bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactions of O− with N2, N2O, SO2, NH3, CH4, and C2H4 and C2H2− with O2 from 300 °K to relative kinetic energies of ∼2 eV
Authors: Lindinger W. , Albritton D. L. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

The energy dependences of the reaction rate constants of O− ions with the neutral species N2, N2O, SO2, NH3, CH4, and C2H4 have been measured from thermal energy to approximately 2 eV relative kinetic energy. The energy dependences of the over-all reaction rate constants do not fit available theories for collision rate constants. The reaction of O− with C2H4 produces four different negative-ion product channels, in addition to the associative-detachment channel. The branching ratios are found to change drastically with reactant energy. The reactions establish the following upper limits on electron affinities: EA(:C=CH2) <0.43 eV and EA(C2H3) ≳0.4 eV. The reactions also establish an upper limit for the heat of formation of the ion C2H3O− (ΔHf°<−0.62 eV).
Differential cross section from an operator viewpoint
Authors: Snider R. F.
Published: February 2003

It is shown that a differential cross section can be identified as a matrix element of a transition superoperator between an observable (density operator) describing unit planar flux and an observable operator for unit spherical flux per steradian. The analysis exactly parallels the usual treatment of scattering of wavefunctions, but is done entirely in terms of density operators. The large distance (far from the scattering center) behavior of the scattered density operator is conveniently described in terms of the equivalent Wigner distribution function. For simplicity of presentation, rearrangement collisions are excluded from discussion, but the treatment may easily be extended to include such effects.
Nonequilibrium velocity distributions and recombination rates in electron–ion and ion–ion recombinations in weakly ionized gases
Authors: Koura Katsuhisa
Published: February 2003

The nonequilibrium velocity distributions and recombination rates in electron–ion and ion–ion recombinations in a heat bath of neutral molecules are investigated by solving the Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distributions and recombination rates from the initial equilibrium to quasisteady states are obtained for the typical electron–ion dissociative recombination e+N+2→N+N in the heat bath of N2 and for the typical ion–ion recombination H++H− →H+H in the heat bath of H2. The velocity distributions and the recombination rates indicate significant deviations from equilibrium. The decreases in the recombination rates from the equilibrium rates are 30% for e+N+2→N+N and 15% for H++H−→H+H at quasisteady states for the heat bath temperature 300 °K and the charge concentrations X∼10−4 and 10−2, respectively. The Monte Carlo result is compared with the Chapman–Enskog solution by Shizgal and Karplus for H++H−→H+H. The Chapman–Enskog solution indicates a too large nonequilibrium for high charge concentration X≳10−4.
The thermomagnetic force in the near-Waldmann regime for CO, N2, HD, and CH4
Authors: Davis Lorne A.
Published: February 2003

The thermomagnetic force exerted on a disk with surface perpendicular to the heat flux has been measured in the gases CO, N2, HD, and CH4 for conditions approximating those assumed by recent theory. Comparison is made with theory and previous experimental work, and the thermomagnetic force is shown to depend upon the surface composition of the disk.
Microwave spectra of e
n
d
o- and e
x
o-2-methylbicyclo[2.1.0]pentane. Methyl group polarity and the sign of the dipole moment in bicyclo[2.1.0]pentane
Authors: Harmony Marlin D. , Wang Charlene S. , Wiberg Kenneth B. , Bishop K. C.
Published: February 2003

The microwave spectra of the endo and exo isomers of 2-methylbicyclo[2.1.0]pentane have been identified and assigned. Rotational constants for the former isomer are A=6257.36±0.01, B=3412.19±0.01, and C=2843.34±0.01, and for the latter, A=7418.75±0.01, B=3101.66±0.01, C=2652.57±0.01, all values in megahertz. Stark effect measurements have provided dipole moment data, the magnitudes of the total moments being 0.295±0.010 D and 0.176±0.001 D, respectively. Abinitio molecular orbital calculations give results in good agreement with experiment and show that the negative end of the dipole moment vector for the ring system is in the endo direction. The experimental data have been used to obtain a quantitative assessment of the polarity changes upon methyl substitution.
NMR study of the pressure effects on the molecular dynamics in the disordered, crystalline phase of neopentane
Authors: Allen W. C. , Liu N. , Jonas J.
Published: February 2003

The NMR proton spin–lattice relaxation times T1 have been measured in the disordered crystalline phase of neopentane as a function of pressure and temperature. The molecular reorientation and self-diffusion contributions to the observed relaxation time are separated and the activation parameters are calculated for the two processes, respectively. The over-all molecular reorientation has a very weak pressure dependence whereas the self-diffusion exhibits a strong pressure dependence. The interesting result that the activation volume for self-diffusion is close to the molecular volume in neopentane is discussed in terms of the diffusion mechanism. The data enable us to estimate the modulated portion of the second moment attributable to the over-all molecular reorientation.
Measurement of the dispersion in polarizability anisotropies
Authors: Alms G. R. , Burnham A.K. , Flygare W. H.
Published: February 2003

