bluuebunny/crossref_metadata_2025 huggingface dataset:
Water droplet growth in a carbon dioxide atmosphere: A case for small sticking coefficient
Authors: Vietti M. A. , Fastook J. L.
Published: February 2003

Water droplet growth was measured in a CO2 atmosphere using an expansion cloud chamber. Sufficient pressure and temperature data were obtained in order to calculate droplet growth which was compared to the experimentally measured growth curves. As the droplets increased in size, it was found that the rate of change of the growth rate was not constant. The sticking coefficient in the droplet growth theory was treated as a parameter, but no single value of this coefficient would yield agreement between theory and experiment over the whole observational period. When the droplets are smaller than about 2.0 μ, a sticking coefficient value between 1.0 and 0.1 yields best agreement. For droplets of larger radius, it was found that an even smaller value of this coefficient, as small as 0.036, was required. The dehydration of the bicarbonate ion appears to be responsible for the lower value of the sticking coefficient.
On positronium formation in binary mixtures of organic compounds
Authors: Wild Ralph E. , Ache Hans J.
Published: February 2003

The intensities of the long-lived component I2 in the positron lifetime spectra were measured for a variety of mono- and dihalogenated benzenes, benzylalcohol, benzaldehyde, benzonitrile, and acetophenone in benzene solution. The results indicate that I2 is not an additive property, e.g., small amounts of bromobenzene (less than 0.1 mole fraction) added to benzene reduce I2 to the value observed in the pure compound. The capability of the various systems to effectively reduce I2 values as well as the minimum I2 values observed in these systems have been evaluated. The results are discussed in terms of the existing models of Ps formation and interactions in binary mixtures.
An asymptotically exact form of the intersecting spheres model of molecules: Formulation
Authors: Antoci S.
Published: February 2003

It is shown that the intersecting spheres model for solving the Xα equations for molecules can be formulated without systematic approximations; therefore, the model allows convergence to the exact solution as the numerical approximations used are made more and more accurate.
An asymptotically exact form of the intersecting spheres model of molecules: Computation of the total energy and of the ESCA spectra of H2+,Li2,N2, F2, and H2O
Authors: Antoci S. , Barino Luisa
Published: February 2003

It is shown that an asymptotically exact form of the intersecting spheres model is useful in practice, since it yields satisfactory predictions for the total energy and ionization spectra of several molecules.
Asymptotic relations for solutions of the Poisson–Boltzmann equation for a spherical particle
Authors: Stokes A. N.
Published: February 2003

An asymptotic series in 1/r for the potential gradient ψ′ at the surface of a sphere in terms of the surface potential ψ is presented: ψ′∼ΣCn(ψ)/rn. For symmetrical, 2–1, and 1–2 electrolytes the first four coefficients are calculated. A useful inverted form with the roles of ψ and ψ′ exchanged is also presented. An integrated form which can be used to calculate free energies and activity coefficients is given. It is shown that the asymptotic expressions give good approximations for the potentials of 3–3 electrolytes, and also for 2–2 electrolytes for all but very dilute concentrations. It is also shown that the asymptotic series can be used in almost all situations where the Debye–Hückel approximation or its improvements fail.
Triplet exciton migration in rigid solution of poly(N-vinylcarbazole) and N-vinylcarbazole–fumaronitrile alternating copolymer
Authors: Yokoyama Masaaki , Tamamura Toshiaki , Nakano Takeshi , Mikawa Hiroshi
Published: February 2003

The migration of triplet excitation energy in poly(N-vinylcarbazole) (PVCz) has been investigated in rigid solution at 77°K. The behavior of triplet energy migration in PVCz is compared with that in N-vinylcarbazole–fumaronitrile (VCz–FN) 1:1 alternating copolymer, in which carbazole chromophores are regularly separated by a main chain segment of five carbon atoms, and in 0.5 mol/1 N-ethylcarbazole, a model compound of PVCz. In every case, the delayed emission spectra are composed of phosphorescence and delayed fluorescence, the latter resulting from the triplet–triplet annihilation between two long-lived mobile triplets. When doped with naphthalene, remarkable difference of the behaviors of triplet energy migration was observed between PVCz and VCz–FN copolymer, i.e., in the emission decay time and in the manner of phosphorescence quenching with naphthalene. In PVCz the phosphorescence lifetime remains 7.6 sec despite the effective intensity quenching of both phosphorescence and delayed fluorescence; thus, the expected relation that the decay time of the delayed fluorescence should be half the phosphorescence time, as is often found in aromatic hydrocarbon crystals, was not found. However, in the case of the VCz–FN alternating copolymer, both intensity and lifetime of phosphorescence are quenched by naphthalene. Such differences in the behaviors of phosphorescence emissions are discussed in terms of the difference of the rate of migration along the polymer chain and the rate of phosphorescence between these polymers. From the quenching experiments with naphthalene, the migration coefficients of triplet energy have been estimated to be 3.2×10−12 for PVCz and 4.9×10−13 cm2 sec−1 for VCz–FN copolymer, respectively. Also, in solid film the triplet energy migration has been demonstrated by the observation of delayed fluorescence. Triplet energy migrates more effectively in solid film than in rigid solution.
Existence of H2−; Additional experimental evidence
Authors: Schnitzer R. , Anbar M.
Published: February 2003

