearity.
Relationship between the Limiting Pressure and the Solid Temperature for the Deflagration of Ammonium Perchlorate
Authors: Watt D. M. , Petersen E. E.
Published: February 2004

A new experimental method has been used to measure the relationship between the limiting pressure for deflagration and the initial solid temperature of single crystals and pressed pellets of ammonium perchlorate (AP). The technique depends upon setting a known linear temperature gradient of 10°–20°C/cm along the length of a sample pressurized in nitrogen or helium so that the warmer end of the AP sample could sustain deflagration, but the cooler end could not. The warmer end is then ignited, and essentially steady-state deflagration propagates through the solid until the limiting solid temperature corresponding to the set pressure is reached. The length remaining unburned determines the limiting solid temperature. At solid temperatures between − 40°– + 50°C the limiting pressure fell essentially linearly from 385–225 psia, and above this solid temperature the limiting pressure appeared to be asymptotically approaching a minimum value. Since the results were the same for AP single crystals and pressed pellets in both nitrogen and helium, this limiting condition appears to be a unique property of ammonium perchlorate. These data provide a basis for uniting theories of AP ignition and deflagration.
Experimental Study of the Equation of State of Liquid Argon
Authors: Streett W. B. , Staveley L. A. K.
Published: February 2004

The density of liquid argon has been measured at 10 temperatures from 100.9° to 143.1°K and at pressures up to 680 atm. The results have been expressed as an equation of state in the form of a double Chebyshev expansion. This equation has been used to estimate, at regular intervals of temperature and pressure, the following properties: molar volume, isothermal compressibility, coefficient of thermal expansion, thermal pressure coefficient, and configurational internal energy. Comparison is made between the present results and previous work on liquid argon, both at the saturation vapor pressure and at higher pressures.
Adsorption of Cs on Tungsten: Measurements on Single-Crystal Planes
Authors: Sidorski Z. , Pelly I. , Gomer R.
Published: February 2004

The adsorption of immobilely deposited and thermally equilibrated Cs layers has been studied on the (110), (211), (100), (111), (103), and (115) regions of a tungsten field emitter. The work-function-vs-coverage data permit resolution into adsorbate charge and effective dipole length, at least on the high work-function planes. The results indicate ionic adsorption on (110) and probably (211) at low coverage, with charge diminishing on all planes as θ increases. Dipole lengths exceed the ionic radius of Cs by 1.4–2 Å, indicating the effect of finite screening at the metal surface. The adsorption energies and their change with θ can be interpreted in terms of the “metallet” model.
Spectral Study of NO2 Fluorescence Excited by 11 Lines of Argon and Krypton Ion Lasers
Authors: Sakurai Katsumi , Broida H. P.
Published: February 2004

High-resolution spectra of the fluorescence of NO2 excited by 11 Ar+ and Kr+ laser lines between 4579 and 5208 Å, have been obtained at NO2 pressures from 10−3–10 torr. Rotational lines as well as vibrational bands are observed superimposed on a continuum extending from the exciting line to the observation limit of 7500 Å. Vibrational assignments of the fluorescent bands have been made, and vibrational constants of the gound electronic state have been determined. The pressure dependence of absorption coefficients of NO2 at the laser wavelengths also was measured; an excited-state lifetime near 10−5 sec was estimated from these absorption coefficients. In addition, the lifetime of excited NO2 was measured to be 5 × 1015 sec from the observation of the increase in visual size of the fluorescence beam at low pressure. Preliminary values of σ2τ (square of collision diameter times lifetime) of the fluorescence excited by the 4880-Å line were measured to be approximately 7 × 10−20 cm2·sec for line emission and 0.5 × 10−20 cm2·sec for the integrated continuum emission, respectively; this difference is attributed to the difference in rates of rotation–vibration collisional transfer and electronic quenching.
Magnetic-Susceptibility Anisotropy, Molecular g Value, and Molecular Quadrupole Moment of 15N15N16O
Authors: Flygare W. H. , Shoemaker R. L. , Hüttner W.
Published: February 2004

The high-field rotational Zeeman effect has been observed in 15N15N16O to give an absolute value of the molecular g value of g⊥ = 0.07606 ± 0.0001 and an anisotropy in the magnetic susceptibility of χ⊥ − χ‖ = (1.68 ± 0.02) × 10−29ergs/G2. Arguments are given which show conclusively that the molecular g value is negative. The molecular quadrupole moment in the center-of-mass coordinate system is Q‖ = − (3.65 ± 0.25) × 10−26esu·cm2. The values of the second moment of the electronic charge distribution in NNO are 〈‖;2〉 = 23.9 × 10−16cm2 and 〈⊥2〉 = 3.0 × 10−16cm2. The results are compared with similar results for CO2, O2, and N2.
Nonresonant Vibration-to-Vibration Energy Transfer Due to Dipole–Dipole Interactions
Authors: Yardley James T.
Published: February 2004

A simple model for nonresonant vibration-to-vibration energy transfer due to dipole–dipole interactions is used to yield analytic expressions for the energy-transfer probability as a function of impact parameter, relative velocity, and deviation from resonance. Where the deviation from resonance is sufficiently large, the probabilities may be integrated over the impact parameter distribution, and in some cases over the velocity distribution, to obtain cross sections for V → V energy transfer.
Dissociation Constants of Excited Phenols from Fluorescence Quenching Data
Authors: Avigal I. , Feitelson J. , Ottolenghi M.
Published: February 2004

