bluuebunny/crossref_metadata_2025 huggingface dataset:
Random coupling models for intramolecular dynamics. II. Kinetic equations for collisionless multiphoton excitation of large molecules
Authors: Carmeli Benny , Nitzan Abraham
Published: February 2003

Multiphoton excitation and dissociation of large molecules under collisionless conditions is discussed in terms of an intercontinuum random coupling model. The mathematical approach described in a previous paper is used to obtain the general solution for a system of consecutively coupled discrete states, quasicontinuous manifolds, and continuous (dissociative) manifolds of molecular levels (eigenstates of the total molecular Hamiltonian), where the radiative coupling matrix elements are assumed to be given as a linear combination of smoothly varying and randomly varying (over level indices in the molecular manifolds) components. In the range of discrete molecular levels the time evolution is coherent and described in terms of the optical Bloch equation. In the quasicontinuous and continuous ranges the time evolution may be described in terms of Markoffian kinetic equations for the number of photons absorbed by the molecule, provided that the intramolecular vibrational relaxation widths associated with the optically active molecular modes is much larger than the Rabi frequency associated with the excitation of these modes. The kinetic evolution itself consists of direct multiphoton excitation processes (simultaneous transitions from the upper discrete levels to all higher energy molecular manifolds) resulting from the smooth component in the radiative coupling, and a consecutive excitation process described by the Pauli master equation with rates given by the golden rule expression. The interaction which enters into the golden rule expression is the variance in the radiative coupling. The direct excitation component contributes a negligible part of the overall excitation even if the random and smooth radiative coupling components are comparable. The resulting incoherent time evolution of the multiphoton excitation process is consistent with available experimental results. Coherent effects in the time evolution are expected for higher radiation field intensities, where the Rabi frequency becomes comparable to the intramolecular vibrational relaxation rate.
Far infrared magnetic resonance of deoxyhemoglobin and deoxymyoglobin
Authors: Champion P. M. , Sievers A. J.
Published: February 2003

We have used FIR magnetic resonance as a direct probe of the low lying electronic states of the high spin (S=2) ferrous ion in deoxyhemoglobin and deoxymyoglobin. In a variety of helium temperature experiments using different samples and different buffers, we have detected a broad absorption band at 3.5 cm−1 that is sensitive to the application of a magnetic field. This band is not observed in samples of oxy-or carbonmonoxy-hemoglobin. The integrated area of the absorption band agrees (via a Kramers–Kronig relation) with the measured values of the static susceptibility of deoxyhemoglobin at 1.2 °K. The large linewidth (3.5 cm−1) of the observed band is thought to reflect the large number of conformational substates associated with slightly different structures of the protein.
Creeping flow translational resistance of rigid assemblies of spheres
Authors: Swanson Eric , Teller David C. , de Haën Christoph
Published: February 2003

The creeping flow translational resistance of dimers and tetrahedral tetramers was measured using spheres glued to each other. The results for the dimer were in excellent agreement with exact theory. Computations based on the modified Oseen tensor differed in the case of the tetramer by 9% from the observed value. Dramatically improved results were obtained by modeling the objects as shells of spherical beads and extrapolation to an infinite number of beads of infinitesimal size. The calculations were extended to other rigid assemblies of spheres.
A comparison between finite element methods and spectral methods as applied to bound state problems
Authors: Duff Marc , Rabitz Herschel , Askar Attila , Cakmak Ahmet , Ablowitz Mark
Published: February 2003

The finite element and spectral methods are applied to two-dimensional bound state problems. A comparison of the spectral method, which requires a global basis set expansion of the wave functions, and the finite element method, which requires no such such expansion, is presented. A procedure is given for formulating the finite element approach and for achieving fast and accurate results. The convergence of the finite element calculations is considered and shown to be well behaved. A discussion of the extension of the finite element method to higher dimensions is also included.
Antisymmetrization effects in bond-orbital models of internal rotation barriers
Authors: Corcoran C. T. , Weinhold F.
Published: February 2003

We exhibit a set of orthonormal CH bond orbitals, having high overlap (?0.998) with those of Hoyland and Sovers et al., but predicting an ethane rotation barrier of incorrect sign when employed in an antisymmetric wave function. Taken together with previous work of Levy et al., this indicates that wave function antisymmetry is neither necessary nor sufficient for the appearance of barrier forces in approximate quantum-chemical calculations.
Quasiclassical close-coupling approximation: Comparison with experimental Ar–HCl differential cross section
Authors: Band Yehuda B.
Published: February 2003

Total differential cross section for Ar–HCl collisions are calculated using the Neilsen–Gordon potential surface within the quasiclassical close coupling approximation (QCCA). Comparison is made with the experimental results of Farrar and Lee.
Low temperature specific heat of Y6Mn23
Authors: Blacklock K. , White H. W. , Hardman K. , James W. J.
Published: February 2003