We have measured the dispersion of the gas phase depolarization ratio of Rayleigh scattered light for 12 linear and symmetric top molecules. Combining these data with known refractive index data we obtain the frequency dependence of the polarizability anisotropy. For all molecules studied we find that the polarizability anisotropy increases more rapidly with increasing frequency than the bulk polarizability. We have correlated this behavior with the oscillator strength and direction of the first electronic transition. We have also compared our zero frequency extrapolated anisotropies with the anisotropies determined from static fields with the Kerr effect and the Stark effect in microwave spectroscopy.
Molecular dynamics study of water microclusters
Authors: Briant C. L. , Burton J. J.
Published: February 2003

We report a molecular dynamics study of water microclusters using a pair potential to describe the water–water interactions. The microclusters were liquid and contained 2–30 molecules. The radial density functions, hydrogen bond patterns, dipole orientations, and surface energies were calculated. The clusters were deficient in hydrogen bonds compared to the bulk phase. Dipole orientation at the surface of the cluster was random for a symmetric potential; for a nonsymmetric potential, there was a tendency for the hydrogens to be turned in toward the center of the cluster. Neither the liquid drop model nor the Tolman model exactly represents the size dependence of the surface energy of the microclusters.
Polymorphism in phenol under pressure: The x-ray powder diffraction patterns of phenol I and II at −190 °C, and the volume compressibilities of phenol I and II at 10 °C
Authors: Bertie John E. , Tremaine Peter R.
Published: February 2003

The correction of the previously reported [J. Chem. Phys. 58, 854 (1973)] volume compressions of phenol I and phenol II at 10 °C for the deformation of the apparatus under pressure is improved. X-ray powder diffraction photographs of phenol I and II at −190 °C and atmospheric pressure are reported. The mean thermal expansivity of phenol I between room temperature and −190 °C is (2.6±0.8) ×10−4 deg−1, and is primarily due to expansion perpendicular to the axes of the hydrogen-bonded chains. The x-ray and density data for phenol II can be explained by an orthorhombic unit cell with a= (10.59±0.01) Å, b= (16.31±0.02) Å, c= (17.39±0.02) Å, Z=24.
Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ-irradiated 10M sodium hydroxide alkaline ice glass
Authors: Narayana P. A. , Bowman Michael K. , Kevan Larry , Yudanov V. F. , Tsvetkov Yu. D.
Published: February 2003

Electron spin echo signals associated with trapped electrons in aqueous matrices show a modulation due to hyperfine interaction with the surrounding magnetic nuclei. The three pulse electron spin echo modulation gives information about the next nearest neighbor deuterons surrounding the electron. Analysis of the phase change in the three pulse modulation gives an effective deuteron interaction distance of 3.6 Å. The two pulse spin echo modulation depends on both the nearest and next nearest neighbor deuterons. Simulations of this modulation support a structural model in which the number of equivalent nearest neighbor deuterons to the electron is 6 at a distance of 2.1 Å and with an isotropic coupling constant of 0.9 MHz (5.8 MHz for protons). The orientation of the water molecules in the first solvation shell is not uniquely determined and two possible models are proposed. In model I there are three first solvation shell water molecules with their molecular dipoles oriented toward the electron so that there are two nearest neighbor deuterons from each molecule. In model II there are six first solvation shell water molecules with one OD bond oriented toward the electron. Model I is approximately consistent with the semicontinuum model for solvated electrons although the experimental distances are 0.2–0.3 Å larger than the predicted point dipole distances.
A theory of the hard sphere solid. II
Authors: Barker J. A. , Gladney H. M.
Published: February 2003

A ’’self-consistent field’’ theory based on the Bethe approximation previously estimated the entropy and single-particle distribution function in good agreement with computer simulations for the three-dimensional hard-sphere solid. In this paper we carry through the theory for the hard disk system, finding results closely similar to those for hard spheres, and numerically in good agreement with asymptotic high-density numerical simulations. Cluster corrections through third-order interactions are calculated and found small. The asymptotic high-density approximation fails, correctly, to lead to a solution for the one-dimensional system of rods in cells. In this case, an exact analytic solution is possible. For this solution, the one-particle distribution function retains finite width in the high-density limit, in contrast to the two- and three-dimensional solutions. At high densities, the free energy of this ’’self-consistent’’ cell model solution corresponds to the known correct result.
The origin of Stark shifts and splittings in molecular crystal spectra. I. The effective molecular polarizability and local electric field: Durene and naphthalene
Authors: Chen Freeman P. , Hanson David M. , Fox David
Published: February 2003

The origin of the linear Stark splittings observed in spectra of polar molecules dissolved in nonpolar host crystals is described. In order to obtain molecular dipole moment parameters from such splittings, the local electric field, among other things, must be known. The effective molecular polarizability and the local electric field are not determined uniquely by the crystal geometry and electric susceptibility for the general case of two or more anisotropic molecules per unit cell. The complete infinite set of solutions to the equations relating these quantities is reported for a crystal with two molecules per unit cell. Consequently, the variation or uncertainty in local field and effective molecular polarizability can be examined for specific cases. The cases of durene and naphthalene crystals are examined here, and it is found that the local field may differ by as much as a factor of two from values used previously in the analysis of Stark splittings.
Towards the development of the quantum mechanics of a subspace
Authors: Srebrenik Shalom , Bader Richard F. W.
Published: February 2003

