bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbation theory using the Yvon–Born–Green equation of state for square-well fluids
Authors: Lincoln W. W. , Kozak John J. , Luks K. D.
Published: February 2003

Using the Yvon−Born−Green integral equation with superposition approximation for the square−well potential with R=σ2/σ1=1.5, values of the coefficients of the reciprocal temperature expansion of the Helmholtz energy are computed. The results compare favorably up to values of n*=0.7 with the machine computations of Alder, Young, and Mark through A4/NkT despite obvious differences in the hard−sphere reference pair correlation functions between the two approaches.
Theoretical treatment of quenching in O(1D) + N2 collisions
Authors: Zahr George E. , Preston Richard K. , Miller William H.
Published: February 2003

It is maintained that quenching of O(1D) by collision with N2 proceeds by formation of a collision complex on the lowest singlet potential surface. Once a collision complex is formed, even the weak spin−orbit interaction in O atom can induce quenching with essentially unit probability (at thermal energies) because the intersection of the singlet [O(1D) + N2] and triplet [O(3P) + N2] potential surfaces is crossed many times during the life of the complex. Rather crude, but qualitatively reasonable potential surfaces for O(1D) + N2 are constructed and classical trajectory calculations carried out to show that the cross section for complex formation is indeed appreciable, ∼40 Å2 at thermal energy; a statistical model is used to determine the quenching probability of the collision complex. Values obtained for the magnitude of the thermal rate constant for quenching, and the fraction of the exoergicity which appears as vibrational excitation of N2, are both in good agreement with experimental results.
Photon correlation study of polydisperse samples of polystyrene in cyclohexane
Authors: Brown Judith C. , Pusey P. N. , Dietz R.
Published: February 2003

Three samples of polystyrene in cyclohexane were studied at the theta temperature by photon correlation spectroscopy. The photon correlation data were analyzed using the moments (or cumulants) method which, under appropriate conditions, yields the Z−average diffusion coefficient ?Z of the solute and one or two higher moments of the diffusion coefficient distribution [D.E. Koppel, J. Chem. Phys. 57, 4814 (1972)]. ?Z can be combined in the Svedberg equation with the weight−average sedimentation coefficient to yield the weight−average molecular weight. The samples studied comprised an essentially monodisperse fraction of polystyrene, a mixture of two monodisperse fractions and a single fraction of broader molecular weight distribution; they had been extensively characterized by other techniques. Our results indicate that, indeed, the Z−average diffusion coefficient is provided by the moments−analysis method. The values of ?W obtained from the Svedberg equation were, within experimental error, in good agreement with those determined by other techniques. The second moment of the diffusion coefficient distribution provides a useful index of polydispersity.
Molecular beam kinetics: Cyanides, isocyanides, and acetyl compounds
Authors: Goldbaum Richard H. , Martin L. Robbin
Published: February 2003

Crossed molecular beam studies have been made on the reactions of K with CH3Br, CH3CN, CH3NC, CH2=CHCH2CN, CH2=CHCH2NC, CH3COCl, CH3COBr, CH3COI, and CH3COCN. All reactions are rebound, with small (3−10 Å2) reaction cross sections for the alkyl and allyl compounds and moderate (15−60 Å2) cross sections for the acetyl compounds. Acetyl cyanide is unusually reactive (∼60 Å2). The latter
Two-photon circular dichroism
Authors: Tinoco Ignacio
Published: February 2003

Equations for two−photon circular dichroism are presented. Two−photon circular dichroism is the difference in absorption coefficient for two left circularly polarized photons and for two right circularly polarized photons. In an unoriented solution only chiral (optically active) molecules can exhibit circular dichroism. The circular dichroism depends on the directions of propagation of the two photons. Parallel propagation, antiparallel propagation, and perpendicular propagation provide independent measurements of the contributions to the circular dichroism. These contributions can similarly be obtained with one circularly polarized photon plus one linearly polarized photon. The two−photon circular dichroism depends on transition electric dipole moments, transition magnetic dipole moments, and transition electric quadrupole moments; it should help in the study of chiral molecules.
The pairwise correlated generalized valence bond model of electronic structure. III. Photoionization of the helium atom
Authors: Wendoloski John J. , Petersson G. A.
Published: February 2003

The calculation of photoionization cross sections for the helium atom provides a simple but sensitive test of the accuracy of correlated wavefunctions and has been used to evaluate the relative importance of orbital splitting and dynamic electron correlation. We find that although dynamic correlation has a greater effect on the total energy, orbital splitting has a greater effect on the transition probabilities. However, if we wish to reduce the errors in calculated transition probabilities below 10%, we must include both orbital splitting and dynamic electron correlation. The pairwise correlated generalized valence bond wavefunction, which includes both effects, gives transition probabilities with errors of about 1%.
Effect of covalency on the electron–nuclear dipolar relaxation in paramagnetic complexes
Authors: Waysbort Daniel , Navon Gil
Published: February 2003

