bluuebunny/crossref_metadata_2025 huggingface dataset:
Effective potential formulation of molecule-molecule collisions with application to H2–H2
Authors: Zarur George , Rabitz Herschel
Published: February 2004

Coupled equations and cross section expressions are derived for the collision of two distinguishable and two identical molecules. A previously developed effective potential (Hamiltonian) formulation of molecular collisions is used throughout this paper. A semiclassical analysis of the total and differential cross sections is presented as an extension of previous elastic scattering studies. The semiclassical analysis is used to discuss conditions for significant quantum statistical oscillations in the elastic and inelastic identical molecule cross sections. Close coupling calculations for collisions of para-para, ortho-ortho, and ortho-para hydrogen up to E = 0.725 eV are presented as an illustration of the general formulation. Various aspects of the H2–H2 potential are investigated. In particular, the different roles of the long- and short-range anisotropies are illustrated. Both total and differential cross sections are presented for a number of transitions. The identical molecule inelastic differential cross sections show considerable quantum statistical oscillations, particularly in para-para hydrogen. The degree of quantum statistical interference is shown to depend on the specific transition considered. The nature and desirability of certain experimental measurements is discussed with regard to determining the energy and angular dependent behavior of the cross sections.
Excitation transfer between fine structure levels: Angular dependence of the sensitized fluorescence
Authors: Chiu Lue-Yung Chow
Published: February 2004

Excitation transfer is studied theoretically as an integral part of sensitized fluorescence, which is a third order time-dependent process, namely the excitation of the absorber atom, followed by the transfer of excitation from this atom to the receiver atom and finally the emission of sensitized fluorescence from the receiver atom. The phase correlation transmitted through such a third order process gives rise to the angular distribution of sensitized fluorescence which is shown to depend on the fine structure levels of both atoms. Long range electric dipole-dipole interaction is used as the mechanism for excitation transfer. Plane wave motion is assumed for the relative motion between two interacting atoms. A general formula is derived for the relative intensities (hence relative cross sections) of the sensitized fluorescence, involving different sets of fine structure levels. This cross-section ratio turns out to be angular dependent but nevertheless is shown to be expressible in a very simple form which can be readily calculated. The ratio calculation is illustrated for the case of K*(42P1/2,3/2) +Rb(52P1/2,3/2). Nuclear spin effect is also considered. For the weak coupling case, the nuclear spin is found to have no effect on the sensitized fluorescence.
Magnetic properties of the trichlorides, tribromides, and triiodides of U(III), Np(III), and Pu(III)
Authors: Jones E. R. , Hendricks M. E. , Stone J. A. , Karraker D. G.
Published: February 2004

Magnetic susceptibilities of uranium, plutonium, and neptunium trichlorides, tribomides, and triiodides were measured from 2.7°K to as high as 240°K with a vibrating-sample magnetometer at applied fields up to 12 kOe. Antiferromagnetic transitions were observed in UCl3, TN=22.0±1.0°K; in UBr3, TN=15.0±0.5°K; in UI3, TN=3.4±0.2°K; and in PuCl3, TN=4.5±0.5°K. A ferromagnetic transition at Tc=4.75±0.1°K was found for PuI3, and a slight hysteresis was measured at 2.7°K. Temperature-independent paramagnetism was observed for NpCl3 below 50°K, for α-NpBr3 below 30°K, and for NpI3 below 15°K. All these latter compounds, and PuBr3, exhibited Curie-Weiss paramagnetism at higher temperatures, with effective moments near the free-ion moments. The experimental magnetic susceptibility of PuCl3 from 10 to 100°K agrees with the susceptibility calculated from crystal field energy levels and wavefunctions.
Kinetic theory and rheology of bead-rod models for macromolecular solutions. I. Equilibrium and steady flow properties
Authors: Hassager Ole
Published: February 2004

The behavior of macromolecules in solutions at rest and undergoing steady flow is considered. The Kramers' freely jointed bead-rod model (the ``pearl-necklace'' model) is used. It is shown that the distribution function for a random walk process is a good approximation to the true equilibrium distribution function for the pearl necklace as obtained by the methods of classical statistical mechanics. The rheological behavior in slow steady flow is solved exactly for the case of two links, and in an approximation for an arbitrary number of links. It is shown how the zero shear rate viscosity of the once-broken rod model may be obtained by a minor modification of the case with two links. The behavior in large deformation rate elongational flow is investigated, including a comparison of rigid and flexible molecules, and an estimate of the elongation rates necessary to produce uncoiling of flexible macromolecules. It is finally shown that a bead-rod model with a large number of links is approximated by a ``corresponding'' bead-spring model.
Crossed beam study of the reaction O+ + N2 → NO+ + N
Authors: Smith Gregory P. K. , Cross R. J.
Published: February 2004

The kinematics of the reaction O+(4S)+N2→NO++N were studied using crossed molecular beams. Product angular and energy distributions were measured for the initial relative translational energy range of 1.5–15.8 eV. The products are formed in their ground electronic states. The product distributions are described, with some deviations, by the spectator stripping model. No evidence was found for a persistent complex or for a second mechanism. A scattering chamber study at 0° lab was made of the related reactions 15N++14NO→15N14N+, 15NO+, 14NO+. Our results are discussed in terms of possible surface hopping effects.
Microwave spectrum, dipole moment, and conformation of 2,3-dihydro-p-dioxin(1,4-dioxene)
Authors: Wells John A. , Malloy Thomas B.
Published: February 2004

