bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibronic coupling effects in magnetic vibrational circular dichroism. A model formalism for doubly degenerate states
Authors: Pawlikowski M. , Keiderling T. A.
Published: February 2003

A formalism for magnetic vibrational circular dichroism (MVCD) of a model system is developed by considering vibronic coupling effects between a nondegenerate ground state and a degenerate excited state. Only one degenerate vibrational coordinate and one degenerate electronic excited state are considered in the model. A diabatic basis set representation was used for the vibronic functions paralleling previous Jahn–Teller effect formalisms. The parametrized model which results is shown to give MVCD A and B terms of a magnitude comparable to those previously observed for substituted benzenes. These model studies are suggestive that vibronic coupling effects are important for complete interpretation of MVCD spectra.
NMR cross polarization in solids using multipulse irradiation
Authors: Quiroga L. , Virlet J.
Published: February 2003

A double resonance NMR scheme (MPSLCP) is presented in which a WHH-4 type multipulse irradiation of the abundant spins I is applied during cross polarization. When using the HW-8 sequence, the locking Hamiltonian for the I spins is provided by the average pulse length error Hamiltonian. Double resonance is obtained with a weak pulse irradiation of the rare spin S, whose amplitude matches (Hartmann–Hahn condition) the average pulse error field. Experimental results on 13C–1H in powdered adamantane are presented. Owing to the high symmetry of both the crystal and the molecular reorientations, the 13C multipulse cross-polarization rise is described by considering one carbon coupled to N equivalent protons and applying the calculations of L. Muller and R. R. Ernst [Mol. Phys. 38, 963 (1979)] for an SIN system with, here N=196 (or better N=235). Such a cross-polarization scheme (MPSLCP) is shown to be applicable to rigid organic solids. For S spins with very low gyromagnetic ratio and/or far from I spins, MPSLCP (i) is selective; (ii) increases the rate of polarization (it is suggested that this might help for quantitative analysis purposes); (iii) provides a very important rf power saving (104 here for adamantane) at the S spin frequency.
The mean bond displacements in O=CF2 and their effect on 19F nuclear magnetic shielding
Authors: Jameson Cynthia J. , Osten H.-Jörg
Published: February 2003

The mass and temperature dependence of the mean bond displacements in carbonyl fluoride have been determined using a modified Urey–Bradley field, by solving the set of coupled equations relating mean bond displacements 〈ΔRCF〉 and 〈ΔrCO〉 to mean square-vibrational amplitudes 〈(ΔRCF)2〉,〈ΔrΔR〉, etc. To these vibrational terms the rotational (centrifugal distortion) contributions are added. The results are applied to the interpretation of the temperature dependence of the 19F nuclear shielding in O=CF2 at the zero-pressure limit. By fitting the observed [σ0(T)−σ0(300)] for T=250–350 K an empirical value of (∂σ/∂ΔrCF)e=−1146 ppm Å−1 is obtained. This derivative is used to calculate the NMR isotope shifts from the mass dependent 〈ΔRCF〉. The calculated value, [σ0(16O12CF2) −σ0(16O13CF2)] =−0.096 ppm, is in good agreement with our measured value of −0.117 ppm. From the fitting to the temperature dependence of σ0 we also get σe−σ0 (300 K)=7 ppm. This and the absolute shielding σ0 (300 K)=221.6 ppm yields an estimate for 19F shielding at the equilibrium configuration, σe≂229 ppm. The errors which are associated with these values are discussed.
Electrical double layers. V. Asymmetric ion–wall interactions
Authors: Valleau J. P. , Torrie G. M.
Published: February 2003

Monte Carlo computations are used to study a primitive-model electrolyte with unequal distances of closest approach of its ions to a charged surface. This asymmetry, like that which must occur in real electrolytes, induces an asymmetric charge-potential relationship, and a concentration-dependent potential of zero charge, due entirely to the diffuse layer. The results provide a test of the Gouy–Chapman-level theory [J. Chem. Phys. 76, 4623 (1982)] for this model, and also display how polarization of a metallic surface will substantially enhance the effects.
On the electrostatic interaction in macroionic solutions
Authors: Sogami Ikuo , Ise Norio
Published: February 2003

A general theory on the electrostatic interaction in macroionic solution is developed. It is assumed that the motion of macroions is adiabatically cut off from that of simple ions and the distribution of simple ions is determined by the Boltzmann distribution. A generalized Poisson–Boltzmann equation is solved with the linearization approximation. The total electrostatic energy of the solution is obtained, from which the Helmholtz free energy of interaction is calculated. The Gibbs free energy is derived by the use of its additivity relation with respect to the number of the ionic species. The two free energies are demonstrated not to be equal to each other for highly charged macroions, in contrast with widely accepted view. It is shown that the electrostatic potential for a pair of macroions predicts strong attraction between the macroions, as was the case with the Levine–Dube treatment, and the Helmholtz pair potential results in purely repulsive interaction, in accordance with the DLVO theory. However, the Gibbs pair potential leads us to repulsion at small interparticle separations and attraction at large distances, creating a ‘‘secondary’’ minimum (with a potential valley deeper than the thermal energy for spherical colloidal particles). Without taking refuge in a van der Waals attraction the theory evidently substantiates the experimental fact recently reported, namely the existence of Coulombic intermacroion attraction through the intermediary of counterions. Thus, the ordering of macroions (synthetic and biological) and charged latex particles in solutions can be accounted for in terms of the present theory: By taking the lattice sum of the pair potential, it is shown that the bcc structure for polymer latex particles is favored in the small κa region (κ−1: the Debye radius, a: the radius of particle), whereas close-packed structures dominate in the intermediate and large κa regions. In the whole regions, the simple cubic structure is disfavored. The interparticle distance (the position of the secondary minimum) is shown to become smaller with increasing κa, while the potential valley takes the deepest value at κa≊1. The interparticle distance is found to decrease with latex concentration and temperature. These theoretical outcomes are compared with experimental data currently available.
Dynamics of polymer chains confined into tubes: Scaling theory and Monte Carlo simulations
Authors: Kremer K. , Binder K.
Published: February 2003