The development of the quantum mechanics for a subspace of a total system is pursued via the derivation or introduction of the following: (1) the variational principle for a subspace; (2) the relationship between the time dependent subspace average of an operator and its associated commutator; (3) the unique set of quantum properties found for a subspace bounded by a surface of zero flux as defined by ∇ρ (r) ⋅n (r) =0, -r-S (r), and denoted as a ’’virial fragment’’; (4) the special variational properties of the zero-flux boundaries in one-electron systems; (5) operators which cause the energy integral to be stationary over a subspace; (6) the fragment virial theorem; (7) the importance of the scaling of the coordinates of a single electron in the determination of subspace properties; (8) a reformulation of the total Hamiltonian for a many-particle system as a sum of single-particle Hamiltonians through the use of a complete set of virial sharing operators; (9) new interpretation of the energy of the equilibrium geometry of a fixed-nucleus Hamiltonian and of the kinetic and potential energies in the molecular virial theorem as purely electronic; (10) a proof that the energy for a molecule in an equilibrium geometry is a sum of one-electron contributions; (11) the crucial nature of the principle stated in (10) for the definition of the energy of a subspace; (12) introduction of the principle of equal sharing of the energy of interaction between identical particles into the definition of a subspace energy; (13) a proof of the nonpartitionability of an atom based on the principle outlined in (12); (14) discussion of the information required to define a fragment and its energy. As a consequence of the above theorems and results there emerges a unique definition of a unit cell in a solid, of an atomiclike fragment in a molecule and of a molecule in a system of interacting molecules, for these are the subsystems bounded by surfaces of zero flux.
Temperature dependence of vibrational relaxation in the HF–DF, HF–CO2, and DF–CO2 systems. II
Authors: Lucht Roy A. , Cool Terrill A.
Published: February 2003

Measurements of vibrational energy transfer probabilities are presented for the temperature range 205–360 °K for HF–DF, HF–CO2, and DF–CO2 gas mixtures. The present results provide an accurate determination of the inverse temperature dependence of the energy transfer probabilities exhibited by these systems. Large deactivation effects caused by HF (DF) polymers were observed for temperatures below 220 °K.
Dependence of spontaneous emission and nonradiative relaxations of Tm3+ and Er3+ on glass host and temperature
Authors: Reisfeld R. , Eckstein Y.
Published: February 2003

Radiative transition probabilities from 3P0, 1I6, 1D2, and 1G4 levels of Tm3+ in glasses were calculated using the matrix elements obtained by intermediate coupling scheme and experimentally obtained intensity parameters. In the absence of self-absorption of fluorescence by the glass host, the fluorescence intensities of Tm3+ and Er3+ increase in the order borate < phosphate < germanate < tellurite. Nonradiative transition probabilities in different glass hosts from 1D2 to 1G4 levels of Tm3+ and from 4S3/2 to 4F9/2 and from 4F9/2 to 4I9/2 levels of Er3+ were calculated. The nonradiative transition rates follow the formula WNR = W (0) exp(−αp), where p is the number of phonons matching the energy gap. The nonradiative transition rates of Er3+ exhibit temperature dependence consistent with the multiphonon relaxation theory. In Tm3+ the temperature dependence of nonradiative relaxation rates is complicated by the existence of self-absorption and transfer of energy from matrix to Tm3+. Based on the above results, quantum efficiencies of luminescence of rare earth in glasses can be predicted.
Singularities of the Born–Oppenheimer exchange amplitude
Authors: Huo Winifred M.
Published: February 2003

Born–Oppenheimer amplitude for exchange collisions of electrons by atoms or molecules is found to possess singularities in the complex k0, kf, and K plane, where k0, kf, and K denote the magnitudes of the incoming, outgoing, and momentum change vectors, respectively. The locations of the singularities are investigated using analytic Hartree–Fock wavefunctions as well as correlated wavefunctions for the target and are found to be related to the asymptotic behavior of the targed electron undergoing exchange.
Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in liquids
Authors: Hwang Lian-Pin , Freed Jack H.
Published: February 2003

It is shown how the equilibrium pair correlation function between spin-bearing molecules in liquids may be incorporated as an effective force in the relative diffusion expressions, and how one may solve for the resulting time correlation functions and spectral densities needed for studies of spin relaxation by translational diffusion. The use of finite difference methods permits the solution no matter how complex the form of the pair correlation function (pcf) utilized. In particular, a Percus–Yevick pcf as well as one corrected from computer dynamics, both for hard spheres, are utilized. Good agreement with the experiments of Harmon and Muller on dipolar relaxation in liquid ethane is obtained from this analysis. Effects of ionic interactions in electrolyte solutions upon dipolar relaxation are also obtained in terms of Debye–Hückel theory for the pcf. Analytic solutions are given which are appropriate for the proper boundary-value problem for the relative diffusion of molecules (i.e., a distance of minimum approach) that has usually been neglected in the spin relaxation theories. Other molecular dynamics aspects of spin relaxation by translational diffusion in liquids are briefly discussed.
Melting properties and ultrasonic velocity of nitrogen to 20 kbar
Authors: Mills R. L. , Liebenberg D. H. , Bronson J. C.
Published: February 2003