Further analysis and experimental evidence is given to show that charge transfer processes cannot account for the experimental findings that have been interpreted as demonstrating the existence of H2− [R. Schnitzer and M. Anbar, J. Chem. Phys. 64, 4332 (1976)].(AIP)
Electric dipole moment function of the X 1Σ+ state of CO: Vibration–rotation matrix elements for transitions of gas laser and astrophysical interest
Authors: Chackerian C.
Published: February 2003

The electric dipole moment function of the ground electronic state of carbon monoxide has been determined by combining numerical solutions of the radial Schrödinger equation with absolute intensity data of vibration–rotation bands. The derived dipole moment function is used to calculate matrix elements of interest to stellar astronomy and of importance in the carbon monoxide laser.
Vibrational relaxation of HF(v=1, 2, and 3) in H2, N2, and CO2
Authors: Bott J. F.
Published: February 2003

The vibrational relaxation times of HF(v=1, 2, and 3) were measured in H2, N2, and CO2 by a laser-induced fluorescence technique. The upper vibrational levels were produced by sequential absorption in which HF(v=0) was pumped first to HF(v=1) and subsequently to HF(v=2) and HF(v=3) by photons from a pulsed multiline HF chemical laser. At T=295 K, the relaxation rates of HF(v=1), HF(v=2), and HF(v=3) in H2 were found to be, respectively, (1.43±0.15) ×10−2, (1.23±0.1) ×10−2, and (1.13±0.1) ×10−2 (μsec Torr)−1; in N2, (1.45±0.15) ×10−4, (8.1±1.0) ×10−4, and (2.92±0.3) ×10−3 (μsec Torr)−1; and in CO2, 0.039±0.004, 0.19±0.02, and 0.38±0.04 (μsec Torr)−1. Values of (7.5±1) ×10−4 and 0.4±0.04 (μsec Torr)−1 were obtained for the relaxation rates of HF(v=3) in O2 and HCl, respectively.
Interference effects in NMR correlation spectroscopy of coupled spin systems
Authors: Ferretti J. A. , Ernst R. R.
Published: February 2003

The nonlinear behavior of coupled spin systems in NMR correlation spectroscopy is studied theoretically and experimentally. The two principal types of effects result from passage with transverse interference and from passage with longitudinal interference. The phase and relative amplitude of the signals strongly depend upon the applied sweep rate and the effective flip angle utilized in these experiments. Specific examples are cited for the case of coupled two-spin systems. Guidance for avoiding nonlinear interference effects in NMR correlation experiments is indicated.
Surprisal analysis of classical trajectory calculations of rotationally inelastic cross sections for the Ar–N2 system; influence of the potential energy surface
Authors: Pattengill M. D. , Bernstein R. B.
Published: February 2003

Rotationally inelastic Ar–N2 scattering on two different empirical potential energy surfaces has been investigated by the classical trajectory method. For each potential surface, state-to-state rotational transition cross sections σj′j (E) have been calculated at five total energies E and several initial rotational quantum states j of the N2. Results obtained from the two potentials differ significantly with respect to final rotational state distributions, but the total inelastic cross sections are very similar. Consideration of the moments of the rotational energy transfer leads to the conclusion that the potential surface of Kistemaker and de Vries is the preferred one to represent the Ar–N2 interaction. A surprisal analysis of the computed cross sections has been carried out. At energies below ?3000 K, near-linear surprisal plots are obtained, as found earlier by Levine, Bernstein, Procaccia et al., thus confirming the exponential gap law of Polanyi, Ding, and Woodall for rotational relaxation. Complete cross section matrices (at a given E) can thereby be generated from a two-parameter surprisal fit of a single column of a σj′j matrix (or even from a classically derived first moment from the state j=0). As expected, the rotational surprisal parameter ϑR is essentially independent of j, but it shows a significant, positive E dependence and differs in magnitude for the two potentials.
Brillouin scattering studies of normal and supercooled water
Authors: Rouch J. , Lai C. C. , Chen S. H.
Published: February 2003

High frequency sound propagation in water has been studied by Brillouin scattering in the entire temperature range of −9°C⩽t⩽100°C. Sound speeds deduced from the data show a peak at t≃70°C in agreement with previous ultrasonic results. Damping constants measured in a temperature range −9°C⩽t⩽20°C are also consistent with the ultrasonic values whenever available. A small negative dispersion of sound velocity is found however in the range 0°C⩽t⩽15°C. We explain this by a simple relaxation model similar to that used by Boon and Fleury in the case of rare gas liquids. We also use a theory of Brillouin scattering which takes into account the temperature fluctuation, and find a small correction term to the expression of the Landau–Placzek ratio. The correction term is calculated and is shown to be nonnegligible for water near 4°C. We believe the data taken in the supercooled regime are reported for the first time and their steep temperature dependence is as yet to be understood theoretically.
Mass spectrometric study of gas-phase clustering reactions: Hydration of the monovalent strontium ion
Authors: Tang I. N. , Lian M. S. , Castleman A. W.
Published: February 2003

Thermodynamic properties of hydrated monobalent strontium ions in the gas phase were measured over the temperature range 38–400°C using high pressure mass spectrometry. Clusters up to 11 water molecules about a strontium ion were observed. The hydration enthalpies and entropies for the clusters, Sr(H2O)n, n?9 were derived from appropriate van’t Hoff plots. The exceptional stability of the clusters observed in the present study indicates the importance of valence electrons of the central ion in pormoting chemical bonding with the clustering water molecules.
Kinetic studies of Ar–N2–SF6 mixtures following proton excitation
Authors: Chen C. H. , Payne M. G. , Hurst G. S. , Judish J. P.
Published: February 2003