The specific heat of the intermetallic compound Y6Mn23 was measured below 20 K. The data were found to be well described by the expression C= αT+γT3 below 7 K, where γ is 212 mJ/mole K4. A Debye temperature of 339 K was calculated for the lattice contribution. The observed behavior and the values for α and γ are consistent with specific heat results of other compounds in the series Y6(Fe1−x Mnx)23.
Theoretical studies of the O+H2 reaction
Authors: Walch Stephen P. , Wagner Albert F. , Dunning Thom. H. , Schatz George C.
Published: February 2003

a b initio theoretical calculations of the potential energy surfaces and rates of reaction of the O+H2 reactions are reported. (AIP)
Reply to ‘‘The time-scale of intraspur muonium formation’’
Authors: Walker D. C. , Jean Y. C. , Fleming D. G.
Published: February 2003

The spur model ofmuonium formation in water is further elucidated. The comments by percival5 on the authors experiments reported earlier, are answered. (AIP)
Continuum resonance Raman scattering of light by diatomic molecules. II. Theoretical study of the Q branches of Δn=1 profiles of molecular bromine
Authors: Atabek O. , Lefebvre R. , Jacon M.
Published: February 2003

The methods developed in Part I for the calculation of continuum resonance Raman scattering amplitudes are applied to the calculation of the Q branches of the Δn=1 profiles of Br2 which have been measured by Baierl and Kiefer [J. Raman Spectrosc. 3, 353 (1975)]. The experimental conditions being such as to imply many initial and final rovibrational states, we first of all study the effect of rotation on the scattering amplitudes. It is observed that while the amplitudes may vary substantially with the rotational number of the initial state, almost no effect is introduced by taking into account the changes in rotational quantum numbers which accompany transitions between different electronic states. This is useful to derive an efficient formula for the scattering cross section. The profiles are then calculated with the potentials available for the 1Π1u and B (3Π+0u) excited states which, as shown by previous workers, interfere strongly in producing the spectra. These calculations make it possible to examine the effect of (1) using semiclassical scattering amplitudes rather than the more accurate coupled channel results, (2) using R-dependent electronic transition moments instead of constant ones, (3) introducing the effect of the angular momentum quantum number J instead of using the amplitudes calculated for J=0, (4) choosing the potentials among those derived from absorption data. The conclusions which emerge from these calculations are that: (a) The semiclassical procedure is fairly accurate. This success indicates that a Raman profile for a giving exciting wavelength represents in fact a very local test (over a range of ∼0.03 Å) of the excited states potentials. Changing the wavelength from 5017 to 4579 Å (two of the lines available from an argon ion laser) amounts to scanning each of the potentials over a region of no more than 0.1 Å; (b) The changes in the scattering amplitudes brought about by the introduction of varying electronic transition moments can be correlated with the values taken by the moments at the radiative crossing points. This is supported both by a detailed analysis of the coupled channel amplitudes and by the form taken by the semiclassical amplitudes; (c) The effect of J on the amplitudes is masked in the profiles as a result of superposition effects; (d) None of the published potentials was able to fit all five profiles. A local downward displacement of the potentials for the 1Π1u and B (3Π+0u) states removes the main unsatisfactory features of the spectra.
Self-avoiding walks with span limitations. I. The mean square end-to-end distance
Authors: Barr Ronnie , Brender Chava , Lax Melvin
Published: February 2003

Self-avoiding walks on the simple cubic lattice of lengths up to 11 steps were exactly enumerated. The mean square end-to-end distance and its mean square components were recorded as a function of the span separation along one principle Cartesian axis. The molecular weight dependence for narrow spans (thin slabs—up to two lattice spacings) obeys that of a two-dimensional walk as conjectured by Wall et al. As the slab thickness increases, one encounters a dimensionality transition in the behavior of the mean separations which is compared with similar transitions observed for the self-avoiding walk in the presence of one interacting barrier. For thick slabs the behavior is three dimensional.
Interaction between two ground-state helium atoms
Authors: Starykh V. V. , Kapyshev V. K.
Published: February 2003

This paper presents the calculations of the energy and interaction potential of two ground-state helium atoms for a wide range of internuclear distances 0.1 Å⩽R⩽1.75 Å, using the one-electron wave functions obtained by numerical solution of the Schrödinger equation for an electron moving in the field of two fixed arbitrary charged nuclei, and assuming compensation of errors made in calculating atomic and molecular energies. The agreement between the calculations and experimental values is within ∼2.2%, this value being by order of magnitude better than the accuracy of any quantum-mechanical method available so far. Both the kinetic and potential energies of the He–He system are calculated. The effects of correlation and polarization of electron density which occurs when atoms come closer are taken into account by means of introducing of variable constants of nuclear screening directly into Schrödinger equations thus providing the high efficiency of the method.
General theory of electronic transport in molecular crystals. I. Local linear electron–phonon coupling
Authors: Silbey R. , Munn R. W.
Published: February 2003

An improved general theory of electronic transport in molecular crystals with local linear electron–phonon coupling is presented. It is valid for arbitrary electronic and phonon bandwidths and for arbitrary electron–phonon coupling strength, yielding small-polaron theory for narrow electronic bands and strong coupling, and semiconductor theory for wide electronic bands and weak coupling. Detailed results are derived for electronic excitations fully clothed with phonons and having a bandwidth no larger than the phonon frequency; the electronic and phonon densities of states are taken as Gaussian for simplicity. The dependence of the diffusion coefficient on temperature and on the other parameters is analyzed thoroughly. The calculated behavior provides a rational interpretation of observed trends in the magnitude and temperature dependence of charge-carrier drift mobilities in molecular crystals.
Variable angle electron-impact excitation of nitromethane
Authors: Flicker Wayne M. , Mosher Oren A. , Kuppermann Aron
Published: February 2003