Simultaneous measurements of the longitudinal sound velocity and relative volume were made on freezing nitrogen at pressures up to 20 kbar in a piston–cylinder apparatus. Results in both the fluid and solid phases were combined with data at lower pressures from the literature to derive equations giving the melting properties of nitrogen at high pressure. The solid in equilibrium with the fluid is apparently β-N2 (hcp) throughout the experimental range. An estimate was made of the transverse sound velocity in β-N2 at high pressure. Analysis of the total entropy change on melting shows that the entropy component due to disordering of the molecules from solid to fluid nitrogen has a fairly constant value of about ln1.7 along the entire melting curve. There is no indication that the melting curve will terminate in a critical point.
Rayleigh scattering in a dense medium
Authors: Patterson G. D.
Published: February 2003

The Rayleigh scattering by a dense medium composed of anisotropic units is calculated directly from Maxwell’s equations in a dielectric. The true scattered field is given by the fluctuations in the total effective field in the medium. A purely classical calculation gives the correct dependence of the scattered intensity on the refractive index of the medium.
On pseudopotential and effective-potential SCF theory and its application to compounds of heavy elements
Authors: Ewig Carl S. , Van Wazer John R.
Published: February 2003

A formulation of pseudopotential and effective-potential theory is developed within the framework of the Hartree–Fock formalism. It is shown that one-electron pseudopotentials may be used for many-valence-electron atoms and molecules. An SCF computational procedure (the NOCOR method) is described that requires a minimal amount of parameterization. Sample calculations are performed on the four halogen atoms and ten dihalogen molecules. These results indicate that the ’’frozen-core’’ approximation is quite accurate at least through the fourth row of the Periodic Table.
Observation of singlet–triplet anticrossings in 4He2
Authors: Miller Terry A. , Freund Robert S. , Zegarski Bernard R. , Jost R. , Lombardi M. , Derouard J.
Published: February 2003

Anticrossings between excited singlet and triplet levels of the He2 molecule have been observed. Measurements have been made on anticrossings which take place between the f (3d)  3Δu and F (3d)  1Δu states both in the v=0, N=2,3,4 and v=1, N=2 vibrational and rotational levels. Analysis of these results yields a new and more precise value for the excitation energy, T0, from the a (2s)  3Σ to the A (2s) 1Σ state. This result, when combined with available optical data, fixes the absolute energy of all He2 excited states. The present experiment establishes that the steady state populations of the 3Δ levels are greater than the 1Δ levels under a variety of excitation conditions. Also, collisional transfer may take place before radiation giving rise to a given anticrossing signal appearing on optical emission lines of several nearby rotational levels.
Relation between charge and force parameters of closed-shell atoms and ions
Authors: Gilbert T. L. , Simpson O. C. , Williamson M. A.
Published: February 2003

Combining rules for parameters which characterize the short-range repulsive forces between closed-shell atoms and ions are used to determine the radius and softness for a number of alkali and halide ions and rare-gas atoms from scattering and spectroscopic data. A corresponding set of charge radii and softnesses are determined from atomic charge densities calculated in both the relativistic and nonrelativistic Hartree–Fock approximations. These characteristic atomic parameters are compared, and it is found that, for species with the same charge belonging to the same column of the periodic table, the relations between charge and force radii and between charge and force softnesses are very nearly linear. The radial distance dependence of the softness is examined, and some areas where further work is needed to follow up this empirical study are indicated.
Recombination dynamics of hydrogen on a tungsten surface
Authors: McCreery Jane Hylton , Wolken George
Published: February 2003

The results of a series of classical trajectory calculations designed to simulate the recombination of two hydrogen atoms adsorbed on tungsten to form a gas phase H2 molecule are reported. (AIP)
Study of hydrogen adsorbed on platinum by neutron inelastic scattering spectroscopy
Authors: Asada Hiromu , Toya Tomiyuki , Motohashi Haruhiko , Sakamoto Masanobu , Hamaguchi Yoshikazu
Published: February 2003

The inelastic neutron scattering spectra of hydrogen adsorbed on platinum is reported and analyzed. (AIP)
Self-diffusion coefficients of a nematic liquid crystal via an optical method
Authors: Hakemi H. , Labes M. M.
Published: February 2003

By observing the textural changes caused when an enantiomer is allowed to diffuse into a racemic nematic liquid crystal of the substituted phenyl-4-benzoyloxybenzoate series, self-diffusion coefficients can be directly evaluated in both the nematic and isotropic phases. The data obtained are shown to be consistent with other determinations by mass-transport techniques, but inconsistent with relaxation methods, and the mechanistic factors causing these inconsistencies are discussed.
Studies in molecular dynamics. XV. High temperature description of the transport coefficients
Authors: Alley W. E. , Alder B. J.
Published: February 2003

The power series expansion of the transport coefficients in the attractive potential relative to the kinetic energy does not exist. However, the first term in this expansion appears to exist and is evaluated indirectly by molecular dynamics at various densities. For the diffusion coefficient this first term is predicted quantitatively at high density by the enhanced local density in the region of the attractive potential relative to that of the hard sphere system. Furthermore, since the enhanced density does not affect the kinetic contribution to the viscosity and thermal conductivity, these transport coefficients compare favorably with the predictions of the Boltzmann equation. An explicit calculation of the first term requires statistical mechanical averaging before expansion, a procedure unsuitable to molecular dynamics computations and which has as yet been unattained analytically at any finite density.
Influence of the Overhauser effect on 14N PNQR T1 measurements on a single crystal of parachloroaniline
Authors: Vega Shimon
Published: February 2003

