bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbational approach to the interaction between two nearly incommensurable polymers
Authors: Imamura Akira , Suhai Sándor , Ladik János
Published: February 2003

The perturbational method developed in the present paper is applied to a few simple model systems, to two interacting hydrogen molecules–polymers and to polymeric systems of hydrogen molecules and hydrogen fluoride molecules. The validity of our method is studied by comparing the results obtained with the perturbation method and the tight-binding SCF crystal orbital method. The obtained total electronic energies and the charge distributions are in good agreement with each other for both methods. This result leads to the conclusion that the present perturbational approach is promising for the application to the interactions between real, incommensurable polymers.
The (H2)2 potential surface and the interaction between hydrogen molecules at low temperatures
Authors: Burton Peter G. , Senff Ulrich E.
Published: February 2003

The (H2)2 rigid rotor interaction potential has been calculated for the intermolecular distance range R = 3.0–11.0 a.u. for six relative orientations to estimate both the isotropic and anisotropic components of the full intermolecular potential. A partially optimized basis set limited in size to 78 independent Gaussian functions was used throughout the energy calculations, which required only very small corrections for basis set unsaturation effects. Correlation effects were computed both at the variational (single and) double excitation PNOCI level and using the CEPA2-PNO approximation to estimate higher order excitation effects. While the latter rigid-rotor surface may overestimate the strength of the H2–H2 interaction in the vicinity of the well by a few wave numbers in the rigid-rotor PNOCI surface from the present study, which we regard as an upper bound to the true rigid-rotor surface, is also slightly deeper than almost all previous theoretical and empirical ’’fit’’ potentials in the well region. Since low energy scattering experiments lack sensitivity to the precise well depth because of the long de Broglie wavelengths appropriate to scattering at energies comparable with the well depth, the significance of the current disagreement on the well depth between the rigid-rotor level of the theory (which puts it at least 27.5 cm−1) and experiment is uncertain. However, coupled with the present uncertainty stemming from the CI formalism of ∼±3 cm−1, is a further uncertainty due to the expected increase in intramolecular zero-point energies due to hindered H2 vibration within the dimer. A zero-point increase within each H2 at Re of 0.1% would effectively raise the rigid-rotor potentials by some 4 cm−1, bringing the well depth into line with experimental estimates. In the low energy part of the repulsive wall of the potential (where the relative importance of these small effects diminishes), which has been probed rather sensitively by recent rotationally inelastic scattering experiments and which strongly influences solid H2 properties, we find that both the present PNOCI and CEPA2 surfaces lie in between the respective semiempirical fit surfaces for the H2–H2 ’’pair’’ interaction. At the zero crossing of the isotropic component of the potential, we find excellent agreement between the present theoretical estimates of R0(2.99±0.03 Å) and the most recent experimental evaluation of the parameter (2.98±0.03 Å) by Buck et al. [J. Chem. Phys. 74, 535 (1981)].
Fourier transform infrared study of the polymer-dopant interaction in polyacetylene
Authors: Rabolt J. F. , Clarke T. C. , Street G. B.
Published: February 2003

Fourier transform infrared (FTIR) measurements of AsF5 doped (CH)x with dopant concentrations which vary over three orders of magnitude have been obtained in both the mid (4000–400 cm−1) and far (400–15 cm−1) infrared. The bandwidths of the strong bands characteristic of the doped polymer have been observed to be invariant with dopant concentration. However, the frequency positions of these new IR bands are found to be a sensitive function of the dopant level. In the far infrared, no evidence of the predicted soliton pinning mode was found for any of the dopant concentrations studied. The consequence of these results, in view of current theoretical models of the polymer-dopant interaction, is discussed.
Infrared study of electron-molecular vibration interactions and phase transitions in the organic conductors (TMTTF)2X (X = BF−4, ClO−4, and PF−6) and TMTTF-bromanil
Authors: Bozio Renato , Meneghetti Moreno , Pecile Cesare
Published: February 2003

The infrared (4000–200 cm−1) absorption spectra of powder samples of (TMTTF)2X (X = BF−4, ClO−4, and PF−6) and of TMTTF-bromanil at temperatures 300 ≳T≳15 K are investigated. Polarized absorption spectra of oriented polycrystalline films of the bromanil complex in the same temperature range are also reported. Spectral features originated by the electron molecular vibration (EMV) interactions involving some totally symmetric modes of TMTTF are identified for all the compounds. For the case of the (TMTTF)2X salts, the intensity of the broad vibronic absorptions has been related to the degree of dimerization of the donor stacks. Analysis of their temperature evolution and comparison with the known behavior of the magnetic and electrical properties suggest that these systems may be viewed as strongly correlated magnetic semiconductors. The role of the commensurate anion sublattice in opening the semiconducting gap and suppressing the (4kF) Peierls instability is stressed. Possible similarities between the electronic properties of the TMTTF salts and of the bromanil complex are discussed. Analysis of vibronic absorptions of (TMTTF)2BF4 according to a ’’dimer charge oscillation model’’ yields semiquantitive estimates of the linear EMV coupling constants of TMTTF which result to be substantially similar to those previously reported for unsubstituted TTF.
Inclusion of frequency shifts with electronic excitation in the calculation of multiple-order resonance Raman profile line shapes from optical absorption data
Authors: Tonks D. L. , Page John B.
Published: February 2003

