bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffraction pattern and structure of amorphous solid water at 10 and 77 °K
Authors: Narten A. H. , Venkatesh C. G. , Rice S. A.
Published: February 2003

X-ray diffraction data show that there are at least two forms of amorphous solid water which differ in density and second nearest-neighbor oxygen–oxygen distribution. (a) The lower density form, made at 77 °K, has a diffraction pattern consistent with a structure that has oxygen–oxygen nearest-neighbor tetrahedral symmetry on average, and a nearest neighbor O–O separation of 2.76 Å with small dispersion. The density of this material is estimated to be 0.94 g cm−3. While it is not possible to uniquely define the structure, the data available support the notion that its fundamental characteristic is the existence of a randomized network of hydrogen bonds with O–O–O angular distribution derived from (i.e., centered about) that of ice Ih. Comparison of neutron diffraction and x-ray diffraction data suggests strongly that the first shell hydrogen bonds are nearly linear and that orientational correlations between water molecules are limited to nearest neighbors. (b) The higher density form, made at 10 °K, has a diffraction pattern similar to, yet distinctively different from, that of the high temperature deposit. The O–O nearest neighbor distance is the same, 2.76 Å, but the dispersion in this separation is larger in the low temperature form. The diffraction pattern shows an extra peak at 3.3 Å, corresponding to about 1.4 molecules, the existence of which is responsible for the estimated higher density, namely 1.1 g cm−3. The data are consistent with several models which share the feature of introducing small O–O–O angles into the structure. We discuss the relationships between our data, and inferences from the data, and the corresponding data for liquid water.
Dissociative excitation of H2: Spectral line shapes and electron impact cross sections of the Balmer lines
Authors: Freund Robert S. , Schiavone James A. , Brader Donna F.
Published: February 2003

The Balmer lines of H, when produced by electron impact dissociative ionization of low pressure H2, are much broader than the Doppler width of thermal H atoms. Indeed, they each have (a) a central peak and (b) broad wings, which result from two different groups of dissociative states. Excitation cross sections show structure as a function of electron energy. This helps to identify the states as follows; Predissociated and directly dissociated Rydberg states produce the central peaks and doubly excited states produce the wings.
The reaction of CO2 with active nitrogen
Authors: Rawlins W. T. , Kaufman F.
Published: February 2003

Atomic oxygen produced by the reaction of CO2 with active nitrogen has been measured by resonance absorption in a discharge-flow system at 300 °K. The initial atomic oxygen production rate is found to be second order in atomic nitrogen concentration, and the effective termolecular rate constant for the process varies inversely with CO2 concentration. The results are analyzed and discussed in terms of a mechanism in which CO2 is dissociated by energetic species of N2 formed by N recombination, possibly N2(A 3Σ+u) in high vibrational states.
Electronic transition moment of the N+2(A 2Πu–X 2Σ+g) system
Authors: Wu H. H. , Shemansky D. E.
Published: February 2003

The variation of the electronic transition moment of the N+2 A 2Πu–X 2Σ+g system has been estimated using measured optical emission rates. The variation is nonlinear and may be described by the equation Re(r̄)/Re(1.0) =33.3[1.00−1.80r̄ +0.83r̄2], 1.02 Å⩽r̄⩽1.14 Å. An accurate transition probability table cannot be established due to lack of emission rate data in the near infrared. The variation of Re(r̄) cannot be obtained from lifetime measurements of the A 2Πu state.
Absolute rate constant for the reaction of atomic hydrogen with acetylene over an extended pressure and temperature range
Authors: 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 absolute rate parameters for the reaction of atomic hydrogen with acetylene, i.e., H+C2H2?C2H3* (1); C2H3*+M→C2H3+M (2). The rate constant for the reaction is strongly pressure dependent and was measured over the pressure range 10 to 700 torr. The reaction was also studied as a function of temperature over the range 193 to 400 °K and the high pressure limit of the rate constant at each temperature was used to obtain the Arrhenius expression k1= (9.63±0.60) ×10−12 exp(−2430±30/1.987T) cm3 molecule−1⋅sec−1. The present results are compared with those of previous studies.
Resonance Raman spectra of manganese (III) etioporphyrin I
Authors: Shelnutt John A. , O’Shea Donald C. , Yu Nai-Teng , Cheung L. D. , Felton Ronald H.
Published: February 2003

Resonance Raman spectra of manganese (III) etioporphyrin I complexes have been obtained with laser excitation in the regions of the bands III and IV, and bands V and Va of the atypical visible absorption spectra of these metalloporphyrins. Excitation profiles in the latter region are interpreted as indicating nonadiabatic coupling between V and Va as well as vibronic activity of some totally symmetric modes. The Raman data support assignment of bands III and IV to the 0–0 and 0–1 components of the Q band. Although bands V and Va are assigned to a charge transfer transition mixed with the main π→π* transition, no selective enhancement of low frequency metal–ligand vibrations attributable to the charge transfer contribution of band V was observed.
Chemical reaction theory for asymmetric atom–molecule collisions
Authors: Walker R. B. , Light J. C. , Altenberger-Siczek A.
Published: February 2003