Spin lattice relaxation measurements of 14N by PNQR pulse experiments on a single crystal and powder of parachloroaniline are reported. In this compound, there are two symmetry related nitrogen sites in the unit cell. The dipolar interaction between nuclei in both sites affects the experimental results by an Overhauser-type effect. This effect on various types of pulse experiments is analyzed. To simplify the analysis, the quadrupole Hamiltonian is expressed in terms of fictitious spin 1/2 angular momentum operators. The density matrix in this representation in the rotating frame of the quadrupole Hamiltonian is discussed, and the signal function for various types of pulse experiments is derived. The experimental results are in good agreement with theory.
Direct transitions among the allotropic forms of boron nitride at high pressures and temperatures
Authors: Corrigan F. R. , Bundy F. P.
Published: February 2003

The direct transition behavior among the graphitic (hBN), wurtzitic (wBN), and zincblende (zBN) crystal forms of boron nitride is investigated as a function of temperature for pressures up to 130 kbar. At pressures in the 45 to 70 kbar range, direct transformation of both the hBN and wBN forms to the zincblende form are observed and at higher pressures (85 kbar and above) direct transformation of hBN to wBN is also observed. A pressure/temperature phase diagram is presented for pressures up to 130 kbar. In this pressure range, the thermodynamically stable solid phases are hBN and zBN, the experimental behavior indicating that wBN is not thermodynamically stable over this range. From temperature/time data, the activation energy for both the hBN to zBN and wBN to zBN transitions is estimated to be about 200 kcal/mole. From these high activation energies it is concluded that the direct conversion processes essentially require disruption of the hBN and wBN lattices before the atoms can re-form into the zincblende structure.
Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: Molecular electron affinities
Authors: Compton R. N. , Reinhardt P. W. , Cooper C. D.
Published: February 2003

The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to ∼400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(−0.60±0.2 eV), COS(+0.46±0.2 eV), and CS2(1.0±0.2 eV). The CO−2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO−2* and with recent theoretical calculations. The lifetime of CO−2* (9±2×10−5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to ∼20 eV c.m.). The fragment ions O−/CO2, O−/COS, S−/COS, and S−/CS2 were detected at a threshold energy which is consistent with the known energetics of the reactions. These results are compared with recent dissociative electron attachment studies.
B-spline expansion bases for excited states and discretized scattering states
Authors: Shore Bruce W.
Published: February 2003

High-order B-splines provide effective basis functions for constructing solutions to the Schrödinger equation. For high-lying levels of the Morse potential septic splines yield an accuracy comparable to harmonic oscillators. They can also approximate hydrogenic wavefunctions; typically one-third of the N eigenvalues of an N×N matrix are more accurate than 0.01%. Matrix eigenvectors can also represent scattering states. The approximation is uniformly valid spatially when basis functions terminate sharply.
Vibrational relaxation in the first excited singlet state of rhodamine in solution
Authors: Ricard D.
Published: February 2003

Using a method which we had previously developed, we have studied vibrational relaxation in the first excited singlet state of rhodamine molecules in solution. After having recalled the principle of the method, we present the results we have obtained by varying parameters such as molecular structure, solvent viscosity, temperature ... and we discuss a few conclusions which can be drawn from this study about the interpretation of the relaxation process.
Molecular structure of XeF6
Authors: Pitzer Kenneth S. , Bernstein Lawrence S.
Published: February 2003

The full array of structure-related experimental data for XeF6 is considered simultaneously. Electron diffraction, infrared, Raman and ultraviolet spectra, electric field deflection of molecular beam, and calorimetrically determined entropy data are all found to be consistent with a structural model involving substantial distortion in the T1u bending mode from octahedral symmetry and a pseudorotation in two dimensions of this distortion. Several other vibration frequencies are assigned, and the mean value of three unobserved frequencies is determined from the entropy. No quantitative value could be given for the potential restricting pseudorotation, but various arguments agree in indicating that it is small. There is no reason to question previous proposals that this peculiar structure arises from a pseudo-Jahn–Teller effect associated with the relatively low energy of excitation of a xenon 5s electron to the 5p level.
Lifetime distribution of electrons in pulse-irradiated liquid propane
Authors: Fueki Kenji
Published: February 2003

A lifetime distribution function for geminate electrons in irradiated liquids is derived and applied to calculations of the lifetime distribution of electrons in irradiated liquid porpane. The results were compared with an experimental determination of the decay of electrons in pulse-irradiated liquid propane and agreement was satisfactory. (AIP)
Mössbauer emission spectrum of 121Sb after the β− decay of 121mSn in SnS2: Nuclear decay synthesis of antimony (V) sulfide
Authors: Ambe S. , Ambe F.
Published: February 2003

The Mössbauer spectrum of 121Sb has been measured after β− decay of 121mSn in SnS2. The results indicate that a covalent antimony (V) compound was formed. This system offers the possibility of synthesizing Sb(V) compounds which have not obtained by other methods. (AIP)
A group theoretical approach to geminal product matrix elements
Authors: Siems W. F. , Poshusta R. D.
Published: February 2003