The electron-impact excitation of nitromethane has been studied at incident electron energies of 25, 55, and 90 eV, at scattering angles from 6° to 80°. The lowest-lying inelastic process which is observed is a previously unreported feature with a maximum intensity at 3.8 eV energy loss. This feature represents at least one singlet→triplet transition. It is likely that this 3.8 eV triplet feature plays a central role in the gas phase photolysis of nitromethane. A weak inelastic process with a peak at 4.45 eV has also been observed, as has a strong transition at 6.23 eV. Both of these excitations are well known from optical spectra, and they are generally believed to represent spin-allowed n→π* and π→π* transitions, respectively. Their assignments are discussed in detail. In addition, seven other transitions, several of which have not been reported previously, have been detected in the 7–12 eV energy-loss range. Three of these transitions, at 8.3, 8.85, and 11.73 eV energy loss, are tentatively assigned to Rydberg excitations of increasingly tightly bound electrons into a 3s Rydberg orbital.
Macroscopic model for solvated ion dynamics
Authors: Chen J. -H. , Adelman S. A.
Published: February 2003

A macroscopic treatment of solvated ion dynamics is developed and applied to calculate the limiting (zero concentration) conductance of cations in several aprotic solvents. The theory is based on a coupled set of electrostatic and hydrodynamic equations for the density, flow, and polarization fields induced in the polar solvent by a moving ion. These equations, which are derived by the Mori projection technique, include crucial local solvent structure (ion solvation) effects through solvent compressibility, and local constitutive parameters. If solvent structure is suppressed, the equations reduce to those derived previously by Onsager and Hubbard [J. B. Hubbard and L. Onsager, J. Chem. Phys. 67, 4850 (1977)]. The macroscopic equations are approximately decoupled into electrostatic and hydrodynamic parts. The decoupled equations are solved assuming a step density, viscosity, and dielectric constant model for the local solvent structure and dynamics. This yields analytic expressions for the viscous, ζV, and dielectric ζD, contributions to the ion friction coefficient. These expressions generalize, respectively, the Stokes and Zwanzig results for the (slip) viscous and dielectric friction so as to account for ion solvation effects. The friction coefficients involve a desolvation function Δ which depends on the local structure (density) and dynamics of the solvent. The drag coefficient results reduce in form to those of Zwanzig (within a flow gradient correction factor of 2/3) and Stokes for both weak (Δ→1) and strong (Δ→0) ion–solvent interaction. For Δ→1 the true ionic radius Ri appears in the drag formulas while for Δ→0 a renormalized solvated ion radius σ=Ri+2Rs (where Rs=solvent molecule radius) appears. The theory is fit to experimental cation conductances in pyridine, acetone, and acetonitrile by representing Δ by a two parameter switching function. Agreement between the model and experiment is satisfactory for all three solvents. Moreover dielectric friction is found to have a negligible effect on ion mobility in these solvents.
Monte Carlo calculations of the equation of state of the square-well fluid as a function of well width
Authors: Henderson Douglas , Scalise Osvaldo H. , Smith William R.
Published: February 2003

Some results of Monte Carlo simulations for the square-well fluid for 1.125?λ?2, where λ-1 is the width of the well, are presented and compared with perturbation theory and, λ=1.85, with the BGY theory. In general, we find perturbation theory to be more reliable than the BGY theory. For λ≳1.5, perturbation theory is generally satisfactory. However, for λ<1.5, the pressures calculated from perturbation theory become less satisfactory as λ is decreased.
Interpretation of ES, CS, and IOS approximations within a translational-internal coupling scheme. II. Application to atom–diatom kinetic cross sections
Authors: Coombe D. A. , Snider R. F.
Published: February 2003

ES, CS, and IOS approximations to atom–diatom kinetic cross sections are derived. In doing so, reduced S-matrices in a translational-internal coupling scheme are stressed. This entails the insertion of recently obtained approximate reduced S-matrices in the translational-internal coupling scheme into previously derived general expressions for the kinetic cross sections. Of special interest is the structure (rotational j quantum number dependence) of the kinetic cross sections associated with the Senftleben Beenakker effects and of pure internal state relaxation phenomena. The viscomagnetic effect is used as an illustrative example. It is found in particular that there is a great similarity of structure between the energy sudden (and IOS) approximation and the previously derived distorted wave Born results.
Dipole polarizabilities of the Group IIb atoms obtained from compact variational trial functions
Authors: Rosenkrantz Marcy E. , Stevens Walter J. , Krauss Morris , Konowalow Daniel D.
Published: February 2003

In order to optimize basis sets for future molecular calculations, the energies and dipole polarizabilities of 1S, 3P, and 1P states of Zn, Cd, and Hg have been determined from Hartree–Fock (HF) and multiconfiguration calculations. These utilize either empirically fitted pseudopotentials or ab initio effective core potentials (ECP). Our calculated polarizabilities for ground state atoms agree within 10% with recommended values so long as either the empirical pseudopotentials or the ECP based on relativistic HF calculations are used. Our calculations agree with the measured anisotropy of the polarizability of 3P1 Hg, but disagree with measured values of the 3P2 state anisotropy. Our calculated 3P2 anisotropy is consistent with both the measured and calculated 3P1 Hg anisotropy; the measured 3P2 values are not. We find calculations based on nonrelativistic ECP to give incorrect polarizabilities.
Two body T matrices derived from strong potentials
Authors: Rosenthal C. M. , Haracz R. D.
Published: February 2003

