bluuebunny/crossref_metadata_2025 huggingface dataset:
Rate of the reaction of atomic hydrogen with propyne over an extended pressure and temperature range
Authors: Whytock D. A. , Payne W. A. , Stief L. J.
Published: February 2003

The technique of flash photolysis coupled with time resolved detection of H via resonance fluorescence has been used to obtain rate constants for the reaction of atomic hydrogen with propyne at temperatures from 215 to 460 K and at pressures in the range 5–600 torr. The rate constants are strongly pressure dependent and the high pressure limiting values give rise to the Arrhenius expression k= (6.00±1.20) ×10−11 exp(−2450±100/1.987 T) cm3 molecule−1⋅s−1. The results are discussed and compared with those of previous studies.
Current fluctuations for Brownian charge carriers in solution
Authors: Harris S.
Published: February 2003

We have calculated the current fluctuations in solution at infinite dilution for the cases where the charge-carrier motion is a classical Brownian motion and a recently suggested generalization of the classical model. The possibility and advantages of utilizing this phenomena as a probe of carrier dynamics is also briefly discussed.
Photoabsorption spectrum of UF6 by electron impact
Authors: Srivastava Santosh K. , Cartwright D. C. , Trajmar S. , Chutjian A. , Williams W. X.
Published: February 2003

The photoabsorption cross section of UF6 in the wavelength region between 206.7 nm (2.5 eV) and 43.5 nm (28.5 eV) has been derived from the UF6 electron energy loss spectra at a 5° scattering angle and at incident electron energies of 75 and 100 eV. The shape of the resulting optical spectrum agrees well with available high-resolution photoabsorption measurements in the 400 nm (3.1 eV) to 200 nm (6.3 eV) region. Below 200 nm, where no photoabsorption data are available, there is strong absorption and appreciable structure present. Absolute values of absorption cross sections have been obtained by normalizing the present relative data to the recently measured photoabsorption value at 225.5 nm.
Perturbation-response treatment of thermodynamic potentials. I. Application to electron and molecular fluids
Authors: MacRury T. B. , Linder B.
Published: February 2003

The perturbation response technique, developed for intermolecular forces, is generalized to include translational fluctuations and applied to an arbitrary system of particles interacting through two-body potentials. The theory takes account of quantum and thermal effects. By neglecting correlations between multiple excitations, a closed-form expression is obtained for the thermodynamic potential or free energy shift in terms of the particle susceptibility and the interaction potential. The result is recast in several equivalent forms which focus on the collective behavior. The theory is specialized to translationally invariant systems and worked out in detail for the electron gas and molecular fluid. The free energy shift for the electron gas is shown to be equivalent to that of Englert and Brout. The molecular fluid free energy shift takes account of quantal translational effects; the classical formula (within RPA) of the Chandler–Anderson theory is obtained as a special limiting case.
Perturbation-response treatment of thermodynamic potentials. II. Application to a van der Waals crystal
Authors: MacRury T. B. , Linder B.
Published: February 2003

The perturbation response technique, developed for intermolecular forces, is applied to the free energy shift of a van der Waals crystal. The treatment is applicable to a system of arbitrary molecules from which the expressions for the point-dipole model can be obtained as a special case. Attention is focused on the problems associated with the finite size of the molecules (or atoms) in a real crystal. Expressions obtained within the dipolar approximation are found to agree with results obtained by other techniques.
Rayleigh light scattering from concentrated solutions of polystyrene in cyclohexane
Authors: Geissler E. , Hecht A. M.
Published: February 2003

The Rayleigh light scattered at 90° by concentration fluctuations in solutions of polystyrene with cyclohexane, in the mass concentration range 0.18–0.56 of polymer, is observed to mix with a heterodyne efficiency close to unity with light scattered elastically by dust particles. A technique is described where by using a digital photon correlator the absolute intensity of the Rayleigh light is measured and the elastic modulus of the uncross-linked polymer gel deduced. The results are in satisfactory agreement with values found in the literature. The measurements of the relaxation time of the concentration fluctuations allow the friction constant between the solvent and the polymer matrix to be calculated.
Evaluation of molecular integrals over Gaussian basis functions
Authors: Dupuis Michel , Rys John , King Harry F.
Published: February 2003

This paper is concerned with the efficient computation of the ubiquitous electron repulsion integral in molecular quantum mechanics. Differences and similarities in organization of existing Gaussian integral programs are discussed, and a new strategy is developed. An analysis based on the theory of orthogonal polynomials yields a general formula for basis functions of arbitrarily high angular momentum. (ηiηj∥ηkηl) = Σα=1,nIx(uα) Iy(uα) I*z(uα) By computing a large block of integrals concurrently, the same I factors may be used for many different integrals. This method is computationally simple and numerically well behaved. It has been incorporated into a new molecular SCF program HONDO. Preliminary tests indicate that it is competitive with existing methods especially for highly angularly dependent functions.
Overtone emission spectroscopy of HF and DF: Vibrational matrix elements and dipole moment function
Authors: Sileo Richard N. , Cool Terrill A.
Published: February 2003