A previously discussed exact relation which allows one to compute first-order resonance Raman intensity profile line shapes directly from optical absorption data within well-defined assumptions is generalized to approximately include vibrational frequency shifts with electronic excitation and is extended to profiles of higher order. For a large class of optical absorption line shapes and sufficiently small frequency shifts and temperatures, the generalization is found to be quite simple. We demonstrate its usefulness and the importance of including frequency shifts by using optical absorption data to compute first-, second-, and third-order profile line shapes for the KClO4:MnO−4 impurity system and comparing the results with measured profiles.
A quasiclassical model for resonance widths in quantal collinear reactive scattering
Authors: Pollak Eli
Published: February 2003

A quasiclassical model with no adjustable parameters is proposed for analysis of resonance widths of collinear atom–diatom reactions. We find two important contributions to the widths. One comes from tunneling through adiabatic exit channel barriers. The other involves the stability frequency of resonant periodic orbits. This frequency, if it is imaginary, is a measure of the nonadiabatic coupling in the system. We find that the resonances of the H+HH exchange reaction are determined by this nonadiabatic coupling. The higher lying resonances of the H+MuH system are controlled by the tunneling mechanism. We find that the resonant periodic orbit of the HMuH reaction is stable over a large energy range. The implications of this stability on analysis of quantal computations are discussed in detail.
Bond shift kinetics in cyclo-octatetraene by dynamic NMR in liquid crystalline solvents
Authors: Naor R. , Luz Z.
Published: February 2003

A complete line shape analysis of the dynamic NMR spectra of cyclo-octatetraene dissolved in nematic liquid crystalline solvents is presented. The analysis involves simulation of the dynamic NMR spectra in the presence of the bond shift reaction. This is performed using a newly developed group theoretical method. Comparison of the simulated and experimental spectra yield the following kinetic parameters for the bond shift process in cyclo-octatetraene: 1/τ (0 °C) = 2.3×102 s−1; ΔE = 10.62 kcal/mol; ΔH≠ = 10.0 kcal/mol; ΔS≠ = −9.7 e.u.
The fluorescence and the absorption spectra of 1,8-diphenyl-1,3,5,7-octatetraene. The origin of the transition moments and the interpretation of anomalous intensity distribution
Authors: Ikeyama Takeshi , Azumi Tohru
Published: February 2003

The fluorescence and the fluorescence excitation spectra of 1,8-diphenyl-1,3,5,7-octatetraene in octane are observed at 77 K. Special effort has been devoted to obtain the correct absorption spectrum from the fluorescence excitation spectrum. The fluorescence and S1←S0 absorption spectra consist of progressions of totally symmetric C = C and C–C stretching vibrations built on the 0–0 band. These spectral features together with the appearance of the 0–0 band indicate that the molecule is distorted from the C2h point group. No mirror image relationship is observed between the S1←S0 absorption and the fluorescence. Especially, the vibronic band in the absorption involving one quantum of the C = C stretching vibration is observed to be anomalously intense. Such anomalous intensity distribution of the S1←S0 absorption and the lack of the mirror image relationship are analyzed in terms of a mechanism in which both the Herzberg–Teller and Born–Oppenheimer vibronic coupling terms contribute to the transition moment. For the 0–0 and all of the vibronic bands in the absorption and fluorescence, the largest contribution to the transition moment comes from the zeroth order term in the expansion in the nuclear coordinate (i.e., nonvibronic) which is ascribed to the reduction of molecular symmetry. In the fluorescence, the Herzberg–Teller vibronic coupling term contributes destructively with the zeroth order term, whereas the Born-Oppenheimer vibronic coupling term contributes only negligibly. In the absorption, on the other hand, the Born-Oppenheimer vibronic coupling term contributes constructively with the zeroth order term, whereas the Herzberg–Teller vibronic coupling term contributes only a little. The anomalous intensity distribution and the lack of mirror image relationship are mainly ascribed to the difference in sign in the two major terms in the transition moment.
The calculation of vibrational frequencies of atoms adsorbed on metal surfaces: H, O, and S on Ni(100) and Ni(111) as examples
Authors: Black J. E. , Bopp P. , Lützenkirchen K. , Wolfsberg M.
Published: February 2003

We present vibrational frequencies calculated for H, O, and S atoms adsorbed on Ni(111) and Ni(100) surfaces. The calculations are done for a single atom adsorbed on nickel clusters, and also for a single atom and periodic arrangements of atoms on an infinite nickel substrate. We are able to indicate under what conditions a simple cluster calculation will yield vibrational frequencies suitable for comparison with experiment.
Spin–orbit coupling and spin splitting in HeNe+
Authors: Cooper David L.
Published: February 2003

Spin−orbit coupling and spin splitting in HeNe+ are considered. (AIP)
Many-electron approximations to the first-order wave function of a system of paired electrons in a magnetic field
Authors: Walnut Thomas H.
Published: February 2003

Many−electron approximation is applied to the first−order wave function of a system of paired electrons is a magnetic field. (AIP)
Calculation of Raman intensities for the torsional vibrations of methylcyclopropane and propylene oxide
Authors: Bocian David F. , Schick G. Alan , Hurd J. Kathleen , Birge Robert R.
Published: February 2003

