bluuebunny/crossref_metadata_2025 huggingface dataset:
IR multiple photon dissociation of C2HCl3: Molecular elimination vs bond fission and efficient dissociation of the C2Cl2 producta)
Authors: Caballero J. F. , Wittig C.
Published: February 2003

The primary step in the IR multiple photon dissociation (IRMPD) of C2HCl3 is molecular elimination of HCl, even with laser fluences as high as 102 J cm−2. A large amount of atomic chlorine derives from the secondary photolysis of the vibrationally excited C2Cl2 produced concomitantly with HCl in the molecular elimination step. This C2Cl2 is dissociated very efficiently ([Cl]/[HCl]=0.6±0.2), since it absorbs radiation readily and is born with considerable vibrational excitation. We point out that the ultimate production of C2 molecules almost certainly involves the IRMPD of C2Cl, which has a low lying A 2A′′ electronic state that facilitates such optical excitation.
The dynamics of infrared photodissociation of van der Waals molecules containing ethylene: An experimental study
Authors: Bomse D. S. , Cross J. B. , Valentini J. J.
Published: February 2003

Infrared (∼950 cm−1) predissociation of ethylene clusters has been studied using a crossed laser beam–molecular beam apparatus equipped with a moveable detector. van der Waals molecules undergo dissociation following absorption of a single infrared photon. Angular distributions, obtained for product molecules (C2H4)n, n=1–3, all show nearly exponentially decreasing product flux with increasing scattering angle. A product flux contour map has been generated for the photolysis reaction (C2H4)2 → C2H4+C2H4. Two isotropic center-of-mass distribution functions yield excellent agreement with experimental results. One is a function of reaction kinetic energy E, with P(E)=exp(−E/80 cm−1); the other is a function of product velocity (momentum) u, with U(u) =u exp(−u/9×103 cm/s). The latter distribution is characteristic of a dissociation pathway with a barrier in the exit channel. Such a barrier could result from centrigufal effects. It is argued that isotropic product scattering can be consistent with a direct dissociation reaction occurring on the subpicosecond time scale. It is not possible to determine, unambiguously, if dissociation is direct or if the reaction proceeds through a long-lived intermediate. Experimental results indicate that only a small fraction of the energy available to products appears as translation, with the remainder appearing as rotational excitation of C2H4. Results are compared with other experimental studies on infrared predissociation of van der Waals cluster as well as a recent theoretical analysis of (C2H4)2 dissociation.
The generalized mobility curve for alkali ions in rare gases: Clustering reactions and mobility curves
Authors: Takebe M.
Published: February 2003

The measurements of mobility values for alkali ions in rare gases at room temperature over a wide range of E/N were completed for all 25 combinations. The experimental mobility curves were compared with a generalized mobility curve calculated from a model potential consisting of an inverse 8th power repulsive term and 6th and 4th power attractive terms, which took into account the core size, and potential well depths being determined for all the ion–gas combinations except for the cases of Rb+–He and Cs+–He from the relation between the observed maximum mobility and the ion energy. Experimental generalized mobility curves for alkali ions in rare gases were obtained using these well depths. It was found that all the experimental mobility curves were unified into a single curve using the model potential including the core size. The rate coefficients were measured for backward clustering reaction: Li+Ar–Ar, Li+Kr–Kr, and Li+Xe–Xe, using a drift tube. It was found that the activation energy is roughly half the well depth, comparing the well depths 0.550 eV for Li+–Ar, 0.710 eV for Li+–Kr, and 0.901 eV for Li+–Xe, with the activation energies obtained by Arrhenius plot for the backward reactions 0.34 eV for Li+Ar, 0.45 eV for Li+Kr, and 0.49 eV for Li+Xe.
Semiempirical calculation of diamagnetic susceptibilities of organomercury compounds
Authors: Gupta R. R. , Kumar M. , Kalwania G. S. , Kishan
Published: February 2003

The semiempirical model is presented to calculate diamagnetic susceptibilities of organomercury compounds with the best-fit parameters. the diamagnetic susceptibility is considered to be contributed by atoms, bonds, and bond–bond interactions. The calculated results are in good agreement with the corresponding experimental values. (AIP)
An improved upper bound for the free energy of a single chain lattice model
Authors: ten Brinke G. , Karasz F. E.
Published: February 2003

A very simple derivation of an upper bound for the free energy of a single chain lattice model is given. It is based on the exact result of Kasteleyn for Hamiltonian walks on a square lattice with Manhattan traffic rule. (AIP)
On representation of electron–electron repulsion energies by simple one-electron functionals
Authors: Gadre Shridhar R. , Bendale Rajeev D.
Published: February 2003

An analysis is presented of the various functional forms representating coulomb energies and the electron-electron repulsion energy in atomic and molecular systems. (AIP)
Placzek-type polarizability tensors for Raman and resonance Raman scattering
Authors: Lee Soo-Y.
Published: February 2003

A systematic derivation, using the time-dependent formulation of Raman scattering, of some Placzek-type polarizability (PtP) tensors—the multidimensional reflection, the semiclassical coordinate space, and the locally linear coordinate-space polarizability tensors—which are dependent only on the nuclear coordinates are presented, and their limitations are discussed. Phase-space pictures are used to show the relationship between the various approximations from which the PtP tensors are derived. These PtP tensors are expected to be accurate, even on resonance, for repulsive, multidimensional excited state surfaces.
Coherent pumping in a weakly coupled quasicontinuum model
Authors: Galbraith H. W. , Ackerhalt J. R. , Milonni P. W.
Published: February 2003