A theoretical framework for describing the quantum dynamics of an atom–diatom system (A+BC) in three physical dimensions is presented. This theory explicitly treats the case where A, B, and C are each distinct, and hence no simplifications arise from the symmetry of a homonuclear diatomic. A natural collision coordinate system, depending primarily on the masses A, B, and C and the asymptotic potential energy surface is engineered appropriate to the system yielding a tractable expression for the kinetic energy operator. Close coupled equations for both free and hindered rotor expansions of the wavefunction are derived and the matching procedure and boundary conditions required to obtain the full scattering matrix (S matrix) are given.
Covalency effects on momentum distributions in compounds: Positron annihilation in Fe3O4
Authors: Chiba Toshinobu
Published: February 2003

A simple method based on the LCAO–MO scheme is presented for the calculation of the momentum distribution in compounds for positron annihilation or Compton scattering. The method is applied to analyze the covalency structrue from the spin-density distribution in Fe3O4 measured by Mijnarends and Singru. The degree of covalency is found to be fσ2=10%, fs2=0.3%, and fπ2=4% for the antibonding orbitals of Fe2+ and Fe3+ in octahedral sites.
A contribution to the thermodynamics of the interstitial solid solution of hydrogen in metals
Authors: Hashino T. , Naito S.
Published: February 2003

A basic physical model has been proposed for the statistical thermodynamics of the interstitial solid solution of hydrogen in metals where the behavior of not only the solute hydrogen but also of the parent metal is considered. The solid phase was represented as a superposition of the metal and hydrogen assemblies. The interaction energy between both the assemblies, which is partitioned into each assembly in an amount just large enough to cancel each other according to the rule of action and reaction, was expressed as a function of the hydrogen content with the aid of the theory of elasticity. The absolute activity for each component was derived from the respective grand partition function. The energy part in the state equation obtained was given in a quadratic expression with respect to the hydrogen content, unlike a linear expression in Lacher’s equation. It has been established that the energy associated with the parent lattice expansion is sufficient to cause the occurrence of two solid phases which coexist in a region of the hydrogen content, as suggested by various investigators. The critical condition, and the relation between the parent lattice expansion coefficient and the interaction energy among nearest-neighbor hydrogen atoms were determined. For simplicity, Hooke’s equation was used to calculate the strain energy, and the results obtained are subject to the limitations of validity of this constitutive equation.
Thermodynamic properties of the system polystyrene-t
r
a
n
s-decalin
Authors: Nakata M. , Higashida S. , Kuwahara N. , Saeki S. , Kaneko M.
Published: February 2003

Osmotic pressure measurements were performed on solutions of polystyrene in trans-decaline in the temperature range from 20° to 90 °C and in concentrations of segment fraction φ2 ranging from 0.06 to 0.26. The cloud-point curves for this system were observed for samples of molecular weights from 3.7 to 270×104. Values of the interaction parameter χ obtained from osmotic pressure data were compared with those calculated from the new Flory theory for liquid mixtures. The behavior of χ was well represented by the simple relation: χ= (127.1+72.6φ2+48.7φ22)/T +(2.32×10−4+2.07×10−4φ2) T, where the terms proportional to temperature and to reciprocal temperature were found to be ascribed mainly to the difference of the liquid properties of the respective components and the exchange energy, respectively. Behaviors of the cloud-point curves were well explained by the above expression for χ.
The reaction of nitric oxide with vibrationally excited ozone. II
Authors: Gordon Robert J. , Lin M. C.
Published: February 2003

The reaction of nitric oxide and vibrationally excited ozone was studied using a CO2 TEA laser to excite O3(001) and a photomultiplier tube to measure the decay of the NO2* chemiluminescence. In experiments with <1% O3, a single, rapid rise rate was observed, suggesting that for ozone lean mixtures the laser-induced signal is caused primarily by the two stretching modes of ozone, with comparatively little V–V transfer to the bending mode. The linear dependence of the NO2* peak height on the laser intensity and background reaction indicates that second order effects do not contribute appreciably. Using a simple model which includes only contributions from the stretching modes and the ground state, the rate constants k3a(308 °K) =4.3±0.7×109 and k3b(308 °K) =5.4±0.7×1010 cm3 mole−1⋅sec−1 were obtained for the laser-enhanced luminescent and nonluminescent reactions, respectively. In addition the rate constants for V–T relaxation of O3(001) by nine buffer gases were measured. It was found that the laser enhancement of the luminescent reaction is less than the thermal enhancement that would be obtained if one quantum of energy (3.0 kcal/mole) were distributed statistically among all reagent degrees of freedom, while for the nonluminescent reaction the laser and thermal enhancements are about equal.
The spin–rotation interaction in the van der Waals molecule KAr
Authors: Freeman Richard R. , Mattison Edward M. , Pritchard David E. , Kleppner Daniel
Published: February 2003

We report a study, by molecular beam methods, of the alkali–rare gas van der Waals molecule KAr. Paramagnetic potassium–argon molecules were produced in a supersonic molecular beam, and their magnetic hyperfine structure probed by resonance techniques. The principle interaction studied is the spin–rotational coupling of the spin angular momentum h/S of the alkali valence electron and the rotational angular momentum h/N of the molecule. The measured value of the interaction constant γ in the (v=0, N=0) molecular state is 240(10) kHz. Limits have been set on the variation of γ with R, the internuclear separation, over the range R=9 to 13 bohr.
Relaxation during photoemission and LMM Auger decay in arsenic and some of its compounds
Authors: Bahl M. K. , Woodall R. O. , Watson R. L. , Irgolic K. J.
Published: February 2003