The calculation of NMR relaxation rates due to electron−nuclear dipolar interaction in paramagnetic complexes was extended to include the distribution of the electronic unpaired spin density on the metal and the ligant orbitals. Numerical calculations were performed for the proton relaxation rates of hexamine ruthenium III ion using various choices for its wavefunctions. While the effect of the distribution of the spin density on the ruthenium d orbitals was small, the spin delocalization to the ligand orbitals increased significantly the calculated dipolar interaction relative to the point dipole value. The numerical values are in good agreement with the available experimental data.
Photodetachment of electrons from phenoxide ion
Authors: Richardson Jeffery H. , Stephenson L. M. , Brauman John I.
Published: February 2003

The relative cross section for the gas phase photodetachment of electrons has been determined for phenoxide ions in the wavelength region 300−530 nm (3.10−2.34 eV). An upper limit to the electron affinity of C6H5O⋅ has been determined to be 2.35±0.06 eV. Evidence is presented for the existence of an autodetaching state in C6H5O−.
The first ionization potentials of neptunium and neptunium monoxide
Authors: Rauh E. G. , Ackermann R. J.
Published: February 2003

The ionization potentials of Np(g) and NpO(g) were determined by electron impact utilizing mass−spectrometric measurements of vapor phase isomolecular equilibria. The results gave good agreement with surface ionization and spectroscopic data. (AIP)
Intercalation and lattice expansion in titanium disulfide
Authors: Whittingham M. Stanley , Thompson Arthur H.
Published: February 2003

The temperature coefficient of expansion has been measured for the a and c axes of titanium disulfide, and of its intercalates with a metal, lithium, and an organic base, s−collidine. The anisotropy in the expansion coefficients is related to the bonding in the structure.
Carbon–carbon bond lengths in cyclopropylamine
Authors: Harmony Marlin D. , Bostrom Robert E. , Hendricksen David K.
Published: February 2003

The natural abundance microwave spectra for the 1−13C and 2−13C species of cyclopropylamine were obtained and the rotational constants calculated. By combining these data with previously obtained rotational constants for the normal species, the C−C bond lengths were obtained. Comparison of these results with bond lengths for other saturated cyclopropyl compounds does not suggest a clear pattern for various substituents. (AIP)
Quantum dynamics of vibrational relaxation in condensed media
Authors: Diestler Dennis J. , Wilson Robert S.
Published: February 2003

The Zwanzig−Mori projection−operator formalism is employed to describe the dynamics of relaxation in a prototypic model well known for its application to a variety of physical problems: a harmonic oscillator coupled to a heat bath. First an exact equation of motion in generalized Langevin form is obtained for G (t), the expectation value of the occupation number of the oscillator, known to be in a definite initial nonequilibrium state. This equation is then solved in the appropriate van Hove weak−coupling, long−time limit for the usual case of physical interest in which the oscillator−bath interaction is linear in the oscillator coordinate. In this case G (t) is found to decay exponentially with a time constant τl = τd/2, where τd is the so called ’’dephasing’’ time, i.e., the time constant associated with the exponentially decaying time correlation function of the oscillator coordinate, 〈Q (0) Q (t) 〉. The approach taken here, which may be easily generalized, leads in a rather natural and straightforward way not only to relations between different relaxation times but also to useful explicit expressions for these times.
Coordination number for the Li+–F− ion pair in water
Authors: Fromm Jacob , Clementi Enrico , Watts Robert O.
Published: February 2003

A cluster of 200 molecules of water containing the Li+−F− ion pair has been studied using Monte Carlo techniques. Two temperatures (T = 298 and 500°K) and three different distances for the Li+−F− separation [R (Li−F) = 2.0, 6.0, and 10.0 Å] have been considered. The water−water potential is obtained from quantum−mechanical Hartree−Fock−type computations corrected by inclusion of dispersion forces; the ion−water potential has been obtained from Hartree−Fock computations for the Li+−F−−H2O complex. The Monte Carlo simulation provides reproducible information on the cluster shape, on the cluster structure, and on the coordination numbers for the Li+ cation and the F− anion, when in presence of one another.
Microwave absorption and molecular structure. LXXXII. The dielectric relaxation mechanisms of benzyl- and benzoylpyridines
Authors: Crossley J. , Johari G. P. , Smyth C. P.
Published: February 2003

Dielectric constant and loss measurements have been made at six frequencies between 0.6 and 150 GHz for 2−, 3−, and 4−benzoylpyridine and 2− and 4−benzylpyridine in benzene solution. The data have been analyzed for mean relaxation times τ0. The τ0 values for the benzoylpyridines indicate that their dielectric absorptions are dominated by the reorientation of the molecule as a whole. It was possible to analyze the data for the benzylpyridines in terms of two relaxation times which may be associated with whole molecule and intramolecular reorientation processes.
Studies of inelastic molecular collisions using impact parameter methods. I. Model calculations
Authors: Fisanick-Englot Georgia , Rabitz Herschel
Published: February 2003

In this paper we explore the behavior of an exponential approximation to the impact parameter amplitudes for model systems with potential matrices of simple form. In these matrices the magnitude of the coupling is governed by the difference in the quantum numbers of the coupled states. In the limit of large numbers of states the amplitude functions may be expressed as single integrals, which in many cases can be evaluated in terms of known mathematical functions. The simple structure of this result allows us to analyze the general features of the amplitude functions and the resulting cross section integrands versus impact parameter. Specific examples for potentials with Gaussian and exponential radial dependence are used for illustration. An exploration of the effect of varying the potential parameters for the Gaussian potential is made. In addition, the behavior of functions related to the generalized cross sections for spectral line broadening are considered. Finally, we explore the generalization of the simple model to more elaborate coupling schemes.
Photoion and photoelectron spectroscopy of CO and N2
Authors: Gardner J. L. , Samson James A. R.
Published: February 2003