We construct and solve a model of laser photon absorption in polyatomic molecules which is related to the onset of effective intramolecular mode coupling (quasicontinuum). We find coherent absorption of the laser photons despite the many-level nature of the system, however, all of the features of intramolecular vibrational relaxation are also evident if the populations of the pure pumped mode states are monitored as a function of time. The model is also found to be quite stable with respect to an asymmetry in the intramolecular mode coupling constant.
A selection rule for M-dependent transitions in collisional excitation of open shell diatomics
Authors: Davis Stephen L. , Alexander Millard H.
Published: February 2003

A selection rule is derived within the collision frame for M=0→M′=0 transitions in collisions between an open shell diatomic of odd multiplicity, e.g., 1Π or 3Σ, and a structureless particle. For 1Π and 3Σ states, the scattering amplitude for the J→J′ transition vanishes unless e/f symmetry is conserved. We then discuss how this selection rule for a 1Π state could be observed experimentally by measurement of the polarized fluorescence intensity ratio I⊥/I∥ in an optical–optical double resonance experiment. This raises the possibility of exploiting our selection rule to study the comparability of cross sections in the collision and laboratory frames.
Ab initio SCF calculations on electrostatically solvated molecules using a deformable three axes ellipsoidal cavity
Authors: Rinaldi Daniel , Ruiz-Lopez Manuel F. , Rivail Jean-Louis
Published: February 2003

The analytical expression of the free energy of solvation of a molecule interacting with a dielectric continuum through a three axes ellipsoidal cavity is used to derive the SCF equations of such a solvated molecule. In this paper, the shape of the cavity is defined after the principal values of the electronic polarizability tensor. Applied to two sets of rotational isomers (trans and gauche 1,2 difluoroethane, and E and Z N methyl formamide), this method confirms that the electronic structure, the molecular geometry, and the equilibrium constant are influenced by the solvent. The energy of solvation depends strongly on the shape of the cavity although the electronic structure appears to be less influenced by a modification of the geometrical characteristics of the boundary surface at constant volume of cavity. Therefore, the computation of the electronic wave function of a molecule interacting with a solvent by electrostatic and induction forces appears to be quite feasible.
Approximate and exact treatments of second-harmonic generation: Regular vs ‘‘stochastic’’ behavior
Authors: Katriel J. , Moiseyev N.
Published: February 2003

The dynamics of the second-harmonic generation Hamiltonian is studied both exactly and using an approximation involving the application of generalized Bose operators. The limits of validity of the approximate treatment, which is of interest in the context of quantum optics, are established. The transition from a regular to a quasistochastic behavior in this nonlinear system is investigated.
Physical properties of nematic mixtures. I. Polar–polar and nonpolar–nonpolar systems
Authors: Osman Maged A. , Schad Hp. , Zeller H. R.
Published: February 2003

The thermodynamic stability, the dielectric, diamagnetic, and elastic properties of mixtures of polar as well as those of nonpolar nematic liquid crystals were investigated and compared to the properties of their components. Small deviations from the ideal behavior were observed and attributed to differences in the packing of unlike molecules which mainly depend on steric factors. Clearing points obtained by the extrapolation method are shown to appreciably deviate from the actual values.
Nitrogen-14 NMR study of molecular motion in rubidium and cesium cyanide
Authors: Wasylishen Roderick E. , Jeffrey Kenneth R.
Published: February 2003

Nitrogen-14 nmr relaxation times T1Z, and T1Q of the cyanide ion in the cubic phase polycrystalline samples of RbCN and CsCN have been measured as a function of temperature. At any given temperature between 210 and 325 K the 14N spin-lattice relaxation times in RbCN and CsCN are the same within experimental error. Assuming the quadrupolar relaxation mechanism, the relationship between T1Z and the rotational correlation time τ2 of the cyanide ion in these salts is calculated. The values of τ2 are short (e.g., ∼0.15 ps at 300 K) and only slightly longer than those calculated using an inertial rotational model. The 14N nmr T1Z data are compared with those previously obtained NaCN and KCN.
On nondiffusive behavior in Brownian motion
Authors: Titulaer U. M.
Published: February 2003

We argue that the slowly decaying discrepancies between certain solutions of the Fokker–Planck and diffusion equations for Brownian motion, found recently by Harris [J. C. P. 75, 3037 (1981)] cannot be ascribed to persistent nondiffusive behavior. Rather, they are surviving memories of a short, initial, nondiffusive stage. (AIP)
Angle-resolved photoelectron spectroscopy of Cl2 as a function of photon energy from 18 to 70 eV
Authors: Carlson Thomas A. , Krause Manfred O. , Grimm Frederick A. , Whitley Timothy A.
Published: February 2003