Measurements of the emission intensities for overtone bands of HF and DF from a chemical laser source have been used to determine the dipole moment function for the ground electronic states of HF and DF for internuclear separations from 1.25–3.1 bohr. Vibrational matrix elements and Einstein coefficients have been determined for the fundamental through fifth overtone bands for all transitions from upper vibrational levels below v=10 for HF and v=13 for DF. The results are compared with recent ab initio and semiempirical dipole moment calculations.
Kinetic study of the reaction of F with H2 and CF3H by ESR methods
Authors: Goldberg Ira B. , Schneider George R.
Published: February 2003

The reactions between F and H2, H and F2, and F and CF3H were studied by using a fast flow reactor with electron spin resonance detection. The reaction rate constant for F+H2→HF+H was determined to be 1.00±0.08×1013 cm3 mole−1⋅sec−1 at 298 °K. The rate of the reaction H+F2→HF+F was determined in a system where known amounts of F and F2 were injected into a stream of H2 in He. Using finite difference techniques to solve the rate equations, the rate constant was found to be 0.26±0.06×1013 cm3 mole−1⋅sec−1. These reaction rates are in good agreement with other published values. The rate constant determined for the reaction F+CF3H→HF+CF3 was 0.90±0.08×1011 cm3 mole−1⋅sec−1. Techniques are presented which permit ESR to be used in studies of complex reaction mechanisms and in nonuniform flow velocities.
Efficiencies for production of atomic nitrogen and oxygen by relativistic proton impact in air
Authors: Porter H. S. , Jackman C. H. , Green A. E. S.
Published: February 2003

Relativistic electron and proton impact cross sections are obtained and represented by analytic forms which span the energy range from threshold to 109 eV. For ionization processes, the Massey–Mohr continuum generalized oscillator strength surface is parametrized. Parameters are determined by simultaneous fitting to (1) empirical data, (2) the Bethe sum rule, and (3) doubly differential cross sections for ionization. Branching ratios for dissociation and predissociation from important states of N2 and O2 are determined. The efficiency for the production of atomic nitrogen and oxygen by protons with kinetic energy less than 1 GeV is determined using these branching ratio and cross section assignments.
Assignments in the electronic spectrum of UF6
Authors: McDiarmid Ruth
Published: February 2003

The absorption spectrum of gaseous UF6 was recorded from 1200–4200 Å (83 300–23 800 cm−1). Eight electronic transitions were observed with intensities ranging from 8–20 000 1/mole⋅cm. Based on correlations between this spectrum and those of UF6−, UCl6−, MoF6, and WF6 and the photoelectron spectrum of UF6, the experimental spectrum was assigned.
C–H vibrational states of benzene, naphthalene, and anthracene in the visible region by thermal lensing spectroscopy and the local mode model
Authors: Swofford Robert L. , Long M. E. , Albrecht A. C.
Published: February 2003

Visible (red) absorption spectra of benzene (h6 and d6), methyl substituted benzenes, naphthalene, and anthracene are presented as seen by thermal lensing spectroscopy. It is shown how this highly sensitive laser-based technique can easily detect transitions having absorption coefficients as small as 10−6 cm−1. The observed transitions are principally due to C–H stretching vibrations at the sixth quantum of excitation. The local mode (LM) model for describing such molecular states is explored in some detail with reference to the C–H excitations in benzene. Based on a two parameter empirical fit, the LM model can give a true C–H stretch ’’mechanical’’ frequency; it successfully predicts isotope effects; and it fully determines the energy level scheme at any quantum of excitation. The LM energy level scheme for the 462 C–H stretching states at the sixth quantum of excitation is presented. A majority of these states are only mildly anharmonic; one state in fact is a perfect harmonic of the fundamental. However, only the most anharmonic state of the 462 is the spectroscopically allowed one. The LM model, already introduced in the literature, is seen to be an excellent zeroth order description of those very same highly excited vibrational states most readily accessible to a sensitive absorption technique such as that offered by the thermal lensing effect.
On metastable H2−
Authors: Durup Jean
Published: February 2003

The 2p3, 4S0 state is suggested as a possible metastable state of H2−.(AIP)
Thermal decomposition of nitrogen trifluoride in shock waves
Authors: Evans P. J. , Tschuikow-Roux E.
Published: February 2003

The thermal decomposition of nitrogen trifluoride has been reinvestigated behind incident shock waves in dilute (1%) NF3/Ar mixtures over the temperature range 1150–1530 K and at 0.80–1.81 atm total pressures. The reaction kinetics were followed by monitoring NF2 radical concentrations in absorption at 260 nm. In several experiments, the infrared emission at 5.17 μ was also monitored. Under the experimental conditions used, the reversible dissociation NF3+M?NF2+F+M is known to be first order in both NF3 and Ar concentrations, and the low pressure limit second-order rate constant was found to be k1(cm3 mol−1 s−1) =1016.61±0.13 ×exp[−(48.0±0.8 kcal mol−1)/RT] using an approximate iterative procedure. A numerical solution of the differential rate equation yielded comparable results. The temperature dependence of the absorption coefficient for NF2 and the equilibrium constant were also determined.
Reduced Green’s functions and perturbed Hartree–Fock calculations. I. Formulation of the theory
Authors: McDowell H. Keith , Porter Richard N.
Published: February 2003