The crossover from chain dynamics described by the Rouse model to reptation behavior is studied for chains confined into straight tubes. Scaling considerations are presented for mean-square displacements parallel and perpendicular to the tube, for the static structure factor, and for dynamic coherent and incoherent structure factors, taking excluded-volume interactions into account. Monte Carlo calculations of long chains (from N=100 up to N=800 links) on the tetrahedral lattice in tubes with diameters in the range from dT=16 to dT=32 lattice spacings are presented and analyzed in terms of the scaling picture. While the scaling predictions are nicely confirmed, the simulations also reveal additional interesting features: (i) For intermediate wave vectors the structure factor exhibits (damped) oscillations. This behavior is discussed in terms of the analogy with a one-dimensional harmonic ‘‘crystal’’ composed of blobs, and differs from the behavior of a nonconfined chain stretched to the same length. (ii) In the center of gravity system of the chain, the monomer displacements along the tube axis for monomers near the chain ends are very much larger than those near the chain center. We also give a preliminary discussion of consequences for the dynamics of melts, if they are modeled in terms of tubes following a random-walk contour.
Disorder-induced piezoelectric and piezo-optic effects. I. The theory of the piezoelectric properties and its application to ice
Authors: Whalley E. , Klug D. D.
Published: February 2003

When a macroscopic elastic strain is applied to an isotropic disordered solid a dipole moment may be generated, but it is usually very small and averages to zero over an ensemble of similar samples. Disordered solids are, therefore, not usually considered as piezoelectric. However, each small volume element has no symmetry, and so a dipole moment is generated in it by an applied stress, but its effect is often not noticeable on a macroscopic scale, and a suitable probe is needed to detect it. Sound waves in typical solids have speeds of several kilometers per second, and sound waves having wavelengths of, say, 50 Å have frequencies of about 1 THz. The dipole moments generated in disordered solids by the spontaneously fluctuating stresses should, therefore, cause absorption of light in the far infrared, and the absorption allows these disorder-induced piezoelectric properties to be measured. The purpose of this paper is to develop the theory of the absorption of far-infrared light by the sound waves of disordered solids in terms of suitable disorder-induced piezoelectric moduli. The theory has been applied to the sound waves of ice and yields the value 50.4×10−6 D2 μm−3 bar−2 (cm s−1)−1 for the quantity d″2l/cl+2d″2t/ct, where d″2l and d″2t are the squared scalar moduli of the disorder-induced piezoelectric effect for longitudinal and transverse waves, respectively, and cl and ct are the speeds of longitudinal and transverse sound, respectively. If the absorption by the transverse waves is assumed to be negligible, the coefficient d″2l has the value 19.5 D2 μm−3 bar−2. A similar disorder-induced piezomagnetic effect should in principle cause a similar absorption, and a related disorder-induced piezo-optic effect should cause the sound waves in disordered solids to be Raman active.
Effect of mass distribution on the isotopic thermal diffusion of substituted benzenes
Authors: Rutherford W. M.
Published: February 2003

Isotopic thermal diffusion factors for chlorobenzene and bromobenzene were measured in a 45 cm liquid phase thermal diffusion column. The data, combined with results obtained earlier for deuterium and 13C substituted benzenes, were interpreted in terms of differences in molecular masses and moments of inertia. The results for five isotopic pairs were represented within experimental accuracy by αT=1.973 (m1−m2)/(m1+m2)+0.973 (I1−I2)/(I1+I2), where αT is the thermal diffusion factor, m1 and m2 are the molecular masses, and I1 and I2 are second moments.
Computer simulation studies of the structure I clathrate hydrates of methane, tetrafluoromethane, cyclopropane, and ethylene oxide
Authors: Tse John S. , Klein Michael L. , McDonald Ian R.
Published: February 2003

We report the results of molecular dynamics simulations on the ice clathrates of methane, tetrafluoromethane, cyclopropane, and ethylene oxide. Although the translational phonon density of states of the host lattice are found to be broadly similar, there are subtle differences which can be correlated to the properties of the guests. The simulations also predict low frequency vibrations that are associated with the rattling and liberations of the guest molecules in the hydrate cages. The occurrence of these low frequency modes may be responsible for the lower thermal conductivities of the clathrate hydrates relative to ordinary ices.
The critical behavior of the Sugden parameter and the surface tension of simple fluids
Authors: Gielen H. L. , Verbeke O. B. , Thoen J.
Published: February 2003