Time resolved studies h!ve b%en made of both the vacuum ultraviolet (vuv) and the uv-visible radiation due to pulsed proton excitation of Ar–N2, Ar–SF6, and Ar–N2–SF6 mixtures. Photon emissions from Ar and from N2 (due to energy transfer from Ar) were both studied, using time resolved spectroscopic techniques. In the Ar–N2 mixtures the decay of N2(C 3Πu→B 3Πg) is much slower than the natural lifetime of N2(C 3Πu). In the case of the (0–1) band for the transition N2(C 3Πu→B 3Πg), the decay follows Ar(3P2); while the decay of the (3–1) band follows that of Ar(1P1). Decay rates of (1–0) and (2–1) are between the decay of Ar(1P1) and Ar(3P2). From least squares fitting, it was found that 68% of v′=2 of N2(C 3Πu) comes from Ar(1P1) and 32% from Ar(3P2); while 86% of v′=1 comes from Ar(3P2) and 14% from Ar(1P1). Since the energy precursor of N2(C 3Πu)v′=3 is Ar(1P1), it is clear that the Wigner spin conservation rule does not hold rigorously in this energy transfer process. Quenching rates of Ar(1P1), Ar(3P2), Ar(3P1) and Ar2(1u) by SF6 have also been determined. Gas laser applications are discussed briefly.
Mean reciprocal distances of short polymethylene chains. Calculation of the translational diffusion coefficient of n-alkanes
Authors: Freire Juan J. , Horta Arturo
Published: February 2003

The reciprocal distance averages of chain molecules 〈R−1ij〉 are calculated by a nonlattice Monte Carlo method. These 〈R−1ij〉’s are needed in the theory of hydrodynamic properties. The calculation is carried out for polymethylene chains of up to 50 C–C bonds. Interactions of short range and of long range are taken into account by means of the relative stability of rotational isomers and of a hard sphere excluded volume. The results obtained are used to interpret literature data on the translational diffusion coefficient of n-alkanes, in benzene, and in carbon tetrachloride.
Raman spectroscopy of gaseous GaCl3 and GaI3
Authors: Drake Michael C. , Rosenblatt Gerd M.
Published: February 2003

Raman spectra of the vapors over GaCl3, GaI3, and Ga–GaI3 mixtures have been obtained at temperatures of 160–960°C, 200–260°C, and 450–650°C, respectively. Three peaks due to GaCl3 are observed: 381 (ν1), 457 (ν3), and 128 cm−1 (ν4). Four peaks are observed for GaI3: 152, 192, 276 (ν3), and 60 cm−1 (ν4). Both GaCl3 and GaI3 are planar (D3h symmetry); the additional polarized peak in GaI3 arises from Fermi resonance involving ν1. The symmetric-stretch vibrations of all Group IIIa trihalides vary linearly with the inverse of the product of the internuclear distance and the square root of the mass of the halide atom, a trend which implies that all these molecules (BCl3, BBr3, BI3, AlCl3, AlBr3, AlI3, GaCl3, GaBr3, GaI3, InCl3, InBr3, InI3) have D3h symmetry. Thermodynamic functions are presented for GaCl3, GaBr3, and GaI3 based upon the present results.
Concentration dependence of the translational friction coefficient for polymer solutions
Authors: Freed Karl F.
Published: February 2003

A theory of the concentration dependence of the polymer translational friction coefficient f (and the sedimentation coefficient) is presented in terms of a multiple scattering representation of fluid scattering by the polymers in solution. The translational friction coefficient emerges as the the average T matrix for scattering of the fluid by the polymers, and the results of the Kirkwood–Riseman theory are recovered for the case of infinite dilution. The translational friction coefficient is evaluated under Θ conditions for concentrations equal to, or greater than, that of the semidilute region and for low but finite concentrations c, where f can be represented as a linear function of c via f=f0(1+ksc). The results obtained for ks are in agreement with experiments on poly(α-methylstyrene) and polystyrene in cyclohexane at their Θ temperatures. An explanation for the general increase in f (as well as the polymer viscosity), as c is raised, is provided in terms of the increased destruction of correlated intrachain scattering of fluid because of the intervention of uncorrelated fluid scatterings by other chains between the successive fluid scatterings by a given polymer.
Quantum chemistry by random walk. H 2P, H+3 D3h 1A′1, H2 3Σ+u, H4 1Σ+g, Be 1S
Authors: Anderson James B.
Published: February 2003

The random-walk method of solving the Schrödinger equation for molecular wavefunctions is extended to incorporate the effects of electron spin in several one- to four-electron systems. Improved calculation procedures reduce computation requirements for high accuracy by a factor of about 10. Results are given for the systems H 2P, H+3 D3h 1A′1, H2 3Σ+u, linear equidistant H4 1Σ+g, and Be 1S.
Methyl NMR relaxation due to dipolar interactions
Authors: Matson Gerald B.
Published: February 2003

The nuclear magnetic relaxation for the three equivalent spin one-half nuclei of a methyl group relaxed by intramolecular dipolar interactions is calculated in terms of three spin systems, the identification of each system being in terms of the symmetry representation and z-component magnetization of the spins. Preservation of the symmetry labels of each system is accomplished through symmetrization of the spin operators. The concepts of auto and cross correlation are shown to disappear in this formalism, to be replaced by spectral densities associated with symmetry-preserving or symmetry-crossing transitions. The relaxation equations become particularly simple in the limits of slow motion and rapid motion for the case of a methyl group attached to a spherical molecule in solution, and these limits are presented in terms of internal and overall rotational diffusion constants.
Single crystal EPR studies of the Cu2Cl84− ions in the complex tris(ethylenediamine)cobalt(III) di-μ-chloro-bis[trichlorocuprate(II)] dichloride dihydrate
Authors: McGregor Kenneth T. , Hatfield William E.
Published: February 2003