Dissociative photoionization processes in CO and N2 have been studied via photoion and photoelectron spectroscopy. Kinetic energy spectra of both the ions and electrons formed in CO and N2 have been recorded with a calibrated energy analyzer at incident wavelengths of 584, 462, and 304 Å. The 304 Å spectra are correlated in energy and intensity to deduce dissociation products. For CO, the 2Σ+ state of CO+ formed by removing a σ2s electron dissociates to the products C+ (2p 2P0) and O (3s′ 3D0). The N2 ion and electron spectra at 304 Å both contain evidence for dissociative transitions involving the D 2Πg state of N2+. Energetic N+ ions produced upon removal of a σg2s electron are best explained by a dissociation limit of N+ (2p3 3P0) and N (2p3 4S0).
Nonlinear effects in the shear viscosity of fluids near the critical point
Authors: Oxtoby David W.
Published: February 2003

We present a microscopic theory for the nonlinear shear viscosity of fluids near the critical point, and find that dependence on shear gradients should be observable in capillary viscometer experiments. We suggest that the leveling off (cusplike behavior) of viscosities that has been observed experimentally may be a result of shear gradient effects. Further experiments to test our theory are suggested.
Photoionization quantum yields. A convenient 147 nm actinometer
Authors: Salomon Delmar , Scala Alfred A.
Published: February 2003

The photoionization quantum yields of trimethylamine, triethylamine, dipropylamine, di−iso−propylamine, and tetramethylene sulfide have been determined, at 147 nm, to be 0.38, 0.46, 0.096, 0.15, and 0.10, respectively. These ionization efficiences permit actinometry based upon saturation current measurements. This physical technique of absolute photon flux measurement is contrasted with the chemical actinometers currently in use at 147 nm.
Intramolecular forces in the water molecule in lithium sulfate monohydrate
Authors: Hayward H. Preston , Schiffer Jerome
Published: February 2003

A study of the low−temperature infrared spectra of varyingly deuterated mulls of lithium sulfate monohydrate has afforded information on the origin of bands due to the vibrations of the water molecule in the solid. Assignments are made for the intermolecularly uncoupled modes which produce these bands in the spectrum and intramolecular coupling force constants are calculated.
Semiclassical three-dimensional model for vibrational relaxation. II.
Authors: Billing Gert Due
Published: February 2003

A three−dimensional semiclassical model for vibrational/rotational transitions has been extended to include an infinite order treatment of the rotational transitions. An analytical expression for the transition probabilities is obtained by using an effective potential and a simple collision trajectory. Numerical results for the Ar−N2 and the I2(B3Π0u+) + He4 systems have been obtained.
Collision-induced Lamb dips in laser Stark spectroscopy
Authors: Johns J. W. C. , McKellar A. R. W. , Oka T. , Römheld M.
Published: February 2003

While observing inverse Lamb dips in infrared laser Stark spectra, we have found unexpected extra dips which lie midway between the ’’genuine’’ dips. These center dips are similar to those observed by Uzgiris, Hall, and Barger in that they are caused by a crossover of two different transitions due to the Doppler effect, but are different in that the two transitions do not share a common level. Thus these newly observed center dips are the result of a four−level infrared−infrared double resonance effect and they provide information on rotational transitions as well as on the velocity changes which occur during intermolecular collisions. Since the transitions which cause the center dips involve different M components of the same rotational level, this method is specially suited for the study of reorientation collisions, that is, collisions in which the direction of the total angular momentum changes without changing its magnitude. A theory is developed in which the relative intensities of the center dips with respect to those of ’’genuine’’ dips are related to the relative importance of reorientation collisions. The experiments were done on NH3, H2CO, and CH3F using the CO and the CO2 lasers. The following conclusions are obtained from analysis of the results: (a) The observation of sharp center dips confirms our previous conclusion (Ref. 3) that, as a result of weak collisions, rotational quantum states of molecules can change without appreciable change in velocity. (b) The variation of relative intensities for CH3F, H2CO, and NH3 indicates that parity changing reorientation transitions are preferred to parity conserving transitions. (c) Analysis of very large signals for J = K levels of CH3F indicates that, for those levels, the rate of ΔM = ±1 reorientation collisions relative to all inelastic collisions is of the order of 70%. The consideration of ΔM ≳ 1 transitions would further increase the relative importance of reorientation collisions. (d) Analysis of ΔM ≳ 1 signals in H2CO indicates that rather large reorientations of molecules can occur during a single collision.
ESR studies of the Cr3+ pair in MgO
Authors: Marguglio J. , Kim Yong Moo
Published: February 2003