The capillary rise method has been used to determine the Sugden parameter for the following simple fluids: argon, methane, nitrogen, oxygen, and carbon dioxide. Special emphasis was given to the critical region. The analysis of the data in terms of power laws gives a common critical exponent φ=0.934±0.010 for the five fluids investigated. Combining the critical amplitudes of the Sugden parameter and of the related surface tension with amplitudes of other singular quantities, it was possible to calculate universal constants in the framework of two-scale-factor universality. Universal, system-independent values were obtained for the fluids, but a difference of nearly a factor of 2 is observed with recent second-order ε-expansions and Monte Carlo simulation results.
Dielectric properties of single crystals of HCl-doped ice
Authors: Takei Iwao , Maeno Norikazu
Published: February 2003

Dielectric properties of single crystals of HCl-doped ice were measured in a frequency range 5×10−4 to 1×105 Hz and temperature range −10 to −150 °C. High-frequency and dc conductivities, σ∞ and σ0, and relaxation times of the Debye and low-frequency dispersions, τD and τL, showed changes of temperature dependencies at a temperature Tc. Values of σ0 at temperatures above Tc were constant, implying the complete dissociation of HCl molecules producing H3O+ ions in the ice lattice, so that σ0 is proportional to the HCl concentration (nHCl). At temperatures below Tc, both of σ∞ and σ0 were found to vary with the square root of nHCl. The behavior of σ0 is in good agreement with earlier results, but that of σ∞ is not. The result suggests that L defects can be liberated from HCl molecules through some activation process in the crystal lattice of ice.
Generalization of atoms-in-molecules theory to include independent scaling of inner and outer shells
Authors: Ellison Frank O. , Chen Cheng
Published: February 2003

Scaled atoms-in-molecules (SAIM) theory is required for obtaining diatomic fragment eigenvectors, and often useful for providing diatomic fragment potential energy curves, needed as input in the scaled diatomic-in-molecules (SDIM) method. Independent scaling of inner shells and valence shells is not admitted in the current formulation of SAIM. A new extension is developed here in which atomic eigenfunctions are partitioned into inner-shell and outer-shell components. These component functions are rigorously defined as solutions of two simultaneous eigenvalue equations; the Hamiltonians in these equations add to yield the original total atomic Hamiltonian. The component Hamiltonians so defined are shown to contain potential energy functions which are approximately homogeneous functions of degree minus one; hence, Coulomb-like. Thus, the inner-shell and outer-shell eigenfunctions may be scaled independently using methods generalized from standard scaled atoms-in-molecules (SAIM) theory. Preliminary applications to LiH, BeH, and Li2, and their positive ions, yield dissociation energies accurate to 7 kcal/mol or better.
Qualitative propagator theory of AX4 Auger spectra
Authors: Ortiz J. V.
Published: February 2003

Nonempirical, parametrized two-electron propagator theory is employed in the characterization of molecular Auger spectra. Based on the Mulliken approximation for many-center Coulomb integrals, the model Hamiltonian requires three parameters for each valence atomic orbital: an exponent for a Slater function, an orbital energy, and an electron–electron repulsion integral. All of these quantities are taken from atomic calculations. Two-electron propagators are derived using the superoperator formalism. The working equations yield some popular expressions that have been used in interpreting molecular Auger spectra as special cases. Partitioning technique reveals qualitative factors that govern configuration mixing in the final state. Electron interaction can play a qualitatively dominant role in some of the final states. Some final states display localization of the holes on the same halogen atom, while others place holes on adjacent halogen atoms. This effect is especially pronounced for valence s orbital holes, but it is also observed for A-X bond orbitals and X lone pair orbitals. Trends in electronegativity differences and orbital sizes determine how much final state localization occurs. Propgator theory permits intensity borrowing between configurations leading to richer predicted spectra for the carbon Auger. The importance of using a theory that qualitatively supersedes molecular orbital theory is emphasized.
The metal–carbonyl bond in Ni(CO)4 and Fe(CO)5: A clear-cut analysis
Authors: Bauschlicher Charles W. , Bagus Paul S.
Published: February 2003

We report an unambiguous analysis of the metal–carbonyl bonding in Ni(CO)4 and Fe(CO)5 which gives new and clear insight into the importance of various contributions to the interaction. Three aspects of the interaction are of particular importance: (i) The frozen orbital repulsion between the metal 4s and the CO is large. This strongly favors the Ni 3d10 and Fe 3d8 occupations as the starting point for the interaction; (ii) The metal to CO π donation is energetically much more important than the CO to metal σ donation. For Fe(CO)5, the CO σ donation is significant because the 3d shell is not filled and it is able to accept charge; for Ni(CO)4, the Ni d shell is filled and the CO σ donation is very small; (iii) The metal 4s and 4p orbitals make a very small contribution <0.4 eV to the interaction energy. The large 4s and 4p populations arise because of artifacts of the population analysis and give misleading information about the nature of the bonding.
Empirical improvement of the anharmonic a
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o force field of methyl fluoride
Authors: Kondo Shigeo
Published: February 2003

