bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the model dependence of Monte Carlo results for the relaxation of the end-to-end distance of polymer chains
Authors: Boots Henk , Deutch J. M.
Published: February 2003

Recently, Verdier and Kranbuehl (VK) have examined by Monte Carlo simulation the relaxation times for the end-to-end vector of a polymer, with excluded volume, on a lattice. The kinetic model employed by VK includes two-bead crankshaft motions in contrast to earlier kinetic models that included only single-bead motions. It is shown that the new VK model contains constraints similar to those discussed by Hilhorst and Deutch for the single-bead model. These constraints lead to artificially long relaxation times not due to the long-range excluded-volume effect.
Magnetic and thermal properties of the Yb3+ ion in tetragonal sites in CaF2
Authors: Neogy D. , Chatterji A. N.
Published: February 2003

Magnetic and thermal properties of the Yb3+ ion in tetragonal sites in CaF2 have been calculated using the crystal field parameters obtained by Baker and Blake from ENDOR experiments. The effective magnetic moment of the ion is found to be smallest compared to two other salts of the ion and also its variation with temperature fastest. The relative anisotropy at room temperature is also found to be largest in the present case and ΔK is negative like that of Yb ethylsulfate. The Schottky specific heat shows a peak around 180 K, which is quite high compared to other salts of the ion. An experimental verification of the thermal variation of the magnetic moment and specific heat is suggested to confirm the crystal field proposed by Baker and Blake.
Force decomposition analysis along reaction coordinate
Authors: Joshi Bhairav D. , Morokuma Keiji
Published: February 2003

An ab initio method for the calculation and analysis of force along the intrinsic reaction coordinate (IRC) is proposed. The force is expressed as a sum of chemically meaningful terms: electrostatic (ES), polarization (PL), exchange (EX), charge transfer (CT), their coupling term (MIX), and the intramolecular deformation force (def). The method has been applied to the model SN2 reaction: CH4+H−→H−+CH4, for which the IRC has become available. Emphasis is placed on the method and the type of results such an analysis yields.
Singlet and triplet exciton percolation in benzene isotopic mixed crystals
Authors: Colson Steven D. , George Steven M. , Keyes Thomas , Vaida Veronica
Published: February 2003

New, closed form, approximate expressions are obtained for the calculation of ideal percolation thresholds which are determined by the migration topology. Excitation tunneling theory is briefly reviewed and extended for application to nonresonant, long range energy migration in mixed molecular crytals. These theories are applied to new singlet and triplet exciton percolation data for benzene isotopic mixed crytals. The differences between these results and the theoretical expectations are used to emphasize the importance of exciton lifetime and exciton trapping in determining the extent of long range energy transport in molecular crytals.
Quantum-mechanical treatment of collision-induced dissociation
Authors: Knapp E.-W. , Diestler D. J.
Published: February 2003

A close-coupling technique for calculating quantum-mechanical probabilities of collision-induced dissociation (CID) of a diatomic molecule by an atom is presented. The internal Hamiltonian (of the diatomic) is first diagonalized in a discrete, square-integrable basis. The lowest several of the resulting discrete eigenstates approximate the true bound states and the remaining (pseudocontinuum) states represent the true continuum. Next, the stationary collision wavefunction is expanded in the diagonal basis to obtain a discrete set of close-coupled equations, which are integrated numerically by standard procedures. The method is applied to a collinear model in which the diatomic is bound by a Morse potential and the interaction is a repulsive exponential. The total CID probabilities appear to be converged to 1% or 2% in most cases. Vibrational ’’enhancement’’ of CID is observed in this model. A very general problem associated with the use of the exponential interaction in conjunction with a binding potential which supports a continuum is discussed.
Classical trajectory study of differential cross sections for Li+–CO and N2 inelastic collisions
Authors: Thomas Lowell D.
Published: February 2003

The quasiclassical method has been used with a CI rigid rotor surface for Li+–CO and an SCF surface for Li+–N2 to compute the broad rotational distributions in the differential cross sections at 4.23 eV and center of mass scattering angles, 35°–50°. Calculations including vibrations have also been done primarily to see the influence on the rotational distributions. The results are compared with experiments and good agreement is found for N2 but unresolved discrepancies are found for CO. The distributions are found to fall off more sharply after the peak than in the experimental results and the reason for this is analyzed in the coplanar case. It is suggested that structure is observed in both the calculated and experimental distributions. Such structure is demonstrated and analyzed in terms of rainbow-type singularities in the coplanar case.
Concentration dependent solute redistribution at the ice–water phase boundary. III. Spontaneous convection. Chloride solutions
Authors: Gross Gerardo Wolfgang , Wong Pui Mun , Humes Karen
Published: February 2003