The energies of the 2p1/2, 2p3/2, 3s, 3p1/2, 3p3/2, and 3d3/2,5/2 photopeaks and of the L2,3M4,5M4,5 Auger peaks have been measured for arsenic metal and 19 of its compounds. By comparing the photopeak and Auger chemical shifts, the changes in the total (static) relaxation energies were determined. It was found that the relaxation energy change provides a useful parameter for characterizing nearest neighbor groups and that electronegativities derived from them offer the advantages that they are independent of the energy reference level and of charging effects.
Ferroelectric mode in ammonium sulfate
Authors: Nordland T. J. , O’Reilly D. E. , Peterson E. M.
Published: February 2003

Proton spin–lattice (T1), rotating frame (T1ρ), and dipolar (T1D) relaxation times are reported in (NH4)2SO4 from 200 to 300 °K. At 236 °K T1ρ is measurably shorter than T1 and T1D? (1/3) T1. These results are in agreement with the results recently reported by Knispel et al. T1 is dependent on frequency at 298 °K in the range 13 to 60 MHz. These effects are interpreted in terms of a ’’ferroelectric mode’’ process that was proposed earlier, and flip times τf are computed that are associated with the two angles of tilt of the distorted NH4+ tetrahedra from the ab mirror plane in the paraelectric phase. At 298 °K, τf=6.7×10−9 sec.
Theory of photochemical isomerization in polyenes
Authors: Kushick Joseph N. , Rice Stuart A.
Published: February 2003

In this paper we investigate the nonradiative transition rate from excited states of molecules which have several internal torsional degrees of freedom. A model is developed for the description of cis–trans photoisomerization in solutions of polyenes. Using as an example potential energy surfaces calculated for butadiene, we assess the effects introduced by dynamical coupling between torsional degrees of freedom. We find that because of the excited state geometry, an electronic excitation will result in no more than one cis–trans isomerization. In addition, the distribution of electronic energy in conjugated polyenes is expected to be quite different from that predicted for molecules with one or several uncoupled torsions. In contrast to the latter cases, the torsional modes of conjugated polyenes are found to be poor acceptors of electronic energy. This energy is thus made available to other vibrational modes, providing a possible mechanism for hydrogen transfer reactions.
Theoretical investigations of excited states of glyoxal and biacetyl
Authors: Leclercq J. M. , Mijoule C. , Yvan P.
Published: February 2003

Excited singlet and triplet states of glyoxal and biacetyl are described by configuration interaction from SCF calculations for the ground state in the CNDO/S approximation. Open-shell SCF calculations have been carried out for the lower triplet states of each compound and completed by a treatment of the electronic correlation using a second-order perturbation calculation which has enabled us to locate the lowest 3Bg state in agreement with recent experimental assignments. The radiative lifetime of the metastable 3Au state and the oscillator strength of the phosphorescence transitions have been calculated by means of a one-center monoelectronic approximation for the spin–orbit Hamiltonian. The spin dipole–dipole and second-order spin–orbit contributions to the zero-field splitting parameters D and E of the metastable 3Au state have been computed and discussed.
Rate constants for the reaction of OH radicals with CHFCl2 and CH3Cl over the temperature range 298–423 °K, and with CH2Cl2 at 298 °K
Authors: Perry R. A. , Atkinson R. , Pitts J. N.
Published: February 2003

Rate constants for the reaction of OH radicals with CHFCl2 and CH3Cl have been determined over the temperature range 298–423 °K using a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained were k1(CHFCl2) =1.75×10−12e−(2490±300)/RT cm 3 molecule−1 sec−1, k1(CH3Cl) =4.1×10−12 e−(2700±300)/RT cm3 molecule−1 sec−1. In addition, the rate constant for the reaction of OH radicals with CH2Cl2 was determined to be k1(CH2Cl2) = (1.45±0.20) ×10−13 cm3 molecule−1 sec−1 at 298.5 °K. These rate constants are in good agreement with literature values.
van der Waals coefficients through C8 for atom–linear molecule interactions. I. CO2–noble gas systems
Authors: Pack Russell T
Published: February 2003

Exact formulas for all the terms in the van der Waals C7 and C8 coeffcients for the interaction of an S-state atom with a Σ-state linear molecule are derived. The angle dependence is presented in convenient Legendre polynominal expansion form. Procedures for the calculation or estimation of the coefficients are proposed, discussed, and applied to CO2–noble gas atom interactions. Simple formulas are found to give good bounds for the angle-dependent part of C6. Although the uncertainties in the estimates of the higher coefficients are large, using them instead of C6 alone improves the long range potential considerably.
Dissipation and fluctuations in nonequilibrium thermodynamics
Authors: Keizer Joel
Published: February 2003