Improvement of the anharmonic ab initio force field of methyl fluoride has been made by the method of ‘‘predicate observations’’ using the observed data on the spectroscopic constants. The newly obtained force field was used to derive the values for the second derivatives of the dipole moment through analysis of the literature values of the effective dipole moment in the excited vibrational states. The result is compared with the theoretical values from ab initio MO method.
Non-orthonormal CI for molecular excited states. II. The zwitterionic states of terminally twisted butadiene
Authors: Petsalakis I. D. , Theodorakopoulos G. , Nicolaides C. A. , Buenker R. J.
Published: February 2003

The proper and accurate description of excited states is often very sensitive to the function space which is used to describe them. This suggests the application of a theoretical approach which is state specific. In cases of valence-Rydberg mixings, of charge localization, or of various transition processes, such an application results in nonorthonormal basis sets describing different parts of the system. The present molecular NONCI method has been applied to the determination of the interaction in the low-lying zwitterionic states of terminally twisted butadiene, as a function of pyramidalization at the twisted-bond carbon atom. The results of the NONCI calculations give negligible mixing between the two states and as a result, they are in agreement with large, orthogonal basis MRD-CI calculations, for the relative ordering of these states and their dipole moment.
Semiclassical spectral quantization: Application to two and four coupled molecular degrees of freedom
Authors: De Leon N. , Heller E. J.
Published: February 2003

Semiclassical quantization of the quasiperiodic vibrational motion of molecules is usually based on Einstein–Brillouin–Keller (EBK) conditions for the quantization of the classical actions. Explicit use of the EBK conditions for molecular systems of K degrees of freedom requires K quantization conditions. Therefore, explicit use of the EBK conditions becomes increasingly difficult if not impossible for polyatomic systems of three or more degrees of freedom. In this paper we propose a semiclassical quantization method which makes explicit use of phase coherence of the de Broglie wave associated with the trajectory rather than the EBK conditions. We show that taking advantage of phase coherence reduces the K quantization conditions to a single quantum condition—regardless of the number of degrees of freedom. For reasons that will become obvious we call this method ‘‘spectral quantization.’’ Polyatomic vibrational wave functions and energy eigenvalues are generated from quasiperiodic classical trajectories. The spectral method is applied to an ABA linear triatomic molecule with two degrees of freedom and to an anharmonic model of the molecule cyanoacetylene. The usefulness of the technique is demonstrated in this latter calculation since the cyanoacetylene model will have four coupled vibrational degrees of freedom.
Semiclassical theory of the helium atomic spectrum
Authors: van der Merwe P. du T.
Published: February 2003

A semiclassical theory of the excitations of two-electron atoms is developed, based on the collective coordinate method and the 1/N expansion, and its application to the s-state helium spectrum is considered. The leading approximation, where quantum fluctuations due to the kinetic term are quenched, while attendant effects are accommodated by an effective potential, provides an inset to delineate spontaneous symmetry rearrangement of the system. The cumulative spectral content of the fragmentation or single-particle regime and the affiliated electron regime as reflected by the doubly excited states, is a Rydberg series within a Rydberg series. The collective regime promotes novel molecular features, such as vibrational s states, due to the underlying geometrical configurations of the affiliated electrons. The work also touches on issues such as quantum defect theory, topological quantization, and rotational effects.
Changes in the electronic structure and vibrational potential of hydrogen fluoride upon dimerization: A well-correlated (HF)2 potential energy surface
Authors: Michael Daniel W. , Dykstra Clifford E. , Lisy James M.
Published: February 2003

A large basis, coupled-cluster level treatment of the electronic structure of the hydrogen fluoride dimer has been used to develop an accurate potential energy surface and dipole moment surface. The vibrational motion of the HF subunits was analyzed using a number of different methods for comparison with recent experimental results. Molecular properties averaged over the vibrational motion were also calculated. The vibrational transition frequencies for the two HF stretches in the dimer agree with experiment to 1%, while the shifts in these frequencies agree with experiment to 10 cm−1. The importance of higher order correlation effects and treatment of vibrational anharmonicities is made clear by these results. Transition moments were calculated and show enhancements for both HF stretches in the dimer.
Polarizability and quadrupole moment for the lowest triplet state of H2
Authors: Rychlewski Jacek
Published: February 2003

Using explicitly correlated wavefunctions, ab initio calculations of frequency dependent polarizability α(ω,R) and quadrupole moment Q(R) for the lowest triplet state of the hydogen molecule are carried out. Results are presented for a range of internuclear distance R and frequencies ω. For the static polarizability anisotropy and the trace of mean polarizability, large differences are found with results based on the configuration interaction method. Behavior of the electric properties is discussed in terms of the internal structure of the molecule and the electron density.
Electron localization in a hard sphere fluid
Authors: Phillips Philip
Published: February 2003

Presented in this paper is a derivation of the resummed density expansion of the Green function for the motion of a single electron in a hard sphere bath. The Green function is constructed by averaging the electron quantum dynamics obtained for a fixed configuration of the hard spheres over all possible configurations of the hard spheres and initial positions of the electron. The electron bath interactions are treated perturbatively through three-body events. It is shown that the bath dissipates the electron energy much the way a bath impedes tunneling in a two-level harmonic quantum system. Furthermore, it is demonstrated from the long time limit of the electron return amplitude that at T=0, the electron is localized above the fluid density ρσ3= (4)/(3) π−1, σ the hard sphere diameter. Dissipation of the electron energy by the bath produces localization.
A study of the symmetric charge transfer reaction H+2+H2 using the high resolution photoionization and crossed ion–neutral beam methods
Authors: Liao C.-L. , Liao C.-X. , Ng C. Y.
Published: February 2003