A method for computing T matrix elements, on and off shell, is derived for strong potentials of Yukawa or exponential form. These include the Morse potentials which have long been used to parameterize chemical interactions. A numerical example involving a single repulsive Yukawa potential with variable potential strength is included to demonstrate how the method works. Good convergence is obtained for potential strengths up to and including 400, in units in which μ/h/2=1, μ being the reduced mass.
An integral equation approach to scaled 1/Z expansion of static multipole polarizabilities
Authors: McDowell Keith
Published: February 2003

A new integral equation is developed which sums to infinite order diagrams which contribute to the scaled 1/Z expansion of atomic coupled Hartree–Fock multipole polarizabilities. Application is made to the dipole, quadrupole, and octupole polarizability of ground state helium. Although sensitive to the choice of scaling parameter, the polarizabilities are found to be in satisfactory agreement with previous values when the scaling parameter is optimized. It is expected that convergence problems found in these calculations will carry over into other scaled 1/Z expansions.
Studies in statistical mechanics of Coulombic systems. IV. The screened-Coulomb charged hard sphere system and its relation to the restricted primitive model
Authors: Larsen B. , Rogde S. A.
Published: February 2003

Monte Carlo results are reported for three liquid states of a model consisting of two species (anions and cations) of charged hard spheres of equal diameter, which interact with a screened Coulomb potential. Results of the mean spherical approximation, the truncated Γ2 approximation, and the EXP approximation are compared with the MC results. Numerical and theoretical relationships between this model and the restricted primitive model (RPM) are established. The effect of the methodological errors involved in MC simulations of the RPM is studied by comparing the MC radial distribution functions (rdf’s) for the RPM with the rdf’s obtained from a perturbation theory, using the screened Coulomb system as the reference system.
Ion clusters in He–Co and Ar–Co glow discharges
Authors: Kaufman Y. , Avivi P. , Dothan F. , Keren H. , Malinowitz J.
Published: February 2003

Positive ion spectra from the positive column of a discharge in a mixture of He–Co with different amounts of O2 are presented. Without O2 the positive ions are CO+ and clusters of the type CO+⋅Co⋅Cn (n=0,1,...,11). The proportion of these clusters is about 75% of the positive charge carriers. Upon addition of small quantities of O2 (few parts per thousand of the total pressure) the ion clusters disappear gradually, starting with the higher mass clusters, while O2+ becomes the dominant positive ion. It has been found that in Ar–Co discharges the addition of Ar causes an increase of cluster density, indicating the importance of three-body processes for cluster formation. A strong dependence of cluster concentration upon gas temperature has been found.
Ionic dissociation of methanol studied by photoelectron–photoion coincidence spectroscopy
Authors: Nishimura T. , Niwa Y. , Tsuchiya T. , Nozoye H.
Published: February 2003

Ionic dissociation of methanol has been investigated by means of photoelectron–photoion coincidence spectroscopy. Breakdown curves and average kinetic energies released in the fragmentations were obtained as a function of molecular ion internal energy. In the CH2OH+ formation about 45% of the excess energy available for the dissociation is converted into translational energy of products. In contrast, the average translational energies released in the formation of CH3+ and HCO+ ions are 0.4 and 2.5 eV, respectively, and these values remain constant irrespective of the excess internal energy over the excited states of ions, 6a′−1, 1a″−1, and 5a′−1. All results including the breakdown curves reveal that the statistical theories of unimolecular decomposition can not give a satisfactory interpretation to mass spectra of methanol. Alternative mechanisms are discussed to account for the observations.
Molecular calculations of moments of the induced spectra for N2, O2, and CO2
Authors: Steele William A. , Birnbaum George
Published: February 2003

Two moments of the induced absorption spectra for gaseous N2, O2, and CO2 are calculated and compared with experiment. The theoretical expressions evaluated are those for spectra due to quadrupole-induced dipoles in molecules with anisotropic polarizability. The intermolecular interaction functions used include the effects of nonspherical shape by modelling the potential as a sum of quadrupolar plus atom–atom energies with parameters that are known to give realistic values for a number of other measured properties. Satisfactory agreement between the calculation and experiment was obtained in the binary interaction limit if hexadecapolar contributions are assumed to be present in the case of O2. These intermolecular potentials are then used together with an approximate theory for the requisite correlation functions in an evaluation of the spectral moments for liquid N2 and O2. It was found that the agreement between experiment and theory for the liquid improved dramatically when three-body contributions to the spectral moments were included in the calculation.
Kinetics of cytochrome c3 reduction with hydrogenase: A Mössbauer effect study
Authors: Utuno Megumi , Ôno Kazuo , Kimura Keisaku , Inokuchi Hiroo , Yagi Tatsuhiko
Published: February 2003