A relationship between dissipation and fluctuations is described which leads to a unified theory of irreversible processes far from equilibrium. The development is based on the principle that dissipation and fluctuations are caused by elementary molecular processes. This permits the formulation of a canonical form for the rate of dissipation of the extensive variables. The canonical form depends on the thermodymanic quantities which are conjugate to the extensive variables, and it is shown that the canonical form leads to the customary transport equations for a variety of linear and nonlinear relaxation processes. Because fluctuations are also caused by molecular events, this formulation of dissipation can be used to examine deviations from the average. The theory associates a nonstationary, Markov stochastic process with fluctuations away from the conditionally averaged extensive variables. This description of nonequilibrium thermodynamics does not require the entropy to be introduced, and for rate processes close to equilibrium, but away from a critical point, reduces to the usual linear theory.
Estimate of the intermolecular polarization contribution to the static dielectric constant of water
Authors: Kress James W. , Kozak John J.
Published: February 2003

In this paper, molecular-structure data derived from recent quantum-chemical studies are used in conjunction with the Mandel–Mazur theory of the static dielectric constant to estimate the importance of intermolecular polarization contributions to the static dielectric constant of water. A scalar expression for the static dielectric constant is identified and then investigated via a multibody decomposition in which the effects of 2-, 3-, ... n-body interactions are displayed and then analyzed numerically. Our main conclusion is that the intermolecular polarization contribution is of crucial importance in determining the static dielectric constant of water. Further, our calculations show that the predicted dielectric constant is extraordinarily sensitive to the geometry of the first hydration shell; thus, the use of a slightly inaccurate potential function may lead to a predicted dielectric constant in significant disagreement with the experimental value.
A comparison between different approaches to the vibronic theory of Raman intensities
Authors: Garozzo M. , Galluzzi F.
Published: February 2003

The conventional vibronic Raman theory of Albrecht and the full quantum approach of Peticolas and co-workers are compared and the physical conditions for their equivalence are discussed. It is found that Peticolas’ theory takes into account also the nonadiabatic coupling and that, under proper assumptions, it is equivalent to the newer formulation of Mingardi and Siebrand.
Electronic-energy exchange cross sections for Ar* (3P) and Kr(1S)
Authors: Winicur Daniel H. , Fraites James L. , Bentley John
Published: February 2003

Differential cross sections for Ar* scattered by Kr are obtained at four relative kinetic energies, 60.6–156 meV. Location and shape of rainbow maxima, observed at all the energies, are used to determine parameters for a double-Lennard-Jones potential. The parameters, ε=9.11±0.25 meV and rm=5.06±0.84 Å, are the same as those for K+Kr and are substantially different from those of Ar+Kr. A second set of maxima, visible at all energies and caused by exchange of electronic energy from Ar* to Kr, are analyzed to determine electronic-energy exchange cross sections. The cross sections together with those obtained from thermal-energy quenching measurements are consistent with an increase from a threshold of 25 meV. Possible mechanisms for the electronic-energy exchange are discussed.
Direct measurements of transition dipole matrix elements using optical nutation
Authors: Shoemaker R. L. , Van Stryland E. W.
Published: February 2003

We describe a method of directly determining transition dipole matrix elements which we have used to make the first measurements of this quantity in an asymmetric rotor. A low pressure gas (a few mtorr NH2D) which is subjected to a cw CO2 laser beam is suddenly Stark switched into resonance. An optical nutation signal is detected whose frequency yields -E0/h/, where - is the transition dipole matrix element of interest and E0 is the optical field strength which may be determined from power and beam profile measurements. Thus an observation of the nutation frequency along with the power measurements yields -. This was done for two transitions of NH2D giving transition dipole matrix elements along two axes in the molecule: 〈ν2=0a‖ (∂μa/∂Q) Q‖ν2=1a〉=0.112±0.009 D and 〈0a‖ (∂μc/∂Q) Q‖1s〉=0.158±0.009 D. The extension of this technique to nonpolar molecules and its advantages over conventional steady state laser methods are discussed.
Dielectric and pressure virial coefficients of imperfect gases. VI. Analysis of results for CF3H and CFH3
Authors: Copeland T. G. , Cole R. H.
Published: February 2003

The dielectric and pressure second virial coefficients of CF3H and CFH3 as measured by Sutter and Cole are reproduced to 5% by a model of pair interactions taking account of central forces, electrostatic interactions, and shape effects. Lennard-Jones 12–6 parameters from viscosity data of Casparian and Cole are used. To secure agreement, effects of molecular quadrupole moments of 4.5 and 2.3×10−26 esu and Buckingham and Pople shape factors −0.10 and +0.25 for CF3H and CFH3 are included. The latter are of proper sign and agree reasonably well with estimates from molecular models by Barnes and Sutton.
Hermiticity of the superoperator Hamiltonian in propagator theory
Authors: Nehrkorn Christoph , Purvis George D. , Öhrn Yngve
Published: February 2003

Spurious terms are shown to appear in finite order perturbation theory treatment of the electron propagator in the so-called equation of motion approach. These terms, which cause the superoperator Hamiltonian not to be Hermitian, are canceled when the reference state of the propagator is improved to the next higher order in Rayleigh–Schrödinger perturbation theory.
Adsorption on heterogeneous surfaces. I. Evaluation of the energy distribution function via the Wiener and Hopf method
Authors: Landman Uzi , Montroll E. W.
Published: February 2003