The microwave spectrum of 2,3-dihydro-p-dioxin(1,4-dioxene) has been studied in the R-band range (26.5–40.0 GHz) using a Hewlett-Packard model 8400 Stark modulated spectrometer. The rotational constants for the ground state and four vibrationally excited states have been derived from fitting b-type, low J R -branch transitions. The rotational constants for the ground state are (in megahertz) A = 5697.703±0.010; B = 4968.410±0.008; C = 2865.181±0.010. The presence of a C2 symmetry axis has been established by analysis of the Stark effect yielding a single nonzero component of the dipole moment μb = 0.939±0.008 D and by the effect of nuclear spin statistical weights on the relative intensities of the rotational transitions. That the symmetry point group is C2 rather than C2v is established by the value of (1/2)(Ia + Ib − Ic) which is clearly too large for the C2v structure. Model calculations to reproduce the rotational constants show that they are particularly sensitive to two structural parameters: the C=C–O angle (123.4±0.5°) and the twist angle, i.e., the angle between the C=C bond and C–C bond (29.9±1.5°). Consideration of the ring angles for a structure derived by diagnostic least squares shows little, if any, ring strain.
Hard-sphere melting model: Melting in argon
Authors: Crawford R. K.
Published: February 2004

A melting model based on the properties of a system of hard spheres is described and used to calculate the melting properties of argon. The results are shown to agree quite well with the experimental data for argon and with the Monte Carlo calculation for the Lennard-Jones potential. The model is then used as the basis for a discussion of various features of the melting data.
Raman scattering of HF in the gas state and in liquid solution
Authors: Duff Y. Le , Holzer W.
Published: February 2004

We have studied the Raman scattering of HF in the gas state and dissolved at low concentration in liquid SF6. For the gas, the absolute intensity of the vibrational band has been measured and the polarizability derivative has been determined to be d α/d r = 1.00 × 10−16 cm2. The system HF perturbed by 23 atm of gaseous SF6 is discussed; we also give some results on the HF polymer bands at 2900–3800 cm−1.
On the problem of hard squares
Authors: Mehta M. L.
Published: February 2004

It is usually thought from computer studies that the partition function of hard squares on a crossed square lattice has all its zeros in the left half complex plane. From the study of a simpler problem we argue that, although not impossible, this is very unlikely.
Theoretical investigations of the reaction dynamics of polyatomic systems: Chemistry of the hot atom (T* + CH4) and (T* + CD4) systems
Authors: Raff L. M.
Published: February 2004

An unadjusted computation of the reaction dynamics in the (CH4 + T*) and (CD4 + T*) systems has been carried out. The six-body potential-energy surface has been obtained from the equilibrium thermodynamic and spectroscopic data for reactants and products, the results of all-valence electron INDO and all-electron, ab initio SCF and CI quantum calculations, and previously formulated three- and four-body valence-bond (VB) potential surfaces. The computed saddle-point geometries for axial abstraction and inversion displacement are in good to excellent agreement with previous ab initio CI calculations. The saddle-point energies are in fair to good agreement. Computed fundamental vibration frequencies for CH4 are in excellent accord with ir and Raman data. Reaction cross sections as a function of relative translational energy for abstraction, displacement, and fragmentation in (CH4 + T*) and (CD4 + T*) systems have been computed by quasiclassical trajectory analysis. Calculated thresholds are in quantitative agreement with experiment. The abstraction and displacement reaction dynamics are examined and discussed. Hot-atom yield ratios in both systems have been determined through solutions of the integral reaction probability equation. Computed results for nuclear recoil tritium incident upon CH4 are in quantitative agreement with experiment. Yield ratios for [CD3T/DT] obtained by photolysis of TBr are in excellent accord with experiment at all photolysis energies. Abstraction yields in CH4 are computed and found to be in good agreement with experiment. The low energy (40–65 kcal/mole) displacement cross sections are found to be too low by a factor of 2–3.
Studies on dielectric relaxation in some rotational isomeric molecules
Authors: Hasan A. , Das A. , Ghatak A.
Published: February 2004

Dielectric permittivities and losses for 1,2-ethanedithiol, 1,2-propanedithiol, and 1,2-dichloropropane in the liquid state have been measured at 1.62, 3.17, and 3.49 cm in the microwave region at different temperatures. The distribution parameter in 1,2-propanedithiol was found to increase with increasing temperature in contrast to that in 1,2-ethanedithiol and 1,2-dichloropropane. A possible explanation for this has been suggested. The energy differences between the trans and gauche isomers in all the liquids obtained from the dipole moment measurements agree fairly well with those obtained from spectroscopic studies.
Vibrational analyses and barrier to internal rotation of CH3CHBr2 and CD3CDBr2
Authors: Durig J. R. , Sloan A. E. , Thompson J. W. , Witt J. D.
Published: February 2004