A diagrammatic analysis is developed to implement the derivation of a set of integral equations for first-order coupled perturbed Hartree–Fock orbitals in the presence of a one-particle perturbation. A general formulation is given in terms of an arbitrary, but well characterized, reduced one-particle Green’s function so that the difficulties of either obtaining a sufficiently complete set of virtual orbitals or of using an explicit form of the reduced Green’s function corresponding to the Fock operator are obviated.
Hartree–Fock density matrix equation
Authors: Cohen Leon , Frishberg C.
Published: February 2003

An equation for the Hartree–Fock density matrix is discussed and the possibility of solving this equation directly for the density matrix instead of solving the Hartree–Fock equation for orbitals is considered. Toward that end the density matrix is expanded in a finite basis to obtain the matrix representative equation. The closed shell case is considered. Two numerical schemes are developed and applied to a number of examples. One example is given where the standard orbital method does not converge while the method presented here does.
Metallic reflection in a crystalline polymethinium dye and in crystalline TCNQ: Theory of the damping
Authors: Johnson Bruce B. , Simpson William T.
Published: February 2003

Metallic reflection occurs in molecular crystals when a stopping band associated with a strong molecular electronic transition emerges. Variations in reflectivity over the band are studied here theoretically for two cases—1,5-bis (dimethylamino)pentamethinium perchlorate (BDP) and tetracyanoquinodimethane (TCNQ). The variations are described by a frequency dependent damping term Γ (ω) in the expression for the polarizability. For the case of one molecule per unit cell (BDP), the observed reflection band shape can be matched using a theoretical expression iΓ (ω) = (γ2/N) Σk 1/(ω−ωk−Δ), where γ is a molecular vibrational relaxation constant and Δ is the frequency of a single active vibration. The variation of ωk with k is approximated by the long-wave method. For the case of two molecules per unit cell (TCNQ) a more complicated expression is obtained which predicts a reflection spectrum richer in structure than in the case of one molecule per unit cell. The number and location of features in the theoretical spectrum match those in the observed spectrum reasonably well.
ESR of Cl2− in various ion pairs at 4° K and the theory of V centers
Authors: de Pinillos J. V. Martinez , Weltner W.
Published: February 2003

ESR spectra have been measured of M+Cl2− ion pairs trapped in solid argon at 4° K, where M=Mg, Ca, Sr, Ba, Li, Na, and K. The derived g and hyperfine tensors of Cl2− are interpreted using the simple crystal field model of Schoemaker, and the spin densities and crystal field parameters E⊥ are compared with those for V centers in alkali halide crystals. From matrix data, it is proposed that the isotropic hyperfine coupling parameter Aσ for interaction with 35Cl in pseudo-’’free’’ Cl2− be taken as 37±1 G. A discussion is given pointing out an approximate linear correlation between the nearest intercation distance (d) and Δg⊥ (or Aσ) among all the VK centers. Constraints imposed by lattices of varying M+/Cl− radius ratios cause variations in d and in the s/pz character in the antibonding Cl2− electron, leading to corresponding changes in Aσ and inverse changes in Δg⊥. VF centers, with minimum constraints, have maximum values of Aσ.
Simple cage process in liquid phase radical reactions
Authors: Dobis O.
Published: February 2003

The cage combination of CF3- and CH3- radicals formed by the photolysis of hexafluoroazomethane and azomethane, respectively, has been investigated in various nonpolar solvents in the temperature range of 0–120 °C [K. Chakravorthy, J. M. Pearson, and M. Szwarc, J. Am. Chem. Soc. 90, 283 (1968); O. Dobis, J. M. Pearson, and M. Szwarc, J. Am. Chem. Soc. 90, 278 (1968)]. In this work, the experimental results are interpreted by using the self-diffusion constants and self-diffusion energies of the solvents as parameters for the radical cage processes. It is presumed that the ratio of the two competitive elementary cage processes—the combination of the geminate radical pair and the escape of radicals by diffusion—is a function of the solvent microstructure. The first approach to separate the two coexistent cage processes is finding a correlation between radical escape probability and the self-diffusion parameters of solvents calculated on the basis of the body-centered geometrical model of liquid microstructure. The coarse graining description of the liquid allows dividing the random motion of the radical pair in the cage into four states of different energy levels. In the steady state limit, this four-state random motion of radicals can be treated according to Markov’s kinetics. The temperature dependence of the radical transition probabilities between the consecutive states points to energetic interaction within the radical pair and a similar interaction between radicals and solvent molecules. The prediffusion and diffusion states of radicals give a good correlation with the quasicrystalline structure of solvents, but the precombination state and the state of the radical pair combination are better correlated with the continuous model of solvent structure.
An R matrix approach to the solution of coupled equations for atom–molecule reactive scattering
Authors: Light John C. , Walker Robert B.
Published: February 2003

We describe in detail a new method for solving the coupled linear differential equations which appear in the theoretical statement of many types of scattering phenomena. Combining the speed advantage of propagation methods with the inherent stability of R-matrix methods, this method is fast and is unaffected by exponential growth of closed channels. We describe the propagation of the R-matrix in terms of a collinear reactive scattering problem, and provide numerical results for several systems, clearly demonstrating the speed, stability, and accuracy of this method.
The a
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o calculation of the spin–rotational coupling in the metastable c 3Πu(1s,2p) state of molecular hydrogen
Authors: Jette A. Norman
Published: February 2003