Next nearest neighbor Cr3+ pairs in MgO single crystals have been studied by ESR techniques. The spectra associated with the spin states S = 1 and S = 2 were observed. The appropriate spin Hamiltonian for the system is H = β H⋅g⋅S + J S1⋅S2 − j (S1⋅S2)2 + Ds{Sz2 − (1/3) S (S + 1) }, where the z axis is along 〈100〉. The axially symmetric D tensors were measured to be Ds = 0.360 cm−1 and Ds = 0.105 cm−1 for S = 1 and S = 2 states, respectively. From the study of intensity change of the resonance line as a function of temperature, the isotropic exchange coupling constant J = 83.3 cm−1 and j = 17.9 cm−1 were also determined. In addition, the temperature dependence of Ds was obtained and was attributed to thermal expansion of the crystal lattice.
Electrical properties of a series of carbazole polymers
Authors: Williams D. J. , Limburg W. W. , Pearson J. M. , Goedde A. O. , Yanus J. F.
Published: February 2003

The electrical transport properties of three vinyl carbazole polymers have been examined to determine if the observed variations in NMR chemical shifts resulting from inter−ring shielding effects could be correlated with carrier mobilities in the polymers. Drift mobilities were measured in films of the polymers overcoated with thin layers of amorphous Se using a xerographic discharge technique. Under the conditions for observing trap free space charge limited currents the hole mobilities were calculated from Child’s law. The three polymers exhibited mobilities varying as the square of the electric field over the range of fields examined. P2VK which showed the overall largest upfield shift of the aromatic protons exhibited a mobility of 1.4×10−6 cm2/V⋅sec at a field of 4×105 V/cm. PVK which exhibit an intermediate degree of shielding had a mobility of 1.4×10−7 cm2/V⋅sec and P3VK which exhibits the smallest shielding effect had a mobility of 2.4×10−8 cm2/V⋅sec both at equivalent fields. Although P2VK showed the highest mobility it also accumulated the largest amount of trapped charge. At present not enough is known about the detailed microstructure of the polymers to identify the reason for this phenomon.
Atomic and molecular calculations with the model potential method. II
Authors: Bonifacic V. , Huzinaga S.
Published: February 2003

The model potentials and the Hartree−Fock−Roothaan wavefunctions of the valence electrons are reported for the third−row atoms, Na through Ar.
Atomic and molecular calculations with the model potential method. III
Authors: Bonifacic V. , Huzinaga S.
Published: February 2003

Theoretical and computational possibilities of the model potential method are examined especially with respect to the feasibility of the semi−ab initio calculation of large molecular systems containing heavy atoms. The separate treatment of the valence and the core regions would enable us to handle more effectively the excitations of valence electrons and also to perform a relativistic calculation on the deep−seated core states.
General treatment of junction potentials in solids using irreversible thermodynamics
Authors: Viktorovitch Pierre , Kamarinos Georges
Published: February 2003

The authors propose a new general method making it possible to obtain the measurable voltage across a chain of metals and semiconductors in thermodynamic disequilibrium. Use is made of the concept of electrochemical potential of free electrons. The interaction between the metal electrons, the interface states and the semiconductor is described by means of chemical reactions. Studying the kinetics and electrochemical affinities of these chemical reactions, and using Onsager’s theorem, it is shown that the measurable voltage across this structure under thermodynamic disequilibrium lies between two extreme values, ΔV12 + [KT/e ln(p/p0)] and ΔV12 − [KT/e ln(n/n0)], where n, p are the densities of free electrons and holes in the volume of semiconductor at the space charge boundary; n0 and p0 are the equilibrium values. ΔV12 is the Dember voltage. Furthermore, the temporal evolution of the measured voltage across the examined structure are investigated and it is found that it may evolve successively through several pseudosteady states before reaching a real and final steady state. The same method allows an original and rigorous definition of the ohmic metal−semiconductor contact. To confirm these conclusions, some experiments performed on metal−Ge contacts are presented. This approach, which describes the system in terms of irreversible thermodynamics, is particularly simple and efficient. It could be a useful tool for investigating the performance of energy−conversion devices (for example, photo−voltaic devices).
Study of the distinguishable electron method
Authors: Chong D. P. , Scott W. R. , Yue C. P.
Published: February 2003

Second−order corrections to first−order properties are calculated by the distinguishable electron method. The rate of convergence is contrary to what one expects from the zeroth− and first−order terms. The quality of the correlated wavefunctions used in DEM is therefore examined and found to be poor for the calculation of first−order properties. These results lead to a reassessment of the distinguishable electron method.
Time-dependent approach to semiclassical dynamics
Authors: Heller Eric J.
Published: February 2003

In this paper we develop a new approach to semiclassical dynamics which exploits the fact that extended wavefunctions for heavy particles (or particles in harmonic potentials) may be decomposed into time−dependent wave packets, which spread minimally and which execute classical or nearly classical trajectories. A Gaussian form for the wave packets is assumed and equations of motion are derived for the parameters characterizing the Gaussians. If the potential (which may be nonseparable in many coordinates) is expanded in a Taylor series about the instantaneous center of the (many−particle) wave packet, and up to quadratic terms are kept, we find the classical parameters of the wave packet (positions, momenta) obey Hamilton’s equation of motion. Quantum parameters (wave packet spread, phase factor, correlation terms, etc.) obey similar first order quantum equations. The center of the wave packet is shown to acquire a phase equal to the action integral along the classical path. State−specific quantum information is obtained from the wave packet trajectories by use of the superposition principle and projection techniques. Successful numerical application is made to the collinear He + H2 system widely used as a test case. Classically forbidden transitions are accounted for and obtained in the same manner as the classically allowed transitions; turning points present no difficulties and flux is very nearly conserved.
Structure of amorphous selenium studied by neutron diffraction
Authors: Hansen F. Yssing , Knudsen T. Steen , Carneiro K.
Published: February 2003