Spontaneous convection in the ice–water system was investigated with temperature and concentration measurements. Earlier work had shown that mild convection causes an apparent concentration dependence of the distribution coefficient that masks any real concentration dependence. Convection was found to be determined primarily by the density inversion at the 4 C isotherm. A simple method of evaluating the height of convection cells was developed which enables one to obtain an order-of-magnitude estimate of the distribution coefficient. Forced convection (stirring at 300 or 1000 rpm) and improved sampling and analysis techniques were used to redetermine the distribution coefficients of chlorides in ice. Solutions of HCl, LiCl, NaCl, KCl, RbCl, CsCl, and NH4Cl were investigated in the concentration range 10−6–10−2M. For the hydrogen and alkali chlorides no effect of the different cations on the chloride distribution coefficient was evident. Its average value is 2.7×10−3. For the ammonium chloride, the distribution coefficient is 1.4×10−2. The chloride distribution coefficient is not affected by solution pH, nor by an electrical interface potential. The chloride distribution coefficient is nearly or completely independent of concentration. This contrasts with the distribution coefficients of alkali fluorides which are strongly concentration dependent and much greater than those of the chlorides. These results point to different structural relationships of chloride and fluoride impurities with the ice lattice. However, dielectric relaxation and electrical conductivity of ice containing chloride and fluoride, respectively, are quite similar. According to a classical concept, each solute particle introduces into the ice lattice electrical point defects of specific type and number. The distribution coefficient enables one to determine these defects. We conclude from our results that this concept needs to be re-examined.
CO2 photodissociation, 1060–1175 Å
Authors: Slanger T. G. , Sharpless R. L. , Black G.
Published: February 2003

The relative O(1S) quantum yield from CO2 photolysis between 1060 Å and 1175 Å has been measured, to sort out the discrepancies between two prior publications on the subject. Whereas the yield is near unity over a considerable portion of the spectral regions, there is an abrupt dip to φO(1S)⩽0.15, centered at 1089 Å. As this wavelength is also the location of a very strong Rydberg transiton in CO2, it appears that the low quantum yield is associated with excitation of the upper level of this transition, the Πu1 state. The O(1D) yield at this wavelength was found to have a minimum value of 0.65±0.1. In conjunction with the lack of any observable fluorescence it is concluded that dissociation on the O(1S)+CO and O(1D)+CO surfaces probably accounts for all the input energy.
Semiclassical dynamics on multiple electronic surfaces: Three-dimensional treatment of reactive F+H2
Authors: Komornicki Andrew , Morokuma Keiji , George Thomas F.
Published: February 2003

The role of electronic transitions in collisions is investigated for the reaction of F+H2 by combining quasiclassical Monte Carlo trajectories with a semiclassical decoupling approximation for the electronic transitions. The interaction potential is represented by three interacting potential energy surfaces constructed from one LEPS and two valence-bond type surfaces. Specific attention is focused on the reaction of excited state flourine atoms reacting to form ground state products. The reactants are initiated in either of the two spin–orbit states of the atom with the diatom in the ground vibrational state and the lowest four rotational states, at relative translational energies of 0.1, 0.2, and 0.3 eV. Cross sections are presented for each of these processes and compared to those from previous single surface calculations and experiment. Even if the reactants are initiated on the excited state surface, the reactive cross sections (which are classically forbidden) are significant. The major dynamical effects of the excited state reaction are the flow of reactant electronic energy into product internal energy. The effects of changing reactant rotational states on the product vibrational distribution was also investigated. By comparing cross sections into specific product vibrational states (v′) we find that there is a decrease in the ratio σ (v′=3)/σ (v′=2) as the rotational quantum number is increased form 0 to 3. This same trend is observed for reactions from both the ground and excited states. The ratio σ (v′=1)/σ (v′=2), however, is seen to increase as the rotational energy is increased.
Nonadditive effects in small beryllium clusters
Authors: Novaro O. , Kol/os W.
Published: February 2003

The SCF analysis of few-body contributions to the energy of Be3, Be4, and Be5 clusters is presented. It is shown that nonadditive terms are decisive for the stability of such clusters. An explicit evaluation of the nonadditive five-body interaction is given.
Stability and relative stability in reactive systems far from equilibrium. I. Thermodynamic analysis
Authors: Procaccia Itamar , Ross John
Published: February 2003

We present in this article an analysis of the stability and relative stability of multiple stationary states in chemical systems driven far from equilibrium. Given the macroscopic kinetic (nonlinear) equations of motion, we postulate that there exists an entropy state function which is maximum for the (time-dependent) solutions (concentrations of chemical species as a function of time) of the kinetic equations. We derive thermodynamic equations of motion for multivariable systems which follow from a thermodynamic variational principle for the power dissipation and are consistent with the kinetic equations. From the thermodynamic equations of motion we obtain conditions for stability, marginal stability, and relative stability of stationary states. The point of coexistence of stationary states, being a special case of relative stability, is identified. We discuss examples of one and two variable systems. The thermodynamic analysis for a one-variable system agrees with a stochastic solution, and with a kinetic analysis presented in the next article.
First order spin–Peierls transition in a quantum antiferromagnetic Heisenberg chain
Authors: Lépine Yves , Caillé Alain
Published: February 2003

The problem of the linear antiferromagnetic Heisenberg chain of elastically coupled spins is investigated in the Hartree–Fock approximation and using mean field theory. At low temperature this system undergoes a phase transition, called spin–Peierls, to a dimerized state. Using an exponential variation of the exchange parameters with the distance between the ions, we show that the transition is of first order for any value of interest of the parameters in the theory. Previous results, using the linear dependence of the exchange parameters, gave a second order transition. The absence of the second order phase transition is attributed to a pseudofermion–pseudofermion interaction in our calculation.
Comparison of nonperturbative variational approximations to the scattering amplitude for a Yukawa potential
Authors: Rosenthal Carey M. , Haracz Richard D.
Published: February 2003