The infrared spectra of gaseous and solid 1, 1-dibromoethane and 1, 1-dibromoethane-d4 have been recorded from 140 to 3500 cm−1. The corresponding Raman spectra of the liquid and solids have also been recorded and depolarization values have been measured. All spectra have been interpreted in detail and the 18 normal vibrations have been characterized on the basis of band contours, isotopic shifts, depolarization values, and normal coordinate calculations. The internal torsional mode was observed at 253 cm−1 in the far infrared spectrum of gaseous CH3CHBr2 and the assignment was confirmed by the shift with deuteration. The threefold barrier to rotation was calculated to be 4.3 kcal/mole. This determination allows a comparison of the barriers for a series of bromoethanes.
Thermodynamic properties of ZrO (g) and HfO (g); a critical examination of isomolecular oxygen-exchange reactions
Authors: Ackermann R. J. , Rauh E. G.
Published: February 2004

The thermodynamic properties of ZrO(g) and HfO(g) have been determined by mass-spectrometric studies of five isomolecular oxygen-exchange reactions of the type M(g)+M′O(g)=MO(g)+M′(g) involving the gaseous atoms and monoxides of yttrium and thorium. Third law values of the standard free energies of formation over the temperature range 2000–2800 K are adequately (within 2%) expressed by the linear equations ΔG°f(ZrO,g)=13 040−16.04T, ΔG°f(HfO,g)=9930−14.68T. Uncertainties of ±1 kcal mol−1 are estimated. Third law analyses of the data yield the enthalpies of formation at absolute zero, ΔH°0(ZrO,g)=21.4, and ΔH°0(HfO,g) = 18.5 kcal mol−1, and the dissociation energies at absolute zero, D0(ZrO)=180.7, and D0(HfO)=188.9 kcal mol−1, with uncertainties of 1–2 kcal mol−1. The equilibrium constants for the isomolecular reactions in this and in previous studies were obtained from ion currents and relative instrumental sensitivities that were approximated from the mean square radii of the gaseous atoms and the squares of the interatomic distances of the gaseous monoxides; the use of these parameters was shown to improve the closure among several thermodynamic cycles by as much as 2 kcal mol−1
Floating ellipsoidal Gaussian orbital computations on small molecules
Authors: Simons Gary , Schwartz Anna K.
Published: February 2004

Electronic structure calculations on H2, HeH+, He22+, HeH−, LiH, BeH+, Li2, BH, CH+, HBH+, HBeH, HLiH−, LiHLi+, HBH−, HCH, and HNH+ are performed using the floating ellipsoidal Gaussian orbital (FEGO) method and optimizing geometries. The resulting electronic energies; nuclear geometries; and shapes, sizes, and locations of orbitals are compared to the corresponding floating spherical Gaussian orbital values. The results suggest that energetics and bond angles are more accurately described by the FEGO computations. Bond lengths are predicted to be longer in the FEGO method. A discussion of the role of lone pair electrons in both methods is given.
Excited molecular terms of the alkali-rare gas atom pairs
Authors: Pascale J. , Vandeplanque J.
Published: February 2004

Numerous excited molecular terms for the various alkali-rare gas atom pairs have been determined. The semiempirical potential model of Baylis has been used with some modifications in the calculation of the molecular terms; in particular, a large number of atomic states are included in our calculation, which ensure the stability of the calculated molecular terms. Our results show that coupling among molecular terms is very important and gives rise to structure in the excited potential energy curves. Comparisons between our results and other theoretical and experimental data are made.
Comparisons of Morse and harmonic oscillator models for vibration-rotation excitation of H2 by Li+
Authors: Kouri Donald J. , Wells Charles A.
Published: February 2004

Matrix elements of a potential for atom-diatomic molecule collision problems in three dimensions have been calculated using two models to describe the rotating-vibrating molecule. These models are the undistorted three dimensional harmonic oscillator model of Eastes and Secrest and a rotating-vibrating Morse oscillator. The most important difference between the two models is the presence of centrifugal stretching in the Morse oscillator model. A comparison of matrix elements is made which is applicable to any atom-diatom potential with a dependence on the vibrational coordinate in the form ξ, ξ2, eαξ, and ξeαξ. In addition, the specific example of Lester's ab initio Hartree-Fock potential for Li+–H2 is considered. Cross sections are calculated for Li+–H2 with limited basis sets for the harmonic and Morse oscillator at 0.1, 2.0, and 3.26 eV relative kinetic energy.
Temperature dependence of the probability of vibration-vibration-rotation energy transfer in HCl (v = 2) + HCl (v = 0) ⇋ HCl (v = 1) + HCl (v = 1)
Authors: Shin Hyung Kyu
Published: February 2004

Energy transfer probabilities for the vibration-vibration process HCl(v = 2) + HCl(v = 0)⇄HCl(v = 1) + HCl(v = 1) + ΔE = 102 cm−1 have been calculated in the temperature range of 300–1000°K assuming that ΔE is supplied or removed through the vibration-rotation process. The interaction potential is assumed as the sum of repulsive, attractive, dipole-dipole, and hydrogen-bond energy terms. It is shown that the probability decreases nearly linearly with rising temperature between 300–600°K; this result agrees with experiment. After reaching a minimum value at 700°K, the probability increases with temperature. The temperature dependence is explained in terms of the contribution of molecular attractions including dipole-dipole and hydrogen-bond terms and the effect of translational motion on the vibration-rotation process.
X-ray scattering from diatomic molecules in the liquid state
Authors: Morrison Paul F. , Pings C. J.
Published: February 2004