An electron transfer process was studied in cytochrome c3 in the presence of a small quantity of hydrogenase using the Mössbauer effect. A rate constant for reduction was derived from the time dependence of the Mössbauer spectrum area ratio, reduced to oxidized cytochrome c3.
The unusual behavior of the A 2Πu state of the O2− ion
Authors: Das G. , Zemke W. T. , Stwalley W. C.
Published: February 2003

The inner wall of a recently calculated potential energy curve of the A 2Πu state of the O2− ion shows a curious falttening. To explain and correct this, various aspects of the model used for the MCSCF calculations are analyzed. The interesting finding is that for the adiabatic potential curve the flattening is real and at distances <2.5 bohrs, a Rydberg-like 2Πu state has a lower energy than the lowest valence 2Πu state. This leads to a distortion of the repulsive wall of the adiabatic A 2Πu state and also to a potential well in the complementary adiabatic B 2Πu state (which is valence in character at short distances and Rydberg-like at large distances). Implications of this avoided crossing for electron scattering from O2 are discussed.
Two-level model of dielectric loss in glasses
Authors: Grout P. J. , Tosi M. P. , March N. H.
Published: February 2003

The two-level model of a glass of Anderson, Halperin, and Varma (AHV) is applied to calculate the dielectric relaxation loss. With the AHV model the loss function tends to a constant at low frequencies in contrast to experiment. Therefore, a modification of their probability distribution is considered, which, while retaining the essential structure, takes account of the fact that the AHV probability distribution overestimates the low frequency modes and does not therefore lead to hydrodynamic behavior. This is done by a low frequency cutoff which is shown not only to restore the correct behavior of the loss function as the frequency tends to zero but also to lead to power law behavior at intermediate frequencies as observed experimentally.
High temperature equilibrium measurements of the yttrium–hydrogen isotope (H2, D2, T2) systems.
Authors: Begun G. M. , Land J. F. , Bell J. T.
Published: February 2003

Equilibrium pressures as a function of concentration and temperature were measured for the systems, yttrium–hydrogen, yttrium–deuterium, and yttrium–tritium. Temperatures were varied from 700–1000 °C and concentrations in the solid yttrium phase ranged from H/Y or D/Y atom ratios of 0.01 to 2.0 and T/Y ratios of 0.03 to 1.1. Equilibrium isotherms have been plotted and Van’t Hoff isopleths calculated for these systems. Relative partial molal enthalpies and entropies have been tabulated for the reactions of the hydrogen isotopes with yttrium as a function of atom ratio in the solid. In the low concentration range (H/Y=D/Y=T/Y<0.3), the data obey Sieverts law where the concentration of hydrogen isotope in yttrium is proportional to the square root of the hydrogen isotope pressure. Sieverts constants have been derived. At constant atom fraction the tritium compound has the highest equilibrium pressure and the protium compound the lowest. Expressions for the isotopic hydrogen pressure ratios have been derived and agree well with a simple theoretical treatment in which the dissolved hydrogen atoms are regarded as independent oscillators held in tetrahedral potential wells in the yttrium lattice.
Rotational predissociation of triatomic van der Waals molecules
Authors: Beswick J. A. , Requena A.
Published: February 2003

In this paper we present analytic expressions for rotational predissociation of triatomic van der Waals molecules X...BC, where X is a rare-gas atom and BC a normal diatomic molecule. The process considered is the decay X...BX(j) →X+BC(j′), where j and j′ designate rotational states of the BC molecule. ’’Body fixed’’ and ’’intermediate’’ rotational basis sets are used in the formulation. Application is made to the X...H2 (X=Ne, Ar, Kr) and Ar...N2 van der Waals molecules. It is concluded that rotational predissociation linewidths for these systems should be amenable to experimental measurement in supersonic beams.
Electrostatic potential–electronic density relationships in atoms
Authors: Politzer Peter
Published: February 2003

Some electrostatic potential (V) –electronic density (ρ) relationships have been investigated for a number of neutral ground-state atoms, the largest being chlorine. The results provide further evidence of the physical significance of the outermost minimum in the radial density function, 4πr2ρ, which has previously been used to define the boundary between the core and valence regions in an atom. It was found, for all of the atoms, that the V–ρ behavior in the core regions agrees remarkably well with the Thomas–Fermi equation, V=4.785ρ2/3. In the valence regions, on the other hand, the Thomas-Fermi expression is no longer obeyed and the atoms show much less uniformity in their V–ρ behavior; a reasonable rough approximation, however, is V=αρ, where α depends upon the atom. By using this more realistic V–ρ relationship in the valence region, three correction terms can be derived which markedly improve the atomic energies obtained with the approximate formula E= (3/7) ZV0, in which V0 is the electrostatic potential at the nucleus. For the atoms K–Kr, the new formula reproduces the Hartree–Fock energies to within an average 0.25%. Further, the density is now predicted to decrease exponentially at large distances from the nucleus, correcting a weakness of the Thomas–Fermi theory.
Multiphoton ionization of azulene and naphthalene
Authors: Lubman D. M. , Naaman R. , Zare R. N.
Published: February 2003