Variational methods for calculating approximations to the scattering amplitude of a spinless particle from a Yukawa potential are compared. These arise from the variation of functionals that depend on trial forms of the Heitler operator K or the transition operator T. The trial forms are nonperturbative, and the variational principle is applied in the free particle and partial wave basis to obtain independent approximations. In addition, no artificial separability constraint is imposed on the momentum space representation of either the trial T or K. The full free particle Green’s function is utilized without basis set truncation in all of the calculations. The different approximations provide a choice of technique in the event that one or the other variational method is unstable. The results at two representative energies are in good agreement with an exact calculation.
Quantum theory of scattering of atoms from solid surfaces
Authors: Metiu Horia I.
Published: February 2003

We present a quantum theory of scattering of an atom from a solid surface. The formalism is constructed by assuming that the solution of two simpler problems is known; one of them is scattering of the atom by the average lattice potential; the other is the motion of the lattice in the absence of the scattering atom. The theory includes the effect of the thermal motion of the surface atoms as well as multiphonon energy transfer processes. The derivation assumes that the thermal fluctuations of the projectile–lattice potential and the energy of the distortion of the lattice by the projectile can be treated as perturbations.
A phase space sampling approach to equilibrium semiclassical statistical mechanics
Authors: Stratt Richard M. , Miller William H.
Published: February 2003

A nonperturbative semiclassical approach to statistical mechanics is reviewed and discussed in terms of the idea of phase space sampling. The approach is then used to treat quantum effects in the vibrational state distribution of HCl and in the two particle radial distribution function for He4 as well as in the case of a sample discontinuous potential. Although further work is indicated for hard wall potentials, the method does seem to provide a facile technique for approximating quantum corrections in situations of chemical interest.
ESR spectra of the MnO, MnO2, MnO3, and MnO4 molecules at 4 °K
Authors: Ferrante R. F. , Wilkerson J. L. , Graham W. R. M. , Weltner W.
Published: February 2003

The molecules MnO, MnO2, MnO3, and MnO4 have been prepared by the vaporization and reaction of manganese atoms with O2, N2O, or O3 and isolated in various inert-gas matrices at 4 °K. ESR has been used to determine magnetic parameters which are interpreted in terms of molecular geometry and electronic structure. MnO is confirmed to have a σπ2δ2, 6Σ+ ground state with g⊥=1.990(7) (assuming g∥=ge) and a zero-field splitting in accord with the gas phase value ‖D‖=1.32 cm−1. Hyperfine splittings due to the 55Mn (I=5/2) nucleus are ‖A∥‖=176(8) and ‖A⊥‖=440(11) MHz. MnO2 is a linear 4Σ− molecule with probable configuration σδ2, ‖D‖=1.13 cm−1 (assuming g∥=g⊥=2.0023), ‖A∥‖=353(11), ‖A⊥‖=731(11) MHz. MnO3 exhibits very large hf splittings ‖A∥‖=1772(3) and ‖A⊥‖=1532(3) MHz indicative of a sdz2 hybrid 2A1 ground state of D3h symmetry. The spectrum of MnO4 is consistent with a C3v molecule distorted from a 2T1 electronic state in tetrahedral symmetry by a static Jahn–Teller effect. g and A tensors are slightly anisotropic: g∥=2.0108(8), g⊥=2.0097(8), ‖A∥‖=252(3), ‖A⊥‖=196(3) MHz. The electron hole is almost entirely in an oxygen π-bonded orbital with one oxygen atom displaced along its Mn–O bond axis.
Atomic multipole polarizabilities in the extended Coulomb approximation
Authors: Lamm Gene , Szabo Attila
Published: February 2003

A new and computationally convenient exact analytic expression for the 2l-pole static polarizabilities of mono and divalent atoms and ions is obtained within the extended Coulomb approximation of Adelman and Szabo. In this approximation the valence electron wavefunction is represented by an asymptotically valid Whittaker function with hydrogenic normalization and the l-symmetry excited orbitals are eigenfunctions of a Coulombic Hamiltonian plus an r−2-type pseudopotential. Our expression for the polarizability contains two parameters which can be obtained from the ionization potentials of the ground state and the first l-symmetry excited state of the system. Illustrative calculations for the alkali and alkaline earth atoms are present. Good agreement is found with recent experimental results as well as those from elaborate calculations using correlated wavefunctions, where such results are available.
NO+O chemiluminescence in low temperature argon matrix
Authors: Fournier J. , Deson J. , Vermeil C.
Published: February 2003

The far uv photolysis of nitrogen dioxide embedded at 0.1% dilution in an argon rigid matrix at 5 K leads to dissociation into NO+O. When the matrix is warmed up, the atomic oxygen diffuses and a thermoluminescence spectrum is observed corresponding to NO2 emission. This emission is also observed in the thermoluminescence spectrum following the photolysis of NO/O2 (1/1) mixture in the same conditions. It follows that the migration of oxygen in argon matrix is already efficient at 9 K.
An accurate intermolecular potential for argon
Authors: Aziz R. A. , Chen H. H.
Published: February 2003