A new method of evaluating the energy distribution function of sites participating in an adsorption process is presented. The method is based on the Wiener–Hopf technique for the solution of integral equations. The method is demonstrated through a derivation of the distribution functions for the generalized Freundlich and Dubinin–Radushkević isotherms, and a new distribution function relating the local Langmuir isotherm to the Javanović isotherm is derived. A prescription for the analysis of experimental data is discussed.
Thermodynamic properties of solid sulfuryl fluoride
Authors: Mountfield K. R. , Weir R. D.
Published: February 2003

The heat capacity of solid sulfuryl fluoride, SO2F2, has been measured over the range 2<T<21 K and in the region of melting with an estimated precision varying between ±1% at the higher temperatures and ±2% at T<5 K. The results agree with those of earlier measurements available above 12 K to within ±1%. However, below 12 K, they significantly affect the values of the residual entropy of SO2F2 based on an extrapolation carried out by Bockhoff et al. [F. J. Bockhoff, R. V. Petrella, and E. L. Pace, J. Chem. Phys. 32, 799 (1960); F. J. Bockhoff and E. L. Pace, J. Chem. Phys. 36, 3502 (1962)]. Analysis of the data indicates that a Schottky anomaly is not present down to 2 K but between 3<T<8 K there is an important contribution to the heat capacity besides that of the lattice vibrations. ΘD(T) curves have been constructed based on different sizes of vibrational unit which allows the coarse features of the lattice frequency spectrum to be determined. The main conclusion reached is that the three components of the spectrum due to translational and rotational vibrations of the SO2F2 molecule and to the intramolecular vibrations are separated. The contribution of the librational modes to the heat capacity can be approximated by a single frequency of 80 cm−1.
State-to-state reaction rates: Ba+HF(v=0,1) → BaF(v=0–12)+H
Authors: Pruett J. Gary , Zare Richard N.
Published: February 2003

The Ba+HF→BaF+H reaction has been studied under single-collision conditions as a function of the reagent and product vibration in order to obtain state-to-state reaction rates. Using a beam + gas arrangement, the HF is pumped by a pulsed HF laser and the BaF product distribution is determined from its laser-induced excitation spectrum. The BaF product is found to retain an average of 64% of the initial reactant vibrational excitation, and the distribution of product states for Ba+HF(v=1) has a broad maximum shifted to v=6 from the value v=1 for Ba+HF(v=0). The implications of these results on various proposed reaction surfaces are discussed.
Fluorescence correlation spectroscopy as a probe of molecular dynamics
Authors: Aragón S. R. , Pecora R.
Published: February 2003

A general theory of fluorescence correlation spectroscopy (FCS), including the effects of translational and rotational motions and chemical reactions, in ideal solutions, is given. The development is carried out explicitly for symmetric rotors of parallel transition moments and generalized to arbitrary configurations of the moments and to asymmetric rotors in the appendices. In addition, the effects of experimental geometry on the measured FCS correlation function are discussed in detail and the optimum geometries, for experiments seeking to measure different quantities, are given. We conclude with a brief discussion of signal-to-noise ratios and prospects for applications.
Molecular beam studies of methyl radical reactions with halogen molecules: Product angular and velocity distributions
Authors: Logan Jennifer A. , Mims Charles A. , Stewart Gerald W. , Ross John
Published: February 2003

Angular and velocity distributions for the products CH3I from CH3+I2, IBr, and ICl and CH3Br from CH3+Br2 have been measured in a crossed molecular beam apparatus. A thermal (∼1000 °K) methyl beam was obtained by pyrolysis and the halogen beam from a supersonic nozzle source. Center of mass cross sections were derived by applying deconvolution procedures, where necessary, to correct for the resolution of the time-of-flight velocity analysis system and the thermal velocity distributions of the reactant beams. In all four reactions the methyl halide product recoils backward with respect to the incoming methyl radical. Although all reactions release similar fractions (∼0.3) of the total energy into translational motion, the energy distributions of the products from the Br2 and I2 reactions indicate product repulsion, whereas those from the mixed halogen reactions indicate more statistical behavior, as shown by a suprisal analysis.
Brillouin scattering in single crystals of β-nitrogen: Determination of the elastic constants in the triple point
Authors: Kiefte H. , Clouter M. J.
Published: February 2003

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants of oriented single crystals of β-nitrogen at temperatures near the triple point (63.15 K). The values obtained were c11=1.8(25), c12=1.1(31), c44=0.32(0), c33=1.9(76), and c13=0.9(80) (in units of 1010 dyn cm−2) with an estimated uncertainty of ±2%. These results indicate that crystals of β-nitrogen are close to being elastically isotropic.
Thermal decomposition of potassium azide
Authors: de Panafieu A. , Royce B. S. H. , Russell T.
Published: February 2003

Experimental studies of the thermal decomposition of single crystal and powder samples of KN3 in an ultrahigh vacuum environment are reported. Mass spectrographic detection was used to monitor the decomposition products and, from the temperature dependence of the N2 partial pressure, three regions of decomposition were found with associated activation energies of 1.0±0.1, 1.35±0.05, and 2.4±0.2 eV in order of increasing temperature. Azide radicals and potassium atoms were also detected in the decomposition products. The application of an electric field across the sample during decomposition was found to increase the decomposition rate for both signs of the voltage on the free surface of the crystal. The relaxation time for the pressure change associated with application of the field was found to be essentially the same as that for the decay of the induced ionic current to an equilibrium value. Models for the decomposition process are discussed.
Activation energy and mass dependence of the coefficient of self-diffusion in liquids
Authors: O’Reilly D. E.
Published: February 2003