A version of the Steele-Pecora equation suitable for use with diatomic molecules has been derived. Substitution of chlorine scattering factor coefficients and Percus-Yevick distribution functions into this equation led to the determination of total scattered intensity functions expressed as sums of gas scattering, spherical, and angular intensity contributions. The angular contributions were shown to be experimentally significant in the regions of the first and second peaks at high densities (ρ*≲ 1.2). Temperature was shown to have only a slight effect on total intensity. g000, g200, and g220 were found to be the principal contributors to the intensity.
Radiative lifetimes of the alkaline earth monohalides
Authors: Dagdigian Paul J. , Cruse Howard W. , Zare Richard N.
Published: February 2004

The radiative lifetimes for a number of electronic states of the alkaline earth monohalide molecules MX, where M denotes Ca, Sr, or Ba and where X denotes F, Cl, Br, or I, have been determined directly from the rate of fluorescence decay with time using a pulsed dye laser as an excitation source. The measured lifetimes in nanoseconds are CaF: A1 21.9(4.0), A2 18.4(4.1), B 25.1(4.0); CaCl: A1 29.4(1.8), A2 28.4(2.6), B 38.2(3.9), C 25.0(2.1); CaBr: A1 34.2(2.9), A2 33.7(1.9), B 42.9(3.1), C1 33.2(4.0), C2 31.8(3.7); CaI: A1 41.7(2.3), A2 41.6(3.4), B 50.9(1.7); SrF: A1 24.1(2.0), A2 22.6(4.7); SrCl, A1 31.3(2.7), A2 30.4(3.6), B 38.8(2.1), C1 26.1(1.9), C2 26.0(1.6); SrBr: A1 34.3(2.3), A2 33.2(1.6), B 42.2(1.6), C1 30.3(2.6), C2 28.1(1.3); SrI: A1 43.3(1.6), A2 41.9(1.3), B 46.0(2.0), C1 36.0(3.7); BaF: C1 23.8(2.3), C2 23.5(0.7); BaCl: C1 17.5(0.8), C2 16.6(0.6); BaBr: C1 17.9(1.0), C2 16.5(1.4); BaI: C1 17.9(1.9), C2 16.0(1.5), where A1, A2, B, C1, and C2 denote, respectively, the A 2Π1/2, A 2Π3/2, B 2Σ+, C 2Π1/2, and C 2Π3/2 states, and where the uncertainties, representing one standard deviation, are given in parentheses. The lifetimes of the CaX and SrX states are seen to be quantitatively similar, while those of BaX are quite different. No vibrational dependence of the lifetime for the BaI C2 state is observed for 0 ≤ ν′ ≤ 39. For other MX molecules the vibrational dependence of the lifetime could not be ascertained because the MX molecule was not produced in a sufficiently wide range of ground state vibrational levels. Various one-electron models for the MX transitions are discussed. It is found that a consistent explanation of the transition moments can be obtained by assuming that the transition is between two nonbonding orbitals centered on the metal atom whose mixing coefficients in terms of a truncated metal atom basis set are adjusted to reproduce a subset of the experimentally determined transiton moments. For the X, A, and B states of CaX and SrX, the nonbonding orbital is primarily n s σ, n p π, and n p σ, respectively, in character; for BaX it is suggested that (n − 1)d π and (n − 1)d σ play an important role in the description of the nonbonding orbital in the A, B, and C states.
Internal optic mode dispersion curves and frequency distribution functions in dipole coupled crystals: Applications to NaN3 and KBrO3
Authors: Frech Roger
Published: February 2004

The internal optic branch dispersion curves in KBrO3 and NaN3 are calculated in the molecular dipole formalism for two selected directions in the first Brillouin zone. It is shown that the dispersion curves of all branches belonging to the same symmetry species in a particular crystal are identical when expressed as a function of a reduced frequency which is introduced for convenience. The role of electronic polarization in the dispersion of the branches is discussed. Frequency distribution functions expressed in terms of the reduced frequency and normalized to a particular branch are then calculated for each crystal, and it is demonstrated that these functions are identical in a particular crystal for branches of the same symmetry. Finally irreducible Brillouin zone segments appropriate to the calculation of frequency distribution functions in these dipole coupled crystals are described.
Complex molecules in cesium-rare earth iodide vapors
Authors: Liu C. S. , Zollweg R. J.
Published: February 2004

The optical absorption of the vapors above neodymium iodide and cerium iodide were studied: (1) individually, (2) in combination with cesium iodide, and (3) in combination with cesium iodide and mercury between 600 and 1100°C. Changes in the spectra show that a stable complex molecule CsI · NdI3 is formed that increases the amount of Nd vaporized. A similar enhancement of the vaporization of Ce appears to occur with the cerium plus cesium iodide system, but no changes in absorption structure were found. At high mercury pressure a weak complex between Hg and CsI · NdI3 apparently forms. Vapor pressures and thermodynamic quantities have been deduced from the data.
Carbon-13 NMR study of molecular reorientation in glycerol
Authors: Burnett L. J. , Roeder S. B. W.
Published: February 2004