A simple precise intermolecular potential is proposed for argon. It was fit to new precise spectroscopic data for the argon dimer and the high temperature viscosity correlation of Maitland and Smith. The long range interaction was constrained within the bounds of Tang et al. and the ’’Hartree–Fock component’’ constrained close to the ab initio SCF calculations at short range due to Wahl. The potential is able to predict many other properties with high precision.
Semiclassical theory of rotational transitions in collisions of diatomic molecules
Authors: Curtiss C. F.
Published: February 2003

In earlier papers, the theory of rotational transitions in collisions of diatomic molecules has been developed in terms of a generalized phase shift. In the present paper, the classical limit of this function is discussed and limiting expressions are developed. These expressions may be used to obtained expressions for the classical limit of various quantities associated with the collisions. In particular, an expression is developed for the moments of the transition probability with respect to the rotational inelasticity.
Theory of resonant Raman scattering. II. Overtones and hot bands
Authors: Hong Hwei-kwan
Published: February 2003

Resonant Raman scattering is discussed in the framework of the Green’s function method in both the strong and weak vibronic coupling limits. We further elaborate on the meaning of each matrix element of the Green’s function, evaluated in the convenient basis set which is commonly referred to as the Longuet–Higgins representation. Cross sections for absorption, Rayleigh, Raman fundamentals, Raman overtones, hot bands, etc., can all be related to the matrix elements of the Green’s function. It is further emphasized that only in the vibronic view elaborated here does a simple and direct correlation exist between the light scattering processes and the Green’s functions. Some typical excitation profiles for overtones and hot bands are calculated and the consequences of strong vibronic couplings are discussed in contrast to the weak vibronic coupling case.
Mass scaling in molecular systems
Authors: García-Sucre M.
Published: February 2003

The scaling of the electron and nuclear masses in molecular systems for which both the electronic and the nuclear motions are quantum mechanically described is examined. Exact closed expressions for both the total contribution to the energy coming from the nuclear motion and for the total energy of any given state of a molecular system are obtained in terms of the nuclear kinetic energy density and the total kinetic energy density functions, respectively. It is shown that the problem of determining the energy levels of a system with n electrons and N nuclei can be reduced to one of determining effective kinetic energy density functions, one for each state of the system, which depend only on three variables. An expression is given for the isotopic effect on the energy of any given state in terms of the exact kinetic energy contribution of the isotopically substituted nucleus. On the other hand, the energy of each state of every system lying on a certain kind of trajectory in a molecular parameter space (to be defined in Sec. I) is given in terms of the energy levels of only one system lying on the same trajectory. Some exact relations between electronic and mesic systems are obtained.
Electric and magnetic properties of N2 and H2O by the equations-of-motion method
Authors: Lamanna U. T. , Guidotti C. , Arrighini G. P.
Published: February 2003

Dynamic multipole polarizabilities and the magnetic dipole susceptibility of N2 and H2O are evaluated in the framework of the equation-of-motion method corresponding to the random phase (RPA) and Tamm–Dancoff approximations (TDA). The results are presented in the form of a Cauchy expansion truncated after the ω4 terms. Nuclear magnetic shielding constants of N, H, and O are obtained as by products of the main calculations. While the RPA results for the magnetic properties compare very well with available experimental data, the computed values for the electric observables are only in moderately good agreement with the experiments, suggesting an inadequacy of the employed basis sets and a different relevance of the correlation effects as well. The dipole–dipole van der Waals dispersion coefficients for the pairs N2–N2, H2O–H2O, and N2–H2O obtained by RPA values of the dipole polarizability at imaginary arguments compare only approximately well with semiempirical estimates.
Force constants for the symmetric stretch motions of acetylene: Accurate a
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o calculations
Authors: Bagus P. S. , Pacansky J. , Wahlgren U.
Published: February 2003

Quadratic force constants for C2H2 have been determined from D∞h potential surfaces obtained using SCF and configuration interaction, CI, wavefunctions. Our best calculation yields quadratic force constants which are of the same accuracy as those derived from experimental data. This calculation is based on a CI model which includes single and double exctitations of all valence electrons and which uses an extended Gaussian basis set with several polarization functions. We have performed calculations using different numbers of polarization functions in the basis set and correlating different numbers of electrons in the CI wavefunctions. The sensitivity of the computed equilibrium geometry and quadratic force constants to these variations have been investigated.
Polariton Green’s functions for semi-infinite spatially dispersive media with applications to molecular crystal reflectivity spectra
Authors: Rimbey Peter R.
Published: February 2003