A new ion–molecule reaction apparatus, which combines the crossed ion–neutral beam method, high resolution photoionization mass spectrometry, and charge transfer detection, has been developed. Using this apparatus, we have examined the relative total charge transfer cross sections of H+2+H2 as a function of the vibrational state of H+2, v′0 =0–4, at the center-of-mass collision energy (Ec.m.) range of 0.38–200 eV. The relative total charge transfer cross sections measured at Ec.m. =8, 16, 22.5, and 200 eV are in general agreement with a recent theoretical calculation based on the semiclassical energy conserving trajectory formulation. The vibrational energy effects on the cross sections for the charge transfer and the H+3+H channels at low collision energies (Ec.m. ≤1 eV) were directly observed. The rotational states, J=0, 1, and 2, of H+2(v0=0) were also selected in this experiment. Within experimental uncertainties, the rotational excitations of H+2(v′0 =0) have no effect on the relative total charge transfer cross sections at Ec.m. =2 and 4 eV. By calibrating the nominal relative total charge transfer cross sections obtained with an ionizing photon energy of 18 eV (688 Å) to absolute total charge transfer cross sections determined previously using low energy electron impact ionization, absolute total charge transfer cross sections for v0 =0 and 1 in the kinetic energy range of Ec.m. =8–200 eV were estimated. The absolute total charge transfer cross sections thus obtained for v′0=0 and 1 are lower than the theoretical values by approximately a factor of 2. However, the kinetic energy dependence of the total charge transfer cross section is in agreement with the theoretical calculation. The final vibrational state distributions of the charge transfer products H+2 from the reaction H+2(v0=0) +H2(v″0=0)  → H2(v′)+H+2(v″) at Ec.m. =4, 8, and 16 eV have been probed by charge transfer reactions H+2(v″)+N2 and H+2(v″)+CO. The results are consistent with the theoretical prediction that approximately 92% and 85% of the product H+2 ions formed at Ec.m. =8 and 16 eV are in the v″=0 state, respectively.
Photochemistry of 1-cyanonaphthalene/triethylamine van der Waals complex: Intermolecular exciplex formation under collision free conditions
Authors: Saigusa Hiroyuki , Itoh Michiya
Published: February 2003

The van der Waals complex of 1-cyanonaphthalene with triethylamine exhibits fluorescence from the intermolecular exciplex upon electronic excitation. The efficiency of exciplex formation is strongly excitation energy dependent. The excitation of the complex into the origin band shows resonant emission, while excitations into vibronic bands with excess vibrational energies >400 cm−1 lead to the exciplex formation. The exciplex formation efficiency is also dependent on isotopic substitution in 1-cyanonaphthalene. The barrier to the exciplex formation is larger for the deuterated complex. The fluorescence excitation spectrum of the exciplex reveals that the exciplex formation process is favored over other intramolecular relaxation processes over the excess energy range 700–1500 cm−1. These results suggest that van der Waals complexes can be important intermediates in the intermolecular exciplex formation in the vapor phase.
Resonances in the photodissociation of OH by absorption into coupled 2Π states: Adiabatic and diabatic formulations
Authors: van Dishoeck Ewine F. , van Hemert Marc C. , Allison A. C. , Dalgarno A.
Published: February 2003

The bound 3 2Π and repulsive 2 2Π states of OH are strongly coupled by the action of the nuclear kinetic energy operator. The process of photodissociation by absorption into the coupled  2Π states is studied theoretically. The adiabatic electronic eigenfunctions and potential energy curves of the 2 2Π and 3 2Π states are calculated using large configuration-interaction (CI) representations and the nuclear radial coupling matrix elements are obtained by numerical differentiation. The coupled equations for the nuclear wave functions of the two states are set up in an adiabatic and in a diabatic formulation and are solved by numerical integration. The electric dipole transition moments connecting the ground X 2Π state to the 2 2Π and 3 2Π states are computed from the CI wave functions and the resulting photodissociation cross sections of OH arising from absorption into the coupled 2 2Π and 3 2Π states are obtained. Two alternative sets of potential curves, coupling matrix elements, and transition moments are employed to provide an assessment of the accuracy of the results. The photodissociation cross section shows a series of resonances superimposed on a broad continuous background. The resonances are located near to the vibrational levels of the uncoupled bound diabatic potential curve. They have asymmetric Beutler–Fano profiles and vary in width from 50 cm−1 for the lowest levels to 2 cm−1 for the higher levels. The accuracy of adiabatic and diabatic approximations, carried to first order in the coupling, is explored and it is demonstrated that the diabatic approximation provides a more satisfactory representation of the photodissociation process. The discrete-continuum configuration interaction theory of Fano is applied in the diabatic formulation and the resonance structures are calculated. The discrete-continuum interaction theory yields profile parameters and level shifts which agree well with the accurate values obtained by solving the coupled equations but its application is considerably more laborious. The contribution of absorption into the coupled 2Π states to the interstellar photodissociation rate is evaluated. In the optically thin limit, the rate is1.5×10−10 s−1. The total rate is 4×10−10 s−1.
Kinetics and mechanisms of the reactions of CH and CD with H2 and D2
Authors: Berman Michael R. , Lin M. C.
Published: February 2003