Molecular beams of azulene and naphthalene are irradiated by the output of a tunable pulsed laser and the charged species produced by multiphoton ionization (MPI) are analyzed using a quadrupole mass spectrometer. A study of the ion fragmentation pattern as a function of laser power and laser wavelength permits these two C10H8 isomers to be readily distinguished. Compared to electron impact (EI) ionization, the MPI process can produce more gentle ionization at low laser powers and more extensive fragmentation at high laser powers. The former permits the study of azulene and naphthalene dimers from which their heats of dissociation are estimated. In the case of azulene, two-color excitation yields a mass spectrum that differs from the mass spectrum obtained by excitation with either color, alone. The ionization efficiency of MPI is very high, exceeding that of EI during the time the pulsed laser is on. A qualitative description involving autoionizing states is presented to account for this efficiency and the growth of the daughter ion yield with increasing laser power.
Use of the multiple-scattering method for calculating the asymmetry parameter in the angle-resolved photoelectron spectroscopy of N2, CO, CO2, COS, and CS2
Authors: Grimm F. A. , Carlson Thomas A. , Dress W. B. , Agron Paul , Thomson J. O. , Davenport James W.
Published: February 2003

Calculations based on the multiple-scattering method have been performed for the asymmetry parameter (β) as a function of photon energy (from the ionization threshold to 2 Ry above threshold) for the gaseous molecules: N2, CO, CO2, COS, and CS2. For the diatomic molecules N2 and CO we have investigated the use of an overlapping spheres transition state model. The calculations were critically compared with experimental data in the literature. In addition, we have carried out angle-resolved photoelectron spectroscopy on the five molecules using a polarized source of He(i) radiation. Although the calculations show in general good predictive capability, there are also several instances where these calculations seem inadequate and suggestions are made for future work.
EXAFS studies of strong electrolytic solutions
Authors: Lagarde P. , Fontaine A. , Raoux D. , Sadoc A. , Migliardo P.
Published: February 2003

Extended x-ray absorption fine structure measurements are reported for aqueous solutions of CuBr2, ZnBr2, CuCl2, NiCl2, and NiBr2 at various concentrations. It is shown that, for highly concentrated solutions of CuBr2, ZnBr2, and CuCl2, some local order, resembling that of the corresponding crystals, exists. When the concentration decreases, this order partly remains, while the hydration increases. In contrast to this behavior, SrBr2 does not show the existence of any bonding, and it is difficult to make conclusions in the case of NiCl2 and NiBr2 solutions. The temperature dependence of the EXAFS spectra of CuBr2 and ZnBr2 is presented also; some change in the low energy region occurs between 45 and 60°C for CuBr2 solutions.
Molecular dynamics studies of Lennard-Jones liquid mixtures. VI. Temperature, density, and unlike interaction dependencies in static and dynamic properties of binary equimolar mixtures
Authors: Nakanishi Koichiro , Narusawa Hiroshi , Toukubo Kazuo
Published: February 2003

Molecular dynamics calculations have been made for pure fluids and binary equimolar mixtures each containing 108 Lennard-Jones particles. The purpose of the calculations is threefold. First, the effect of the attractive term on the dynamic properties of liquids has been examined at fixed temperature and number density with equimolar mixtures of three models in which different combining rules for the energy parameter are assumed. Next the temperature and density dependences in static and dynamic properties of Lorentz–Berthelot mixture have been investigated. The variation of the velocity autocorrelation function with temperature and number density has been examined in detail. The activation energy for self-diffusion at constant number density is almost independent of the strength of interparticle interaction and is estimated to be 0.35 kcal mol−1, which is comparable with that for argon. In spite of remarkable change in the pattern of velocity autocorrelation function in the density range studied, self-diffusion coefficient changes linearly with density. Finally, a preliminary calculation on the microscopic process of mixing has been described.
Conduction- and localized-electron spin resonance in the lithium–methylamine system: Inferences for the existence of the metallic compound tetramethylaminelithium(zero), Li(CH3NH2)4
Authors: Edwards Peter P. , Lusis Andrejs R. , Sienko M. J.
Published: February 2003

Electron spin resonance (ESR) studies of fluid and frozen solutions of lithium in anhydrous methylamine are reported. The composition and temperature dependence of the ESR properties (electron spin relaxation times, electronic ge factor) are indicative of metallic compound formation in the lithium–methylamine system at low temperatures. Marked changes in both the electron spin relaxation properties and the ESR line shapes in the temperature interval 156–158 K are consistent with those expected for a metallic system moving through the melting point. Electron spin relaxation characteristics of the corresponding metal–ammonia compounds Li(NH3)4 and Ca(NH3)6 have been adequately interpreted in terms of a nearly free-electron (NFE) picture for these low electron density materials. In contrast, electronic properties of the compound tetramethylaminelithium(zero), although nominally metallic (σ∼400 Ω−1 cm−1 for a 22 mole% metal fluid solution), cannot be described in the context of a NFE material. In particular, conduction electron spin relaxation properties reveal a breakdown in the Elliott treatment of spin relaxation in pure metals. We take this as evidence for incipient localization in the very strong scattering electronic regime. It is shown that an enhancement of the electron spin relaxation time relative to the predicted Elliott value in Li(CH3NH2)4 arises quite naturally if one considers the increasing importance of electron–electron and electron–phonon interaction in the approach to the metal nonmetal transition. This changeover in electron spin relaxation characteristics in lithium–methylamine solutions is correlated with the corresponding changes in the electronic conductivity. We suggest that the enhancement in the electron spin relaxation time can be used as an indicator, albeit qualitative, of the degree of localization of the electronic wave function in a nominally metallic system. This behavior contrasts markedly with that of nuclear spin relaxation in the lithium–methylamine system, where incipient localization gives rise to an enhancement in the nuclear relaxation rate relative to that predicted for a metallic system.
On the orbital description of the 4s3d
n+1 states of the transition metal atoms
Authors: Dunning Thom. H. , Botch Beatrice H. , Harrison James F.
Published: February 2003