A complete set of polariton Green’s functions are deduced for the half-space. The symmetry conditions used by Fuchs and Kliewer and Rimbey and Mahan placed on electromagnetic fields in a semi-infinite homogeneous isotropic medium are shown to correspond to external current densities in an infinite anisotropic medium. Solutions to Maxwell’s equations in spatially dispersive media are delineated for arbitrary polarization. Components of the Green’s dyadic are related to the surface Green’s function derived by Garcia-Moliner and Rubio. The normal incidence reflectivity spectra of 1-5 bis(dimethylamino) -pentamethinium perchlorate is calculated near the intense (f∼3.31), 4090 Å molecular transition using the linear response theory of Davydov, and the reflectivity theories of Mahan and Obermair, Rimbey and Mahan, and Fuchs and Kliewer. A one-dimensional interacting exciton–phonon system is used as the model Hamiltonian. The general features of the metallic reflection spectrum, i.e., bandwidth, phonon shoulder, and peak height, are reproduced satisfactorily. The formalisms of Mahan and Obermair and Rimbey and Mahan reproduce the spectrum more accurately than the Fuchs–Kliewer reflectivity indicating the ABC P (0+) =0 is appropriate for this Frenkel exciton system.
Chemi-ionization in collisions of metastable argon with sodium
Authors: Neynaber R. H. , Magnuson G. D.
Published: February 2003

Absolute and relative cross sections were obtained for the Penning-ionization (PI) reaction Ar*+Na→Ar+Na++e and the associative-ionization (AI) reaction Ar*+Na→ArNa++e. The studies were made by a merging-beams technique for a relative kinetic energy W of the reactants from nominally 0.01 to 10 eV for PI and 0.01 to 0.4 eV for AI. The Ar* represents a composite of Ar(4s 3P2) and Ar(4s 3P0). The PI is directed and occurs predominantly when the difference between the relative kinetic energy of the heavy products and the reactants is approximately 0.1 eV. Cross sections for AI are roughly proportional to W−1. Cross sections for total ionization (AI plus PI) vary as W−1/3 for the lower W.
Forced diffusion of molecules with internal rotation
Authors: Warchol Mark P. , Vaughan Worth E.
Published: February 2003

The dipolar correlation function for a system of independently relaxing molecules containing rotatable groups is calculated by solving the forced diffusion equation. It is found that the correlation function has the same form as a previous result for spheroids. This result follows from the assumption that the equilibrium distribution function is constant, an assumption made to allow linearization of the diffusion equation. Procedures for relieving the assumption are outlined and applied to a hindered internal rotation case. The presence of a barrier to internal rotation makes the correlation function decay more slowly. The relation between the multimolecular correlation function and the complex dielectric permittivity is investigated.
EPR spectroscopy of delocalized and localized charge-transfer excitons in phenanthrene–PMDA single crystals
Authors: Keijzers C. P. , Haarer D.
Published: February 2003

Detailed EPR investigations of delocalized and localized charge-transfer excitons in phenanthrene–PMDA single crystals are reported. The ionicity of the excited charge-transfer state is investigated by two different methods, by a detailed analysis of the fine structure tensor and by an analysis of the hyperfine tensors of the donor protons. The fine structure analysis is carried out on the basis of a ’’point charge’’ model calculation; it yields a degree of charge transfer of 76%±5%. This value is more reliable than the number derived from the hyperfine structure analysis due to uncertainties in available spin densities for the donor and acceptor molecule. The anisotropy of the exciton hopping rate parallel and perpendicular to the crystal stacking axis is at least 100:1 with a hopping rate parallel to the stack axis of ν∥?108/sec at 130°K.
Absolute infrared intensities of methyl bromide and all deuterium derivatives
Authors: van Straten A. J. , Smit W. M. A.
Published: February 2003

The integrated infrared intensities of all the fundamentals of CH3Br, CH2DBr, CHD2Br, and CD3Br are experimentally determined using nitrogen broadening pressures of 37 atm. The results are interpreted in terms of dipole moment derivatives with respect to the symmetry coordinates (∂μ/∂S) and in terms of bond charge parameters. These molecular polar quantities derived from the observed intensities are compared with the corresponding values from molecular orbital calculations.
Spin coupling of two identical complex ions. III. Singlet–triplet splitting
Authors: Amoretti G. , Varacca V.
Published: February 2003

The theoretical treatment in a preceding paper is applied to the singlet–triplet splitting of the three compounds under consideration. The results of the numerical calculations are discussed and the suitableness of the theory to interpret the coupling mechanism in such type of dimeric complexes is pointed out. A comparison with preliminary experimental results shows an encouraging agreement and confirms the validity of the approximations introduced.
Photoabsorption cross sections of CH4, CF4, CF3Cl, SF6, and C2F6 from 175 to 770 Å
Authors: Lee L. C. , Phillips E. , Judge D. L.
Published: February 2003

Using synchrotron radiation as a continuum background, the photoabsorption cross sections of CH4, CF4, CF3Cl, SF6, and C2F6 were measured photoelectrically in the 175–770 Å region. Discrete structure was observed and attributed to the Rydberg states converging to the known states of the ions: In CH4, Rydberg states converging to the (2a1)−1 ion state with vibrational frequencies varying from 1800 to 2200 cm−1 were observed in the 563–627 Å region. In CF4, Rydberg states converging to the (4a1)−1 and (3t2)−1 ion states with vibrational frequencies varying from 550 to 730 cm−1 were observed in the 559–611 Å region. ’’Window’’ type Rydberg states converging to the (4a1)−1 ion states were also observed. In CF3Cl, discrete structure with energy spacings varying from 540 to 700 cm−1 was observed in the 614–666 Å region. The assignment of this structure to Rydberg states was not attempted because the vibrational structure of the CF3Cl ion states is not known. In SF6, structure was observed in the 630–676 Å region and was attributed to the Rydberg states converging to the (1t2g)−1 ion state.
Brillouin scattering studies of orientationally disordered sodium cyanide and potassium cyanide single crystals
Authors: Wang C. H. , Satija S. K.
Published: February 2003