An ab initio calculation of the spin–rotational coupling C for the c 3Πu(1s,2p) state of H2 gives a result within 7% of the experimental constant. The primary contribution to the spin–rotational coupling is found to be the 1sσg one electron molecular orbital. (AIP)
Comment on ’’Existence of H2−, a relatively long-lived doubly charged negative atomic hydrogen ion’’
Authors: Aberth William
Published: February 2003

It is maintained that the recent report of the existence of H2− [Schnitzer and Anbar, J. Chem. Phys. 64, 2466 (1976)] can be interpreted as arising from electron attachment and detachment collisions of the H beam ions with the background gas. (AIP)
Comment on ’’Existence of H2−, a relatively long-lived doubly charged negative atomic hydrogen ion’’
Authors: Vestal Marvin L.
Published: February 2003

An experimental method for distinguishing between two conflicting hypotheses [R. Schnitzer and M. Anbar, J. Chem. Phys. 64, 4332 (1976) and W. Aberth, J. Chem. Phys. 65, 2466, (1976)] concerning the possible existence of H2− is given.(AIP)
T
1(ππ*) →S radiationless transitions in aromatic molecules with nonbonding electrons
Authors: Kanamaru N. , Lim E. C.
Published: February 2003

The matrix elements for T1(ππ*) →S0 radiationless transitions in aromatic molecules with nonbonding electrons are examined within the framework of pure spin adiabatic Born–Oppenheimer basis set, in order to deduce the relative importance of various ’’mechanisms’’ of the intersystem crossing. It is concluded that, unless forbidden by symmetry, the mechanisms involving vibronic coupling in the triplet manifold are much more important than those involving vibronic coupling in the singlet manifold. Unlike in aromatic hydrocarbons, the mechanisms induced by nonadiabatic coupling can be competitive when compared to those induced by adiabatic coupling. The experimental data are compared with qualitative theoretical predictions for a number of nitrogen heterocyclic and aromatic carbonyl compounds.
Matrix perdeuteration effects on the 3ππ→S phosphorescence of p-chlorobenzaldehyde at 4.2°K. I. Phenomenology
Authors: Khalil Omar S. , Goodman Lionel
Published: February 2003

The effect of matrix perdeuteration and variation of cooling rate on the phosphorescence vibrational structure of p-chlorobenzaldehyde (PCB) are studied in methylcyclohexane (MCH) and p-xylene. PCB shows very different phosphorescence spectra in slowly cooled MCH-h14 and MCH-d14, generally broad spectra in fast cooled samples, and a mixture of the two phosphorescences (observed in the slow cooled sample) in intermediate cooled MCH-d14. In p-xylene, no change in the phosphorescence vibrational structure is observed on matrix perdeuteration. These observations are interpreted by postulating two crystalline modifications for methylcyclohexane, one of them stable in slowly cooled MCH-h14, the other stable in slowly cooled MCH-d14. The spectra of PCB is different in the two modifications. The anomalous response of the PCB phosphorescence vibrational structure to the crystalline modifications of MCH is indicative of a large degree of distortability in its 3ππ* state. The distortability is interpreted as originating from vibrational–electronic interactions between the closely spaced 3ππ*–3nπ* states. Support for this view is found in the phosphorescence spectra of various deuterated derivatives of PCB in perprotonated and perdeuterated MCH. The apparent distortability of the emitting state varies with the extent of deuteration.
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o SCF and CI studies on the ground state of the water molecule. II. Potential energy and property surfaces
Authors: Rosenberg Bruce J. , Ermler Walter C. , Shavitt Isaiah
Published: February 2003

Self-consistent field and configuration interaction calculations for the energy and one-electron properties of the ground state of the water molecule were carried out with a (5s4p2d/3s1p) 39-function STO basis set. The CI treatment included all single and double excitation configurations (SD) relative to the SCF configuration, and a simple formula due to Davidson was used to estimate the energy contribution of quadruple excitations and thus produce a set of corrected (SDQ) energies. The calculations were carried out for 36 molecular geometries (22 symmetric and 14 asymmetric) in the neighborhood of the equilibrium geometry. Least-squares analysis was used to determine theoretical equilibrium geometries and to derive quartic and cubic expansions for the energy and one-electron properties in terms of internal displacement coordinates. Considerable improvement was found in the SD and particularly in the SDQ results relative to SCF, with the SDQ equilibrium geometry being in excellent agreement with experiment and the harmonic force constants also showing much better agreement than other ab initio results. Results for higher-order force constants are difficult to assess because of the large uncertainties in both theoretical and experimental results. The dipole moment function obtained in these calculations should be quite useful in the assessment and analysis of infrared intensity data and isotope effects.
The fragmentation of propane and deuteropropane molecular ions
Authors: Stockbauer Roger , Inghram Mark G.
Published: February 2003

Fragmentation of molecular ions as a function of internal energy has been studied for the propane molecules, C3H8, CH3CD2CH3, CD3CH2CD3, and C3D8 using threshold photoelectron–photoion coincidence mass spectrometry. Within the energy range covered (0–6 eV internal energy) the experimental fragmentation curves are compared with those calculated using the quasiequilibrium theory. The general features of the experimental curves are reproduced by the calculations but significant deviations from exact calculations are apparent. The experimental curves for the d2 and d6 propanes show that hydrogen scrambling processes occur and are of sufficient importance that they must be included in future calculations.
Heteronuclear dipolar modulated chemical shift spectra for geometrical information in polycrystalline solids
Authors: Stoll M. E. , Vega A. J. , Vaughan R. W.
Published: February 2003