The quenching effects of a set of carboxylic acid anions on the fluorescence of some phenol derivatives are analyzed. It is found that the quenching rate constants, obtained from the corresponding Stern–Volmer plots, satisfy the Bro/nsted general-base catalysis law. Extrapolation of the Bro/nsted plots to water as base yields values for the pKa's of the various phenols in their lowest excited singlet state. The results are discussed and compared to pKa* values previously obtained from an alternative “thermodynamic” procedure.
ESR Study of Interconversion in Substituted Piperidine Iminoxyls
Authors: Windle J. J. , Kuhnle J. A. , Beck Barbara Hardin
Published: February 2004

ESR spectra of iminoxyl radicals formed photochemically from the following six-membered alicyclic hydroxylamines, 1-hydroxypiperidine, 1-hydroxy-4-methylpiperidine, 1-hydroxy-4-phenylpiperidine, and 4-hydroxymorpholine, have been obtained in methylene chloride over a temperature range of −130° to +120°C. The main features of the ESR spectra result from hyperfine coupling to the nitrogen nucleus and to the four β protons. The nitrogen hyperfine interaction is independent of temperature over the range studied in contrast to the β-hyperfine interaction which depends upon the conformational behavior of these radicals. From the variations of the ESR spectra with temperature the barrier to ring inversion, the rate of interconversion, the conformational preferences, and structural parameters for these molecules are calculated. It is concluded that these radicals are in the chair conformation. At low temperature, hyperfine coupling to γ protons are detected and are shown to result from coupling to the axial pair. For 1-hydroxypiperidine coupling to one δ proton is found. This coupling is attributed to the equatorial δ proton and is equal in magnitude to the γ-proton hyperfine interaction in this radical.
Energy Dependence of Two-Photon-Absorption Cross Sections in Anthracene
Authors: Webman Itzchak , Jortner Joshua
Published: February 2004

Two-photon-absorption cross sections were determined in the energy range 3.10–3.56 eV for crystalline anthracene and for the anthracene molecule in solution. The experimental method was based on the measurement of the excitation spectrum for fluorescence induced by two-photon absorption from a Raman-shifted ruby laser. From these results we conclude that: (a) a symmetry-forbidden two-photon transition corresponding to the A11g→B12u excitation was observed both in the crystal and in the solution; (b) this transition is vibronically induced by a nontotally symmetric vibration of the frequency 1200 ± 400 cm−1; (c) a crystal state is observed at 3.5–3.6 eV which has no counterpart in the solution spectrum; (d) this crystal state is tentatively assigned to a g-symmetry-type charge-transfer state; (e) the possible observation of a quadrupole two-photon A11g→B12u to the 0–0 band seems likely since the dipole two-photon transition is forbidden and vibronic mixing within the true 0–0 band does not occur.
Molecular g Values, Magnetic Susceptibility Anisotropies, and Molecular Quadrupole Moments in Fluorobenzene
Authors: Hüttner W. , Flygare W. H.
Published: February 2004

The first- and second-order molecular Zeeman effect has been observed in fluorobenzene to give the molecular g values of gaa = − (0.0670 ± 0.0008), gbb = − (0.0397 ± 0.0015), gcc = + (0.0266 ± 0.0017), and the magnetic susceptibility anisotropies of 2χaa − χbb − χcc = + (52.9 ± 0.8) × 10−6erg/G2·mole and 2χbb − χcc − χaa = + (63.6 ± 1.5) × 10−6erg/G2·mole. The a axis passes through the CF bond and the b axis is also in the molecular plane. Only the relative signs of the g values are obtained experimentally. However, by combining our magnetic susceptibility anisotropies with the average magnetic susceptibility we can show that the signs must be as assigned above. The diagonal elements in the total magnetic susceptibility tensor are χaa = − (40.6 ± 0.8) × 10−6, χbb = − (37.1 ± 0.8) × 10−6, and χcc = − (97.2 ± 2.0) × 10−6erg/G2·mole. The diamagnetic and paramagnetic susceptibilities are given as χaad = − (293.3 ± 1.7) × 10−6, χaap = (252.7 ± 0.7) ± 10−6, χbbd = − (509.7 ± 3.6) × 10−6, χbbp = (472.6 ± 2.6) × 10−6, χccd = − (732.4 ± 5.0) × 10−6, and χccp = (635.2 ± 4.0) × 10−6erg/G2·mole. The molecular quadrupole moments are Qaa = − (1.9 ± 0.8) × 10−26, Qbb = + (5.1 ± 1.5) × 10−26, and Qcc = − (3.2 ± 1.5) × 10−26esu·cm2. The values of the second moment of the electronic charge distribution are 〈a2〉 = (111.8 ± 0.7) × 10−16, 〈b2〉 = (60.8 ± 0.7) × 10−16, and 〈c2〉 = (8.4 ± 0.7) × 10−16cm2.
Radiationless Transitions in Isolated Molecules. The Effects of Molecular Size and Radiation Bandwidth
Authors: Rhodes William
Published: February 2004

Light absorption and emission characteristics of an isolated molecule are examined in terms of the stationary states of the molecule and the properties of the incident light (e.g., coherence and bandwidth). The use of various correlation functions and correlation times for the molecule and radiation field permits a detailed description of the time dependence of various excitation processes. The formalism developed, when applied to a simplified model, provides an explanation for the observed differences in fluorescence behavior between large and small molecules. The conditions of validity of the usual description of molecular excitation (followed by internal conversion) in terms of Born–Oppenheimer (B–O) states is discussed in terms of the molecular stationary states and radiation bandwidth. It is shown in terms of the model that if the radiation bandwidth is sufficiently large excitations occur to an electric-dipole-allowed B–O state and that photon emission is determined in a simple way by the relaxation function for that state. The distinction between large and small molecules depends on the density of final B–O states relative to their coupling strength with the allowed B–O state. Comparisons are made with the work of Jortner and Berry [J. Chem. Phys. 48, 2757 (1968)], who have studied this problem using a different formalism.