Expressions enabling systematic compilation of Hamiltonian and overlap matrix elements for an antisymmetrized multiterm geminal product trial function are derived, using double coset (DC) decompositions and subgroup adapted irreducible representations of the symmetric group, SN. The trial function may describe an even electron atomic or molecular system in any total spin eigenstate, and the geminals may be nonorthogonal, have arbitrary permutational symmetry, and be explicit functions of interelectronic distance. A DC decomposition is used to factor out permutations not exchanging particle labels between geminals (elements of the interior pair group, Sn2, n≡N/2). This reduces the sum over N! permutations to a sum over DC generators. If the irreducible representation λ (S) of SN is adapted to Sn2 each geminal is projected into its singlet or triplet component. The DC generators are chosen such that each has the form QP, where Q permutes odd particle labels only and P is a permutation of geminals (element of the exterior pair group, S?). With the aid of matrices called DC symbols an algorithm for these generators is derived, and used to find explicit sets for N=2, 4, 6, and 8. The N-electron Hamiltonian and overlap integrals arising with a particular DC generator QP are factored into products of smaller integrals, called cluster integrals, according to the cycle structure of Q. The cluster integrals are of only three main types—overlap, one-cluster energy, and two-cluster energy (analogs of orbital overlap, 1-electron, and 2-electron integrals) —and are further classified by order (number of geminals), geminal permutational symmetry, and in some cases pattern of connection. Matrix element compilation is systematic in that all N-electron integrals are products of a relatively small number of different types of cluster integral, and that N-electron integrals with similar factored forms are collected together in the summation. From counting the different cluster integrals required, it is concluded that a geminal product calculation not using orbital expansion is feasible only for systems with eight or less electrons. In some cases semiempirical calculations with correlated geminals might be considered, for the more complex cluster integrals (those of high order) are quite small for a system approximating a collection of localized electron pairs. The matrix element expressions are specialized for three cases—all geminals being singlets, strongly orthogonal geminals, and identical geminals. Comparison is made with a recently developed diagrammatical method.
Chemiluminescence from thallium–fluorine reactions
Authors: Maya J. , Nordine P. C.
Published: February 2003

Chemiluminescence from reactions of thallium with fluorine has been observed in a dilute flame experiment. Atomic emission from the highest observed Tl (6 2D5/2,3/2) states (103.5 kcal above ground state) is shown to arise from energy transfer processes. The role of the metastable Tl (6 2P3/2) state in energy transfer processes is discussed. Weak molecular emission from TlF (A 3Π–X 1Σ, B 3Π–X 1Σ) has been observed.
Elastic and nonelastic cross sections for Ar* (3P)+HBr(X 1Σ)
Authors: Fraites James L. , Winicur Daniel H.
Published: February 2003

The energy dependence of the differential scattering of metastable Ar* (3P) by ground state HBr(X 1Σ) has been studied at four relative kinetic energies from 60–160 meV over an angular range of 4–120 deg c.m. The position and curvature of rainbow maxima, which are observed at each energy, are used to obtain LJ(12,6) potential parameters. The position of the minimum rm=4.44±0.8 Å is the same as that for K+HBr, and the well depth, ε=25.2±1.1 meV, is about 4% larger than ε (K+HBr). The scattered intensity shows a distinct falloff at large angles compared to that expected for elastically-scattered Ar*. This depletion, caused by quenching of Ar*, is analyzed in terms of an optical model to determine the threshold and energy dependence of the quenching cross section. The nonelastic cross section increases from a threshold of 25 meV to a nearly constant value of 41 Å2 at 160 meV. The optical analysis predicts a maximum of 42.5 Å2 at 215 meV.
Reactions and quenching of vibrationally excited hydroxyl radicals
Authors: Streit G. E. , Johnston H. S.
Published: February 2003

Rate constants for the deactivation of vibrationally excited hydroxyl radicals by the inert gases Ar, O2, N2, H2, and D2 have been determined. Additionally the rate constants for the reaction HO(v)+O3→ products have been measured and found to decrease from 11×10−12 to 3.7×10−12 cm3 molecule−1⋅ sec−1 from v=9 to v=4. Hydroxyl radicals were generated in a large tank at low pressures from the reaction of hydrogen atoms and ozone. The radicals were formed in excited vibrational states (v?9), and the excited radicals, v=4 to 9, were observed by visible light emission from high overtone transitions. Vibrational populations and the rate constants for loss processes were deduced from observations over a wide range of wavelength and pressure fitted to a comprehensive model of the system.
Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide
Authors: Lee J. H. , Timmons R. B. , Stief L. J.
Published: February 2003