Single crystal electron paramagnetic resonance studies have been carried out on the exchange-coupled pairs of copper(II) ions contained in the title compound. EPR measurements were made at X-, K-, and Q-band microwave frequencies at selected temperatures in the range 2–77 °K. The angular variation of the Q-band spectra taken at 4.2 and 77 °K were used to diagnonalize the appropriate spin Hamiltonian. The Zeeman interaction and the zero-field interactions were found to have different principal coordinate systems. Although the dipolar interaction dominates the zero field splitting, a small but finite contribution from the anisotropic exchange interaction is present. The temperature dependence of the spectrum indicates that an interdimer exchange interaction is also present. The interdimer interaction causes the fine structure of the triplet excitations to be averaged into a single resonance line at 77 °K. The temperature of coalescence of the fine structure is 17 °K. Although the fine structure is resolved at liquid helium temperatures, owing to the lower density of excitations, the hyperfine interaction is averaged out by motion of the excitations produced by the effective interdimer exchange. The spin Hamiltonian parameters of the isolated excitations and the interdimer exchange frequency have been determined. These results are discussed in terms of the crystal and molecular structure of the complex.
Cross sections for 3064 Å band emission produced in collisions of 50–250 keV protons with water molecules in water vapor
Authors: Nussbaum Gilbert H. , Cathers Arthur R.
Published: February 2003

Emission by excited products of collisions between 50–250 keV protons and H2O molecules in water vapor has been studied in the wavelength region 1050–3300 Å. The relative single collision cross sections for emission of 3064 Å (? 2Σ+–? 2Π: O–O) band radiation by excited OH have been measured for proton kinetic energies of 50,150, and 250 keV. Measurements were also made of the relative cross section for emission of 3914 Å (B 2Σ+u−X 2Σ+g:O–O) band radiation by excited N2+ produced in collisions of 50 keV protons with N2. Comparison of the relative OH emission cross sections with that obtained for N2+, and normalization of the 3914 Å N2+ emission data to the absolute value for this cross section obtained by other investigators, yields values of the cross section for 3064 Å band emission by OH in H++H2O collisions of (2.82±1.42) ×10−18 cm2 at 50 keV, (1.79±0.93) ×10−18 cm2 at 150 keV, and (1.69±0.87) ×10−18 cm2 at 250 keV proton projectile energy.
Anisotropic fluorine and proton hyperfine interaction and motional effects in the electron spin resonance spectrum of the β,β,β-trifluoroethyl radical (CF3CH2⋅) in an argon matrix at 4.2 K
Authors: Jinguji M. , Lin K. C. , McDowell C. A. , Raghunathan P.
Published: February 2003

The electron spin resonance spectrum of the β,β,β-trifluoroethyl (CF3CH2⋅) radical has been measured in a polycrystalline argon matrix at 4.2 K. The radical was produced in situ by photolysing the corresponding iodide. An analysis of the spectrum leads to a complete evaluation of the anisotropies in the g and hyperfine interaction tensors of the spin Hamiltonian for the radical. The analysis is confirmed by a computer simulation of the observed line shapes, and the following magnetic parameters were evaluated: gx=2.0021, gy=2.0020, gz=2.0026, AxH(α) =23.7±0.1, AyH(α) =22.3±0.1, AzH(α) =30.9±0.1, AxF(α) =17.7±0.1, AyF(α) =20.4±0.1, and AzF(α) =48.0±0.1 G. A discussion of the above results provides an understanding of the mechanism of electron spin delocalization to the β-fluorine atoms, as well as additional information on the conformation of the α-substituent nuclei and the effects of their motion on the line shapes of the spectrum.
A molecular-statistical theory of the temperature-dependent pitch in cholesteric liquid crystals
Authors: van der Meer B. W. , Vertogen G. , Dekker A. J. , Ypma J. G. J.
Published: February 2003

An interaction between chiral molecules is derived in terms of the orientations of their long molecular principal axes. In analogy with Maier–Saupe and Goossens the derivation is based on the electric multipole expansion. The molecules are assumed to behave as if they were cylindrically symmetric, i.e., the system is locally nematic. As an example the chiral molecules are represented by Kuhn models, and the relevant coupling constants are calculated explicitely. An expression for the free energy is obtained in the molecular field approximation. It is shown that, if only the induced dipole–dipole and the dipole–quadrupole dispersion energies are taken into account, a temperature-independent cholesteric pitch is obtained. This result is traced back to the symmetric character of the interaction. In order to explain the experimental situation one has to introduce, in analogy with Keating, an asymmetry producing interaction. The proposed model is discussed and its thermodynamic properties are calculated in the molecular field approximation. It is found that the magnitude of the reciprocal pitch varies nearly linearly with temperature in agreement with experiment. Interpretational difficulties related to the use of the multipole expansion are discussed.
Stability of the NO+⋅N2 ion cluster
Authors: Turner D. L. , Conway D. C.
Published: February 2003