Raman intensities are calculated for the torsional vibrations of methylcyclopropane and propylene oxide using an anisotropic atom-point dipole interaction model to calculate the elements of the molecular polarizability tensor. It is predicted that electrical anharmonicity contributes substantially to the intensities of both the Δv = 1 and δv = 2 transitions. The calculated intensity distribution of the Δv = 1 transitions of propylene oxide is in excellent agreement with experiment. The calculated intensities of the fundamental transitions of methylcyclopropane are not in as good agreement with experiment as those of propylene oxide; however, the observed transitions of the former molecule are extremely broad and difficult to measure accurately. The calculations predict that the Δv = 1 transitions of both molecules are substantially more intense than the Δv = 2 overtones. The intensity of the overtones cannot be measured accurately because they are partially obscured by intense in-dand out-of-plane bending vibrations of the methyl groups. (AIP)
Total energy of atomic ions related to low-order ionization potentials and to diamagnetic susceptibility
Authors: Pucci R. , March N. H.
Published: February 2003

We explore the representation of the energy E(Z,N) in terms of the ionization potential I and the diamagnetic susceptibility, proportional to 〈r2〉. For one-electron ions, one finds immediately ln I = −ln 〈r2〉+ln(3/2) and for N electron we demonstrate that ln I = α(N) ln 〈r2〉+β(N) is an excellent approximation. The energy is finally represented in terms of the first three ionization potentials and 〈r2〉 as E(Z,N) = IN+IN-1+IN-2 +ΣN-3n = 1〈r2〉α(n)Z,n exp[β(n)].
A quantum stochastic theory for nonadiabatic processes in condensed phases and on surfaces
Authors: Wassam William A. , Freed Jack H.
Published: February 2003

A quantum stochastic theory for nonadiabatic processes in condensed phases and on surfaces is given. The present theory is primarily concerned with processes involving the fragmentation of two-particle systems and/or the ’’collision’’ of two particles with internal structure, i.e., electronic, spin, and nuclear degrees of freedom. A fully quantum mechanical joint energy space–quantum phase space master equation for two-particle systems is presented. A simpler form of this equation emerges in the limit where the length of thermal spatial fluctuations along the relative coordinates is much greater than half the wavelength for thermal momentum fluctuations associated with relative motion. (This limit is equivalent to fulfilling a ’’thermal Heisenberg uncertainty’’ relation: Δp̄Δq̄>>ℏ/2.) The simpler master equation is converted into a nonlinear quantum Fokker–Planck equation for nonadiabatic systems. Introducing simplifying assumptions, we obtain a quantun analog of the stochastic Liouville equation augmented with irreversible kinetic terms, which describes nonadiabatic transitions and the interruption of coherence in energy space at different points in quantum phase space. The final results are appropriately modified to incorporate the influence of applied time-dependent electric and magnetic fields. The present work provides a formal theoretical framework for examining the role of quantum effects that are neglected in ’’semiclassical’’ kinetic and ’’diffusional’’ theories for nonadiabatic processes.
Cluster theory for concentration dependence of shear viscosity for suspensions of interacting spheres. I
Authors: Freed Karl F. , Muthukumar M.
Published: February 2003

We present a general divergent-integral-free cluster theory for the concentration dependence, to any order in concentration, of the shear viscosity of a suspension of interacting colloidal spheres. Treating the solvent as a viscous hydrodynamic continuum, the steady state limit of vanishing shear rate is considered, so the sphere–sphere interactions enter through the Mayer cluster expansion of the various sphere equilibrium distribution functions. The spheres are taken to move such that the net hydrodynamic forces and torques acting on them identically vanish. We present general formulas for all coefficients in a virial type expansion of the suspension shear viscosity in powers of the concentration for any prescribed spherically symmetric direct interaction potential between the spheres. Previous work has been limited to the two-sphere contributions only. Every virial coefficient in our complete expansion involves integrals of the sphere equilibrium distribution functions multiplied by functions of hydrodynamic origin. These virial coefficients are perfectly convergent because of the nature of the various equilibrium distribution functions present in the integrals for the virial coefficients.
Molecular interaction of dense water at high temperature
Authors: Ree Francis H.
Published: February 2003

Experimental shockwave data of water were analyzed using intermolecular potentials based on quantum mechical ab initio calculations. A central pair potential obtained by averaging the ab initio result according to the Perram and White method was found to predict accurately the shockwave thermodynamic properties at temperatures between approximately 1300 and 3000 K. At higher temperatures, the theory overestimates the single-shock pressures and temperature by 10% to 20%. This deviation is attributed to the shock-induced ionization of water molecules. As a result of our calculations, we suggest the molecular interaction of water becomes simpler under a high-temperature (≳1300 K) and high-pressure (≳8 GPa = 80 kbar) environment, except for a small ionization correction, and it is possible to carry out reasonably reliable statistical mechanical calculations under such conditions. Similar comparisons are also presented using potentials developed by Stillinger and Rahman and by Watts.
The analysis of the surface reactions of ammonia synthesis on iron using infrared spectroscopy. II
Authors: Nakata Toshiko , Matsushita Sanjuro
Published: February 2003