Neutron diffraction measurements on amorphous selenium have been performed at 293 and 80 K. Careful analyses of the instrumental corrections were made to avoid systematic errors in the measured structure factor S (κ) in the wave vector region 0 ? κ ? 12 Å−1. As a result of the data treatment, the neutron scattering cross sections of selenium are determined to be σcoh = 8.4±0.1 b and σinc = 0.1±0.1 b. Using the fact that S (κ) for large κ’s is determined by the short distances in the sample, a new method for extrapolation of the experimental S (κ) until convergence is proposed. This allows an accurate determination of g (r) showing peaks at distances that agree well with the previous x−ray experiment by Kaplow et al. for distances up to 11 Å. However, only partial agreement is obtained when finer details of g (r), e.g., the width of the first peak, are considered. Finally, we give a brief discussion of the different models for the structure of amorphous selenium, taking both diffraction measurements and thermodynamic considerations into account.
Scattering of electromagnetic waves by an optically active spherical shell
Authors: Bohren Craig F.
Published: February 2003

The scattering of electromagnetic waves by a spherical shell with intrinsic optical activity is calculated exactly from electromagnetic theory using the constitutive relations for an optically active, isotropic medium. The solution so obtained reduces to the standard solution in the limit of equal refractive indices for left− and right−circularly polarized waves. Expressions are given for single−particle cross sections and scattering matrix elements as well as circular dichroism and optical rotation for an aggregate of shells. These expressions are potentially applicable to the calculation of scattering contributions to optical rotatory dispersion and circular dichroism spectra for those particles of biological origin which can be modeled by an optically active shell.
Augmented random phase approximation
Authors: Zwicker Judith O. , Yaris Robert
Published: February 2003

An infinite set of ladder diagrams are summed to yield an augmented random phase approximation (ARPA) to the Bethe−Salpeter amplitude equation. The final equation is of the easily solved form of the RPA amplitude eigenvalue equation but with a more complete (augmented) vertex which is the product of a matrix and an inverse matrix. The diagonal ARPA vertex is obtained by taking only the diagonal terms of the inverse matrix, making inversion necessary. The diagonal ARPA should include the most important hole−hole and particle−particle terms which are left out of the RPA. The final ARPA equation is derived two ways, by direct diagram summation and from the integral equation for the ladder vertex.
A limited basis molecular orbital calculation on H2O and H2O+
Authors: Smith Jack A. , Jo/rgensen Poul , Öhrn Yngve
Published: February 2003

An MO−SCF calculation on five states of H2O+ and on the ground state of H2O is shown to yield molecular geometries, force constants, ionization energies, and for the neutral ground state, relative infrared intensities in good agreement with experimental results, and, where available, with more sophisticated calculations.
The spline representation: II. Shell orbitals and exponential spline bases
Authors: Gilbert T. L.
Published: February 2003

The cardinal spline expansion method for atomic calculations, introduced in a preceding article, is extended from linear to exponential splines. These are obtained by replacing the spline basis functions χp(r) by χp(u), where u = e−ζr. An orbital multiplying factor rn−1e−ζnlr is introduced for each shell in order to improve convergence. The Hartree−Fock equations for closed−shell atoms are transformed to equations for nodeless orbitals localized in atomic shells (shell orbitals), and the construction of shell orbitals for atoms embedded in molecules is discussed briefly. Explicit expressions for the one−center matrix elements are derived. A major advantage of the linear spline representation, viz., that the number of two−electron integrals increases only as the square of the basis size and the off−diagonal integrals can be factored, is retained. A comparison is made of the relative advantages and disadvantages of linear and exponential spline representations with regard to probable storage requirements, computing time, and program complexity. No numerical results are given, for the purpose of this work is to assess the optimum strategy for subsequent numerical studies. The linear spline representation, which can also be used for calculating shell orbitals, appears to be the most promising approach for further work.
Delta-function model for short-range interatomic forces: Correlation scheme for closed-shell atoms and ions
Authors: Sondergaard N. A. , Mason E. A.
Published: February 2003

A correlation scheme is developed for the short−range interaction energies between rare−gas atoms and positive and negative ions with rare−gas configurations. The exponential form of the correlation and the relation between the exponental and pre−exponential constants are predicted by a delta−function model. The exponential constants are found to correlate with the softness parameters obtained from Hartree−Fock−Slater calculations on single atoms and ions. Comparisons are made with experimental beam scattering data, ab initio quantum−mechanical calculations, and the electron−gas model of Gordon and Kim. The over−all agreement is good.
Predicted ionization potentials for 70 alkali metal triatomic molecules containing Li, Na, K, Rb, and Cs
Authors: Hart George A. , Goodfriend P. L.
Published: February 2003

Predictions are made of ionization potentials for 70 alkali metal triatomic molecules containing Li, Na, K, Rb, and Cs using a perturbation method based upon the use of Simons type model pseudopotentials. The usefulness of the results to experimentalists is pointed out.
Microwave spectra of 1,3-difluoroallene and fluoroallene
Authors: Ellis P. D. , Li Y. S. , Tong C. C. , Zens A. P. , Durig J. R.
Published: February 2003