Brillouin spectra of acoustic phonons in the orientationally disordered phase in KCN and NaCN have been studied. The phonon frequencies have been determined as a function of temperature. The frequency of the transverse acoustic phonon propagating along (110) and polarized along (001) corresponds well with mean field theory for the phase transition for the two cyanide crystals. The elastic constants C11, C12,, and C44 have been determined as a function of temperature in the cubic disordered phase. The Cauchy relation for cubic crystals is found to be invalid in KCN and NaCN. The two types of phase transitions in KCN and NaCN are discussed in terms of the vanishing shear modulii C44 and (C11−C12)/2, each of which induces lattice instability when it becomes small. The integrated intensities of the shear modes in KCN and NaCN were also measured as a function of temperature. The intensity enhancement upon approaching the phase transition temperature has been found to be due to the decrease of C44. The relative values of the elasto-optical constant P44 in KCN and NaCN have also been determined as a function of temperature.
Monte Carlo studies of self-replicating macromolecules
Authors: Frisch H. L. , Bishop Marvin , Roth Jonathan
Published: February 2003

Preliminary results of Monte Carlo computer studies of the temporal evolution of self-replicating copolymers from adsorbed monomers are presented here. Various kinetic parameters have been studied and some of the effects of specificity and cooperativity have been noted.
Many-body analysis of long-chain interactions. Inclusion of quadrupoles
Authors: Rabenold David A. , Linder Bruno
Published: February 2003

A many-body theory, based on time-dependent Hartree approximation, is developed for the van der Waals energy between two parallel linear chains. The treatment takes account of nonadditive many-body interactions to all orders within the dipolar approximation and includes the major contributions from dipole–quadrupole and quadrupole–quadrupole couplings. The theory is applied to Drude chains of varying lengths and numerical results are presented for chain systems, each of which contains between two and 10 polarizable units. It is concluded that for the model parameters considered the effects of quadrupolar interactions are small and that the overall interaction is substantially the same as the many-body van der Waals interaction based on dipolar couplings developed earlier [J. Chem. Phys. 63, 4079 (1975)].
A partial approximation in calculations of second-order perturbation energies
Authors: Hiroike Eiko
Published: February 2003

In calculations of molecular problems, only a part of excited electronic states corresponding to low excitation energies can be obtained with numerically reliable accuracy. Then, in the series of the second-order perturbation energy, some of the intermediate excited states corresponding to low excitation energies are retained explicitly but others are eliminated by use of a mean excitation energy and completeness relations. The appropriate value of the mean excitation energy is discussed for the case of the proton spin–spin coupling in CH4.
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Authors: Kertész Miklós , Koller Jo?e , A?man Andrej
Published: February 2003

A b initio ’’exact exchange’’ LCAO Hartee–Fock (HF) energy bands are given for a polyene model with equal bond lengths. In addition to the symmetric wavefunction a broken-symmetry solution of charge density wave (CDW) type with lower total energy occurs. This instability of the metallic-type wavefunction with respect to the charge-density wave was overlooked by André and Leroy in their ab initio HF treatment of polyene. We wish to discuss the nature of the CDW and the screening effect of the σ electrons, which tend to diminish the amplitude of the CDW of the π electrons. Comparison with a model with alternant bond lengths shows the latter to be energetically more stable.
Determination of absolute photon yields under single-collision conditions
Authors: Dickson C. R. , George S. M. , Zare R. N.
Published: February 2003

The experimental procedure is presented to measure the absolute photon yield, the percentage probability of emitting a visible photon per reactant molecule consumed, for chemiluminescent reactions under single-collision conditions. Using a well-defined metal beam directed into a scattering gas at submillitorr pressures, this procedure is applied to the reactions Sm+N2O, Sm+F2, Ba+N2O, and Ba+NO2 to obtain the photon yields in the 350–800 nm range of 0.39%, 11.8%, 2.4%, and 0.18%, respectively, where the estimated uncertainty is about 50%. The absolute photon yields for each of these reactions initially increases with scattering gas pressure, demonstrating that secondary collisions ’’feed’’ radiating states from dark, reservoir states. It is suggested that other relative photon yields can be put on an absolute basis by comparison with the Sm+N2O chemiluminescent reaction.
HTFFR kinetics studies of Al+CO2→AlO+CO from 300 to 1900 K, a non-Arrhenius reaction
Authors: Fontijn Arthur , Felder William
Published: February 2003