In their preceding papers, the present authors ascertained, by an IR technique, reaction intermediates NH2 and NH of ammonia synthesis, and determined the heat of reaction of NH2⇄NH+H on the surface. Based on these results, the analysis expressed in the title is carried out in this paper. The widely accepted reaction mechanism of stepwise hydrogenation starting from adsorbed nitrogen atoms and the theorem of the forward and reverse reactions following the same route were presupposed. By combining the above heat with already known heats of adsorption of the participating gases, the rises and falls of energy in all the intermediate states along the reaction route were made clear. This made it possible to compare the reaction rates of all steps on the route (by expressing them with simple absolute rate equations) and to determine the slowest step under given conditions. The main features of all the facts observed in this series of works could be explained in order of magnitude in a unified way using a single simple model. The energy curve determined was found to be basin-shaped, the activation energy being largest at both end-steps; however, when the activation entropy is taken into account, the slowest step is always the adsorption or desorption of nitrogen. Such conclusions are generalized. Additional experimental facts, an interpretation of so-called compensation effects and others are presented in relation to the main reasoning.
Correlation functions of the Π–Σ and the Σ–Σ vibration–rotation bands of a triatomic linear molecule mixed with dense rare gases
Authors: Dreyfus C. , Berreby L. , Nguyen-Tan T.
Published: February 2003

The degenerate and nondegenerate rotation–vibration bands of OCS mixed with rare gases have been scanned in the density range 15–300 amagat. The related correlation functions have been corrected for the hot band perturbations and compared with usual rotational models. None of the models fit both correlation functions. A comparison with experimental correlation functions arising from other linear molecules mixed with rare gases shows that a convenient rotational relaxation model would have to consider explicitly the mean square torque at least in the range of density explored. Such a requirement is fulfilled by using a memory function proposed by Steele. The correlation functions are then very well predicted for both vibrations using the same values of the parameters. The adjusted mean square torque values are then successfully compared with those determined directly by a spectral moment measurement method.
Theoretical calculation of the intensity modification of HCl rotation–vibration lines by rare gases at moderate pressures
Authors: Piollet-Mariel Edmée , Boulet Christian , Levy Armand
Published: February 2003

The formal expression giving the integrated intensity of a rotation–vibration line observed in the spectrum of a linear active molecule immersed in a bath of foreign gas perturbers has been determined in the case of moderate densities. From this expression, the rate of intensity variation due to the perturbation can be calculated, provided the intermolecular potential is known. Two possible effects of the interaction are pointed out: induction contributions and, most important, deformation of the wave functions during the collisions. Application of this formalism is performed for the system HCl–Ar, by using several of the presently available potential surfaces and comparison is made with our previous experimental results. Correct agreement is observed in most cases and the specific role of the P1 (cos ϑ) part of the anisotropic potential is stressed. In the case of HCl–Xe, no accurate potential is available. As a first attempt, some approximate forms have been generated from the HCl–Ar models. The general m dependence of the intensity variation is correctly reproduced, but the quantitative agreement is not yet quite satisfactory.
Raman and infrared reflectivity spectra of 6LiNaSO4 and 7LiNaSO4
Authors: Teeters Dale , Frech Roger
Published: February 2003

The polarized Raman and infrared reflectivity spectra of single crystal LiNaSO4 are reported. Symmetry-based vibrational band assignments are given for both the internal and external optic models. The spectra of 6LiNaSO4 clearly identify higher frequency external modes as originating in lithium ion motion essentially decoupled from the lower frequency external optic modes.
Infrared intensities of methyl fluoride: Determination of the signs of the dipole moment derivatives
Authors: Kondo Shigeo , Saëki Shinnosuke
Published: February 2003

Since the problem was presented by Abbate and Gussoni, resolution of the confusion on the infrared intensities of methyl fluoride has been hoped for. In the present study, the experimental data reported so far on this subject have been reinvestigated to find finally that the intensity value reported on the ν6 band of the CD3F molecule was very much in error and therefore caused essentially the present problem. By using the new data, an intensity analysis has been carried out again, and a unique set of solutions for the infrared intensity parameters has been determined for this molecule. The resulting parameter values are found to be in perfect accord with what chemical intuition predicts them to be.
On the origin of some anomalous luminescence behavior of isomeric cyanopyridines
Authors: Sarkar S. K. , Ghoshal S. K. , Kastha G. S.
Published: February 2003