The 13C spin-lattice relaxation time T1 has been measured as a function of temperature by the PRFT technique in a sample of carefully dried glycerol. Data were taken between 267 and 339°K and the results fitted to a theoretical description of relaxation involving unlike spins which are undergoing anisotropic reorientation. The best fit gave a ratio of rotational diffusion coefficients of 1:5:200 and an activation energy for molecular reorientation of 2.41±0.05 kcal/mole. The results indicate that it is not necessary to invoke a continuous distribution of correlation times in order to describe the relaxation behavior. A speculative model for rotational and translational motion, consistent with the results of previous studies, is presented.
Electron attachment and cesium collisional ionization studies of tetrafluorosuccinic and hexafluoroglutaric anhydrides: Molecular electron affinities
Authors: Cooper C. D. , Compton R. N.
Published: February 2004

The electron affinities of tetrafluorosuccinic anhydride(I) and hexafluoroglutaric anhydride(II) were measured to be 0.5 ± 0.2 eV and 1.5 ± 0.2 eV, respectively, using a cesium collisional ionization technique. Dissociative electron attachment to I produced the negative ions: C2F4CO2−* (≥ 5 msec), C2F4CO−* (124 μsec), C2F4−* (14 μ sec), and F−; while C3F6CO2−*  (≥ 1.2 msec), C3F6CO−* (80 μsec), C3F6−* (760 μ sec), and F− were observed from II. The asterisk indicates that the ion is metastable with respect to autodetachment, and the numbers in parentheses are the measured autodetachment lifetimes for incident electrons at the maximum in the cross section. Very large cross sections (∼ 10−14 cm2) for production of C2F4CO2−* and C3F6CO2−* were observed for dissociative attachment of thermal electrons to I and II, respectively. The parent negative ions C4F4O3− and C5F6O3−, as well as all of the ions (with the exception of C2F4−) which were observed for dissociative electron attachment, were observed from collisions between fast cesium atoms and I and II.
Effect of the anisotropic chain motion in molten polymers: The solidlike contribution of the nonzero average dipolar coupling to NMR signals. Theoretical description
Authors: Cohen-Addad J. P.
Published: February 2004

A model of chain fluctuations analogous to that proposed by Rouse is used to describe NMR properties of molten polymers. As a general property of the magnetic relaxation of a single proton pair, it is shown that within the motional narrowing approximation, the nonzero average dipolar coupling gives rise to a free procession signal with two components in the rotating frame. One is in quadrature with the pulse radio-frequency field, as is usual in ordinary liquids. However, its time evolution is typical of the presence of a nonzero average dipolar coupling. The other one is in phase with the rf field, as is usual in ordinary solids; its amplitude is proportional to the nonzero average of dipolar coupling within the proton pair. It is also shown that this component can be observed by using suitable pulse sequences already proposed to study spin systems in solids. In the same approximation, it is shown that the high resolution resonance spectrum of such a proton pair may be strongly modified in the presence of low-speed rotation of the sample. These effects are exactly described. Furthermore, by taking into consideration the properties of the model of molten polymers presently chosen, the NMR signals are expressed as a function of the essential parameters of the polymer system. The observation of a signal in phase with the rf field is predicted to reflect closely the presence of permanent chain constraints exerted on the polymer chains in the molten system. Short numerical and experimental illustrations are given. It is shown that a signal in phase with the rf can be observed on molten cis - 1,4-polybutadiene at room temperature.
Diffusion coefficient and interaction potential of the (H, H2) system
Authors: Tang K. T. , Wei P. S. P.
Published: February 2004

Binary diffusion coefficients of atomic and molecular hydrogen are calculated from the collision integrals. The interaction potential between the atom and the molecule used in the present calculation is quite different from the Lennard-Jones, Buckingham, and exponential potentials commonly used in the transport studies. It is generated from a H3 potential surface in the inner region and joined to the van der Waals attraction in the outer region. The results from various potentials are compared. The present results, which are larger than those of previous calculations, seem to agree generally with the most recent measurements.
Quantum mechanical close coupling approach to molecular collisions. jz -conserving coupled states approximation
Authors: McGuire Paul , Kouri Donald J.
Published: February 2004

New coupled equations describing collisions of an atom and a diatomic molecule are derived in this paper. By utilizing a description of the collision in terms of rotating coordinates, all coupling in the z component of angular momentum is isolated into purely kinematic effects. By neglecting these couplings, one is led to approximate equations for which the jz component of angular momentum for the molecule is conserved. In addition, the scattering cross sections are formulated by neglecting the effect on the wavefunction of the rotation of the coordinate axes so that in place of Wigner rotation matrices dmmJ (Θ) appearing, one deals with simple Legendre polynomials and the orbital angular momentum l2 is approximated by l(l + 1) ℏ2. It is noted that the procedure involves no approximations so far as the potential matrix elements are concerned. Furthermore, the number of equations remaining coupled is drastically reduced and a completely quantum mechanical description of the dynamics of both internal states and relative motion is retained. The physical implications of the approximations are examined, and it is seen that the neglect of intermultiplet coupling gives rise to consideration of only transitions where both the orientation and magnitude of the rotor angular momentum change. Further, the neglect of transformation effects on the wavefunction is expected to be least accurate for the inelastic forward scattering and best for backward scattering and the j =0→0 elastic scattering. Finally, the present simplest version of the approximation obviously is not intended for treating processes dependent on mj transitions, e.g., NMR relaxation in He–H2. Next the formalism is applied in test calculations to He–H2 collisions using the Krauss-Mies potential energy surface. Numerical results for elastic and inelastic integral and differential cross sections are compared with exact quantum mechanical close coupling solutions of the standard coupled channel equations. Over the energy range studied (from 0.1 eV up to 0.9 eV), agreement to within a few percent is obtained. Additional coupled states calculations are reported at 1.2 eV and computation times are compared against those required for a full close coupling solution. Calculations for the Roberts He–H2 surface are also reported to illustrate the independence of the approximations on the strength of the coupling (so long as the inelastic scattering is predominantly in the backward direction). The dramatic savings afforded by the present approach are such as to make possible fully converged calculations at collision energies typically studied in molecular beam experiments. Thus, for elastic and inelastic nonreactive collisions, involving a repulsive-type interaction, the approach makes the a priori quantum mechanical description of the scattering of a diatom by an atom practical.
Memory functions and recurrences in intramolecular processes
Authors: Lefebvre R. , Savolainen J.
Published: February 2004