High-temperature fast-flow reactors (HTFFR) were used to obtain the rate coefficients k1 (and their accuracies) for the reaction Al +CO2→AlO+CO. At 310, 490, 750, 1500, and 1880 K, k1 is found to be (1.5±0.6) ×10−13, (6.9±2.7) ×10−13, (1.6±0.7) ×10−12, (9.0±3.8) ×10−12, and (3.8±1.5) ×10−11, respectively (all in ml molecule−1 s−1 units). For this temperature range k1(T) may be expressed by the curve fitting equation k1(T) =2.5 ×10−13 T1/2 exp(−1030/T)+1.4×10−9 T1/2 exp(−14 000/T). The data also indicate a wall-oxidation process of zeroth order in [CO2] with γAl of 10−3 to 10−2, not measurably dependent on T. Factors affecting the accuracy of the measurements are discussed. Over the 310–750 K range k1(T) obeys an Arrhenius expression, with an activation energy of 2.6±1.3 kcal mole−1, which implies D(Al–O) ?122 kcal mole−1. Above 750 K, k1(T) increases much more rapidly with T. This behavior cannot be described on the basis of simple transition state theory alone; the most probable additional factors involved are the opening of a second reaction channel leading to AlO(A 2Π) and preferential reaction of Al with CO2 in bending modes.
Restricted self-consistent field method for excited states
Authors: Ohmichi Naoto
Published: February 2003

A restricted self-consistent field method for excited states is proposed. Assuming that the excited-state Hartree–Fock space is divided into three subspacies orthogonal to each other, we minimize the energy of a single configuration excited state. By using the ground-state Hartree–Fock orbitals as the basis functions, the three coupled pseudoeigenvalue equations obtained from the minimization reduce to the simple secular equations, only two of which are solved iteratively. The optimized excited state guarantees the orthogonality with the HF ground state and the Brillouin-type theorems satisfied in EHP method of Morokuma and Iwata. In addition, the method guarantees two other important Brillouin-type theorems. With an additional assumption this method reduces to Huzinaga’s method. In order to demonstrate the usefulness of this theory in the perturbation treatment, we examine the contribution to the transition probability beyond the independent-particle model using the excited-state wavefunction obtained from the restricted SCF method as the unperturbed wavefunction of the excited state. An approximate expression for the off-diagonal matrix element which determines the theoretical electronic transition probability taking into account the electron correlation is given.
Calculation of the magnetic susceptibility of H2 by the distinguishable electron method
Authors: Pflug Donald R. , Palke William E. , Kirtman Bernard
Published: February 2003

The magnetic susceptibility of H2 is calculated by the distinguishable electron method (DEM). Using a distorted atoms in molecules initial approximation an accuracy comparable to that of the Karplus and Kolker uncoupled Hartree–Fock method is achieved. We conclude that DEM shows promise as a simple way of computing reasonably accurate magnetic susceptibilities.
On the Sturmian representation of the Coulomb Green’s function in perturbation calculation
Authors: Rahman N. K.
Published: February 2003

The Sturmian representation of the Coulomb Green’s function is examined for calculations in many-body perturbation theory. Some representative calculations are worked out and the prospects with this representation are discussed.
Homogeneous linewidth of the S1←S transition of free-base porphyrin in an n-octane crystal as studied by photochemical hole-burning
Authors: Voelker S. , Macfarlane R. M. , Genack A. Z. , Trommsdorff H. P. , van der Waals J. H.
Published: February 2003

The homogeneous linewidth of the 0–0 band of the S1←S0 transition of free-base porphyrin (H2P) in an n-octane matrix and its temperature dependence (T=1.5–4.2 K) have been measured by means of photochemical hole-burning. The linewidth extrapolated to T=0 is ∼9 MHz for the two types of sites which have been investigated, and it appears to be determined entirely by the decay time of the emitting state S1 (17 ns). The temperature dependence of the linewidth is strongest for the thermodynamically less stable sites. The increase in linewidth with temperature may arise from a relaxation process involving a low frequency resonance mode (∼5 cm−1) localized at the H2P molecules. At 77 K the homogeneous linewidths are comparable to the inhomogeneous widths of 3–4 cm−1; hence, hole–burning is no longer observed.
Photoelectron spectrum of the Xe2 van der Waals molecule
Authors: Dehmer P. M. , Dehmer J. L.
Published: February 2003

Photoelectron spectrum of Xe formed in a supersonic expansion is reported. The results show that excited states of Xe+2 have dissociation energies of up to 0.37 eV. (AIP)
Reaction product identification from O(3P)+benzene, toluene, and 1,3,5-trimethylbenzene collisions in crossed molecular beams
Authors: Sloane Thompson M.
Published: February 2003

Although absolute rate coefficients have been measured for a number of reactions of aromatic molecules with O(3P) atoms, very little is known about the mechanisms of these reactions. In order to identify the products of O atom–aromatic molecule reactions, the products of single reactive collisions between oxygen atoms and benzene, toluene, and 1,3,5,-trimethylbenzene have been observed in crossed molecular beam experiments. The products were detected with a quadrupole mass spectrometer. Two product paths were observed for O+benzene: (1) the O atom–benzene adduct, which is most likely phenol, and (2) carbon monoxide and a C5H6 hydrocarbon which is probably 3-penten-l-yne. An additional path involving the formation of (methyl-substituted) benzaldehyde and H2 was observed for the reaction with methyl-substituted benzenes. Only the adducts, the benzaldehydes, and the olefins were observed directly. The identities of the corresponding products were inferred from the difference between the mass-to-charge ratio of the observed product and that of the C6H6O, C7H8O, and C9H12O adducts. Arguments based on electronic state symmetry indicate that the carbon monoxide-olefin path involves the decomposition of vibrationally excited phenol in its ground electronic state. This state is reached by a spin-forbidden transition from the triplet reactant state. The benzaldehyde products can be explained by a mechanism analogous to one of the reaction paths of O(3P)+ethylene.
Rotational excitation of linear molecules by collisions with atoms: Comparison of classical and quantum methods
Authors: Chapman Sally , Green Sheldon
Published: February 2003

Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO–He, CS–H2, OCS–H2, HCl–He, and HCl–Ar at collision energies up to 500 cm−1. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.
An SCF-stabilization approach to excited states embedded in the continuum
Authors: Bender C. F. , McKoy Vincent , Davidson Ernest R.
Published: February 2003

By using SCF and stabilization-like procedures, we have located a (π, π*) singlet resonance-like state in ethylene at 10.21 eV. This state is embedded in the ionization continuum and carries an oscillator strength of 0.46 and is probably the analog to the spectroscopic V state in Hartree–Fock theory. Implications of these results for other systems are discussed.
Raman intensities, depolarization ratios, and polarizability derivatives: Matrix notation, relation to other molecular constants and simple isotopic rules
Authors: Müller A. , Mohan N.
Published: February 2003

New matrix relations for the Raman intensities Ii, the polarizability derivatives (∂?/∂Qi)2 as well as (∂γ/∂Qi)2, and the depolarization ratios ρi are presented. These matrix formulations have been utilized to relate the Raman data to other molecular constants such as the mean square amplitudes of vibration, Coriolis constants, etc. In order to serve as a check on quantum mechanical calculations the extremal properties of the derivatives of the polarizability components with respect to the symmetry coordinates Si are derived. The Jacobians related to the Raman intensities are presented with a view to study the sensitivity of the force constants to the Raman intensities. Based on the first order perturbation theory a new isotopic rule involving the change in each Ii, (∂?/∂Qi)2, and (∂γ/∂Qi)2 is presented to study the isotopic substitution of heavy atoms in general.
The electronic structure of the ground and excited states of Mg+2 and Mg2
Authors: Stevens W. J. , Krauss M.
Published: February 2003

The ground and excited valence states of Mg+2 and Mg2 are calculated using the multiconfiguration self-consistent-field method. The energy curves of the X 1Σ+g, 3Πg, 3Σ+u, 3Πu, 3Σ+g, 1Πg, 1Σu+, 1Πu, and 2 1Σ+gμ states of Mg2 and the 2Σ+u and 2Πμ curves of Mg+2 are obtained for internuclear separations between 4 and 15 bohr. All of the excited states except the 3Πu and 3Σ+g are found to be bound while the ground state is a weakly bound van der Waals molecule. Comparison between the calculated and experimentally known X 1Σ+g and 1Σ+u energy curves shows good agreement for the equilibrium internuclear separation but the calculated De (620 cm−1) for the best approximation of the X 1Σ+g state exceeds the experimental value by about 40%, and the calculated De for the 1Σ+u state is smaller than the experimental value of 9387 cm−1 by about 1800 cm−1. Analysis of the electronic structure of the excimers shows that the triplet states and the 1Πg state are qualitatively described by covalent molecular orbital theory but all of the other bound singlets require more complicated descriptions including covalent configurational mixing, charge transfer, and Rydberg–covalent configuration mixing. Only the 1Πu state can be unambiguously characterized as a charge transfer state. The Mg2 states were calculated to provide a model of the Group II excimer systems, which are candidates for the active media of lasers.
The temperature dependence of chemical shielding in diatomic molecules: CO, F2, ClF, HBr, and HCl
Authors: Jameson A. Keith , Schuett Karla , Jameson Cynthia J. , Cohen Sheila M. , Parker Harriet
Published: February 2003

The temperature dependence of chemical shielding in CO, F2, ClF, HBr, and HCl gas has been measured. The functions σ0(T) and σ1(T) in the virial expansion of chemical shielding σ =σ0(T)+σ1(T) ρ+⋅⋅⋅ are reported here for 13C, 19F, and 1H nuclei. The largest shifts with temperature are observed for F2.
Recombination of alkali positive and halogen negative ions
Authors: Janev R. K. , Radulović Z. M.
Published: February 2003

The process of mutual neutralization of alkali positive and halogen negative atomic ions is considered at low energies (0.01 eV–10 keV). The exchange interaction between ionic and covalent states at the diabatic curve-crossing point is calculated for each alkali–halogen pair using the Landau–Herring method. The neutralization cross sections, as well as the rate constants for these reactions, are calculated.
Statistical theory of entanglement density in linear polymers and fiber systems. III. Three-dimensional entanglement of rigid and flexible chains
Authors: Acierno Domenico , Ziabicki Andrzej
Published: February 2003

The statistical theory of entanglement is applied to a three-dimensional model with random distribution of orientations and motions. The entanglement coefficient ε has been computed numerically using a Monte Carlo procedure for two different cases (i) rigid and slippery chains, and (ii) flexible and rough chains. The results have been compared with earlier data obtained for a two-dimensional model and with available experimental estimates of the entanglement density in polymer solutions.