An expression for the activation energy entering into the quasilattice model of self-diffusion in liquids is derived. The explicit mass dependence of the coefficient of self-diffusion is given and compared with experiment for liquid argon and benzene.
Mössbauer spectra and magnetic susceptibility of neptunium tetrabromide
Authors: Stevens J. L. , Jones E. R. , Karraker D. G.
Published: February 2003

Magnetic properties of NpBr4 were studied from 4.2 to 20.0 °K with the Mössbauer resonance of the 59.54-keV γ ray in 237Np. Magnetic susceptibility was measured from 2.2 to 70.5 °K using a vibrating sample magnetometer at an applied field of 10 KOe. An antiferromagnetic transition was found at 12.5±0.5 °K. Above the Néel temperature, NpBr4 obeys the Curie–Weiss law, χ=C/(T+ϑ), with ϑ equal to 16.2 °K and an effective moment of 2.60 μB. The Mössbauer spectra exhibited magnetic hyperfine splitting over the temperature range 4.2 to 20 °K with g0μnHeff=10.4 cm/sec at 4.2 °K. A small decline in g0μnHeff was observed for temperatures above 12 °K.
Oscillator strengths for transitions involving excited states not lowest of their symmetry oxygen I and oxygen II transitions
Authors: Luken William L. , Sinanoğlu Oktay
Published: February 2003

Theoretical f values and transition rates we calculate using the nonclosed shell many electron theory (NCMET) of Sinanoğlu for the OI 792 Å transition and the OII 538 and 581 Å transitions are reported and compared to experiment. Good agreement is obtained resolving the discrepancies which existed in previous literature. Results are also reported for the OI 811 and 878 Å transitions and the OII 617 and 673 Å transitions.
Vibration–translation energy transfer in anharmonic diatomic molecules. II. The vibrational quantum-number dependence
Authors: McKenzie Robert L.
Published: February 2003

A semiclassical model of the inelastic collision between a vibrationally excited anharmonic oscillator and a structureless atom is used to predict the variation of thermally averaged V–T rate coefficients with temperature and initial-state quantum number. Multiple oscillator states are included in a numerical solution for collinear encounters. The results are compared with CO–He experimental values for both ground and excited initial states using several simplified forms of the interaction potential. The numerical model is also used as a basis for evaluating several less complete but analytic models. Two computationally simple analytic approximations are found that successfully reproduce the numercial rate coefficients for a wide range of molecular properties and collision partners. Their limitations are also identified. Finally, the relative rates of multiple-quantum transitions from excited states are evaluated for several molecular types.
Equilibria among point defects in GdF3–doped CaF2
Authors: Franklin Alan D.
Published: February 2003

EPR and dielectric relaxation measurements, reported earlier, have been combined to produce estimates of the mole fraction (n′) of pairs formed of substitutional Gd3+ ions and F− interstitials as a function of total GdF3 mole fraction (M) and temperature at which the crystals are annealed prior to rapid cooling. Over the range of M from 1×10−4 to 27×10−4, n′ rises to a saturation value when M increases to about 10−3. At constant M, n′ goes through a maximum at anneal temperatures from about 1073 to 1273 K. The rate of the annealing reaction increases with M and with decreasing sample size, and the density increases with increasing M faster than the interstitial model predicts. A qualitative model of the annealing process is suggested, in which F− interstitials react with an atmospheric constituent (probably HF) to form an immobile complex that provides remote charge compensation for the cubic Gd3+ ions. The remaining free F− interstitials are trapped by Gd3+ ions during quenching to form the pairs.
Nuclear spin–lattice relaxation of coupled spin systems in liquids
Authors: Wang C. H. , Grant David M.
Published: February 2003

A new method based upon the Zwanzig–Mori projection operator technique has been introduced to treat the spin–lattice relaxation problem of coupled nuclear spin systems in liquids, with emphasis on high resolution NMR spectroscopy. The important quantities required for the use of the method are a set of orthogonal operators which can readily be constructed and given physical meaning in the eigenstate representation for any particular spin system. Some of the expectation values of these orthogonal operators may be related to the measurable quantity in high resolution NMR experiments. The time rate of change of each observable may then be formulated in terms of the trace of various operator functions, allowing the calculation to be carried out in any representation. The computational advantage of using simple spin-product functions is obvious. The prescription for constructing irreducible orthogonal tensor operators is given, and a variety of such tensor operators are explicitly given for several common spin systems. In the present method each orthogonal operator has a definite symmetry property under spin inversion which can be used to separate the coupled equations into a symmetric and an antisymmetric set. The problem of multiexponential decay in the time evaluation of the longitudinal magnetization, owing to effects of interferences between pairwise dipolar interactions, becomes clear in the present formalism. The present paper is a generalization of earlier work on the relaxation behavior of multiplet spectral structure in high resolution NMR.
The spectrum of DO2 near 60 GHz and the structure of the hydroperoxyl radical
Authors: Beers Yardley , Howard Carleton J.
Published: February 2003