The equilibrium constant K for the reaction NO++2N2?NO+⋅N2+N2 has been determined mass spectrometrically from 178–273 °K at N2 pressures of 3–12 Torr. Small quantities of O2 were added to the gas stream to catalyze the charge exchange between N4+ and NO. By analysis of the K data it was found that ΔH°=−5.04±0.09 kcal/mole and ΔS°=−18.5±0.3 cal/mole °K at 298 °K. By extrapolation of the enthalpy data to 0 °K, the NO+–N2 bond energy was found to be 4.98±0.12 kcal/mole.
The visible emissions of molecular oxygen in rare gas solids
Authors: Richards John L. , Johnson Philip M.
Published: February 2003

The vacuum ultraviolet-excited emission spectra of molecular oxygen in low temperature rare gas solids have been recorded and analyzed. The observed transitions have been reassigned using isotopic substitution and are found to be C 3Δu→X 3Σg− in argon and krypton and c 1Σu−→a 1Δg in xenon. These transitions have not been observed in the gas phase and the transition origins established here confirm previous tentative assignments of vibrational numberings of the upper states by Herzberg and Degan, respectively. The emission in xenon is very unusual for a solid phase spectrum in that emission occurs from several excited vibrational levels, which are apparently being populated directly by intersystem crossing. It is also unusual in that the lower state of the emission is not the ground state. No significant chemical interaction between the molecule and the rare gas is seen for any of the observed states.
Energy loss spectra of Li+ ions (0.5–1.0 keV) in collision with N2 molecules
Authors: Sato Y. , Niurao K. , Takagi H. , Inouye H.
Published: February 2003

Energy loss spectra of Li+ ions scattered by N2 molecules were obtained at incident energies from 0.5 to 1.0 keV and at scattering angles from 1° to 7°. Peaks due to vibro–rotational excitation, electronic excitation, and ionization of N2 were observed in the energy loss spectra. The gross feature of the vibro–rotational excitation as a function of incident energy and scattering angle can be interpreted by a classical impulse approximation. The electronic excitation of N2 provided two energy loss peaks corresponding to the a 1Πg←X 1Σg+ transition and the c′ 1Σu+, c 1Πu←X 1Σg+ transitions, suggesting that the σg2p electron is most likely to be promoted. The vibro–rotational excitation in the electronic excitation and ionization is similar to that in the electronically elastic process. This suggests that the mechanism of vibro–rotational excitation is insensitive to the coupling of potential surfaces.
Ground X 2Σ+ state potential well, and excited state dynamics of diatomic XeF in solid Ne and Ar
Authors: Goodman Julie , Brus L. E.
Published: February 2003

The ’’shape’’ of the ground state potential well of XeF in solid Ne has been obtained from an analysis of D (1/2) →X 2Σ+ resonance fluorescence. The constants ωe″=247 cm−1 and ωexe″=10.2 cm−1 give an extrapolated Morse oscillator De?1500 cm−1. The ground state well is only slightly deeper in the solid than in the gas phase (De?1160 cm−1). In solid Ar, the Te values for the excited D (1/2) and B (1/2) states undergo unprecedented red shifts of ?8% and ?13% respectively, apparently reflecting the charge transfer nature of these excited states. The extrapolated gas phase radiative lifetimes are τr=13±1 nsec for D (1/2), and τr=8±2 nsec for B (1/2). Polarized fluorescence studies prove that the gas phase 3500 Å lasing transition is B (1/2) →X 2Σ+. The influence of the Te red shift upon the excited state dynamics, and upon the X 2Σ+ well depth in the solid, is discussed. Unassigned emission spectra are also observed from other van der Waals complexes among Xe and F atoms.
Decomposition of isolated molecules: A transition state treatment
Authors: Kay Kenneth G.
Published: February 2003

It is argued that, contrary to the assumption of RRKM theory, reactant states near the critical surface frequently may not be at statistical equilibrium with the bulk of reactant states. The main objective of this work is to examine conditions under which the RRKM specific unimolecular rate constant expression nevertheless remains valid. The analysis proceeds by casting the general, time-dependent, decomposition rate of isolated molecules into time-independent form and applying a transition state approximation similar to that introduced by Miller for bimolecular reactions. In the process of carrying out this program, the following is achieved: (a) A potentially useful unimolecular rate expression, analogous to Miller’s transition state theory rate formula for bimolecular reactions, is obtained; (b) a criterion for choosing critical configurations similar to the Bunker–Pattengill recipe is derived when classical mechanics is obeyed; (c) features of a recent calculation of unimolecular reaction dynamics are explained.
The low-lying states of hydrogen fluoride: Potential energy curves for the x 1Σ+, 3Σ+, 3Π, and 1Π states
Authors: Dunning Thom H.
Published: February 2003

Generalized valence bond (GVB) and derived configuration interaction (CI) calculations have been carried out on the states of hydrogen fluoride (HF) which arise from ground state atoms: X 1Σ+, 3Σ+, 3Π, and 1Π. The GVB plus all single and double excitation calculations yield (with experimental constants in parentheses): De=5.72 eV (6.12 eV), Re=0.920 Å (0.9168 Å), and ωe=4158 cm−1 (4138.73 cm−1). The polarization CI (POL−CI) wavefunction is found to reproduce well the results of the more extensive calculation. The POL−CI wavefunction takes into account the internal and semi-internal correlation effects of open-shell many electron theory. A simple formula, involving Morse functions, is proposed to account for the remaining error in the ground state potential curve.
Kinetic studies of N2 and N2–SF6 following proton excitation
Authors: Chen C. H. , Payne M. G. , Hurst G. S. , Judish J. P.
Published: February 2003