Multiconfiguration Hartree–Fock calculations are presented for the 3F state of Ti and the 2S state of Cu. (AIP)
Ultraviolet resonance Raman excitation profiles of pyrimidine nucleotides
Authors: Blazej Daniel C. , Peticolas Warner L.
Published: February 2003

A semiempirical theoretical method for calculating the resonant Raman excitation profiles (RREP) from the measured absorption profile is developed and applied to measurements on pyrimidine mononucleotides. The method is general and provides a way of checking the consistency of a measured RREP with a measured absorption profile. From this treatment one obtains Δej, the shift in the equilibrium position of the normal coordinate Qj when the molecule is excited to the eth (resonant) electronic state. This quantity is useful in the estimation of the excited state geometry. Some error in our calculated values of Δej may result because changes in the excited state normal-mode frequencies have not been included in our evaluation of the Franck–Condon factors.
Nonequilibrium time dependent theory of hot atom reactions. II. The hot 18F+H2 reaction
Authors: Shizgal B.
Published: February 2003

The kinetics of the hot atom 18F+H2 reaction are considered. For both equilibrium nonequilibrium conditions, a high temperature steady state is found to exist for this reaction. The high temperature steady state that occurs is due to the particular form of the elastic and reactive cross sections. The steady state temperature is, however, not useful in characterizing the chemical kinetics of this reaction since the steady state temperature is attained only after much reaction has occurred. The departure of the velocity distribution function from Maxwellian that occurs as a result of the temperature relaxation and the reactive process is found to be significant.
Collisions of xenon (n
f) Rydberg atoms with ammonia
Authors: Kellert F. G. , Smith K. A. , Rundel R. D. , Dunning F. B. , Stebbings R. F.
Published: February 2003

In thermal collisions between Xe(nf) Rydberg atoms and NH3 collisional depopulation of the nf state occurs through at least three distinct mechanisms: n-changing collisions, l-changing collisions, and ionizing collisions. Using selective field ionization we have measured the total collisional depopulation rate constants kd for laser-excited Xe(nf) atoms in the n range of 22 to 39. The magnitudes of kd are large, corresponding to reaction cross sections comparable to the geometric size of the Xe(nf) atom at n∼22. In the n range of 25 to 40, Xe+ ion production provides an absolute measure of the rate constants for collisional ionization. Approximate rate constants for n- and l-changing collisions are also presented for the 31f state. In these experiments we find that the state-changing collision rate constants are fairly independent of n and are determined largely by the l-changing collisions. The collisional ionization rate constants measured are smaller than the state-changing rate constants and increase with increasing n, ranging between 0.2 and 4.6×10−7 cm3/sec for n between 25 and 40. The present results are compared to those of recent theoretical calculations.
Theory of structural phase transitions in crystalline poly(vinylidene fluoride)
Authors: Banik N. C. , Boyle F. P. , Sluckin T. J. , Taylor P. L. , Tripathy S. K. , Hopfinger A. J.
Published: February 2003

A mean-field theory capable of predicting structural phase transitions in a linear polymer is presented. The intrachain and interchain potentials, derived from conformational analysis based upon molecular mechanics, are combined in a transfer–integral approach to calculate free energies as a function of temperature and applied uniaxial stress. The theory is applied to poly(vinylidene fluoride) and is successful in yielding several observed phases of this material and in predicting the effect of uniaxial stress on the most stable phase. There are no adjustable parameters in the theory.
Calculated barrier to hydrogen atom abstraction from CH4 by O(3P)
Authors: Walch Stephen P. , Dunning Thom. H.
Published: February 2003

The barrier height and transition state geometry have been calculated for the reaction CH4+O(3P) →CH3+OH using POL–CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be of C3v symmetry with the CH and OH bonds stretched by ∼0.27 Å (25%) and ∼0.21 Å (21%), respectively. The CH bond lengths of the CH3 group change only very slightly (<0.01 Å) during the reaction, while at the transition state each CH bond is ∼13.1° out of the plane perpendicular to the CHO axis and containing the carbon atom (19.5° for CH4 and 0.0° for CH3). The calculated barrier height is 14.4 kcal/mole using a [3s2p1d/2s1p] basis set. From comparison to similiar calculations for O(3P)+H2→OH+H the basis set error in ΔEb is estimated to be ∼2.4 kcal/mole leading to a predicted barrier height of ∼12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole (3A′) and 10.1 kcal/mole (3A″) as compared to experimentally derived activation energies of 9.0–11.4 kcal/mole.
Investigations of static properties of model bulk polymer fluids
Authors: Bishop Marvin , Ceperley David , Frisch H. L. , Kalos M. H.
Published: February 2003