Spectra for the lowest rotational transition in DO2, N=1←0, have been observed near 60 GHz. These data have been used to calculate the sum of the rotational constants B+C for DO2=60 467.7±3 MHz. This result was combined with other measurements on HO2 to compute a structure for the hydroperoxyl radical: r (H–O) =0.977×10−10 m, r (O–O) =1.335×10−10 m, and <H–O–O=104.1°. Also we have found for DO2 that (εbb+εcc)/2=−191±3 MHz for elements of the electron–spin rotation tensor and a value of −4.2±1.5 MHz for the Fermi contact interaction parameter σ.
Statistical properties of scattered field from nematic liquid crystals in dynamic scattering mode
Authors: Yamaguchi T.
Published: February 2003

Statistical properties of scattered field from nematic liquid crystals in the dynamic scattering mode were studied by using the intensity correlation technique. Experimental results showed that an increase of the external electric field gives rise to a decrease of the correlation time, and the shape of the correlation function changes from an exponential shape to a Gaussian shape. This phenomenon is discussed by developing the theory of light scattering that takes into account a fluctuation of the dielectric tensor. It was found that the phenomenon can be interpreted due to the statistical effect of the simple stochastic process, i.e., the motion of the principal axis of the dielectric tensor being randomly modulated.
The enumeration of Goldstone diagrams in many-body perturbation theory
Authors: Rossky P. J. , Karplus Martin
Published: February 2003

By use of basic results of diagrammatic perturbation theory and combinatorial arguments, a formula is derived for the number of topologically distinct, connected, time ordered Goldstone diagrams contributing to the nth order perturbation energy. A general perturbation consisting of one- and two-particle operators is considered. The result is extended to an effective one-particle operator that implicitly includes all equal-time contractions and to multiple one-particle perturbations that enter only a restricted number of times in any order; the former is important for a Hartree–Fock zero order function and the latter for a system with external or internal field perturbations. It is pointed out that the method for computing the number of disconnected diagrams is quite general and independent of the nature of the perturbation and that the connected topologies which possess unusual symmetry (i.e., those for which all sets of vertex space–time labels are not distinct) are of a very specific and easily identified type. Numerical results for the various cases considered are given through 5th order.
Theory of collision induced intersystem crossing
Authors: Freed Karl F.
Published: February 2003

A theory is presented of the pressure dependence of collision induced intersystem crossing. A density matrix formulation is employed to retain all information about the initial and final states in collisions. Within the impact approximation the rates are expressed in terms of thermally averaged products of S-matrix elements. The general theory reduces to that of the ’’mixe-state’’ model only at very low pressures and in small molecules when Born collision cross sections are applicable. The general theory demonstrates the important influence of the rotational sublevels in the collision induced crossing rates, in collisional quantum interference effects, and in the nature of the phenomena at higher pressure. A discussion is also presented of the role of any intermolecular spin–orbit coupling.
Deactivation of CO2(010) and CO2(001) by hydrogen and deuterium
Authors: Stricker Josef
Published: February 2003

The probabilities of deactivating the bending and asymmetric stretching modes of CO2 by hydrogen and deuterium are calculated semiclassically over the temperature range of 300–2000 °K. It is assumed that the molecular interaction potential consists of a superposition of atomic pair repulsion forces. For the CO2(010) –H2 case, an inverse temperature dependence of probability was found over the 300–500 °K range; no inverse temperature dependence was found for CO2(010) –D2. The calculations indicate that it is possible to attribute the vibrational relaxation to energy transfer between the vibrations of CO2 and the rotation as well as translation of H2/D2. The different temperature dependences are essentially due to the difference between H2 and D2 in rotational energy matching with CO2 vibrations. The calculated temperature dependence of the deactivation probabilities is in good agreement with experimental data. However, the measured absolute values are lower than the calculated ones.
Oscillations in chemical systems. XII. Applicability to closed systems of models with two and three variables
Authors: Noyes Richard M.
Published: February 2003

Most computations on models of chemical oscillators have maintained constant concentrations of major reactants while concentrations of intermediates were allowed to vary. Such simplified computations are applicable to closed chemical systems only if reactants are depleted by small fractions during each cycle. Existing models that involve only two intermediate species are generally unsatisfactory for modeling closed systems. Thus, the Lotka mechanism (which does not generate a true limit cycle) can not model even infinitesimally small oscillations in a closed system unless the rate constant for predator–prey interaction is very large. The Brusselator model can not model closed system oscillations unless the various rate constants are confined to very restricted ranges. Any other model with only two intermediates must contain a step at least third order in those intermediates. By contrast, the Oregonator model with three variables and only first- and second-order processes, can model a closed system in which the major reactants are depleted by only a small fraction during each cycle. By suitable coupling approximations, some of the mathematical advantages of models involving only two variables can be retained when the Oregonator is used as a model.
Muonium chemistry in gases: Mu+Br2
Authors: Fleming D. G. , Brewer J. H. , Garner D. M. , Pifer A. E. , Bowen T. , Delise D. A. , Crowe K. M.
Published: February 2003