Low intensity proton pulses have been used to excite pure N2 and N2–SF6 mixtures. By accumulating time and wavelength resolved information on the fluorescence that arises from a very large number of pulses, we have obtained data that are free from effects due to superelastic collisions with electrons or other nonlinear effects such as the interactions between ions or excited neutral species. For N2 the natural lifetimes of N2(C 3Πu) and N2+(B 2Σu+) were determined by monitoring N2(C 3Πu→B 3Πg) and N2+ (B 2Σu+→X 2Σg+) states, respectively. The quenching rates of N2(C 3Πu) and N2+(B 2Σu+) in different vibrational states by pure N2 and by SF6 were obtained. The addition of small concentrations of SF6 altered the late time behavior of the N2(C 3Πu→B 3Πg) transitions, and the addition of much higher concentrations of SF6 caused a large increase in the intensity of the 3371 Å (C→B) transition.
A theory of molecular velocity correlations in liquids
Authors: Phillipson Paul E.
Published: February 2003

It is proposed that the frequency spectrum (Fourier transform) of the single particle velocity correlation function for a classical liquid is of the form ρ (ω) = (ωc2−ω2)1/2f (ω) + (ωcω−ω2)1/2g (ω), where ωc is identified with the intermolecular collision frequency. The rationale for this construction is that the features of a cutoff frequency and nonanalytic points characteristic of the phonon spectrum of a solid should be reflected in the associated liquid: ωc defines the cutoff frequency and ρ (ω) is nonanalytic at ω=0. As a consequence, a general analytic expression for the correlation function as a superposition of Bessel functions is developed, whose damped oscillatory behavior parallels that deduced from neutron scattering experiments and molecular dynamics computer simulation studies of liquids. The Bessel function basis set for the correlation function implies an expansion of the frequency spectrum in terms of a Gegenbauer polynomial basis set, both of which can provide analytical tools for molecular dynamics investigations. For long times such that ωct≫1 the correlation function decays as t−3/2[A+B sin(ωct−π/4)]. The first term is identified with the shear viscosity from hydrodynamics, and it is argued that the second term should also exist as a residuum lattice contribution not deriveable from hydrodynamics. A specific two parameter model spectrum is considered to establish a connection between the present formulation and the Langevin theory of Brownian motion.
Dynamic orientational pair correlations in symmetric tops and static orientational pair correlations in anisometric molecules
Authors: Gierke T. D.
Published: February 2003

The theory describing the depolarized spectrum of light scattered from symmetric tops is re-examined. On the basis of molecular symmetry it is shown that, in the rotational diffusion limit, dynamic orientational pair correlations do not affect the depolarized spectrum of scattered light. Molecular symmetry arguments are also applied to the calculation of the depolarized spectrum of light scattered from asymmetric tops and binary solutions of symmetric tops. It is found that orientational pair correlations may affect the calculated spectrum in these two cases in a rather complex fashion. Effects which had previously been attributed to the affect of dynamic orientational correlations may actually be explained on the basis of static orientational correlations alone. The experimental data on nitrobenzene and binary solutions of hexafluorobenzene/benzene are discussed explicitly. The possibility of obtaining information about the static orientational pair correlations of more than one axis in anisometric molecules is suggested.
Nonequilibrium velocity distribution and reaction rate in hot-atom reactions
Authors: Koura Katsuhisa
Published: February 2003

The nonequilibrium velocity distribution and reaction rate in the hot-atom reactions are studied by solving the time-dependent Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distribution, temperature, and rate constant of hot atoms from initial to steady states are obtained for a simple model system, where hot atoms are dispersed in the heat bath of surrounding molecules without internal degrees of freedom and the cross sections are chosen as a simple hard sphere model. The low- or high-temperature steady state exists in consistence with the prediction of Keizer with the Maxwell distribution approximation. The hot-atom velocity distribution for the high-temperature steady state is, however, different from the Maxwell distribution. The nonequilibrium velocity distribution yields a much smaller value of the lower limit of the ratio of reactive to elastic cross sections for the existence of the high-temperature steady state and a smaller rate constant than those of the Maxwell distribution approximation by Keizer.
Collision-induced dissociation of N2+ in X 2Σg+, A 2Πu and excited metastable states in N2+–N2 interactions
Authors: Moran T. F. , Wilcox J. B. , Abbey L. E.
Published: February 2003

The collision-induced dissociation of 0.65–5.0 keV N2+ has been examined in an apparatus with an extended flight path through which ions take a long time to travel. Short lived states in the reactant ion beam decay radiatively prior to reaction. Reactant ions have been produced in an electron impact ion source. Collision-induced dissociation of N2+ has been studied as a function of the ionizing electron energy. Electronic and vibrational state distributions of reactant ions as functions of ionizing electron energy have been estimated from published spectroscopic data and the dissociation cross sections determined for N2+ (X 2Σg+,υ and A 2Πu,υ) ions. The relative contribution of a higher metastable ion state or states to the dissociative channels becomes more important as the incident ion kinetic energy is lowered. An energetic threshold of approximately 22.5±1.5 V has been measured for this long-lived state leading to N+ in dissociative collisions.
Weyl’s theory applied to predissociation by rotation. I. Mercury hydride
Authors: Hehenberger M. , Laskowski B. , Brändas E.
Published: February 2003