The results of a comparative study of the absorption and emission properties and the triplet yields of the isomeric cyanopyridines (CNP) and benzonitrile under various environmental conditions are reported. All the three CNP isomers are found to be nonfluorescent but exhibit a fairly strong phosphorescence emission of π, π character in rigid polar media at 77 K. ΦP values of the CNP are found to be sensitively dependent on the rigidity (or viscosity) and crystalline structure of the solvent medium. Whereas the 3-CNP molecule phosphoresces with moderately strong intensity in hydrocarbon solvents, the 2 and 4 isomers are completely nonluminescent in glassy or highly disordered crystalline phases of hydrocarbons, e.g., n-alkanes, cyclohexane (CH), and methylcyclohexane (MCH). A dramatic reappearance of the phosphorescence emission from these two isomers (with anomalously large red-shifted spectral positions) has, however, been observed in crystalline CH and MCH matrices. Unlike the 3 isomer, both the 2- and 4-CNP have been found to exhibit phosphorescence in pure crystalline form at 77 K. The n, π electronic states associated with the pyridinic nitrogen atom have been identified in the absorption (or excitation) spectra and shown to be responsible for the nonfluorescent nature and the high triplet yields (∼0.8) of CNP molecules. A tentative interpretation of all the diverse emissive characteristics of cyanopyridines has been offered in terms of the predominance of the effect of pseudo-Jahn–Teller distortion and the matrix potential in governing the radiative and radiationless deactivation of the lowest triplet state of these molecules. The distinctive differences observed in the emissive characteristics of the 3-CNP and other two isomers have been attributed to the differences in the distortability of the conformational mobility of the CNP isomers arising primarily as a result of inductive effects of the pyridinic nitrogen atom and the nitrile group.
Energy transfer in CaWO4: Sm3+
Authors: Smith W. Bryan , Powell Richard C.
Published: February 2003

The interaction between Sm3+ ions in CaWO4 crystals has been investigated using high resolution dye laser spectroscopy techniques. The results are found to be different from those obtained using either broadband or intermediate resolution laser excitation sources. At low temperatures, both concentration quenching through cross-relaxation and resonant energy transfer processes are found to take place by exchange interaction. At high temperatures, energy transfer between Sm3+ ions in nonequivalent crystal field sites takes place by a two-phonon diffusion process with a diffusion coefficient of the order of 4.9×10−10 cm2 s−1.
Calculations of ESR line shapes of triplet molecular aggregates in solid matrices. II. Dimers and trimers in disordered matrices
Authors: Lemaistre Jean-Pierre , Kottis Philémon
Published: February 2003

It is shown that the unusual temperature dependence of ESR spectra of excited triplet aggregates, embedded in a glassy matrix, definitely cannot be interpreted by a single, temperature dependent, transfer rate k(T), which induces the usual pattern of ’’exchange broadening,’’ shifting, coalescence, and narrowing of the ESR lines. The experimental data strongly suggest rather the existence of many hopping excitons in competition which exchange intensity among them as the temperature varies. In this work, we analyze models for two limit situations where such experimental data may be encountered: (i) undistorted aggregates included in an ordered matrix, where the excitons are associated with Boltzmann populated triplet libronic site excitations, and they diffuse ’’adiabatically’’ with rates depending on the libration quantum number and (ii) more or less distorted aggregates, embedded in a disordered matrix, where slow and rapid excitons are not in Boltzmann equilibrium as they correspond to cage effects. In the latter case, the temperature, by diminishing the cage constraint, appears as ’’transferring population’’ from slow excitons (associated with deeply distorted aggregates) to rapid excitons (associated with less distorted or resonant aggregates) and show the way the cage effect diminishes as temperature increases. This effect is proper to weakly coupled aggregates, and it is shown that, because of the high spin–spin anisotropy, an ESR experiment may select different magnetic species (monomers, dimers, and trimers) where optical excitation cannot be selective because of the small exchange electronic interaction hidden under the inhomogeneous broadening. Simulations of the experimental data show that our interpretation of the temperature effect on the competing and population exchanging excitons is very satisfactory.
Energy transfer as a function of collision energy. IV. State-to-state cross sections for rotational-to-translational energy transfer in HF+Ne, Ar, and Kr
Authors: Barnes J. A. , Keil M. , Kutina R. E. , Polanyi J. C.
Published: February 2003

State-to-state cross sections for rotationally inelastic collisions of HF (v,J) with Ne, Ar, and Kr have been measured. Laser pumping of the molecular beam to the initial states v = 1, J = 1–6, and v = 2, J = 2, followed by infrared fluorescence, permitted measurements of relative cross sections with ‖ ΔJ ‖⩽8. The collision energy was varied between 4 and 16 kcal/mol. These cross sections could be fitted well using an inverse-power dependence on the rotational energy gap [due to Pritchard and co-workers; J. Chem. Phys. 70, 4155 (1979)] for rotational energy transfers of up to 55% of the initial translational energy. The energy-corrected sudden approximation was used to determine an ’’effective’’ collision length for rotationally inelastic scattering. The scattering is thought to occur predominantly on the repulsive wall of the intermolecular potential, except for the J = 1→J′ = 0 transition, which is shown to be sensitive to the depth of the van der Waals attractive well.
A microscopic theory of compressive wave-induced reactions in solid explosives
Authors: Coffey C. S. , Toton E. T.
Published: February 2003

A simple physical model is developed for the interaction of molecules with compressive waves in condensed explosives. Multiphonon excitation of the internal vibrational levels of the molecules is shown to be possible and to exceed thermal dissociation rates. A calculation of the threshold condition for which a self-sustaining reaction can occur is given.
Theoretical studies of photoexcitation and ionization in H2O
Authors: Diercksen G. H. F. , Kraemer W. P. , Rescigno T. N. , Bender C. F. , McKoy B. V. , Langhoff S. R. , Langhoff P. W.
Published: February 2003

Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H2O employing Franck–Condon and static-exchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground-state equilibrium geometry, and previously devised moment-theory techniques are employed in constructing the continuum oscillator-strength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact–excitation cross sections, and dipole (e, 2e)/(e, e+ion) and synchrotron-radiation studies of partial-channel photoionization cross sections. The various calculated excitation series in the outer-valence (1b−11, 3a−11, 1b−12) region are found to include contributions from valence-like 2b2 (σ*) and 4a1 (γ*) virtual orbitals, as well as appropriate nsa1, npa1, nda1, npb1, npb2, ndb1, ndb2, and nda2 Rydberg states. Transition energies and intensities in the ∼7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b−11) X 2B1, (3a1−1)2A1, and (1b2−1)2B2 partial-channel cross sections are found to be largely atomic-like and dominated by 2p→kd components, although the 2b2(σ*) orbital gives rise to resonance-like contributions just above threshold in the 3a1→kb2 and 1b2 →kb2 channels. It is suggested that the latter transition couples with the underlying 1b1→kb1 channel, accounting for a prominent feature in the recent high-resolution synchrotron-radiation measurements. When this feature is taken into account, the calculations of the three outer-valence channels are in excellent accord with recent synchrotron-radiation and dipole (e, 2e) photoionization cross-sectional measurements. The calculated inner-valence (2a1−1) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final-state configuration-mixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a1 oxygen K-edge electron energy-loss profile in H2O are seen to be clarified fully by the present calculations.
Solubilities of inert gases and methane in H2O and in D2O in the temperature range of 300 to 600 K
Authors: Crovetto Rosa , Fernández-Prini R. , Japas María Laura
Published: February 2003

The solubility of inert gases and methane in H2O and D2O has been measured between room temperature and 600 K. The calculation of Henry’s constants kH, from the solubility data is analyzed in detail; if due account is taken of the nonideality in the gas phase, they can be determined unambiguously up to 520 K. Above this temperature, the ambiguity in kH increases sharply as contributions of third and higher order virial coefficients to the equation of state of the gaseous mixture become more important. The differences of gas solubilities in light and heavy water essentially disappear above the temperature of minimum solubility of the gases. The characteristic thermodynamic features of the aqueous solutions of gases (i.e., large values of −ΔS02 and of ΔC0p2) are still present at 520 K. It is shown that mean-field theories can account for the
Theory of the dielectric constant of solutions
Authors: Friedman H. L.
Published: February 2003

The dielectric constant ερ that appears in the effective force between two ions in solution is expressed in terms of molecular direct correlation functions for quite general mixtures of nonpolarizable molecules, some of which may bear electric charges. For general mixtures there is flexibility in the definition of ερ, the most apt definition depending on the intended application. For nonconducting mixtures one particular definition of ερ corresponds to the dielectric constant seen by an externally applied electric field. In this case the present calculation of ερ, which begins with Adelman’s formulation in term of an effective direct correlation function, agrees with the most general results for the dielectric constant of mixtures which have been reported on the basis of quite different theories. Particular attention is given to the linear coefficient in the equation ερ = ε0+ε(1)ρ+⋅⋅⋅, where ε0 is the solvent dielectric constant and ρ is the total concentration of solutes. The theory of ε(1) is given in terms of molecular direct correlation functions and in terms of second moments of site–site correlation functions hαβ(r), all for quite general solutes. Finally, a cluster expansion for the contribution of nonpairwise components in the McMillan–Mayer (i.e., solvent averaged) Hamiltonian for an ionic solution is shown to lead to the same expression for ε(1) in terms of direct correlation functions, as was obtained from Adelman’s prescription, and quite directly to a cluster expansion for ε(1) in terms of short range contributions to the solvent–solvent and ion–solvent molecular pair correlation functions hab(1 2).
Solute structuring in iron(III) chloride solutions. II. Evidence of polynuclear complex formation in the FeCl3⋅6H2O melt by the ‘‘isoelectronic solutions’’ method
Authors: Magini Mauro
Published: February 2003

Two iron(III) chloride solutions, prepared in suitable chemical conditions, have been examined by the x-ray diffraction technique. The two solutions are isoelectronic (i.e., they have the same average scattering power per chlorine atom) with the hydrated FeCl3⋅6H2O melt previously examined [J. Chem. Phys. 71, 4255 (1979)]. A comparison of the radial distribution function of the melt with those of the two solutions (reference solutions) leads to the conclusion that polynuclear complex formation occurs in the melt and that a bitetrahedral Fe2Cl6 molecule is the most probable polynuclear species formed in this system. An analysis of the Fe–Cl association in the reference solutions has been also performed and the results substantially agree with those obtained previously.
On the one-dimensional steady-state Ising problem
Authors: Hill Terrell L.
Published: February 2003

The exact linear flux–force relation, near equilibrium, is found for a one-dimensional, steady-state, two-state Ising system with nearest-neighbor interactions that affect the rate constants of the two-state cycle. The physical interpretation of the linear flux–force coefficient is shown to be a one-way cycle flux at equilibrium. A new approximation is introduced for the same system at an arbitrary steady state.
Time resolved energy transfer from electronically excited 3B3u pyrazine molecules to planar Ag and Au surfaces
Authors: Rossetti R. , Brus L. E.
Published: February 2003