We report a precise measurement of the rate of reaction of muonium atoms with bromine molecules in an argon moderator gas at a atm and 23 °C. The bimolecular rate constant is k= (2.4±0.3) ×1011 l/mole-sec, ten times higher than that for the analogous reaction of hydrogen atoms. Since muonium can properly be treated as a light isotope of hydrogen, this comparison has potential significance for the theory of reaction rates. The technique is described and the results discussed.
Intermolecular potential models for anisotropic molecules, with applications to N2, CO2, and benzene
Authors: MacRury T. B. , Steele W. A. , Berne Bruce J.
Published: February 2003

Three representations of the angle-dependent intermolecular interactions of nonspherical molecules are compared with one another and with experimental data for nitrogen, carbon dioxide and benzene. The models are the atomic, Kihara core, and overlap. The effect of including quadrupolar terms in the potential functions is also considered. It is found that the theoretical virial coefficients are not greatly altered when the quadrupole terms are added, at least at the temperatures corresponding to the experimental ranges for the molecules chosen. However, the theoretical solid state heat of sublimation and, to a lesser extent, the crystal lattice parameter, do change significantly. Satisfactory fits to the experimental second virial coefficients and the solid state data were obtained by varying ε, σ, the well-depth, and range parameters, while fixing the other constants appearing in the models (such as the parameter specifying the nonsphericity) at reasonable values. Quantities of interest, including the equilibrium distance, angular configuration, and dissociation energy of the van der Waals’ dimer, are estimated from the best-fit potential functions. It is concluded that all three models are approximately equivalent when the molecules are only slightly nonspherical and that there is sufficient flexibility in the models to give good fits to the data even for strongly nonspherical molecules. It appears that the representations of the N2, CO2, and benzene interactions derived here are sufficiently realistic to warrant their use in future work.
Librational excitations in ordered and in partially ordered molecular crystals
Authors: Michel K. H. , Kroll D. M.
Published: February 2003

A projection operator technique is used to calculate the matrix of time correlation functions describing the librational dynamics of molecular crystals in ordered and partially ordered phases. Starting from a general multipolar potential and using the symmetry properties of the dynamic variables, coupled dynamic equations are derived for the angular displacements, their conjugate momenta and the energy density. The frequency matrix and memory kernels that enter the equations describe the libronic excitation frequencies and damping, respectively. The nonharmonicity of the kinetic energy is taken into account self-consistently in evaluating the libron frequencies. New transport coefficients arising from the nonharmonicity of the kinetic energy are discussed. In addition, for solid CD4II, coupling of the librational excitations to the rotationally disordered central molecules is taken into account. This coupling can lead to an overdamping of the librons. A new central resonance due to the relaxation of the central molecules is found. Comparison with recent neutron scattering experiments is made.
Ultrasonic investigation of cholesteric liquid crystals
Authors: Kor S. K. , Pandey S. K.
Published: February 2003

This paper reports experimental results for the velocity and absorption of ultrasonic waves in the mesomorphic states of four cholesteric liquid crystals, namely, cholesteryl nonanoate, decanoate, oleate, and linoleate. The absorption has been measured by a pulse technique and velocity by an acoustic interferometer, both at a single frequency of 2 MHz. The results obtained for these materials furnish distinct similarities with the behavior of pure normal materials in the critical region. Anomalous increase in absorption and decrease in velocity observed across the isotropic–mesomorphic transition temperature in each case are interpreted in terms of pretransitional effect.
Absolute elastic differential electron scattering cross sections in the intermediate energy region. II.—N2
Authors: Srivastava S. K. , Chutjian A. , Trajmar S.
Published: February 2003

Utilizing a recently developed method, the ratios of elastic differential cross sections (DCS) of N2 to those of He have been measured at electron impact energies of 5, 7, 10, 15, 20, 30, 40, 50, 60 and 75 eV covering an angular range of 20° to 135° at each energy. The absolute values of N2 DCS (which, in the present work, also include the DCS for pure rotational excitation) have been obtained from these ratios by the use of absolute He DCS of McConkey and Preston. From the differential cross sections, elastic integral and momentum transfer cross sections have been evaluated. The results of the present experiment are compared with some previous measurements.
CI study of the water dimer potential surface
Authors: Matsuoka O. , Clementi E. , Yoshimine M.
Published: February 2003

The potential energies for the water dimer in various geometrical configurations have been calculated with a configuration–interaction method. The computed dimerization binding energies corresponding to the potential minima for the linear, cyclic, and bifurcated configurations are −5.6, −4.9, and −4.2 kcal/mol, respectively; the correlation effects account for −1.1, −1.2, and −0.9 kcal/mol, respectively, of the total binding energy for these three dimeric forms. The correlation effects for the entire potential surface have been analyzed in terms of inter- and intramolecular effects; the substantial coupling found between these effects, particularly in the vicinity of equilibrium position, is discussed. The computational technique employed, in particular an analysis on the selection criteria for the configuration state functions, is discussed, and its reliability is assessed. Two analytical expressions for the water dimer potential surface obtained by fitting the calculated energies are presented. The potential surface given here is being used to determine the structure of liquid water (in the pairwise approximation and with Monte Carlo techniques); this latter work will be reported elsewhere.