The microwave spectra of 1,3−difluoroallene and fluoroallene have been recorded from 12.4 to 40.0 GHz. For the 1,3−difluoroallene molecule, both R−branch and Q−branch assignments have been made for the ground vibrational state for the b−type transitions. The rotational constants were found to be: A = 25101.15±0.12, B = 2170.02±0.07, and C = 2117.40±0.04 MHz. The value of the dipole moment was obtained from Stark splittings to be 1.77±0.01 D. For the fluoroallene molecule, only R−branch a−type transitions were assigned from which the B and C rotational constants were determined to be 4300.11±0.05 and 4039.34±0.05 MHz, respectively. The dipole moment components for fluoroallene were found to be: μa = 1.44, μb = 1.34, and μt = 1.97 D from the Stark splittings. The dipole moments are compared to those of similar molecules.
Studies by electron paramagnetic resonance of K- and Na- bromides doped with Eu2+
Authors: Aguilar S. G. , Murrieta S. H. , Rubio O. J. , Muñoz P. E.
Published: February 2003

The electron paramagnetic resonance of Eu2+ in orthorhombic sites of NaBr and KBr has been observed at 1.2 and 300 K using an X−band spectrometer. The spin−Hamiltonian parameters for Eu2+ ions in NaBr have been determined for the first time. Some disagreement between the results obtained at room temperature and those reported recently by other workers for the KBr host is noted. Absolute signs of the spin−Hamiltonian parameters were determined from the relative intensities of the resonance lines at liquid helium temperatures. In both studies the sign of the b02 term was found to be positive, and is therefore in agreement with the sign reported by Nair et al. for Eu2+ in KCl and by Aguilar et al. for Eu2+ in NaCl. The behavior of the second−order spin−Hamiltonian paramerer b02 as a function of the lattice size is discussed.
Many-body calculation of the atomic hyperfine coupling constant: The lithium atom
Authors: Hata Jiro
Published: February 2003

The atomic hyperfine coupling constant of the lithium atom in the 2S ground state is calculated by taking into account a correlation based on the wavefunctions of Clementi, Roothaan, and Yoshimine, including higher−order Exclusion−Principle−Violating (EPV) diagrams of first and second kind. The result is 2.91±0.02 a. u., compared with the experimental value 2.9096. Detailed comparison is made with the work by Chang, Pu, and Das who have calculated the coupling constant as 2.89±0.02 a. u. According to the comparison with the work of Chang et al., there is good agreement between individual diagrams, considering the difference in potentials used and the total result, the small difference in the latter being due to the higher order EPV diagrams effect considered by the present calculation.
PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. II. The molecules BeH2, BH, BH3, CH4, CH−3, NH3 (planar and pyramidal), H2O, OH+3, HF and the Ne atom
Authors: Ahlrichs R. , Driessler F. , Lischka H. , Staemmler V. , Kutzelnigg W.
Published: February 2003

PNO−CI and CEPA−PNO calculations are performed in a systematic way for the molecules BeH2, BH, BH3, CH4, CH−3, NH3, H2O, H3O+, HF, and the neon atom. Two types of Gaussian basis sets are used: the ’’small basis,’’ which contains one set of polarization functions on the heavy atom and one on the hydrogen atoms, and the ’’standard basis,’’ which contains two d and one f set on the heavy atom and two p sets on the hydrogen atoms. For neon, in addition, a ’’large’’ basis is used. The improvement of the energy due to the different polarization functions is discussed in detail. The computed correlation energies are analyzed in terms of quantities defined in Paper I, in particular, in terms of the IEPA (independent electron pair) correlation energies ℰIEPAμ and the error ΔEIEPA of the IEPA approximation. The pair interaction contributions Δℰμν to EIEPA are usually smaller in absolute value in the localized than in the canonical representation. In the canonical description Δℰμν values of either sign occur so that because of a partial cancellation, ΔEIEPA is usually smaller in absolute value than for localized pairs where almost all Δℰμν are positive. Nevertheless the localized representation turns out to be generally preferable, mainly because it is more economical. The correlation energies accounted for in calculations that give rigorous upper bonds are in the order of ?85% of the exact correlation energies for the standard basis sets and ?70% as obtained with the small basis sets. The force constants and equilibrium geometries of BH, BeH2, and NH3 are calculated using the small basis sets. The role of the correlation energy for the inversion barrier of NH3 (and the isoelectronic species CH3− and OH3+) is discussed in detail.
An acoustical method for determining the thermal and momentum accommodation of gases on solids
Authors: Shields F. Douglas
Published: February 2003