Angle-resolved photoelectron spectroscopy using synchrotron radiation has been carried out on gaseous chlorine as a function of photon energy from 18 to 70 eV. From this data the partial cross sections and angular distribution parameter β have been derived for the first three orbitals. For the lone pair orbitals 2πg and 2πu, distinct minima are found in both the partial cross sections and β values in the region of photoelectron energies from 30 to 40 eV. Less well-defined minima are also seen in the case photoionization for the nonlone pair orbital 5σg. These features are believed related to the Cooper minimum that arises from an atomic 3p subshell. Calculations have also been carried out on the partial cross sections and β values using the multiple scattering Xα method. Comparison with experiment gives excellent qualitative agreement with regard to the ‘‘Cooper minimum.’’ The nature of the Cooper minimum in molecules and its usefulness in understanding photoelectron dynamics is discussed.
The E, F 1Σ+g double-minimum state of hydrogen: Two-photon excitation of inner and outer wellsa)
Authors: Marinero E. E. , Vasudev R. , Zare R. N.
Published: February 2003

Low-lying vibrational levels (v=2 and 3) of the outer well of the double-minimum E, F 1Σ+g state of molecular hydrogen are experimentally observed for the first time permitting a rotational analysis. The E, F state is excited by two-photon absorption using tunable (∼195 nm) Raman-shifted radiation of a frequency-doubled dye laser. The nonlinear absorption is detected by monitoring the subsequent photoionization of the E, F state. Homogeneous perturbations are observed between the vE=1 and vF=2 levels of the inner and outer wells, respectively. The molecular parameters for these levels (derived by a deperturbation analysis) as well as those for the unperturbed vF=3 level are compared with recent ab initio calculations and small deviations are noted. The rotational intensity distribution of the E–X (0,0) band mirrors the ground state rotational population, whereas the corresponding distributions of the E–X (1,0) and (2,0) bands as well as the F-X (4,0) band do not follow this pattern. This is interpreted in terms of vibronic coupling between the inner and outer wells of the E, F state. A comparison between the experimental rotationless intensities and those calculated from ab initio ground and excited state vibrational wave functions suggests that photoionization from the outer well is more effective than from the inner well.
Kinetic processes of electron beam generated XeF* and Xe2F* excimers
Authors: Sauerbrey R. , Walter W. , Tittel F. K. , Wilson W. L.
Published: February 2003

Kinetic processes of XeF and Xe2F have been investigated experimentally in electron beam excited Ar/Xe/NF3 mixtures. In Xe/NF3 mixtures no vibrational relaxation was observed in the XeF(B, C) states due to the rapid two-body quenching of XeF(C) by Xe. Two- and three-body quenching constants of XeF(B, C) were measured for several collision partners. An investigation of the Xe2F production process showed that XeF(C) is the precursor of Xe2F. Formation and quenching rate constants for Xe2F have been determined. The kinetic processes of the XeF–Xe2F system can be described by a relatively simple kinetic model.
Comparison of variational transition state theory and quantum sudden calculations of three-dimensional rate coefficients for the reactions D(H)+BrH → DBr(HBr)+H
Authors: Clary D. C. , Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

We calculate rate coefficients for the three-dimensional reactions H+BrH → HBr+H and D+BrH → DBr+H using two different dynamical methods but with the same potential energy surface. One method is a three-dimensional quantum mechanical technique in which the energy sudden approximation is used for the entrance reaction channel and the centrifugal sudden approximation is applied to the exit reaction channel. The second method is improved canonical variational transition state theory with small-curvature-tunneling semiclassical adiabatic ground-state transmission coefficients. The potential energy surface is an empirically adjusted diatomics-in-molecules surface which has a very narrow barrier to reaction. The rate coefficients predicted by the two very different dynamical theories are in excellent agreement—they differ by less than 20% over the temperature range from 150 to 500 K.
Thin double layer approximation for electrophoresis and dielectric response
Authors: Fixman Marshall
Published: February 2003

An earlier thin double layer approximation for the dielectric response of spherical macroions is extended to allow calculation of the electrophoretic mobility and inclusion of convective polarization of the ion atmosphere. Comparison is made with exact numerical results and with other thin double layer approximations. The approximation gives fairly accurate values of the dielectric response at macroion radii down to 50 D lengths, for the range of zeta potentials 0≤ζR≤8, where ζR is the reduced zeta potential and D is the Debye length. The accuracy worsens gradually with decreasing size and increasing zeta potential. Electrophoretic mobility results are considerably better.
Theory of multiple phase separations in binary mixtures: Phase diagrams, thermodynamic properties, and comparisons with experiments
Authors: Goldstein Raymond E. , Walker James S.
Published: February 2003

The lattice-gas models of phase separating binary liquid mixtures, introduced by Walker and Vause, are studied in detail and generalized within a high-temperature series expansion. This approximation allows for a straightforward study of rather complex, orientationally specific pair interactions, like those found in real systems. These theories can predict much of the complex miscibility phenomena often found in these mixtures, which are characterized by hydrogen-bonding interactions. Such phenomena include up to five critical solution points as a function of temperature. By comparisons with experiments, we determine the model parameters, thus mapping these experiments onto the global phase diagrams. These experiments include studies of the dependence of liquid/liquid miscibility on temperature, pressure, concentration of electrolytes, and addition of a dilute third component. Specifically, we make direct comparison with various experiments on the binary systems 2-butanol+H2O, 3-methyl pyridine+H2O(D2O), gylcerol +o- methoxy phenol and ethanol+H2O+electrolytes. Very simple and often easily interpreted trends in the parameters are found and quantitative agreement with experiments is possible with minimum parametric freedom. Explicit predictions of critical exponent renormalization in several systems are made. In addition, suggestions are made for a number of light scattering and specific heat experiments, some of which may demonstrate incipient critical behavior, such as the onset of long range correlations, in systems not undergoing phase separation.
Third-order optical susceptibility determination by third harmonic generation. I
Authors: Meredith G. R. , Buchalter B. , Hanzlik C.
Published: February 2003