A double resonance multiple pulse nuclear magnetic resonance experiment is proposed and demonstrated which allows the use of heteronuclear dipolar interactions to determine geometrical, orientational, and motional information in polycrystalline solids. 13C–1H double resonance experiments are performed in polycrystalline benzene and calcium formate to demonstrate the capabilities of the technique in its present form.
Magnetic properties of the nearly two-dimensional ferromagnets [C6H5(CH2)nNH3]2CuCl4 with n=1,2,3
Authors: Dupas A. , Le Dang K. , Renard J.-P. , Veillet P. , Daoud A. , Perret R.
Published: February 2003

Magnetic susceptibility and nuclear magnetic resonance of Cu and Cl nuclei have been studied in monocrystals of [C6H5(CH2)nNH3]2CuCl4 with n=1,2,3 below 4.2 K. These compounds are two-dimensional ferromagnets with a dominant XY-type anisotropy. For the compound with n=1, the Ising anisotropy is extremely small. The NMR measurements of the temperature and field dependence of the magnetization are consistent with a two-dimensional spin-wave model and allowed us to determine the value of the intralayer exchange integral.
Graph theory and the statistics and dynamics of polymer chains
Authors: Forsman W. C.
Published: February 2003

The random-flight model for flexible, branched polymer molecules can be considered as a directed graph of (n+1) points and n vectors with an associated (n+1) ×n matrix G. The matrix incorporating entropy spring effects in the Rouse approach of describing chain dynamics is shown to be given by A=GTG. Likewise, the matrix used in the Zimm approach is given by Z=GGT. Moments of the distribution of the radius of gyration and its orthogonal components are given by the eigenvalues of F= (n+1)A−1. The distributions of the radii can be computed from these same eigenvalues or by performing integrations utilizing properties of a matrix P=GT, where T is an (n+1) ×n matrix formulated to eliminate the coordinates of one of the n+1 masses (since there are only 3n degrees of freedom in a coordinate system with its origin at the center of mass). The contribution to the mean segment density about the center of mass ρi as a function of distance r of any mass i is given by ρi= (βi/π 1/2)exp(−βi2r2), where βi2=nβ02(Σn+1j=1Qi,j2)−1 and Qi,j are the elements of Q=P−1, and β0 is the parameter in the end-to-end distribution of a linear chain of n statistical segment. The total mean density of segments about the center of mass is thus given by the expression ρ=Σn+1i=1=ρi. All of the statistical and dynamic characteristics of the assembly of chains can thus be determined from its graph representation and are independent of how the elements of the graph are numbered.
Effects of electric fields on the ultrasonic attenuation in liquid crystals
Authors: Hatakeyama T. , Kagawa Y.
Published: February 2003

Effects of electric fields on the ultrasonic attenuation in nematic and cholesteric liquid crystals are investigated. When ultrasound is applied parallel to the electric field, the change of the attenuation in the nematic liquid crystal MBBA first decreases with the increase of the electric field intensity as it is small, then begins to increase above a certain intensity, and finally saturates. When the sound is applied perpendicular to the electric field, the change is in the opposite direction as MBBA is electrically negative. On the other hand, for the nematic mixture which is electrically positive, the change of the attenuation varies monotonously with the electric field intensity and reaches saturation. The phenomenon can be explained by the molecular alignment due to the electric field together with the ultrasonic scattering due to the density fluctuation in the liquid crystal caused by the ion injection. The attenuation characteristics of the cholesteric liquid crystals COC, CO, and cholesteric mixture are also measured, whose magnitude of the attenuation is less than a quarter of that of the nematic liquid crystals.
Prigogine–Defay ratio for systems with more than one order parameter
Authors: Gupta Prabhat K. , Moynihan Cornelius T.
Published: February 2003

The Prigogine–Defay ratio, Π≡ΔCp Δκ/TV (Δα)2, has been derived for systems whose states need to be specified in terms of a number of order parameters z by considering the conditions of tangency of the equilibrium free energy surface and the free energy surface for constant values of the order parameters. It is shown that Π?1. The equality applies for the case of a single order parameter or for the case of more than one order parameter which satisfies the constraint (∂V/∂zi)/ (∂S/∂zi) = (∂V/∂zk)/(∂S/∂zk). The inequality applies for the case of more than one order parameter which does not satisfy this constraint. The relevance of the Prigogine–Defay ratio to the kinetics of the structural relaxation in glass is discussed.
Vibronic coupling by two active modes: A simple, exactly solvable model for the Duschinsky effect
Authors: Gregory A. R. , Silbey R.
Published: February 2003