The lifetime of 3B3u pyrazine near Ag surfaces has been measured down to a separation of ∼35 Å, where it has decreased by a factor of ∼600 from the isolated molecule value. There is no evidence for saturation of the energy transfer rate. The technique and apparatus are described. Numerical solutions to the classical double interface theory of energy transfer show energy transfer rates should be sensitive to adsorbed gas on top of the excited molecule.
CO on Ru(001): Island size and disordering
Authors: Williams Ellen D. , Weinberg W. H. , Sobrero A. C.
Published: February 2003

Low-energy electron diffraction (LEED) has been used to study the influence of steps and of temperature variation on the formation of ordered islands of CO on the (001) surface of ruthenium. The clean surface was found to have steps two atoms (one hcp unit cell) high separated by terraces with an average width between 135 and 270 Å. Widths of the beam profiles for the CO overlayer were measured as a function of coverage at 100 and 310 K. The coverage dependence requires that there be increasing numbers of islands per terrace at decreasing coverages. The temperature dependence of the overlayer was measured also. The frequency of the frustrated translational motion of the CO admolecules parallel to the surface is estimated to be 45 cm−1. At fractional (ϑ) coverages up to 1/6, the ordered islands of CO disorder substantially below the desorption temperature. At ϑ = 1/3, disordering is much less marked. The disordering behavior depends strongly on the distribution of sizes of islands in the overlayer. The correct distribution was determined and used to calculate the island sizes as a function of coverage. The mean number of CO molecules per island is 1000 at ϑ = 1/6, 500 at ϑ = 0.14, and 300 at ϑ = 0.12.
Thermodynamics of sulfur chemisorption on metals. III. Iron and cobalt
Authors: McCarty Jon G. , Wise Henry
Published: February 2003

Isosteric heats of chemisorption of sulfur on iron and cobalt have been evaluated from measurements of the equilibrium ratio of H2S/H2 gas mixtures in contact with the powdered metals over a temperature range from 677 to 855 K. At low surface coverage of sulfur on cobalt, the heat of chemisorption with respect to 1/2 S2(g) is −236±15 kJ mol−1. With increasing coverage it decreases to −200±10 kJ mol−1. The heat of sulfur chemisorption on iron is −197±15 kJ mol−1. For equivalent coverage, the thermodynamic activity of sulfur chemisorbed on cobalt exceeds that of iron by approximately one order of magnitude. At low surface coverage (<0.5 saturation coverage), the bond strength of sulfur on the various metals is found to vary in the order: Ni≳Co≳Ru≳Fe.
Quasitrapping and rainbow mechanisms in model rigid-rotor–rigid- surface scattering
Authors: Bowman Joel M. , Park Seung
Published: February 2003

The quasiclassical trajectory method with a Lennard-Jones–Devonshire potential is used to investigate N2 collisions with LiF surfaces. (AIP)
Alternative derivation of the free energy in the mean spherical approximation
Authors: Rosenfeld Yaakov
Published: February 2003

The closed form expressions for the free energy and pressure, of Ho/ye and Stell for the MSA, and of Rosenfeld and Achcroft for the ’’soft’’ MSA are derived by a new and simple approach.(AIP)
19F NMR spectrum of solid xenon hexafluoride
Authors: Leffler Amos J.
Published: February 2003

The NMR spectra of high purity xenon hexafluoride in the solid phase are reported.(AIP)
Semiempirical calculation of diamagnetic susceptibilities of organotin compounds
Authors: Gupta R. R. , Kumar M. , Kishan
Published: February 2003

A semiemperical model is presented to calculate diamagnetic susceptibilities of organotin compounds. Diamagnetic susceptibilities are considered to be contributed by the atoms, bonds, and bond–bond interactions. An excellent agreement between semiemperically calculated and experimental values of diamagnetic susceptibilities is reported. (AIP)
Unimolecular reactions with intense radiation
Authors: Pritchard H. O. , Vatsya S. R.
Published: February 2003

A model is described for a unimolecular reaction taking place in the presence of imposed infrared radiation; the model includes allowance for collisional relaxation and for variation of absorption coefficient or of randomization rate with the degree of excitation. An efficient algorithm is derived for the calculation of the steady-state rate constant for reaction under conditions where the collisional relaxation rate is small.
Generalized symmetrization of the Feshbach–Rubinow single variable variational method
Authors: Rosenthal C. M. , Haracz R. D.
Published: February 2003

A method is presented for incorporating full symmetrization of the trial wave function in the Feshbach–Rubinow variational approach. This new technique avoids the severe restrictions on the variational function that had previously been imposed by symmetrization. In an application to the H− ion, considerable improvement over previous results is achieved.
Monte Carlo variational study of Be: A survey of correlated wave functions
Authors: Moskowitz Jules W. , Schmidt K. E. , Lee M. A. , Kalos M. H.
Published: February 2003

Using the Metropolis Monte Carlo integration technique, we calculate upper bounds to the correlation energy of a Be atom for a variety of wave functions. With this method, it is simple to treat unconventional wave functions, including those which depend on the interelectronic distance rij. We obtain about 40% of the correlation energy by using only a simple two-parameter Jastrow function of rij with a single Slater determinant of Hartree–Fock orbitals. A four configuration wave function with this Jastrow function yields 87% of the correlation energy. Several wave functions derived from nonvariational methods are shown to give no correlation energy when used in a strictly variational computation.
Saddle point region of the a 4A″ first excited state of N2O+
Authors: Hopper Darrel G.
Published: February 2003