A method for determining the third-order nonlinear susceptibility of liquids using third harmonic generation (THG) is presented. A review of molecular polarizability and discussion of the relationship to susceptibilities are presented. All materials, including air and windows of the liquid cell, can make substantial contributions to the total THG intensity; this technique accordingly accounts for their effects. The method relies on use of a cell with wedged windows and a wedged liquid chamber which when placed in a vacuum chamber and translated across the optical path generates complex THG intensity fringes. By analysis of the latter, the liquid susceptibility is extracted. Several simple neat liquids are used to demonstrate the method.
A study of carrier generation mechanism in x-metal-free phthalocyanine
Authors: Popovic Zoran D.
Published: February 2003

Combined pulsed photoconductivity and electric field induced fluorescence quenching measurements have been used to study carrier generation mechanism in x-metal-free phthalocyanine. It was found that at low fields fluorescence quenching is a quadratic function of the applied field and saturates at unity at very high fields. Carrier generation is finite at zero applied field and has a linear component at low fields. Linear correlation between carrier generation and fluorescence quenching is observed for fields exceeding 60 V/μm. The results are interpreted by assuming field dependent charge transfer (CT) state formation which either dissociates into carriers or decays to the ground state. Fluorescence quenching data are in excellent agreement with theoretical predictions, while carrier generation at low fields cannot be explained by continuum theory of CT state dissociation probability. It is suggested that the development of a new theoretical framework may be needed which will take into account the discrete nature of charge hopping in the vicinity of the counterion.
Classical and quantum-mechanical models of thermal and flash desorption of atoms from surfaces
Authors: Goodman Frank O.
Published: February 2003

Two models, one classical and the other quantum-mechanical, for nonequilibrium thermal and flash desorption of atoms and molecules from surfaces are proposed, and their predictions are compared with those of previous (classical) models (of thermal desorption). Deviations from the equilibrium dependencies, on desorption angle, of (a) the desorbing atomic flux, (b) the average energy of desorbing atoms, and (c) the desorbing atomic speed ratio are found, these deviations being generally less pronounced than, and sometimes qualitatively opposite from, those predicted by previous models. The classical model lends itself to relatively easy calculations of atom-surface sticking probabilities, and several examples of these results are presented; apart from their intrinsic interest, they also serve as useful checks on the model calculations. Analogous calculations are not possible on the present form of the quantum-mechanical model.
Experimental studies of the vapor phase nucleation of refractory compounds. II. The condensation of an amorphous magnesium silicate
Authors: Nuth Joseph A. , Donn Bertram
Published: February 2003

Measurements of the critical partial pressure of SiO, Pc, required for the initiation of avalanche nucleation, as a function of ambient temperature in the system Mg–SiO–H2 have been obtained over the temperature range 750<T<1015 K. These are compared to similar measurements of Pc for the SiO–H2 system. It is found that the presence of magnesium lowers the partial pressure of SiO required to initiate avalanche nucleation in the Mg–SiO–H2 system for T≲925 K, but does not effect the onset of condensation at the higher temperatures.
1
3C and 18O isotope enrichment by vibrational energy exchange pumping of CO
Authors: Bergman R. C. , Homicz G. F. , Rich J. W. , Wolk G. L.
Published: February 2003

Measurements of preferential vibration-to-vibration (V–V) pumping of high vibrational states of 13C 16O and 12C 18O in optically excited CO gas are reported. It is found that the v=22, 25, 27, 30, and 32 states of 13C 16O and the v=8, 10, and 12 states of 12C 18O are substantially overpopulated compared to the same states in 12C 16O in strongly V–V pumped CO. Such mixtures are observed to react, forming products enriched in 13C. The results are in reasonable agreement with an analytical kinetic model of V–V pumping in binary mixtures of diatomic gases.
Mechanism of thermal electron attachment to O2 as studied by observing isotope effects of attachment rates for 18O2 systems
Authors: Shimamori Hiroshi , Hotta Hiroshi
Published: February 2003

Thermal electron attachment in pure 18O2 and its mixtures with H2, D2, N2, and CO2 has been studied at room temperature. The three-body attachment rate constants for pure 18O2, and its H2, D2, and CO2 mixtures are twice or more greater than those for the corresponding 16O2 systems, but slightly smaller than those expected from the Bloch–Bradbury mechanism. Analyses of the results suggest that the attachment in those systems mostly follow the Bloch–Bradbury mechanism but involve a partial contribution of the attachment to van der Waals molecules containing oxygen. The result for the 18O2–N2 mixture, in which the three-body rate constant increases only by a factor of 1.3 compared to that for the 16O2–N2 mixture, strongly suggests a significant role of the attachment to van der Waals molecule (O2⋅N2) in this case. The difference in the rate constant between the H2 and D2 mixtures may primarily be interpreted in terms of the difference in the collisional-stabilization probability between O−2*−H2 and O−2*−D2 collisions.
Product vibrational state distributions in thermal energy associative detachment reactions: F−+H,D → HF(v), DF(v)+e−
Authors: Smith Mark A. , Leone Stephen R.
Published: February 2003