The properties of a simple model for the Duschinsky effect are investigated. The model consists of two electronic states coupled by two harmonic oscillators. Weak and strong adiabatic and nonadiabatic coupling limits are defined. If an alternative phase convention is adopted the two ways of approaching the strong nonadiabatic coupling limit lead paradoxically, in the one-oscillator case, to adiabatic Born–Oppenheimer wavefunctions that are exact eigenstates of the system. Expansion of the adiabatic coupling matrix element as a power series in the normal coordinates in the weak nonadiabatic coupling regime is not possible in the two-oscillator case unless a constraint is placed on the Duschinsky parameter.
Single crystal ESR of VO2+ in hexa-urea aluminium perchlorate
Authors: Sathyanarayan S. G. , Krishnan V. G. , Sastry G. Sivarama
Published: February 2003

Electron spin resonance studies have been made on VO2+ in hexa-urea aluminium perchlorate single crystals and VO2+ is found to take up substitutional Al3+ sites. The V–O bond takes six possible orientations in the lattice and each of these are at 45° to the crystallographic c axis. Molecular orbital coefficients have been evaluated using the spin Hamiltonian parameters and optical absorption data.
On the triplet energy transfer mechanism in isotopic mixed molecular crystals
Authors: Tudron Frank B. , Colson Steven D.
Published: February 2003

The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady state and pulsed excitation sources. It has been demonstrated for the first time that the excitation–tunneling transfer mechanism [H. Sternlicht, G. C. Nieman, and G. W. Robinson, J. Chem. Phys. 38, 1326 (1963)] predicts the correct dependence of the transfer rate on trap depth. Furthermore, it is shown that the Perrin approximation, which grossly simplifies the treatment of the experimental data, is a reasonable albeit not perfect approximation for this system. A large fraction of the energy transfer is completed during times much shorter than the benzene triplet state lifetime. This is consistent with the strong distance dependence of the transfer rate constant predicted by a tunneling mechanism. Using the Perrin approximation, it has been shown that the use of a linear chain tunneling mechanism predicts the excitation exchange integral to be of the correct magnitude, a conclusion that is also consistent with a strong distance dependence of the transfer rate constant. (Energy can be transferred through 2–3 intervening host molecules.) In this system there is no evidence for the existence of the intermolecular triplet energy relaxation mechanism recently shown to be operative in chemically mixed crystals [H. C. Brenner and C. A. Hutchison, Jr., Chem. Phys. 58, 1328 (1973)].
Absorption and emission spectra of matrix-isolated XeF, KrF, XeCl, and XeBr
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

Argon matrix samples of Kr/F2, Xe/F2, Xe/Cl2, and Xe/Br2 have been photolyzed at 20 K. Emission spectra using uv excitation and uv absorption spectra following photolysis are assigned to the KrF, XeF, XeCl, and XeBr molecules. Vibrational spacings on two KrF and two XeF absorption bands are near RbF and CsF values, which supports the ionic model for the excited states. The vibronic KrF and XeF absorptions, and two very sharp XeCl bands, indicate ground state potential function minima near the ionic excited state values. This suggests that chemical bonding effects are significant in ground state KrF, XeF, and XeCl.
Sequential impluse model of direct reactions
Authors: Mahan Bruce H. , Ruska W. E. W. , Winn John S.
Published: February 2003

A model in which the reaction A(BC, C) AB occurs as the result of a sequence of two hard sphere elastic impulses between the A–B and B–C pairs is analyzed. It is shown that the limits of the product velocity vector distribution can be readily obtained from the analytic geometry of the impulse sequence, and that an analytical expression for the detailed product distribution for all mass combinations and energies can be deduced. The results show a propensity for processes in which the velocity of the C atom is changed very little and which therefore lead to products near the spectator stripping velocity. The angular distribution is dependent on, but relatively insensitive to, the ratio of the mutual hard sphere diameter of B and C to their bond distance. The predictions of the model are generally consistent with the features found in the experimental investigations of high energy ion–molecule reactions.
Ion induced luminescence of alkali halides with CN impurity
Authors: Bazhin A. I. , Rausch E. O. , Thomas E. W.
Published: February 2003

During a study of luminescence induced by low energy (20 keV) light ion (H+, He+) impact on certain alkali halides (LiCl, NaBr, NaCl, KBr, KCl) we have observed a band spectrum in the region 230 to 350 nm. The bands are separated by 0.26 eV and have a half-width of 0.12 eV; no fine structure is observed. Identical bands of much increased intensity are observed when samples deliberately doped with CN− are bombarded. The phenomenon is not found with KBr crystals. We suggest that the observed bands are due to transitions in the CN radical, possibly the D 2Π1–X 2Σ+ transition that is observed as a very weak transition in discharges. The mechanism whereby CN is formed on the samples remains unclear.
Vibrational energy exchange between H2(v=1) and D2, N2, HCl, and CO2
Authors: Bott J. F.
Published: February 2003

The vibrational relaxation of H2(v=1) by D2, N2, HCl, and CO2 was studied at 295°K by a laser-induced fluorescence technique. HF added in small quantities to the gas mixtures and vibrationally excited by a pulsed HF TEA laser produced H2(v=1) by V–V transfer; HF also served as the tracer for H2(v=1). The results were interpreted to be primarily V–V rates; the values being (1.25±0.15) ×10−3, (3.14±0.4) ×10−4, (2.1±0.3) ×10−6, and (4.4±0.9) ×10−4(μsec Torr)−1 for H2(v=1) transfer to HCl, D2, N2, and CO2, respectively. The data for D2 and N2 were correlated with the V–V exchange probabilities for other diatom–diatom collisions by the parameter μ1/2ΔE, where μ is the reduced mass of the collision partners and ΔE the energy mismatch. This parameter also correlates V–V exchange probabilities for CO2–diatom collisions. A separate correlation of V–V exchange probabilities for hydrogen halide–homonuclear diatom collisions was obtained by plotting the probabilities versus ΔE/hν.
On representative orbital angular momentun quantum numbers in the definite parity j
z CCS approximation
Authors: Shimoni Yair , Kouri Donald J.
Published: February 2003