The reactions CH+H2→ k1products and CD+D2→ k2products were studied using a laser photolysis/laser-induced fluorescence (LIF) technique. The room-temperature rate constant for reaction (1) was found to be pressure dependent between 25 and 600 Torr. Reactions (1) and (2) were studied as a function of temperature in the range 159–658 K at a total pressure of 100 Torr. Two pathways dominate these reactions. Below 400 K, addition followed by collisional stabilization of the adduct predominates. The rate constants display negative temperature dependences and are described by k1=(2.37±0.43)×10−12 exp[(1041±85)/RT] and k2=(3.51±0.58)×10−12 exp[(1030±90)/RT] cm3 molecule−1 s−1 with Ea in units of cal mol−1. Above 500 K, production of CH2+H or CD2+D with an activation energy of 4±2 kcal mol−1 dominates. In the reactions of CH+D2 and CD+H2, anomalously fast removal of the parent radical is observed because of formation and dissociation of chemically activated addition complexes which lead to isotopically substituted parent radicals. LIF spectra of these product radicals were obtained. A transition-state-theory model was applied to a mechanism in which collision stabilization and product formation proceed through a single complex.
Coherent elimination of isotropic scattering invariants in coherent Raman and hyper-Raman spectroscopy
Authors: Desiderio R. A. , Hudson B. S.
Published: February 2003

In both spontaneous Raman and Hyper-Raman scattering from isotropic systems, scattering due to one of the spherical tensor invariants cannot be eliminated. It is shown that these invariants can be eliminated through use of an appropriate experimental geometry in coherent Raman and hyper-Raman scattering. Applications of this technique are presented.
Electronic spectroscopy of perylene–rare-gas van der Waals complexes
Authors: Leutwyler Samuel
Published: February 2003

van der Waals (vdW) complexes of perylene (P) with rare-gas atoms R (R=Ne, Ar, Kr, and Xe) were synthesized in seeded supersonic free jet expansions and studied by laser-induced fluorescence spectroscopy. P ⋅ Rn complexes with n=1–4 were identified for all rare gas solvents. The P ⋅ R2 complexes with R=Ar, Kr, and Xe were found to form two spectroscopically distinct species, which are assigned as vdW isomers. The P ⋅ R and P ⋅ R2 intermolecular interactions were calculated using a model calculation based on atom–atom pair potentials, yielding potential energy surfaces, equilibrium structures, and vdW binding energies. The results for P ⋅ R2 support the existence of two different equilibrium structures with almost equal binding energies. The observed microscopic red shifts of P ⋅ R complexes are in excellent agreement with the predictions of microscopic solvent-shift theory based on a second-order perturbative treatment. The microscopic red shifts of the larger P ⋅ Rn complexes can be systematized by additivity rules which reflect their geometric configurations. Intermolecular vibrational excitations in the S1 state were observed for P ⋅ Ar, P ⋅ Kr, P ⋅ Xe, and P ⋅ Xe2 with fundamental frequencies in the range from ν′z =47.2 cm−1 (P ⋅ Ar) to νz =38.7 cm−1 (P ⋅ Xe). The observed frequencies are in excelllent agreement with the values for the out-of-plane modes calculated on the basis of the model intermolecular potentials.
Effects of high pressure on molecular electronic spectra
Authors: Kesarwani R. N. , Varshni Y. P.
Published: February 2003

The Drickamer–Franck–Slichter model for the effects of high pressure on electronic spectra is reformulated by incorporating the Kratzer–Fues potential to allow for anharmonicity. The configuration-coordinate parameters for the ground and excited states are shown to be readily derivable from the experimental data. The proposed model is applied to six cases for which experimental data are available.
Densities of vibrational states of given symmetry species. Linear molecules and rovibrational states of nonlinear molecules
Authors: Lederman Steven M. , Marcus R. A.
Published: February 2003

A simple statistical expression is given for the density of states of any symmetry species for linear molecules. Molecules with one and two pairs of doubly degenerate bending modes are considered. The results of our previous paper for vibrational states of nonlinear molecules are also extended to include density of rotational-vibrational states by symmetry species. The various expressions are tested by comparing with exact counts of states.
Photoionization of vapor phase thallium and indium monohalides in the ultraviolet: Absolute cross sections and photofragment spectroscopy by photodetachment of I−
Authors: Geohegan D. B. , McCown A. W. , Eden J. G.
Published: February 2003