The exact amplitudes of the initial state in some models used in the theory of intramolecular processes (both sequential and nonsequential but all involving a set of equidistant discrete states and one or more continua) are determined. The method is based on the derivation of a memory function for the initial state amplitude. An important feature of the formulas is that although they involve an infinite sum, for any finite time only a finite number of terms is effective, owing to the presence of step functions depending on times that are retarded by a integer number of recurrence periods h/ε, where ε is the level separation in the discrete set. The conditions for the validity of the exponential law can be given more precisely than in the previous discussions of this problem: (a) The decay is rigorously exponential for an observation time t smaller than the recurrence time, whatever values are given to the six parameters defining the models; and (b) if the damping constant of the discrete levels is much larger than the level separation, the law is exponential for an arbitrary t, in contrast with previous assertions where t was limited to t ≪τmax, where τmax is the lifetime of one of the discrete levels. The role of the first recurrence is examined. The term contributed to the amplitude interferes with the initial exponential either in a destructive or in a constructive way. Under some conditions it is possible for the recurrence effect to shorten instead of lengthen the decay of the initial state.
Analysis of intensity correlation spectra of mixtures of polystyrene latex spheres: A comparison of direct least squares fitting with the method of cumulants
Authors: Bargeron C. B.
Published: February 2004

This paper presents a comparison of direct least squares fitting with the method of cumulants for the analysis of intensity correlation spectra of a mixture of two sizes of polystyrene latex spheres of diameter ratio 1:2. It is demonstrated that both methods lead to mean values of the particle diameters and scattering amplitude ratio which agree with each other within about 5% or less. However, the cumulant analysis depends critically on the choice of the coincidental background for its success. It is observed that for the type of correlation function involved in this study, the formulas necessary to calculate the parameters mentioned above from the cumulants can result in substantial error propagation. Therefore, the cumulants must be known with high precision in order to avoid large uncertainties in the results.
Ion-dipole scattering in classical perturbation theory
Authors: Gentry W. Ronald
Published: February 2004

The classical theory for scattering of ions by dipolar molecules is derived in the perturbation limit applicable at high translational energy and small scattering angles. In this approximation the differential cross section and the dependence of the rotational excitation energy on kinetic energy, impact parameter, and orientation of the dipole are given by simple analytic expressions, and the orientation-averaged quantities of interest for comparison with experiment are easily calculated. The range of validity for this treatment is explored by comparison of the results with those obtained by a less restricted impact-parameter method and by direct numerical integration of the full classical equations of motion.
Theoretical studies of heavy-atom magnetic shielding in some small polyatomic molecules
Authors: Appleman B. R. , Tokuhiro T. , Fraenkel G. , Kern C. W.
Published: February 2004

Perturbed Hartree-Fock calculations are performed on the molecules CH4, C2H2, C2H4, C2H6, CH3F, CH3OH, H2O, NH3, and C6H6 to obtain the heavy-atom NMR shielding constants and magnetic susceptibilities. Slater-type atomic orbitals (STO's) are used as the basis set. Various aspects of this model, including the dependence on basis-set size and on the origin of the vector potential, are examined. The results are compared with the available experimental data and with other ab initio and semiempirical treatments.
ESR spectra of BrO3 and BrO32−
Authors: Byberg J. R. , Kirkegaard B. S.
Published: February 2004

ESR spectra of x-irradiated single crystals of KBrO3 show that the paramagnetic species previously identified as BrO3 has a large, nonaxial quadrupole interaction and nonparallel principal axes for the g, A, and Q tensors. These findings suggest that the ESR signal should be assigned to a BrO32− ion having Cs symmetry rather than to BrO3. ESR spectra of x-irradiated crystals of KNO3 doped with bromate ions exhibit a signal from a bromine-containing species having a cylindrical spin Hamiltonian analogous to that of ClO3. We assign this signal to the species BrO3. Both assignments are supported by a modified extended Hückel calculation of field gradients and spin densities at the bromine nuclei of BrO3 and BrO32−.
Magnetic circular dichroism of the 4A 1, 4E region in the antiferromagnetics MnF2 and K2MnF4
Authors: Schwartz R. W. , Spencer J. A. , Yeakel W. C. , Schatz P. N. , Maisch W. G.
Published: February 2004