The velocity and absorption of sound in He, Ne, and Ar have been measured at room temperature and pressures ranging from 6.0−0.3 torr. The gases were contained in a tungsten tube 0.953 cm i.d. Flashing this tube at approximately 1900°C in a high vacuum before the measurements produced a marked increase in the sound velocity and a marked decrease in the sound absorption. The effect has been attributed to a decrease in the thermal and momentum accommodation coefficients produced by removing the adsorbed gases from the tungsten surface. In the presence of mischmetal getter, the accommodation coefficients for the surface after flashing remained constant for a period of two weeks. When oxygen was admitted to the system momentarily to a pressure of a few torr and then pumped out, the sound velocity and absorption in the noble gases returned to values corresponding to higher accommodation coefficients. The effect was repeatable. Comparison of the measured absorption and velocity with theoretical curves places reasonable limits on the permissible values for the energy and tangential−momentum accommodation coefficients.
Dynamics of molecular reorientational motion and vibrational relaxation in liquids. Chloroform
Authors: Rothschild Walter G. , Rosasco G. J. , Livingston R. C.
Published: February 2003

Vibrational and rotational (dipole and second−order tensor) correlation functions were obtained by Fourier inversion of infrared and Raman vibrational band contours of the three ∥ and one ⊥ fundamental of liquid CHCl3, CDCl3, and isotopically pure CH35Cl3. All correlation functions are nonexponential at short times and approximately exponential for long times. The symmetry axis of the molecule reorients by ’’free’’ jumps of about 1/3 rad, turning through a root−mean−square angle of 1 radian within 2psec by about 13 orientational jumps. Computer simulations show that J diffusion is too fast beyond 1 psec and that M diffusion fits the data up to 4 psec (τJ = 0.12 psec); thereafter, M diffusion is too slow. The Raman rotational correlation time is approximately equal to the NMR quadrupolar correlation time; the infrared rotational correlation time is only 0.75 of a corresponding dielectric relaxation time. Vibrational relaxation in the symmetric near−infrared carbon−hydrogen stretch is of the same order of importance as rotational relaxation; however, the dynamics of the vibrational relaxation of this mode do not support the presence of ’’hydrogen bonding’’ in the neat liquid. In the symmetric far−infrared carbon−chlorine deformation mode, vibrational relaxation is of considerably lesser significance than rotational relaxation throughout the whole time domain, whereas the rate of vibrational relaxation of the symmetric midinfrared carbon−chlorine stretch is intermediate to those of the other two symmetric fundamentals. None of these modes obeys vibrational energy dissipation or resonance vibrational energy transfer mechanism induced by dipole−dipole interaction. The ratios of the derived polarizability tensor elements, which are required to evaluate the rotational correlation function of the degenerate mode (carbon−hydrogen deformation), were computed from formulations relating them to the bond polarizabilities: It appears that its Raman and infrared correlation function does not contain the same vibrational correlation function and that the respective contour is determined essentially by nonorientational relaxation processes. An extensive analysis of the experimental errors inherent in our Raman band contour determinations is presented, as well as a critical comparison of our conclusions with previous results in the literature.
Theoretical description of the diimide molecule
Authors: Winter Nicholas Wilhelm , Pitzer Russell M.
Published: February 2003

The N2H2 molecule is only metastable and as a consequence is not well characterized experimentally. Therefore, we have carried out extensive Hartree−Fock calculations in order to determine equilibrium geometries, one−electron properties, and the relative energies of the cis and trans isomers. In addition, Hartree−Fock and multiconfiguration calculations were carried out to determine the mechanism for isomerization. The trans isomer was found to be 6.6 kcal/mole more stable than the cis form using a basis set which included polarization functions. The lowest energy path found for isomerization occurred by inversion about one nitrogen (rather than rotation about the NN bond) with an activation energy of 47 kcal/mole. Excitation energies for the lowest singlet and triplet excited states are also presented.
Born-like scaling of elastic electron scattering cross sections for elements near Z = 79 at 200–500 eV
Authors: Riley Merle E.
Published: February 2003

The elastic, electron−Hg scattering measurements of Bromberg show a Born−like scaling in the 200−500 eV energy range. The calculations presented here show that this scaling is present in the static approximation and that the general feature is a shoulder in the energy dependence of the small−to−moderate−momentum−transfer differential cross section. This shoulder for Hg is predicted to be a part of a general structure in the elastic, electron−scattering cross sections for elements near Au in the periodic table.
Symmetry breaking in water molecule interactions
Authors: Stillinger Frank H. , Lemberg Howard L.
Published: February 2003

We discuss the phenomenon of symmetry breaking observed in minimum−energy structures for water dimers. Based on two explicit representations of the intermolecular pair potential, the ST2 and PKC forms, lowest energy dimers are evaluated as a function of oxygen−oxygen separation, and it is concluded that a critical distance Rc exists which separates mechanically stable dimers of differing symmetries. For small distances, the familiar linear hydrogen bond is manifested, while for large distances dipole−dipole forces promote alignment of the molecular axes. At Rc, eight equivalent small−distance minima of low symmetry and four large−distance high−symmetry configurations undergo confluence. Thorough investigation of constrained minima illustrates how this transition occurs and invites comparison with the theory of critical phenomena. The second−order phase transition of the Ising ferromagnet provides a useful analogy to the structural symmetry breaking of the water dimer. Finally, we consider the relation of the present case to the larger issue of interactions in condensed phases, and to the interaction of more complex molecular pairs; one thereby discovers analogs for (a) Ising models in external fields, (b) upper and lower consolute points in liquid mixtures, and (c) first−order phase transitions.
Some models for lipid bilayer and biomembrane phase transitions
Authors: Scott H. L.
Published: February 2003