A detailed study of the role of representative orbital angular momentum quantum numbers ? (J,j) in a definite parity jz CCS approximation (DPjz) is presented. First the DPjz equations are derived by a procedure which establishes the validity for kjR≫Jj in place of kjR≫[(J+j)/2]2 in the original derivation of Kouri, Heil, and Shimoni. Then expressions for integral and differential cross sections within the DPjz approximation are derived. This analysis points up the importance of choice functions ? (J,j), which determine the representative orbital angular momentum quantum numbers associated with given values J,j. Computations for the He+H2 system are carried out for various choices of ? (J,j) and results are compared with exact close coupling (CC). Analyses of trends in the results suggest (a) the median choice ?=max(J,j) gives best results at the integral cross section level (if one disregards parity), (b) elastic cross sections for given j depend on the numerical value of ? and not on the l̄′ values occurring for other rotor states j′, (c) averages of both partial and integral cross section results with respect to choice function ? (j,J) lead to excellent agreement with CC results, (d) by noting (b) above, an alternative average over ? values can be achieved by relabeling amplitudes by ?,j rather than J,j, (e) this last average can be made using the results obtained with a single choice function.
Phase transitions in the Gaussian core system
Authors: Stillinger Frank H.
Published: February 2003

Some aspects of phase transition behavior have been studied for a classical system of particles which interact in pairs via repelling Gaussian potentials. In a specific low-temperature, low-density limit, this model acts as though it were composed of rigid spheres. Study of lattice sums suggests that at T=0, a fcc crystal stable at low density transforms under compression to a bcc crystal. It is shown that sufficient compression at any fixed T≳0 always produces a fluid phase. Under suitable temperature and density conditions, the model can exhibit positive, negative, or vanishing volumes of melting.
Cross sections for symmetric charge transfer reactions of NO+ in selected vibrational and translational energy states
Authors: Squires Larry , Baer Tomas
Published: February 2003

The cross sections for symmetric charge transfer reactions of NO+ in selected vibrational and translational energy states have been measured in a single chamber experiment using a combination of photoion–photoelectron coincidence and time of flight ion analysis. The ion vibrational states and translational energies investigated were v=0–5 and 1–60 eV, respectively. A test study of the Xe charge exchange reaction demonstrates the ability of this experiment to produce accurate absolute rates. In the symmetric charge transfer reaction of NO+(v), the influence of the ion vibrational state was found to be most predominant at low translational energies. The cross sections for the reactions of ions in the first three vibrational states tend to follow Franck–Condon factors, however at higher vibrational states additional reaction channels evidently compensate thus increasing the cross sections. The data are compared to recently reported theoretical cross sections using the multistate approximation. The theory overestimates the cross sections by a factor of approximately 2.0. Although the theory correctly predicts the relative rates of v=0, 1, and 2, it fails to account for the high cross sections of v=3 and 4 relative to those for the lower ion vibrational states.
ESR study of oxidation product in irradiated α-amino acids: Nitrogen centered π radical in α-glycine
Authors: Nunome Keichi , Muto Hachizo , Toriyama Kazumi , Iwasaki Machio
Published: February 2003

The structure of the oxidation product H2Ṅ+CH2CO2− in irradiated α-glycine has been studied based on the hyperfine coupling tensors which have been determined at 4.2°K by single crystal ESR measurements. It has been found that the radical initially possesses a nearly planar structure at the nitrogen atom with a framework similar to that of the undamaged molecule and then changes its conformation into the form previously found by Sinclair. The principal axes of the nitrogen coupling tensor suggest that the selective proton or hydrogen atom transfer from the NH3+ group takes place across the shortest hydrogen bond forming the radical.
Polarized Raman spectra and infrared reflection spectra of lithium potassium sulfate
Authors: Hiraishi Jiro , Taniguchi Nagahisa , Takahashi Hiroaki
Published: February 2003

Polarized Raman spectra and infrared reflection spectra of LiKSO4 are observed in the frequency region lower than 2000 cm−1. The frequencies of transverse and longitudinal modes for the internal and external vibrations are obtained directly from the polarized Raman spectra and from the classical dispersion analysis of the infrared reflection spectra. The assignments of the observed bands are discussed.
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o effective core potentials: Reduction of all-electron molecular structure calculations to calculations involving only valence electrons
Authors: Kahn Luis R. , Baybutt Paul , Truhlar Donald G.
Published: February 2003