Nascent product vibrational state distributions are obtained by the method of spectrally resolved infrared chemiluminescence for the associative detachment reactions: F−+H → HF(v≤5)+e−, ΔH=−238.3 kJ mol−1 and F−+D → DF(v≤7)+e−, ΔH=−245.3 kJ mol−1. These reactions are carried out under thermal energy conditions in a flowing afterglow. The nascent distribution for HF(v) is Nv=1=0.0+0.06−0.0, Nv=2=0.09±0.01, Nv=3=0.21±0.01, Nv=4=0.41±0.02, Nv=5=0.30±0.02 with an average fraction of energy deposited into vibration, 〈fv〉=0.72±0.03 and for DF(v): Nv=1=0.08+0.01−0.07, Nv=2=0.09±0.01, Nv=3=0.15±0.02, Nv=4=0.11±0.02, Nv=5=0.15±0.01, Nv=6=0.24±0.03, Nv=7=0.18±0.02 with 〈fv〉 ≤0.61±0.04. Simple kinematic effects based on angular momentum constraints are not able to explain the broader distribution observed for DF as compared to HF. Several possibilities for this difference are discussed. In an argon buffer, which is much less effective than helium for rotational relaxation, the DF emission exhibits highly nonthermal rotational excitation.
Ab initio potential curve for the b 3Σ− state of CH+
Authors: Saxon Roberta P. , Liu B.
Published: February 2003

An accurate potential energy curve has been obtained for the b 3Σ− state of CH+ by a convergent sequence of multiconfiguration self-consistent field and configuration interaction calculations. Spectroscopic constants derived from the calculated potential curve are in excellent agreement with experimental data. A long range potential barrier of 0.05 eV is predicted which results from charge-quadrupole repulsive interaction. The calculations show that, in an MCSCF+CI approach, it is most appropriate to view the b 3Σ− state of CH+ as a ground state C atom perturbed by a proton.
The convergence of the cluster model for the study of chemisorption: Be36H
Authors: Bagus Paul S. , Schaefer Henry F. , Bauschlicher Charles W.
Published: February 2003

A three layer cluster (with 14, 8, and 14 atoms comprising the three layers) has been used to model the chemisorption of atomic hydrogen on the (0001) surface of beryllium. Ab initio molecular electronic structure theory was employed at the self-consistent field level with a minimum basis set (Be, 1s, 2s, 2p, and H 1s functions). The properties of the Be36 cluster are discussed in some detail, as is the adsorption of H on each of four different high-symmetry surface sites. Significant differences are reported for chemisorption on this model surface relative to the results obtained previously with a two-layer Be22 cluster. Changes in chemisorptive bond energies range from 0.2 to 9.7 kcal, while BenH distances change by 0.01 to 0.04 Å in going from Be22 to Be36. It is concluded that some properties related to chemisorption converge reasonably rapid with respect to cluster size, e.g., bond distances, while others, e.g., dissociation energies, converge more slowly.
Structure of a binary mixture under shear: Non-Newtonian effects from computer simulation
Authors: Hanley H. J. M. , Evans D. J. , Hess S.
Published: February 2003

A binary equimolar dense fluid mixture is subjected to a shear. The orientational distribution of particles of type i around particles of type j (i, j=1, 2) and the distortion of the radial distribution function is discussed for planar Couette flow. Results are presented in terms of a mixture of soft spheres, for which one species differs substantially in size and mass from the other, simulated on the computer using the technique of shear nonequilbrium molecular dynamics. Transport coefficients, including those associated with normal pressure differences, are given for the mixture and for the species in the mixture. Non-Newtonian phenomena are observed.
Lattice dynamics of solid nitrogen with an ab initio intermolecular potential. II. Anharmonic librations in the α phase
Authors: Kuchta Bogdan , Luty Tadeusz
Published: February 2003

Using an intermolecular N2–N2 potential from ab initio calculations, we have performed calculations of frequencies for anharmonic librational modes at zero wave vector in the α phase of solid nitrogen. In the first step, the librations (Eg, Tg, Tg) were considered as uncoupled oscillators with frequencies calculated from an effective one-dimensional anharmonic potential being a section of the crystal potential surface along a direction specified by harmonic eigenvectors of a given mode. Within the approximation, very good agreement with the experimental frequency for the Eg mode has been found. Finally, the coupling between two Tg librational modes has been introduced and the calculated frequencies were found to be almost identical to the experimental ones.
Shock-induced changes in the electronic spectra of liquid CS2
Authors: Ogilvie Kendal M. , Duvall George E.
Published: February 2003

Experimental techniques are described for electronic time-resolved reflection and transmission spectroscopy in a thin liquid CS2 sample undergoing successive shocks by reverberation. Transmission spectra in the near UV and visible show progressive shifting of the red edges of absorption bands with shock pressure. At 55 kbar, extinction of the transmitted signal is complete for wavelengths between 2500 and 4100 Å, but absent at longer wavelengths. At 120 kbar extinction extends through the visible as well. Reflection experiments suggest that extinction is due to coherent scattering. Relaxation in both reflected and transmitted light at 120 kbar is in accord with other observations relating to shock-induced decomposition.
Electronic structure and photoionization cross sections of Li2, Na2, and K2
Authors: Davenport J. W. , Cosgrove G. J. , Zangwill A.
Published: February 2003