Absolute rate constants for the reactions of O(3P) atoms with dimethyl sulfide and episulfide have been measured over the temperature range 268 to 424 °K. The kinetic data were obtained using the flash photolysis–resonance fluorescence method. The O(3P)+CH3SCH3 reaction exhibited a negative temperature dependence and the Arrhenius expression obtained was k1= (1.42±0.07) ×10−11 exp(727±31/RT) cm3 molecule−1⋅sec−1. The episulfide reaction was essentially temperature independent, the corresponding Arrhenius expression being k2= (1.34±0.09) ×10−11 exp(−35±40/RT) cm3 molecule−1⋅sec−1. Use of a fast flow system with direct mass spectrometric analysis of reaction products provides evidence that the reaction mechanisms involved are O(3P)+CH3SCH3⇄[CH3SOCH3]*→CH3SO+CH3, O(3P)+CH2–SCH2⇄[CH2–SOCH2]*→CH2=CH2+SO. The very great difference in reaction rates and mechanisms between these sulfides and the corresponding oxygen analogs CH3OCH3 and ethylene oxide is discussed.
Chemi-ionization reactions in accelerated uranium–O2 crossed molecular beams
Authors: Young C. E. , Dehmer P. M. , Cohen R. B. , Pobo L. G. , Wexler S.
Published: February 2003

Chemi-ionization and electron transfer reactions in accelerated uranium-atom–oxygen−molecule collisions have been studied by crossed molecular beam techniques: U+O2→UO+2+e, associative ionization (1); U+O2→UO++(O+e), rearrangement (reactive) ionization (2); U+O2→U++(O2+e), electron transfer (3). Cross sections for the three ionization channels were measured as a function of relative collision energy from 0.2 to 60 eV. The thresholds observed for channels (2) and (3), which are closed at thermal energies, are in good agreement with values calculated from thermochemical data. The cross section for associative ionization varied inversely with vr, the relative collision velocity, as predicted for weak autoionizing transitions. With the use of a rough absolute calibration and a 1/vr extrapolation to lower energies, the UO+2 cross section was found to be in agreement with the previously reported value measured with thermal beams. In the case of rearrangement ionization, the cross section rose rapidly at threshold, peaked near the threshold for the electron transfer process, and then declined with increasing collision energy. The electron transfer cross section increased with a power-law dependence of the excess energy above threshold throughout the range investigated. The maximum absolute cross section measured was 1.4×10−17 cm2. The dynamics of ionization processes (2) and (3) were elucidated from the energy dependences of the cross sections and from measurements of the angular and kinetic energy distributions of the product ions. The results support the conclusion that rearrangement ionization proceeds by a ’’spectator-stripping’’ mechanism. Both the UO+ from this reaction and the U+ from electron transfer are scattered predominantly in the forward direction.
Transverse relaxation in heteronuclear coupled spin systems: AX, AX2, AX3, and AXY
Authors: Vold Regitze R. , Vold Robert L.
Published: February 2003

All infrared and Raman active lattice modes of crystalline benzene were calculated using a six-center hexagonal potential function. The interaction was expressed in terms of one parameter, d, the side of the hexagon. The agreement is good for the translational modes and reasonable for the librational ones.
A study of molecular pair potentials; A semiempirical approach
Authors: Ellis R. L.
Published: February 2003

An expression is developed for the short range molecular pair potentials within the framework of the Heitler–London approach for molecules in a closed shell state. Assumptions common with the CNDO/2 semiempirical method are employed in evaluating the molecular integrals. The final expression depends only on quantities related to the atoms making up each molecule. There are four such quantities; (1) the atom charge density Pm; (2) the atom core charge Zcm; (3) the distance between atoms rmn; and (4) the value of two electron repulsion integrals between atoms γmn. In terms of these quantities the final expression has the simple form E(1)ab= (−1) Σm(a)Σn(b) [PmPnγmn(Zcm/Pm+Zcn/Pn −1)−ZcmZcnr−1mn]. Although several approximations are introduced to obtain this expression these are related to the evaluation of integrals, and no adjustable parameters are required for the evluation of E(1)ab. This potential is in good agreement with the results of ab initio calculations for the system He/H2, Ne/H2, and HF/HF. A comparison is also made with the results of the Gordon–Kim method for the system HF/HF. Both E(1)ab and the Gordon–Kim method are found to give comparable results, but the method presented in this work is found to be computationally ∼3000 times faster per data point.
Calculations on the HCl–Ar van der Waals complex
Authors: Dunker Alan M. , Gordon Roy G.
Published: February 2003

Previously determined anisotropic intermolecular potentials for HCl–Ar are tested for comparison with the recent molecular beam resonance experiments on the van der Waals complexes. The Schrödinger equation for the coupled rotational and vibrational motion of the complex is solved accurately by a close coupling method. Energy levels, and expectations of the dipole moment, cosϑ, and cos2ϑ, where ϑ is the Ar–HCl center of mass-H angle, have been computed for the lower states of 40Ar complexes with H35Cl, H37Cl, D35Cl, and D37Cl. Rotational and centrifugal distortion constants were also computed for the lowest level. The assignment of the angle ϑ is discussed, and our results support the acute choice (i.e., H on the average between the Cl and Ar nuclei). The shift in angular expectations with HCl isotope distinguishes between the potentials tested here, and comparison with the experiments suggests that the true potential has an absolute minimum near ϑ=0°. However, none of the available potentials fits both the scattering data and the bound state spectroscopic data quantitatively. This demonstrates that the simultaneous analysis of scattering and bound state data constrains the potential much more than either alone.
Gas phase measurements of the Kerr effect in some n-alkanes and cyclohexane
Authors: Buckingham A. D. , Sutter H.
Published: February 2003