Liquid helium temperature magnetic circular dichroism (MCD) data are presented and analyzed in some detail for the 4A 1, 4E region of MnF2 and K2MnF4 with attention focused on the magnetic dipole origins. The MCD technique is ideal in this regard since it shows very prominent pseudo-A terms for the π-polarized origins while effectively ``filtering out'' almost all of the much more intense electric dipole absorption. In this experiment, the 4A 1 origin of MnF2 has been directly observed for the first time about 143 cm−1 to the blue of the 4Eθ origin, as predicted. The signs of the A terms provide an independent confirmation of the accepted order of the two states. The observed magnitudes of the MCD and absorption parameters can be rationalized on the basis of a simple octahedral model if substantial mixing (85%/15%) of the 4E θ and 4A 1 states is assumed. In K2MnF4 two very sharp ([inverted lazy s]1 cm−1) magnetic dipole origins are observed in the π and axial spectrum separated by 25.1 cm−1 which show very prominent A terms of opposite sign. Theoretical analysis of the MCD shows that the higher energy of these must correspond to the 4E ε origin. The lower energy origin is assigned to 4E θ, and a prominent A term about 350 cm−1 to the blue can be assigned with reasonable confidence to the 4A 1 origin, which is not evident in the absorption spectrum. The A terms that have been assigned to the three origins vanish above the spin-flop field as expected. The absorption and MCD parameters for 4E ε are substantially smaller relative to 4E θ and 4A 1 than is predicted on the basis of a simple octahedral model.
Rotational energy for spherical tops. I. Vibronic ground state
Authors: Michelot F. , Moret-Bailly J. , Fox K.
Published: February 2004

The rotational energy for spherical top molecules in their vibronic ground state is given in a general form to all orders. The sixth-order result is developed explicitly.
Energy levels and potential energy curves for H2, N2, and O2 with an independent particle model
Authors: Miller K. J. , Green A. E. S.
Published: February 2004

Energy levels for valence and Rydberg states are calculated for H2, N2, and O2. An independent particle model (IPM) with an effective one-electron potential whose nuclear charge depends on the electron position is utilized to approximate the Coulomb and exchange interactions. An analytical generalization of the potential parameters to reflect a change in internuclear separation yields accurate average molecular potential energy curves E(R) when experimental ion energies E+(R) are added to IPM molecular orbital energies ε(R), viz., E(R)=E+(R)+ε(R). A convergence formula for incrementing the energy in the numerical integration procedure is presented.
On the theory of Brownian motion. VII. A hot particle in a dense medium
Authors: Mazo Robert M.
Published: February 2004

Starting from the Liouville equation and using projection operator techniques, we derive a Fokker-Planck equation for a Brownian particle that has a temperature T0 different from the temperature of the ambient medium. The medium is assumed to be dense. The resulting equation has the form of a Fokker-Planck equation for a nonspherical particle in a medium at T0, with two additional terms and two new friction coefficients. These new coefficients have the structure of the autocorrelation function of the intermolecular force (respectively, torque) with a microscopic analog of the rate of entropy production due to heat flow.
Frequency dependent transport coefficients in binary mixtures
Authors: Haus Joseph W.
Published: February 2004

The results of the modified Kubo technique are applied to the transport coefficients characterizing a binary mixture of fluids in order to provide a coherent treatment of systems that relax through their vibrational degrees of freedom. The transport coefficients are used in equations for the sound velocity and attenuation and the generalized structure factor in scattered light intensity. The results are consistent with those of previous investigators, and the technique may be generalized to multicomponent systems and to systems exhibiting other relaxing modes.
Raman spectrophotometric study of zinc thiocyanate solutions
Authors: Strommen Dennis P. , Plane Robert A.
Published: February 2004

The well-known ambidentate nature of the thiocyanate ion is exemplified by its complexes with the Group IIb metal cations, in which zinc tends to be N bonded, mercury to be S bonded, and cadmium, a borderline case, has been shown to be bridged in aqueous solution (Cd–NCS–Cd). An investigation was initiated in order to more closely study the aqueous zinc/thiocyanate system, in particular to see if zinc can be induced to show either a change in bonding preference or bridging. Raman spectroscopy combined with the method of continuous variations was used to study a series of aqueous zinc thiocyanate solutions in which the ratio (R) of thiocynate to zinc was varied from 0.25 to 9. Under these conditions, there is no spectroscopic evidence for bridging thiocyanate ions or for a change in the bonding preferences of zinc. However, the results obtained were consistent with the existence of four distinct complex species corresponding to the mono-, di-, tri-, and tetra-substituted zinc ion. Furthermore, there is strong evidence that the two lower species are octahedrally coordinated with water as the other ligand while the higher species are tetrahedral. Comparison of spectra from the C–N stretching region of aqueous solutions with high R values [where the Zn(NCS)42− species is predominant] with similar spectra of Zn(NCS)42− in organic solvents reveals that the A1 mode in aqueous solutions is more intense than the F2 stretching mode while the converse is true in the organic systems. If the additivity of bond polarizabilities were rigorously obeyed, this reversal of intensities would not be expected. In certain cases this type of behavior has been explained by means of the resonance Raman effect. However, this does not seem to be the explanation in the present case.
Electron paramagnetic resonance studies of transitionmetal oxalates and their photochemistry in single crystals. I. K3[Cr(C2O4)3] · 3H2O in K3[Al(C2O4)3] · 3H2O
Authors: Doetschman D. C.
Published: February 2004