We have calculated the photoionization cross sections and oscillator strengths for the strongest bound–bound transitions of Li2, Na2, and K2 using multiple scattering theory. The three dimers strongly resemble the corresponding isolated atoms. There are, however, significant molecular effects. In Li2 the photoionization cross sections are split in a manner consistent with a cylindrical perturbation. There is no Cooper minimum in the photoionization cross section in analogy with the atom. In Na2 and K2 there is a very large enhancement of the cross section over the atomic one within a few volts of threshold. This enhancement has been observed experimentally. For K2 our cross sections are in good agreement with experiment while for Na2 they are too low. The Cooper minimum in Na2 is broadened considerably over atomic Na because the matrix elements for the σ and π channels pass through zero at different photon energies. We find no Cooper minimum in K2. We also do not find molecular shape resonances of the type found for N2 and CO. It is possible that atomic peaks observed in our cross sections may evolve into molecular resonances in the heavier dimers.
Interdependence of radiative and nonradiative decay rates of the lowest excited singlet state as revealed by the fluorescence of dinaphthylalkanes
Authors: Lim B. T. , Lin S. H. , Lim E. C.
Published: February 2003

It is shown, for dinaphthylalkanes, that the greater radiative rate constant of the lowest excited singlet state is accompanied by proportionally greater rate constant of the competing singlet → triplet intersystem crossing. The linear relationship betwen the two decay rates is rationalized in terms of the substituent-induced mixing of the lowest excited singlet state with higher energy singlet states, which possess (relative to the lowest excited singlet state) much greater transition moments to the ground electronic state, and much greater electronic matrix elements of radiationless transition to low-lying triplet states.
Emission spectra of CF3 radicals. III. Spectra and quenching of CF3 emission bands produced in the VUV photolyses of CF3Cl and CF3Br
Authors: Suto Masako , Washida Nobuaki , Akimoto Hajime , Nakamura Masatoshi
Published: February 2003

Fluorescence in the photolysis of CF3Cl and CF3Br was investigated using various vacuum ultraviolet light sources of atomic lines (Ar, Kr, H, and O). Both UV and visible emission bands of CF3 were observed as in the case of the photolysis of CF3H. Lower v′ levels of the emission bands were excited when CF3X was photolyzed by the lower energy incident light. In the case of CF3Cl, the emission bands were observed by the hydrogen Lyman alpha line, but no emission was observed by the krypton lamp with a CaF2 window. The threshold energy required to produce the UV and visible emission bands should be in the region 10.03–10.20 eV for CF3Cl. The energy separation between the fluorescing upper state and the ground state of CF3 is smaller than 6.52 eV. The emission bands were quenched by the addition of N2. Vibrational relaxation was predominant in the emission bands when He was added. Total intensity of the UV emission bands decreased and that of the visible emission bands increased by the addition of He. The results show that the UV and visible emission bands of CF3 would not originate from the same electronically excited state. Collision induced cross transfer from the state associated with the UV emission to that associated with the visible emission was observed.
Infrared–microwave double resonance spectroscopy of the antisymmetric amino wagging band of hydrazine in the 10 μm region
Authors: Jones Harold , Takami Michio
Published: February 2003

The antisymmetric amino wagging vibrational state of hydrazine in the 10 μm region was investigated by infrared–microwave double resonance. Nearly 200 microwave transitions were measured in the frequency range of 6–60 GHz using N2O and isotopic CO2 lasers. Among the microwave transitions 27 were assigned as those of the ground vibrational state and nine as those of the vibrationally excited state. About 20 infrared absorption lines were assigned based on the double resonance measurements. The excited state rotational constants, the internal rotation splittings, the inversion splittings, the Coriolis splittings, and the band origin were determined accurately.
Scattering of thermal He beams by crossed atomic and molecular beams. V. Anisotropic intermolecular potentials for He+CO2, N2O, C2N2
Authors: Parker Gregory A. , Keil Mark , Kuppermann Aron
Published: February 2003

Experimentally measured differential cross sections are presented for the interactions of He with the linear molecules CO2,N2O, and C2N2. These data show pronounced dampening of the diffraction oscillations, and provide an indication that the intermolecular potentials have large anisotropies. Indeed, central-field analyses of the experimental data, even in terms of very flexible models, are shown to be inadequate. However, an anisotropic analysis using the infinite-order sudden approximation yields physically reasonable intermolecular potentials, and provides excellent fits to the scattering data. Neither the spherical averages of these empirical anisotropic potentials, nor the spherically symmetric potentials obtained by the central-field analyses, reproduce the experimental differential cross sections. For He+CO2, the anisotropic potential is extended to highly repulsive interactions, to which the present data are insensitive, by combining theoretical calculations and measurements of gas-phase bulk properties. Estimates for the reliability of the fitted anisotropic potentials are given, and convenient parametrizations are recommended.
Diffusion theory and picosecond atom recombination
Authors: Langhoff C. A. , Moore Brian , DeMeuse Mark
Published: February 2003