Biological membranes consist to a large extent of lipid bilayers, some of which exhibit fairly sharp phase transitions near body temperatures. We describe here a model for the main observed phase transition in pure lipid monolayers and bilayers. The model contains approximate treatments of hard−core repulsions and weak attractive forces between molecules. We believe that the model accounts qualitatively for many of the experimental properties that the model accounts qualitatively for many of the experimental properties of of monolayers and bilayers. Improvements which should enhance quantitative agreement are also discussed.
Vibrational excitation in the DECENT approximation: The large-angle scattering of Li+ from H2
Authors: Gentry W. Ronald , Giese Clayton F.
Published: February 2003

We have applied the DECENT model, which exploits the correspondence between the classical and quantal equations of motion for a forced harmonic oscillator, to the calculation of quantum vibrational transition probabilities and differential cross sections for large−angle scattering of Li+ from H2, using the Lester potential energy hypersurface. The results are in good agreement with the experimental data of David, Faubel, and Toennies, but they show the necessity for taking into account concurrent rotational excitation, which has not yet been resolved experimentally in this energy and scattering angle regime. If the trajectories are calculated using the spherically averaged potential, rather than the results obtained first using the full anisotropic potential and then orientation−averaged, the same angle dependence of the classical differential cross section is obtained but the vibrational transition probabilities are severely underestimated. A collinear trajectory also gives very small vibrational excitation energy. We show that large vibrational excitations are associated with a particular class of noncollinear trajectories in which the vibrational and rotational motions are strongly coupled. The vibrational excitation process can therefore be adequately described only by a three−dimensional calculation using the complete potential energy hypersurface, including anisotropic terms.
On the theory of energy distributions of products of molecular beam reactions involving transient complexes
Authors: Marcus R. A.
Published: February 2003

Theoretical energy distributions of reaction products in molecular beam systems are described for reactions proceeding via transient complexes. Loose and tight transition states are considered for the exit channel. For a loose transition state and the case of l ≫ j, the result is the same as of Safron et al. For the case of a tight transition state exit channel effects are included analogous to steric effects for the reverse reaction. It is shown how, via one mechanism, bending vibrational energy of that transition state can contribute to the translational energy of the reaction products. Expressions are derived for the energy distributions of the products when l ≫ j and j ≫ l.
Primary radiation products in x-irradiated dihydromethyluracil
Authors: Budzinski Edwin E. , Box Harold C.
Published: February 2003

ESR and ENDOR measurements have been made on single crystals of 5,6−dihydro−6−methyluracil and its ND analog, x−irradiated at 4.2°K. The radiation products are identified by analysis of the hyperfine splitting. (AIP)
Multimedia dispersion relation for surface electromagnetic waves
Authors: Ward C. A. , Bhasin K. , Bell R. J. , Alexander R. W. , Tyler I.
Published: February 2003

We have derived a general, n−media dispersion relation for surface electromagnetic waves propagating on isotropic layers with complex dielectric functions. The equation is presented in a convenient, compact form for ease of application.
Spectroscopic constants of the Rittner potential evaluated by the Dunham method
Authors: Finn Edward J.
Published: February 2003

Two major terms of the Rittner potential depend on the polarizability of the ions. The model investigated in this paper develops an equation for the Dunham constants, an, which depend on experimental values of ωe and Be for the molecule and the ionic polarizabilities of the constituent ions. The predicted dissociation energies of 20 alkali halides are in excellent agreement with experimental evidence. The model is therefore expected to be a satisfactory predictor of other spectroscopic parameters. The paper cites theoretical values of Y20 and Y11 for the 20 alkali halides which compare favorably with available data.
Optical study of defect clustering in NaCl:Sm2+
Authors: Sundberg Michael N. , Lauer Howard V. , Fong Francis K.
Published: February 2003

In this paper, we report a detailed spectroscopic investigation of the NaCl:Sm2+ system. In sharp contrast to the homologous KCl:Sm2+ and KBr:Sm2+ systems, whose narrow−line fluorescence spectra mainly arise from Sm2+−cation−vacancy pairs, the NaCl:Sm2+ system gives rise to spectroscopic properties that are dominated by cluster formation. In addition to the usual narrow 5D0 → 7FJ fluorescence lines originating from single Sm2+−cation−vacancy pairs a host of f → f and d → f lines attributable to small and large Sm2+−defect clusters is observed. The site origins of the fluorescence lines are determined by Zeeman anisotropy fluorescence, lifetime, and polarization measurements. The cluster origins of many intense lines have been ascertained through a high−temperature annealing−quenching procedure. The prominent role played by defect clustering in NaCl:Sm2+ is qualitatively explained in terms of the sizable difference in the ionic radii of the Sm2+ and the Na+ ions.
Small periodic cluster calculation on point defect problems in hexagonal layered solids
Authors: Zunger Alex
Published: February 2003

A model is proposed for discussing deep defect levels in covalent solids, based on the representation of the one−electron energies of the crystal by the eigenvalue spectrum of a small periodic cluster of atoms. Calculations of this model by semiempirical MO−LCAO methods for a vacancy problem in hexagonal boron nitride and graphite and substitutional boron impurity in graphite, yield satisfactory results when compared with experimental EPR, thermoluminescence, and thermally−stimulated−currents data.