Absolute cross sections for the photoionization of thallium iodide (TlI) and thallium bromide (TlBr) at 193 nm (ℏω=6.4 eV) have been measured to be 2.4×10−17 and 2.2×10−17 cm2, respectively. An upper bound for the ionization cross section of indium monoiodide (InI) at 193 nm was determined to be 7×10−18 cm2. At 248 nm (ℏω=5.0 eV) and 351 nm (ℏω=3.5 eV), the TlI cross section has fallen to 2.6×10−18 and 1.5×10−19 cm2, respectively. The large cross section for TlI at 193 nm arises from photoionization of a bound molecular state. For InI at 6.4 eV and TlI at 5.0 and 3.5 eV, the molecule rapidly dissociates and the smaller cross sections are associated with photoionization of the metal atom. Further experiments combining two excimer lasers with a microwave absorption apparatus were conducted in order to determine the fraction of TlI molecules absorbing a 193 nm photon that produce Tl+–I− ion pairs. By photodetaching electrons from those I− ions remaining after an initial ultraviolet laser pulse, the branching ratio for ion pair production in TlI at 193 nm was measured to be 2%. Approximately 36% of the 193 nm photons absorbed by ground state TlI molecules result in the production of the Tl(7 2S1/2) species and 62% directly produce an excited TlI molecule. The implications of these results for existing atomic photodissociation lasers (Tl: 378 and 535 nm; In: 451 and 410 nm) and column IIIA metal film deposition are discussed.
The occurrence and stability of trivalent zirconium in orthophosphate single crystals
Authors: Abraham M. M. , Boatner L. A. , Ramey J. O. , Rappaz M.
Published: February 2003

EPR spectroscopy has been used in investigations of the unusual 3+ valence state of zirconium. The EPR spectrum of Zr3+, which was initially observed as a parasitic signal in Fe-doped single crystals of ScPO4, was definitively identified by preparing single crystals of ScPO4 doped with isotopically enriched (94.6%)91Zr. Single crystals of LuPO4 and YPO4 doped with both naturally abundant and isotopically enriched Zr were also grown and investigated. The EPR spectrum of Zr3+ could be observed at room temperature in the ‘‘as-grown’’ single crystals of both ScPO4 and LuPO4; and, accordingly, the 3+ state is stable at room temperature in these hosts. The spectrum of Zr3+ in the YPO4 host could only be observed following a γ irradiation at 77 K, and this spectrum decayed rapidly with increasing temperature. Axial spin-Hamiltonian parameters were determined for Zr3+ in all three hosts at a sample temperature of 77 K and at room temperature for the ScPO4 and LuPO4 hosts. In the case of the latter two hosts, the spin-Hamiltonian parameters were found to exhibit a small temperature dependence. The observed g values were not accounted for by previously published second-order expressions indicating that additional coupling mechanisms are operative.
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9F nuclear magnetic shielding scale from gas phase studies. II
Authors: Jameson Cynthia J. , Jameson A. Keith , Honarbakhsh Jila
Published: February 2003

The absolute 19F nuclear magnetic shielding at 300 K is reported for 13 molecules containing 21 chemically different 19F nuclei.
Perturbation theory for the Lennard-Jones diatomic fluid. II. Thermodynamic and quasithermodynamic properties
Authors: Nezbeda Ivo , Smith William R. , Labik Stanislav
Published: February 2003

The RAM theory is used to calculate the spherical harmonic expansion coefficients with respect to the sites, and these are used in turn to calculate the Helmholtz free energy, pressure, internal energy, mean squared force, mean squared torque, and g2 factor for a range of homonuclear Lennard-Jones diatomic fluids. The Helmholtz free energy, pressure, and some representative correlation functions are also calculated by using a recently proposed median reference system potential. All results are compared with computer simulation data and with the results of other currently available theories. The accuracy of the RAM theory ranges from good to excellent for all properties except the pressure at high densities.
Depression of the liquid crystal-isotropic transition temperature by a magnetic field
Authors: Subramanian R. , DuPré D. B.
Published: February 2003

A reduction in the temperature of the transition to the biphasic and clearing points by the application of a magnetic field in excess of the critical unwinding field has been observed in a cholesteric liquid crystal. This observation qualitatively is in accordance with the phenomenological expansion of the excess free energy in the order parameter S. Inclusion of a twist term in the gradients of S leads to a rescaling of T* which is inversely proportional to the equilibrium pitch P0. When the pitch of the cholesteric liquid crystal is increased or removed by a magnetic field, a reduction in the transition temperature is expected. The observed effect is apparently insignificant in thermotropic liquid crystals but may be more pronounced in the lyotropic liquid crystal of this study due to its very small latent heat.
A shock tube study of vibrational relaxation of O2 in argon by small amounts of H2 (860–1290 K), D2 (890–1070 K), and He (1000–1500 K)
Authors: Subba Rao V. , Skinner Gordon B.
Published: February 2003

Vibrational relaxation times of O2 in a mixture of 2% O2 in Ar were measured behind reflected shock waves at temperatures of 1000–1600 K and total pressures of 2–3 atm. The effects of small amounts (0.05%–0.3%) of H2 (860–1290 K), D2 (890–1070 K), and He (1000–1500 K) on the relaxation times were measured, the effects being consistent with the linear mixture rule. At the D Ly-α wavelength (121.6 nm), new values of the absorption cross sections for vibrationally excited O2 are σ1 (for v=1)=(3.4±1)×10−19 cm2, and σ2 (for v=2)=(5±2)×10−18 cm2.
Symmetry violations in partially oxidized one-dimensional (1D) transition metal polymers. The Ni(CN)2−4 system
Authors: Böhm Michael C.
Published: February 2003