We attempt to describe our picosecond iodine recombination data using simple diffusion theory. The physical picture is simple. Recombination of iodine atoms is the result of the diffusive motion of two iodine atoms, with such parameters as viscosity and temperature together with the diffusion theory governing the motion. The difference between most tests of diffusion theory and the present one is the extremely short time scale of our experimental data. We are thus close to, if not already in, the regime where the solvent structure is noncontinuous. In this regime the fundamental hypothesis of diffusion theory is invalid and one would not expect diffusion theory to be valid. We compare several different formulations of the diffusion equation solutions using different boundary conditions. The comparisons of each of these treatments with the experimental data fails. A discussion of the implications of this result, as well as some suggestions for improving the applicability of diffusion theory is presented.
Effects of intermolecular interaction on crystal structure of helical polymers
Authors: Saruyama Yasuo
Published: February 2003

Intermolecular interaction energy in a crystal composed of helical polymers was calculated analytically without specifying a material. It was found that the interaction energy is lowered when the molecular conformation is a commensurate helix (helical conformation with translational symmetry along the helix axis) with small translational period. Even a small change of the molecular conformation from the commensurate one, which is unimportant for intramolecular interaction, increases the intermolecular interaction energy abruptly. Because of this abrupt change of the intermolecular interaction energy, the actual molecular conformation in the crystal will not be what gives the lowest intramolecular interaction energy but what corresponds to one of the minimums of the intermolecular interaction energy. The obtained results assure translational periodicity of the crystal structure along the molecular axis; the translational periodicity is not directly required by the postulates of the molecular conformation presented by Natta and Corradini.
Hydrodynamic resistance and diffusion coefficients of segmentally flexible macromolecules with two subunits
Authors: Harvey Stephen C. , Mellado Pelayo , García de la Torre José
Published: February 2003

We present an algorithm for the calculation of the resistance coefficients and diffusion coefficients for the translational and rotational motions of a particle composed of two rigid subunits joined at a single point of articulation. By modeling each subunit as a collection of hydrodynamic resistance elements or beads, the method allows the full inclusion of all hydrodynamic interactions. The generalized coordinates have been chosen in such a way that it is easy to treat cases with seven, eight, or nine degrees of freedom (the subunits being joined at a hinge, a universal joint, or a swivel), as well as the intermediate cases where the internal motions are opposed by restoring forces, and analytic expressions are given for the resistance and diffusion coefficients for these cases. The examination of the dependence of the various coefficients on the choice of the origin of the coordinate system reveals that the rotational diffusion coefficients are independent of the location of the origin, so any convenient point (such as the point of articulation) can be used for the analysis of rotational diffusion. On the other hand, the calculated translational diffusion coefficient does depend on the choice of origin, and we discuss the still unresolved problem of how to find the origin that will give a value corresponding to the experimentally observed translational diffusion coefficient. One specific application of the method is examined in detail, that of a particle made up of three identical spheres with one sphere for each arm and the central sphere serving as the joint. The various rotational diffusion coefficients of the model particle are given, and it is seen that ignoring hydrodynamic interactions produces large effects in the calculated values. Calculations on this same model also demonstrate the sensitivity of the calculated translational diffusion coefficient both to the effects of ignoring hydrodynamic interactions and to the choice of coordinate system.
Dynamical properties of the structure I clathrate hydrate of xenona)
Authors: Tse John S. , Klein Michael L. , McDonald Ian R.
Published: February 2003

Dynamical properties of the structure I clathrate hydrate of xenon are studied theoretically by the method of molecular dynamics. Comparison is made with recent experimental far-infrared data.
Comment on ‘‘Few-atom 3He–4He mixed molecules’’
Authors: Harada M.
Published: February 2003

It is pointed out that a recent work by Nakaichi et al. on few-atom 3He–4He mixed molecules is based on incorrect values of Θ2/M3 and Θ2/M4, where M3 and M4 are the masses of the 3He and 4He atoms. This is shown to affect their conclusion on the 3He(4He)2 system seriously.
Laser intensity effects in the IR multiphoton dissociation of CF2HCl and CF2CFCl
Authors: Stephenson John C. , King David S.
Published: February 2003

CO2 laser pulses of 2, 10, and 50 ns duration, for which the temporal profile was approximately rectangular, were used in the multiphoton dissociation of low pressure CF2HCl and CF2CFCl. Probing a region of well-defined CO2 laser intensity, laser excited fluorescence determined the yield of CF2 formed in the v=0 and in the high vibrationally excited v2=5 (Evib=3335 cm−1) levels as a function of fluence (F) and intensity (I) over a factor of 100 variation. In the dissociation of CF2HCl by pulses of a given F, increasing I by a factor of 25 (50 vs 2 ns pulse) typically increased CF2(v=0) yield by factors of 8; this I dependence is probably due to power broadening of the discrete levels. The CF2(v=0) yield from CF2CFCl was almost independent of I over this range, which may reflect the coincidence of the 1079 cm−1 R(24) laser frequency with a CF2CFCl Q branch head at 1080 cm−1. The ratio of CF2(v2=5)/CF2(v=0), which is insensitive to discrete levels effects in the excitation process, increases with I for both reactants. This ratio may be expressed as a vibrational temperature, Tv for the CF2 fragments, and varied from about 1400 to 2600 K and from 900 to 1400 K for CF2CFCl and CF2HCl reactants, respectively, as I increased from 55 MW/cm2 to 3.3 GW/cm2 for the 50 ns laser pulses. Arguments are presented relating these results to the establishment of steady-state excitation conditions and to absorption cross sections in the continuum levels.
Empirical force field calculations for bridged annulenes. II. 1,6-ethano-8,13-methano- and 1,6:7,12-bismethano-‖14‖annulenes
Authors: Favini Giorgio , Simonetta Massimo , Todeschini Roberto
Published: February 2003