The static properties of continuum, multichain systems are investigated by a ’’reptation’’ Monte Carlo algorithm. All beads interact via a repulsive (shifted) Lennard-Jones potential. In addition, nearest neighbors along chains are linked by a quasiharmonic potential which permits limited pair extensions. Chain lengths of 5, 10, 20, 32, 50, and 70 beads have been studied. Studies at densities of 0.1, 0.3, and 0.5 demonstrate that chain dimensions are compressed as the concentration is increased. Both the mean square end-to-end distance, 〈R2〉, and the mean square radius of gyration, 〈S2〉, have a power law dependence upon l-1, the number of bonds, with exponent approximately 1.16 for ρ=0.1 and 1.07 for ρ=0.3 and 0.5. 〈R2〉 and 〈S2〉 scale with density as ρ−γ where γ∼−0.22±0.02 for long chains, in reasonable agreement with the scaling prediction of −0.25. The asphericity ratios, the pair correlation functions of the center of masses, and the extent of chain overlaps indicate the nonideal behavior of these systems.
Multiphoton ionization spectra of benzene, fluorobenzene, and chlorobenzene resonant with S–S1 transitions by use of nozzle beam method
Authors: Murakami Jun-ichi , Kaya Koji , Ito Mitsuo
Published: February 2003

Multiphoton ionization technique combined with nozzle beam method was applied to the observation of two-photon absorption spectra of benzene, fluorobenzene, and chlorobenzene in S0–S1 valence shell excitations. Strong two-photon activity of ν14 (b2u or b2) vibration throughout these molecules was established. Details of the multiphoton ionization spectrum of chlorobenzene which was observed for the first time will be given. It was concluded that the two-photon resonant four-photon ionization spectra obtained in the present work reflect correct two-photon absorption cross sections. The vibrational temperature of a lower frequency vibration was found to decrease preferentially in an expanded low temperature nozzle beam.
On the rotational relaxation of hydrogen halides
Authors: Levine R. D.
Published: February 2003

The pure rotational relaxation of hydrides due to collisions with inert buffer gas is considered. Rate constants for rotational relaxation are derived. (AIP)
On the rotational relaxation of hydrogen halides
Authors: Koura Katsuhisa
Published: February 2003

The comment of Levine on the rotational relaxation of hydrogen halides are answered. In particular the information theoretic predictions for the rotational transitions rate constant are considered. (AIP)
A
b i
n
i
t
i
o study on the lone pair contributions to the infrared intensities of ammonia
Authors: Smit W. M. A. , van Dam T.
Published: February 2003

Dipole moment derivatives and lone pair moment derivatives with respect to symmetry coordinates have been calculated for ammonia from a near Hartree–Fock wave function (13, 8, 2/8, 2→ 8, 5, 2/4, 1). The dipole moment derivatives are compared with earlier ab initio and CNDO results. Two distinct definitions of the lone pair moment have been used, one based on the highest occupied MO, the other on the appropriate LMO. The calculations reveal large lone pair moment contributions to all dipole moment derivatives. The ab initio results for the lone pair moment and lone pair moment derivatives are compared with the simple hybridization model introduced by McKean and Schatz.
Photoelectron spectroscopy of iodine-doped polyacetylene
Authors: Salaneck W. R. , Thomas H. R. , Bigelow R. W. , Duke C. B. , Plummer E. W. , Heeger A. J. , MacDiarmid A. G.
Published: February 2003

X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used in a study of the electronic structure of iodine-doped polyacetylene. Core level and valence band spectra are used to assess the nature and the consequences fo the iodine doping. The spectra are compared with our previous results on pure and AsF5-doped polyacetylene, and other published work on iodine-doped polyacetylene.
Avoided-crossing molecular-beam spectroscopy of methane
Authors: Itano Wayne M. , Ozier Irving
Published: February 2003

An avoided-crossing RF resonance technique has been used to study methane (12CH4) in the (J=2) rotational state with a conventional molecular- beam magnetic resonance spectrometer. In zero external field the (I=1) levels are about 8 MHz above the (I=0) levels because of the tensor centrifugal distortion interaction. The (I=1) levels with negative nuclear Zeeman effect can be made nearly degenerate with corresponding (I=0) states by the application of a magnetic field of about 2 kG. Mixing of the levels by nuclear hyperfine interactions gives rise to avoided crossings. Electric and magnetic dipole transitions were observed near these avoided crossings. The method is discussed in detail. Values were obtained for the scalar and tensor spin–rotation constants ca and cd, and for the tensor spin–spin coupling constant d. By using a value of the tensor centrifugal distortion splitting Δ2=60 (DT+6 H4T+36 L4T) calculated from a combined double-resonance and microwave absorption set of distortion constants, a value of the rotational g-factor gJ was obtained. Alternatively, the value of gJ measured in studies of the rotational-moment molecular-beam spectrum can be used, in conjunction with the present work, to obtain Δ2. The values and their estimated uncertainties (90% confidence limits) are ca=10.372 (83) kHz, cd=18.370 (23) kHz, d=21.17 (32) kHz, gJ=0.31317 (28), and Δ2=7970.30 (66) kHz. These results for (J=2) are in agreement with previous determinations, which (except for Δ2) represent averages over many values of J. The application of the present measurements to calculations of the nuclear magnetic shielding constants and of the molecular magnetic susceptibility is discussed.