Gas phase measurements of the molar Kerr constant, mK, of cyclohexane and the n-alkanes from propane through decane have been made over the temperature range 313–461 K and pressure range 0–8 bar. When examined in conjunction with previous light scattering measurements our results lead to the following conclusions: hyperpolarizability contributions to mK are of the same order of magnitude as anisotropy of polarizability contributions; the assumption of the additivity of bond polarizability tensors seems to fail for the higher n-alkanes in the gas phase; values of mK and the depolarization ratio obtained from solution measurements are much less than the gas phase values.
Elimination of polarization bias in fluorescence intensity measurements
Authors: Mielenz Klaus D. , Cehelnik Edwin D. , McKenzie Raymond L.
Published: February 2003

It is proposed theoretically and confirmed experimentally that, if the fluorescence emission from a polarized sample is viewed in the horizontal plane at an angle of 45° (or 135°) to the direction of propagation of the exciting radiation with a polarizer set at 54.75° (or 125.25°) from the vertical direction, then, the reading obtained is proportional to the total flux emitted by the sample and is independent of the state of polarization of the exciting radiation and the emission anisotropy of the sample. The polarization sensitivity of the emission detection system then becomes a factor which is included in the spectral sensitivity curve of the emission detection system.
Hydrogen abstraction reactions by atomic fluorine. IV. Temperature dependence of the intermolecular kinetic isotope effect for the nonthermal 18F+H2 reaction
Authors: Grant Edward R. , Root John W.
Published: February 2003

The nuclear recoil systems 18F vs H2/C3F6 and 18F vs D2/C3F6 were investigated from 273–347 °K over the full range of mixture compositions at a constant pressure of 1000 Torr. The results were systematized and interpreted within a nonthermal bimolecular rate constant framework. The phenomenological rate constants as well as their purely non-Boltzmann components exhibited significant composition and ambient temperature dependences. Most, if not all, of this latter effect was attributable to the 18F*+C3F6 reaction channel, either through an internal state coupling mechanism or through secondary decomposition of excited CF3ĊFCF218F radicals by 18F-atom loss. Preferred values for the thermal reaction fractions in extrapolated pure H2 (or D2) and C3F6 systems were 20±10% and 20±20%. The corresponding average intermolecular kinetic isotope effect for the pure non-Boltzmann 18F+H2 reaction was 1.3±0.1.
Scattering of argon and neon from W(112)
Authors: White Ralph E. , Ehrhardt Jean-Jacques , Merrill Robert P.
Published: February 2003

The angular scattering distributions of thermal Ne and Ar beams incident in the [11̄0] azimuth of the W(112) surface are presented as a function of angle of incidence, surface temperature, and beam temperature. For Ne, the distributions are rainbow patterns similar to those observed for Ne on LiF for small angles of incidence. They coalesce into a broad lobe at an incidence angle of 70°. Argon scattering is unilobular and similar in all details with Ar scattering from the smoother W(110) surface. Ne appears to be in the inelastic regime while Ar is trapping dominated.
A low-temperature source for the generation of uranium vapor
Authors: Lee Henry U. , Zare Richard N.
Published: February 2003

Uranium is a highly refractory metal, boiling at 4135 °K and having a vapor pressure of 1 torr at 2729 °K. At these elevated temperatures, the corrosiveness of uranium poses severe materials problems in its vaporization. An alternative approach for the production of uranium atoms is to make a volatile chemical derivative of uranium and then to find a simple means of reducing this compound to elemental uranium in the gas phase. The generation of uranium vapor from the dissociation of uranocene by collisions with metastable argon atoms is reported. (AIP)
Spline bases for atomic calculations
Authors: Altenberger-Siczek A. , Gilbert T. L.
Published: February 2003

The use of spline bases has been proposed for ab initio electronic structure calculations. They offer the apparent advantage that no nonlinear parmameters occur, so that exponent optimization is unnecessary, and the number of two-electron integrals in SCF calculations increases only as n2 rather than n4 where n is the number of basis functions. The results of test calculations on the helium atom are reported in this note. Analysis of the computer time required to achieve accuracy comparable to other methods indicates that spline bases are not suitable for SCF calculations. (AIP)
Picosecond dynamics of electron localization in metal/methyl amine solutions
Authors: Huppert D. , Rentzepis P. M.
Published: February 2003

Photoionization of the M− band of sodium/methyl amine by a picosecond pulse reveals the following: (1) The time of electron localization is less than 5 psec. (2) The bleaching of the solvated electron absorption band in the 1000 nm region with a relatively narrow bandwidth ∼100 cm−1 picosecond pulse at (1060 nm) is found to be proportional to the extinction coefficients. (3) The relaxation time of the excited state of the solvated electron was determined to be ∼2×10−13 sec.
Temperature dependence of the spin–orbit relaxation of lead, 6p2(3P2) and (3P1)
Authors: Trainor Daniel W. , Ewing J. J.
Published: February 2003

Rate constants for the collisional deactivation of Pb 6p2(3P2) and (3P1) metastable states are reported over the temperature range 300 to 600 °K. Quenching data over this temperature range were obtained for relaxation by Ar, Xe, H2, D2, N2 and by Hg at 373 °K. These results were obtained in a temperature controlled, flash photolysis apparatus using tetramethyl lead as a precursor and time resolved atomic resonance absorption techniques for monitoring the 3P2 and 3P1 metastable states of interest.