The potential energy surface for the a 4A″ state of N2O+ has been scanned at 120° over the space 1.8⩽RNN⩽3.2 a.u. and 1.8⩽RNO⩽3.0 a.u. at the ab initio SCF and CI levels with double-zeta plus polarization one-electron basis. This surface exhibits a saddle topology cresting at RNNsp=2.29 a.u. and RNNsp=2.51 a.u. in the CI calculations. Within the high interaction region, the minimum energy pathway proceeds smoothly from RNN≃Re(N2), at RNO=3.0 a.u., through a remarkably elongated saddle-point region to RNO≃Re(NO+), at RNN=3.2 a.u. A bending curve for RNN=RNO=2.2 a.u. starting from ϑNNO=120° and encompassing angles up to 180° and down to 90° has shown that (a) the Cs(C∞v) state correspondence is a A″(1 4Π) and (b) the purely angular barrier to the collinear configuration is 2.6 eV, relative to a most stable saddle-point angle of ϑNNsp= 118°.These results are discussed in relation to the electronic structure of N2O+, especially the X 2Π and A 2Σ+ states; and the O+(N2, N) NO+ reaction. Two principle conclusions are established on the strength of the SCF/CI results: (a) the a 4A″ state is the first adiabatically excited state of the N2O+ molecular ion and (b) the potential energy hypersurface for this state provides a smooth, adiabatic, low-barrier Cs pathway for the ion-molecule reaction. Incorporation of the MCSCF/CI results described below leads to the establishment of a lower limit to the a 4A″ potential barrier to reaction: 3.9 kcal/mol. It is apparently the availability of this novel pathway which causes the O+(4S)+N2→NO++N reaction to play such a critical role in ionospheric chemistry. The excitation energies 1 4Π←X 2Π and A 2Σ+←X 2Π are obtained by optimizing MCSCF and MCSCF/CI wave functions in a double-zeta plus polarization one-electron basis for each of these three states at the N2O(X 1Σ+) geometry. The resulting Te(A←X) transition energy is calculated to be 4.17 eV, compared to the zero-point corrected spectroscopic value of 3.42 eV. The Te(1 4Π←X 2Π) spacing is predicted to be 6.56 eV with an estimated uncertainty of 0.75 eV. No Te(1 4Π←X win 02Π) experimental value is available for comparison.
Wigner phase space method: Exact solution of a harmonic oscillator with time dependent force constant
Authors: Sebastian K. L.
Published: February 2003

The Wigner phase space method, suggested by Heller, is applied to the problem of a harmonic oscillator with a time dependent force constant, which may be thought of as a model for collisions involving harmonic oscillators. The method leads to an exact solution of the problem.
Relationship between the area of L2,3 x-ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium
Authors: Horsley J. A.
Published: February 2003

The relationship between the intensity of the L2 and L3 x-ray absorption threshold resonances and d orbital occupancy has been investigated for a series of compounds of platinum and iridium and the pure metals, using Xα-SW molecular orbital calculations to obtain the unoccupied d orbital states. The change in d orbital occupancy caused by formation of the core hole was found to be fairly constant in different compounds and the pure metal. The difference between the d orbital occupancy in platinum metal and the d orbital occupancy in a given platinum compound agrees well with the calculated effective charge on the platinum atom in the compound. The areas of the threshold resonance lines were obtained by a deconvolution of the absorption edge into a Lorentzian component and an underlying ’’step’’ representing the onset of absorption to continuum states. For a series of platinum compounds, a linear relationship was obtained between the unoccupied d orbital states calculated in the core hole potential, and the sum of the areas of the L2 and L3 threshold resonance lines (corrected for the different degeneracies of the initial state). However, this relationship could not be applied to IrO2 because of the absence of transitions to empty d states at the top of the valence band.
On the atomic kinetic energy functionals with full Weizsacker correction
Authors: Gázquez J. L. , Robles J.
Published: February 2003

The functional proposed by Acharya, Bartolotti, Sears, and Parr for representing the kinetic energy of the ground state of an N electron atom or ion in terms of its electron density, namely T[ρ] = Tw[ρ] +γ(N, Z) TTF[ρ] [TW is the Weizsacker term, TTF is the Thomas–Fermi term and γ(N, Z) = 1−C0/N1/3] is derived by: (1) considering explicitly the variation of the charge density in an atom, and (2) approximating the pair correlation function for parallel spin electrons by that of a uniform free-electron gas, but including the corrections to the momentum at the Fermi level and the appropriate boundary conditions that result from taking into account that the number of electrons in an atom is finite. The first consideration leads in a natural way to the full Weizsacker correction, while the second consideration yields γ(N, Z) = (1−2/N)(1−C0/N1/3−C1/N1/3) times the Thomas–Fermi term. Thus, the functional obtained is exact for one-electron atoms and two-electron Hartree–Fock atoms, yields an adequate functional derivative, is in agreement with the leading correction to Thomas–Fermi, and provides an excellent representation of the kinetic energy of atoms 2⩽Z⩽54 when Hartree–Fock densities are used.