Weyl’s theory for singular ordinary second order differential equations is presented as a numerical method for analyzing the spectral properties of the Schrödinger operator associated with predissociation by rotation in diatomic molecules. It is pointed out that the poles of Weyl–Titchmarsh’s m function characterize both the discrete eigenvalues and the resonances in the continuous spectrum, thus allowing a unified treatment of the entire spectrum. The conventional hard core treatment of the origin is compared to two alternative approaches, both applicable to more general singular behavior of the potential. A detailed discussion of the other singularity, infinity, is given and Kodaira’s theorem is generalized to an expression involving Wronskian’s between the regular and the asymptotic solutions. The calculation of the Weyl–Titchmarsh m function is based on numerical Runge–Kutta integrations of both independent solutions, combined with asymptotic information derived from a Riccati expansion. Im(m) is shown to have its physical significance as flux of predissociating particles. The resonances in the continuous spectrum are obtained by numerical analytical continuation of Im(m) across the real energy axis. The method is applied to Stwalley’s isotopically combined (IC1) potential for HgH. All resonance energies and lifetimes are reported and compared to Stwalley’s phase shift results. Excellent agreement is found for sharp resonances whereas considerable discrepancies occur for the broader ones. This latter fact is attributed to the failure of the Breit–Wigner fit to account for asymmetric non-Lorentzian shapes.
Excitonic interaction among three chromophores: An application to the purple membrane of H
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Authors: Kriebel Arnd N. , Albrecht Andreas C.
Published: February 2003

The spectroscopic properties of the purple membrane of Halobacterium halobium in the visible region are examined within the framework of an exciton model for the retinal chromophores in bacteriorhodopsin (bR). Both earlier x-ray and electron diffraction studies as well as more recent circular dichroism (CD) measurements indicate that considerable chromophore interaction is likely in the purple membrane. This interaction disappears when the protein is solubilized. The exciton model accounts for the red shift of the visible absorption band in the membrane from the absorption peak seen in solubilized bR. It explains the approximate conservation of absorption intensity as well as the changes in the observed CD spectrum when bR is solubilized. The dipole moments of the Cl− salt of the protonated N-butyl Schiff base of all-trans retinal in its electronic ground and first dipole allowed excited state have recently been measured. If these are introduced into the exciton model an estimate of the distance of adjacent retinal chromophores in purple membrane is obtained.
Further Mössbauer study of iron in human transferrin
Authors: Tsang C. P. , Bogner L. , Boyle A. J. F.
Published: February 2003

Mössbauer spectra of human serium transferrin have been observed in high magnetic field at 4.2 K and at temperatures as low as 175 mK. These spectra were fitted to the calculated spectra based on a rhombic spin Hamiltonian for a high spin ferric ion. Good fittings of the spectra were obtained and the parameters of the electronic part of the spin Hamiltonian were found to be D=0.3 cm−1 and λ=0.26. Effects of the Mössbauer spectra due to higher order spin Hamiltonian terms were investigated and used to explain the discrepancies between EPR and Mössbauer results. Changes in the Mössbauer spectra in the presence of perchlorate ions have also been investigated.
Quantum mechanical reactive scattering for three-dimensional atom plus diatom systems. II. Accurate cross sections for H+H2
Authors: Schatz George C. , Kuppermann Aron
Published: February 2003

Accurate three-dimensional reactive and nonreactive quantum mechanical cross sections for the H+H2 exchange reaction on the Porter–Karplus potential energy surface are presented. Tests of convergence in the calculations indicate an accuracy of better than 5% for most of the results in the energy range considered (0.3 to 0.7 eV total energy). The reactive differential cross sections are exclusively backward peaked, with peak widths increasing monotonically from about 32° at 0.4 eV to 51° at 0.7 eV. Nonreactive inelastic differential cross sections show backwards to sidewards peaking, while elastic ones are strongly forward peaked with a nearly monotonic decrease with increasing scattering angle. Some oscillations due to interferences between the direct and exchange amplitudes are obtained in the para-to-para and ortho-to-ortho antisymmetrized cross sections above the effective threshold for reaction. Nonreactive collisions do not show a tendency to satisfy a ’’jz-conserving’’ selection rule. The reactive cross sections show significant rotational angular momentum polarization with the mj=m′j=0 transition dominating for low reagent rotational quantum number j. In constrast, the degeneracy averaged rotational distributions can be fitted to statistical temperaturelike expressions to a high degree of accuracy. The integral cross sections have an effective threshold total energy of about 0.55 eV, and differences between this quantity and the corresponding 1D and 2D results can largely be interpreted as resulting from bending motions in the transition state. In comparing these results with those of previous approximate dynamical calculations, we find best overall agreement between our reactive integral and differential cross sections and the quasiclassical ones of Karplus, Porter, and Sharma [J. Chem. Phys. 43, 3259 (1965)], at energies above the quasiclassical effective thresholds. This results in the near equality of the quantum and quasiclassical thermal rate constants at 600 K. At lower temperatures, however, the effects of tunneling become very important with the quantum rate constant achieving a value larger than the quasiclassical one by a factor of 3.2 at 300 K and 18 at 200 K.
Dynamics of carbon monoxide binding to protoheme
Authors: Alberding N. , Austin R. H. , Chan Shirley S. , Eisenstein L. , Frauenfelder H. , Gunsalus I. C. , Nordlund T. M.
Published: February 2003