The spin-Hamiltonian parameters and axis directions of two inequivalent Cr(C2O4)33− complexes in single K3Al(C2O4)3 · 3H2O crystals have been determined by EPR, and a search for EPR evidence of photochemical reactions in the crystals has been made. The complexes and their environments are asymmetric. The Cr(C2O4)33− orientations in the crystal have been obtained from the axis directions. The Cr(C2O4)33− substitutions, structure, and orientations are compared with the available crystallographic information. The existence of two inequivalent Cr(C2O4)33− complexes is related to the degree of hydration of the crystal. No Cr(C2O4)33− photochemical reactions in the crystals are observed, and the maximum quantum yields consistent with these observations are very small.
Photoionization of atomic nitrogen
Authors: Dehmer P. M. , Berkowitz J. , Chupka W. A.
Published: February 2004

Relative photoionization cross sections were measured for atomic nitrogen from 870 to 600 Å at a wavelength resolution (FWHM) of 0.83 Å. Five members of the autoionizing Rydberg series 2s 2p3(5S0)np 4P were observed, and values of the line shape parameters q, Γ, and ρ2 were determined for the two lowest energy transitions (n = 3 and 4). Oscillator strengths were estimated for both transitions after normalizing the present data to absolute photoionization cross section measurements. The results are q = −1.7±0.1, Γ=0.0275±0.003 eV ρ2=0.51±0.04, and f =0.00580±0.00109 for the n = 3 transition and q = − 1.7 ± 0.1, Γ = 0.010 ± 0.003 eV, ρ2 = 0.58 ± 0.15, and f =0.00240±0.00100 for the n =4 transition. The photoionization cross section showed no other structure in the wavelength region from the NI ionization threshold to 600 Å.
Free radicals in an adamantane matrix. IX. Optical excitation and emission spectra of benzyl radical and its monofluorinated derivatives
Authors: Lloyd Roger V. , Wood David E.
Published: February 2004

The electronic emission and excitation spectra of benzyl radical and its monofluorinated derivatives have been obtained in an adamantane matrix. The spectra obtained for benzyl radical were similar to those found in other condensed phases; in addition, adamantane offers advantages in ease of sample preparation and handling, of radical identification by EPR, and of variable-temperature spectral studies. The electronic spectrum of the α-fluorobenzyl radical and the complete spectra of the o -, m -, and p -fluorobenzyl radicals in a condensed phase are reported for the first time. The 0–0 bands for the first three electronic transitions (4500, 3100, and 2600 Å) in adamantane are, respectively, (in cm−1) for benzyl 21 682, 31 566, 38 095; 0-fluorobenzyl 21 650, 31 546, 37 807; m -fluorobenzyl 21 505, 31 211, 37 594; p -fluorobenzyl 20 938, 32 051, 37 244; α-fluorobenzyl 21 478, 31 153, 35 714. The values for all of the fluorobenzyls are red shifted relative to benzyl except for the 3100 Å transition of p -fluorobenzyl. We have observed and analyzed vibrational fine structure in the 4500 and 3100 Å systems of all of the radicals.
Electronic excitations of benzene from the equations of motion method
Authors: Rose John B. , Shibuya Tai-Ichi , McKoy Vincent
Published: February 2004

We have used the equations of motion method to calculate the excitation energies and intensities of several transitions in benzene. The ordering of the singlet and triplet states of B2u, B1u, and E1u symmetry agrees with experiment and the error in the calculated frequencies ranges from 3% to 25%. This error range is reasonable considering the relatively small basis set used. The most extensive calculation included 10 hole and 28 particle states and shows the effect of changes in the sigma core for each transition. The calculated transition moment of 1.74 a.u. for the 1A1g→1E1u transition agrees well with the experimental value of 1.61 a.u.
Space and time dependent polarization fluctuations in dielectric media
Authors: Titulaer U. M. , Deutch J. M.
Published: February 2004

Molecular dipole moments in a dielectric medium are statistically correlated over large distances. In this paper we derive an expression for the correlation function of two molecular dipoles at different times and at different points in space in the limit of large separation. The result is obtained by means of a purely macroscopic treatment. For the case of equal times our results agree with those obtained by Nienhuis and Deutch from a microscopic theory of a fluid of rigid dipoles; in particular we see the same dependence on the shape of the dielectric sample and on the nature of its surroundings. The methods used in deriving the expression for the dipole correlation function may also be used to calculate the fluctuations in the higher electric multipole moments of small spherical regions in a dielectric sample. A number of results of this type are presented.
Excess chemical potential of dilute solutions of spherical polyelectrolytes
Authors: Phillies George D. J.
Published: February 2004

The excess chemical potential of a dilute solution of spherical polyelectrolytes is calculated in the limit of the linear Debye-Hückel theory. The charge distribution within each polyelectrolyte is expanded in terms of its spherical harmonics. The calculation includes the effect of the finite radius of both of a pair of polyelectrolytes on the screening of their electostatic interactions. It is found that the term in the excess chemical potential of a minimum-salt protein solution that depends on the square root of the protein concentration arises from fluctuations in the net charge of each protein molecule. Fluctuations in the dipole moment of each protein molecule give rise to a term linearly proportional to the protein concentration. Interactions among the net charges Ze of protein molecules in a minimum-salt solution give rise to a concentration-independent term in the chemical potential of the form KT|Z̄|.