A study of some new bridged ‖14‖annulenes by empirical force field calculations is described. A comparison between the theoretical results obtained for the isolated molecules and few experimental results allows us to aim towards unambiguous interpretations. Some considerations are also performed on the aromaticity character of the studied ‖14‖annulenes.
Point-group selection rules for ΔS=2 multiplicity change: Application to catalysis and organo-transition metal reaction
Authors: Chiu Ying-Nan , Chow Chiu Lue-Yung
Published: February 2003

Electron spin–spin and second-order spin-orbit interaction operators are expanded as products of irreducible representations of symmetry point groups (Oh, Td, D5d, D6d, and C4v). From the transformation of the separated orbit and of the spin part, the selection rules for off-diagonal matrix elements may be deduced by taking direct products of the ‘‘initial’’ and ‘‘final’’ states. The special ΔMl selection rule for the orbital part of spin–spin interaction after expansion is also discussed. Emphasis is given to the ΔS=2 change connected by these operators. Possible examples of ΔS=2 change in d4, d5, and d6 configurations under the above mentioned point groups are given. As illustrations of the selection rules, the matrix elements for ΔS=2 and ΔMs=2 for these configurations are evaluated in the decoupled representation and given in terms of common parameters. The relevance of these multiplicity change to catalysis and reaction of organo-transition metal complexes is briefly alluded to.
Ultrasonic measurements and liquid structure of DMSO–water mixture
Authors: Madigosky W. M. , Warfield R. W.
Published: February 2003

The ultrasonic absorption of a 30% mole fraction mixture of dimethyl sulfoxide (DMSO) and water was measured at frequencies from 5 to 200 MHz by a pulse echo technique and over the temperature range −80 to 25 °C. At temperatures above −55 °C the observed absorption is greater than the classical value and is due to volume viscosity. To a very good approximation the ratio of the volume viscosity ηv to the shear viscosity ηs is 2/3. Below −55 °C a simultaneously occuring shear and volume relaxation is observed and the data in this region can be fit with a single relaxation time with an activation energy of 7.3 kcal/mol in agreement with that obtained for other hydrogen bonded liquids. No evidence of a distinct DMSO–water complex has been found.
Critical behavior of the Yukawa fluid in the mean spherical approximation
Authors: Cummings P. T. , Stell G.
Published: February 2003

Using the analytic solution of the mean spherical approximation (MSA) for a three-dimensional fluid of particles with hard-sphere cores plus pairwise attractive potentials of Yukawa form, we calculate some critical exponents characterizing the liquid–gas transition. From the compressibility equation of state, we find the expected three-dimensional spherical-model exponents (η=0, γ=2, δ=5); in contrast, classical exponents follow from the virial and energy equations of state. Our analysis illuminates the mechanism responsible for the quite different Percus–Yevick (PY) adhesive hard-sphere results found by others, and leads us to conclude that for the potential we consider here the PY exponents will be the same as those found here for the MSA. The results are all fully consistent with the general conclusions that emerged from earlier studies of Ornstein–Zernike systems by one of us.
Solvent dependence of apparent third-order hyperpolarizability in paranitroaniline
Authors: Meredith G. R. , Buchalter B.
Published: February 2003

Optical third harmonic generation has been used to characterize solutions of paranitroaniline (pNA) in five different solvents. Results were analyzed to extract an apparent third-order hyperpolarizability of pNA. Substantial variation of this quantity was observed, far exceeding the uncertainty of each solution determination. This unusual behavior was attributed to cascaded second-order interactions through a second harmonic local field. The analysis is in substantial agreement with electric field induced second harmonic (EFISH) generation characterization of the second-order hyperpolarizability of pNA when corrected via molecular orbital calculations for differences in dispersion and anisotropies. Solutions of 2-methyl-4-nitroaniline were also studied and the cascading model suggests that the increase of second-order nonlinearity over pNA is not generally as large as previous EFISH measurements suggest.
The quasichemical approximation for a lattice gas model with nonadditive lateral interactions
Authors: Milchev Andrey
Published: February 2003

The traditional assumption of a nonvariable energy bond between adatoms of a model lattice gas system is replaced with a physically more realistic one, namely, that the strength of an energy bond which links a certain atom with any of its nearest neighbors considerably depends upon the rest of the nearest neighbors being present (or absent) on their sites in the first coordination sphere of the atom. A method is proposed which enables one to take effectively into account the actually nonadditive character of the lateral forces within the framework of the quasichemical approximation (QCA) and requires a slight modification only of some conventional formulas for the thermodynamic functions. It is shown that the inclusion of nonadditivity in the lateral interactions between the atoms of a lattice gas system strongly influences the predictions of the theory with regards to the critical parameters of the system, although the qualitative behavior of the thermodynamic functions with the variation of temperature is not changed.