Protoheme rebinding of carbon monoxide after photodissociation has been observed at temperatures from 5 to 340 K for times from 2 μs to 1 ks. Below 80 K, binding is nonexponential in time and CO-concentration independent, above 230 K exponential and the rate is CO-concentration proportional. A model is proposed in which the carbon monoxide, moving from the solvent to the binding site at the ferrous heme iron, encounters two successive barriers. The outer is formed by the solvent, the inner is a property of the heme and probably connected to the motion of the iron from the spin-2 deoxy to the spin-0 carbon monoxide state. The temperature dependence of the two processes yields all activation enthalpies and entropies for the two barriers. The nonexponential rebinding observed at low temperatures implies that the inner barrier possesses distributed activation enthalpy and entropy. The enthalpy spectrum and the entropy spread are determined. The spectrum demonstrates that heme exists in many different conformational states. At low temperatures, these states are frozen; above about 230 K, rapid conformational relaxation renders rebinding exponential. Below 15 K, quantum-mechanical molecular tunneling dominates. The tunneling rate yields the width of the innermost barrier. Earlier experiments on carbon monoxide binding to myoglobin had provided evidence for four barriers. The present results imply that the innermost barrier in myoglobin is caused by the heme, the outermost by the solvent, and the two intermediate ones by the globin.
Analytical evaluation of multicenter integrals for Slater-type orbitals. II. Three-center nuclear attraction integrals
Authors: Guseinov I. I.
Published: February 2003

Convenient formulas have been obtained for three-center nuclear attraction integrals with an arbitrary combination of quantum numbers of Slater-type atomic orbitals. New auxiliary functions have been introduced by means of which these integrals are expressed. The auxiliary functions are discussed in detail, and recurrence relations and analytical formulas given for their computation.
Crossed molecular beam metathesis: Alkaline earth dihalides+Cl2 and HCl
Authors: Freedman Andrew , Behrens Richard , Parr Timothy P. , Herm Ronald R.
Published: February 2003

Exploratory crossed molecular beam studies of bimolecular reactions of alkaline earth dihalides with halogen containing compounds are reported. The c.m. product distribution is symmetric about ϑ=90° for the BaI2+Cl2 halogen-exchange reaction indicating formation of a collision complex which persists for a least a few rotational periods. The data are consistent with, but do not establish, formation of a similar complex in the SrI2+Cl2 reaction and with a statistical loose transition state energy partitioning in both reactions. The BaI2+HCl→BaICl+HI reaction is direct, favoring backscattering of the metal atom. Relative total reaction cross sections for the MX2+Cl2 halogen-exchange reactions decrease as 1.0:0.45:<0.007 when M is varied from Ba to Sr to Mg. All of these observations support an ion-pair adduct model which was previously advanced to interpret facile four-center exchange reactions of cesium halides with diatomic halogens. This model, which qualitatively correlated observed trends with reactant electronic structure, pictures these present reactions as (MI)+I−+Cl2→ (MI)+(Cl−I−Cl)−→products and (MI)+I−+HCl→ (MI)+(I−H−Cl)−→MICl+HI.
The interconversion of the zwitterion and the uncharged form of γ-aminobutyric acid (GABA) at low temperatures
Authors: Huong P. V. , Cornut J. C.
Published: February 2003

The infrared spectra of γ-aminobutyric acid (GABA) in the solid state and inert gas matrices at several temperatures and concentrations have been analyzed. Evidence of a neutral molecular form of GABA is shown for the first time; this compound was known until now only in its zwitterionic or cationic forms. The intermolecular mechanism is also established for the formation of the GABA zwitterion. This final step is shown to be preceeded by hydrogen-bonded intermediates.
Comment on ’’Isotope effect in solutions of hydrogen and deuterium in niobium’’
Authors: Westlake D. G.
Published: February 2003

The thermodynamic properties of niobium metal and the effects of hydrogen and deuterium in the metal on its thermal properties are discussed. (AIP)
Reply to ’’Comment on ’isotope effect in solutions of hydrogen and deuterium in niobium’ ’’
Authors: Steward S. A.
Published: February 2003

The thermal properties of niobium metal and the effect of the presence of hydrogen and deuterium in the metal are discussed. (AIP)
Product magnetic deflection slotted disk velocity analysis molecular beams kinetics: Li+SnCl4, PCl3, and SF6
Authors: Behrens Richard , Herm Ronald R. , Sholeen Charlotte M.
Published: February 2003

Recoil velocity spectra of the diamagnetic Li-containing products reactively scattered from crossed thermal beams of Li and SnCl4, PCl3, or SF6 have been measured by positioning a combination of an inhomogeneous deflecting electromagnetic and slotted disk velocity analyzer between the collision zone and detector. Transformation of these data into the center-of-mass coordinate system provides contour maps of the differential reaction cross sections. Although data on Li+SF6 are very noisy, they are consistent with a long-lived collision complex and energy equipartitioning in a loose transition state (LTS) mechanism. At some scattering angles, two peaks are observed in the measured product recoil spectra for both Li+SnCl4 and Li+PCl3. For Li+SnCl4, indirect kinematic arguments suggest that this bimodal structure is due to LiSnCl3+Cl and LiCl+SnCl3 product channels proceeding via an LTS mechanism. For Li+PCl3, this bimodal structure is attributed to production of LiCl+PCl2 products via an LTS and a direct mechanism. These results are discussed in relation to previous work on these systems and interpreted in terms of the electronic structure of the ion-pair intermediates.
Pressure dependence of fluorine chemical shift in CaF2
Authors: Lau K-F. , Vaughan R. W.
Published: February 2003

The flourine chemical shift of a single crystal CaF2 has been measured as a function of external pressure up to 4 kbar at room temperature using multiple pulse NMR techniques. The pressure dependence of the shift is found to be −1.7±1 ppm/kbar, while a theoretical calculation using an overlap model predicts a shift of −0.46 ppm/kbar.