A formalism is developed for obtaining ab initio effective core potentials from numerical Hartree–Fock wavefunctions and such potentials are presented for C, N, O, F, Cl, Fe, Br, and I. The effective core potentials enable one to eliminate the core electrons and the associated orthogonality constraints from electronic structure calculations on atoms and molecules. The effective core potentials are angular momentum dependent, basis set independent, and stable against variational collapse of their eigenfunctions to core functions. They are derived from neutral atom wavefunctions using a pseudo-orbital transformation which is motivated by considerations of the expected accuracy of their use and of basis set economy in molecular calculations. Then the accuracy is demonstrated by multiconfiguration Hartree–Fock calculations of potential energy curves for HF, HCl, HBr, HI, F2, Cl2, Br2, and I2 and one-electron properties for HF and HBr. The differences between valence-electron calculations employing the present effective core potentials and all-electron calculations are smaller than differences due to basis set choices, even though the basis sets are extended ones. Thus the effective core potentials are quite successful. In addition larger configuration mixing calculations are performed for HBr and Br2 (1637 and 3396 configurations, respectively) and again the effective core potentials are judged to perform well.
Pair correlation function of dense electrolyte system: Restricted primitive model
Authors: Mou Chung Yuan , Mazo Robert M.
Published: February 2003

The solution of the Ornstein–Zernike equation with core condition, and direct correlation function of Yukawa form is analyzed to give analytic expressions of pair correlation functions for dense electrolyte system. A method for seimempirical determination of Yukawa parameters is proposed. Agreement of the correlation functions with Monte Carlo calculations is good.
Saturable absorption and rotational relaxation in CO2
Authors: Hays G. N. , Fisk G. A.
Published: February 2003

Experimental data demonstrating saturated optical absorption of HF P2(4) laser radiation by CO2 is presented and analyzed in terms of a simple kinetic model. Within the framework considered, relatively slow rotational relaxation in the (00°0) vibrational level of CO2 is shown to be responsible for saturated absorption. The relation between the rotational relaxation rate constant appropriate to these experiments and that determined in thermal experiments is discussed.
Weyl’s theory applied to predissociation by rotation. II. Determination of resonances and complex eigenvalues: Application to HgH
Authors: Hehenberger Michael , Froelich Piotr , Brändas Erkki
Published: February 2003

A numerical scheme for analytic continuation of a complex function using a fit to a rational fraction is described. The procedure is applied to Weyl–Titchmarsh’s m function for the case of predissociation by rotation in mercury hydride. It is shown that analytic continuation of the full complex m function, as compared to treating its imaginary part only, is preferable for very broad and nonsymmetric resonance shapes. This situation is particularly pronounced for the close-lying interacting resonances E (3,9) and E (4,9) for the HgH system, as calculated from Stwalley’s IC1 potential.
Quantum mechanical reactive scattering for planar atom plus diatom systems. I. Theory
Authors: Kuppermann Aron , Schatz George C. , Baer M.
Published: February 2003

A method is presented for accurately solving the Schrödinger equation for the reactive collision of an atom with a diatomic molecule on a space-fixed plane. The procedure consists primarily of two steps. First, the Schrödinger equation in each of the three arrangement channel regions is transformed into a set of coupled differential equations and numerically integrated in each of these regions to generate primitive solutions. The rotational part of the vibration–rotation basis functions involved is not changed from its asymptotic form during this propagation, but the vibrational eigenfunctions as well as the integration variable are changed periodically so as to follow the vibrational motions in a nearly adiabatic manner. In the second step, the primitive solutions generated in each of the three arrangement channels are smoothly matched to each other on a set of appropriately chosen matching surfaces. The resulting solutions are then linearly combined to satisfy the proper asymptotic boundary conditions, and the scattering matrix, scattering amplitudes, and cross sections are determined. Application of this procedure to the special case of the H+H2 reaction is discussed in detail including simplifications arising from the additional symmetries involved, and the inclusion of effects resulting from indistinguishability of identical particles.
Quantum mechanical reactive scattering for planar atom plus H2 * diatom systems. II. Accurate cross sections for H+H2 *
Authors: Schatz George C. , Kuppermann Aron
Published: February 2003

The results of an accurate quantum mechanical treatment of the planar H+H2 exchange reaction on a realistic potential energy surface are presented. Full vibration–rotation convergence was achieved in the calculations, and this, together with a large number of auxiliary convergence and invariance tests, indicates that the cross sections are accurate to 5% or better. The reactive differential cross sections are always backward peaked over the range of total energies from 0.3 to 0.65 eV. Nonreactive j=0 to j′=2 cross sections are backward peaked at low energy (0.4 eV) shifting to sidewards peaking for E≳0.5 eV. Quantum symmetry interference oscillations are very significant in the j=0 to j′=2 para-to-para cross sections for E?0.6 eV. Reactive integral cross sections show two distinct kinds of energy dependence. At low energy (<0.5 eV), barrier tunneling gives them a largely exponential energy dependence while above 0.5 eV (the effective threshold energy) the cross sections vary nearly linearly. Comparison of collinear and coplanar transition probabilities indicates similar 1D and 2D energy dependence but with a shift in energy from 1D to 2D due to bending motions in the transition state. An analysis of rotational distributions indicates surprisingly good correspondence with temperaturelike distributions. The results of a one-vibration-approximation calculation are examined, and errors of as much as three orders of magnitude are found at some energies. Shapes of angular distributions are, however, accurately predicted by this approximate method. Additional analyses include comparisons with previous distorted wave and coupled-channel results, and calculations of thermal rate constants.