The band structure of tetracyanonickelate (II), Ni(CN)2−4 1 has been studied in the unoxidized state and in a partially oxidized form with one electron removed per two Ni(CN)2−4 sites. Computational tool for this investigation is a semiempirical INDO (intermediate neglect of differential overlap) crystal orbital (CO) model based on the tight-binding formalism and combined with the grand canonical (GC) averaging procedure. This approximation allows for the formation of average values over certain manifolds of one- (or many-electron) states and thus makes possible the investigation of violations of the spatial symmetry separated from metal–insulator transitions and spin-dependent phenomena. The oxidation process in 1 is metal centered (3dz2 states) and leads to a broken symmetry CDW (charge density wave) solution with trapped valences at the Ni sites within the allowed variational space in the Hartree–Fock (HF) CO formalism. The charge separation between the transition metal centers ΔqNi amounts to ∼0.87e. The band structure shows some analogies to the dispersion curves encountered in semiconductor superlattices. The physical origin for this valence trapping is analyzed and compared with violations of spatial symmetries in finite transition metal complexes, clusters as well as simple solids. The title compound belongs to a class of organometallic polymers in the 3d series where the covalent metal–metal interaction (kinetic energy operator) is negligibly small in comparison to the strongly repulsive two-electron part. The symmetry adapted HF state of the partially oxidized backbone with averaged populations at the Ni sites is thus unstable against a condensation into an arrangement with alternating oxidation states (trapped valences).
An approximation for the two particle distribution of a hard-sphere fluid near a hard wall at low densities
Authors: Navascués G. , Bragado R.
Published: February 2003

A suitable mean density average for the two particle distribution function g(2)(r1,r2) of a nonuniform simple fluid is proposed. The mean takes into account all the intermediate densities of the system between the r1 and r2 points. The approximation is tested in a hard-sphere fluid near a hard wall at low densities where exact results are available. The approximation gives better results for the structure of the density profile, adsorption and surface tension than any other approach (scale particle theory, Percus–Yevick, and hypernetted chain closures and superposition approximation).
Raman scattering topology in an anisotropic conjugated molecule: A novel approach through single crystal Raman
Authors: Kuehl Scott A.
Published: February 2003

A comprehensive investigation into the Raman scattering topology of a polyene chain is carried out through polarized Raman spectroscopy on a single crystal of diphenyloctatetraene. By using the crystallographic data of the monoclinic modification of diphenyloctatetraene, the relative magnitudes of the components of the Raman scattering tensor for those normal modes subject to resonance enhancement have been obtained. These vibrations correspond to the double bond and some single bond stretch modes and two phenyl ring modes, these being the most intense bands observed in the Raman spectrum enhanced by lasing in the preresonance region. Intensity measurements were carried out in the principal coordinate system of the electrical susceptibility tensor. The intensity data obtained are appropriately interpreted with an oriented gas model. Furthermore, recognizing the high anisotropy of the scattering topography, a novel simplification is used enabling a transformation which results in the relative magnitudes and signs of the complete molecular Raman scattering tensor in molecular fixed coordinates.
Laser-pulsed thermal chemistry: UV laser-activated oxidation of niobium
Authors: Marks R. F. , Pollak R. A.
Published: February 2003

Surface oxidation of niobium metal has been studied under pulsed UV laser (λ=308 nm) exposure at one atmosphere of oxygen. Diffusion apears to dominate this laser-pulsed thermal chemistry (LPTC) reaction. X-ray photoelectron spectroscopy reveals a range of niobium oxidation states Nb+5, Nb+4, Nb+2, and Nb metal from the surface to the bulk, respectively. LPTC oxidation depth increases monotonically with repeated laser irradiation unlike infrared laser-induced pulsed plasma chemistry, which is self-limiting. Possible electronic contributions to the LPTC process are considered as well as effects due to surface crystal orientation.
A model microemulsion
Authors: Widom B.
Published: February 2003

The microemulsion model of de Gennes et al., which is a modified version of the earlier model of Talmon and Prager, is modified further by imposing a microscopic cell-size cutoff. Two free-energy minima compete and lead to a composite free-energy surface consisting of two intersecting sheets. The microemulsion phase arises from one sheet and the oil- and water-rich phases with which it may be in equilibrium arise from the other. The Schulman condition, according to which the surfactant-film pressure equals the oil–water interfacial tension, is found to hold to good approximation in the middle-phase microemulsion that contains comparable amounts of oil and water. The oil- and water-filled domains in that microemulsion are found to be about 75–80 Å across. That phase is of such high osmotic compressibility that it would be opalescent or turbid due to fluctuations of composition. The model shows a range of phase equilibria like that seen in experiment, including two- and three-phase equilibria, critical points, and critical endpoints. The critical points are tricritical, and each is associated with a locus of second-order transitions.
Vibrational line shapes in liquids. The role of resonant intermolecular coupling
Authors: Knapp E. W.
Published: February 2003

The role of intermolecular coupling on vibrational line shapes is studied in terms of a mapping procedure relating the full spectrum with a spectrum corresponding to vanishing coupling. This spectral transform is based on a single site approximation. It exhibits an asymmetric line, exchange narrowing, and a shift of the line center depending on the average intermolecular coupling. Fluctuations of the total amount of intermolecular coupling at a reference molecule lead to line broadening. Fluctuations of the intermolecular coupling leaving the total amount of coupling at a reference molecule constant, invoke motional narrowing. The consequences of the model are compared with available data on isotopic dilution experiments.