bluuebunny/crossref_metadata_2025 huggingface dataset:
Colloidal crystal photothermal dynamics
Authors: Kesavamoorthy R. , Jagannathan S. , Rundquist Paul A. , Asher Sanford A.
Published: July 2002

Localized heating of a colloidal crystal by absorption of high intensity laser radiation (>25 W/cm2 ) results in compression of the hot region. The time dependent variations in the lattice spacing of the crystal due to this local heating has been studied by monitoring the transmission profile of a weak laser beam incident on the crystal at an angle close to the Bragg angle. The dynamics of the compression phenomenon and its subsequent relaxation after removing the pump laser beam are analyzed using the screened Coulomb pair potential formulation. A simple linear model is developed to explain the deformation of the crystal during both the heating and cooling processes. Using this model we estimate the time dependent temperature rise in the crystal for any given pump power. During the heating process and the early stages of cooling the change in the crystal lattice spacing lags behind the rate of temperature change, while during the later stages of cooling it follows the rate of cooling. We demonstrate the necessity for the renormalization of the surface charge of colloidal particles when using the screened Coulomb pair potential.
Unusual photofragmentation dynamics in the multiphoton ionization of Cr(CO)6 /methanol van der Waals heteroclusters
Authors: Peifer William R. , Garvey James F.
Published: July 2002

Mixed van der Waals clusters containing Cr(CO)6 and methanol are generated in the free-jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of solvated Cr(CO)6 are strikingly different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula SnCr(CO)+x (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula SnCr(CO)+x (x=5,6), first appears in the mass spectrum at n+x=7.

We discuss two possible dynamical schemes for MPI of Cr(CO)6 /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)6 -containing clusters gives rise to solvated Cr(CO)+6 primary photoions, which subsequently photodissociate to yield the observed coordinatively unsaturated daughter fragments; and an alternative scheme, in which initial photodissociation in the neutral manifold gives rise to one or more coordinatively unsaturated primary photoproducts, each of which subsequently undergoes photoionization. We consider, in a qualitative fashion, the mass spectral fragmentation patterns predicted by these two alternative schemes under conditions of extremely high laser fluence (where essentially all one-photon processes are saturated), and compare these predictions with the observed MPI mass spectra following high-fluence irradiation at both 248 and 350 nm. Assuming that any secondary photodissociation in the ionic manifold occurs statistically, our high-fluence results are inconsistent with a dynamical scheme in which MPI precedes photodissociation.

We suggest that the dynamics are correctly described by initial photodissociation in the neutral manifold, followed by MPI of the coordinatively unsaturated primary photoproduct clusters. Intracluster stabilization of nascent photoion internal energy, either through evaporative cooling or collisional V–V energy transfer, is suggested to strongly influence subsequent metal–ligand bond dissociation.
Photodynamics of extended molecular systems. I. A theoretical approach for strongly coupled primary and secondary regions
Authors: Srivastava Deepak , Micha David A.
Published: July 2002

A general time-dependent quantum mechanical approach to the interaction of visible and UV light with extended polyatomic systems is presented. It is treated as a two-step process: a photon absorption excites electronic transitions in the target system, and this is followed by a dynamical evolution of the system on the excited potential energy surface. The time evolution in a large polyatomic system is treated within a molecular time-correlation function (TCF) approach. For a general two-surface electronic excitation problem, an extension of these molecular TCFs from real to complex times is introduced to facilitate their computation. Time-dependent self-consistent field (TDSCF) equations for transition amplitudes are derived from a variational approach and are used for large polyatomic systems to factor the molecular TCFs into primary and secondary region TCFs. The primary and secondary regions are modeled by considering a general primary motion coupled to many harmonic degrees of freedom in the secondary region. In the strong coupling case, the Hamiltonian has a general dependence on the variables of the primary region, whereas it has linear and bilinear terms in the variables of the secondary region. A weak coupling limit is obtained by dropping the bilinear terms in the coupling. The complex time propagators for the secondary region dynamics are constructed analytically for both cases. The present treatment provides a quantal version of the generalized Langevin equations of molecular dynamics. It allows for the derivation of a fluctuation–dissipation theorem similar to the classical one, by introducing a displaced Boltzmann distribution and identifying dissipation and fluctuation forces within the TDSCF approximation.
Planar diffusion and discrete jumps in pyramidic liquid crystals
Authors: Zamir S. , Luz Z. , Poupko R. , Alexander S. , Zimmermann H.
Published: July 2002

Deuterium nuclear magnetic resonance (NMR) spectra are presented for homologues of two series of pyramidic liquid crystals, viz., hexaalkyloxytribenzocyclononatriene, I-n, and hexaalkanoyloxytribenzocyclononatriene, II-n, (where n represents the number of carbon atoms per side chain). The homologues of series I-n exhibit a single hexagonal columnar mesophase PA, while those of series II-n exhibit one or two columnar mesophases (depending on n); a low-temperature biaxial phase PD, and a high-temperature hexagonal phase PC. NMR measurements were performed in the mesophase region on compounds specifically deuterated at the crown methylene groups of the tribenzocyclononatriene core. The spectra in both systems exhibit dynamic features typical of molecules reorienting about their C3 axes within the columnar structures. The experimental spectra were compared with simulated line shapes calculated on the basis of two reorientation mechanisms, (i) symmetric threefold jumps and (ii) planar diffusion about the molecular symmetry axis. It was found that the two compounds of series I-n studied, I-7 and I-10, exhibit spectral line shapes which fit best the planar diffusion model, as was found earlier for the I-8 homologue of this series, while the spectra of the three compounds of series II-n, II-13, II-14, and II-15 are only consistent with the symmetric threefold jump model. Possible reasons for the difference in behavior between the two series are discussed. In an Appendix the asymptotic method for calculating frequency-domain spectra is described.
Observation of far-wing absorption bands of the Ba 6s2 1S–5d6p 1P1 line broadened by rare gases
Authors: Ueda K. , Sotome H. , Sato Y.
Published: July 2002

We have determined reduced absorption coefficients on far wings of the Ba 6s2 1S0–5d6p 1P1 line broadened by He, Ne, Ar, Kr, and Xe. Using the unified Franck–Condon theory, we analyzed the blue satellite band observed for each of the He, Ne, and Ar perturbers and obtained the position and the height of the extremum of the difference potential-energy curve to be 6.5±1.5 A and 251±4 cm−1 for He, 7.6±2 A and 114±2 cm−1 for Ne, and 7.4±2 A and 70±2 cm−1 for Ar. We have also observed the collision-induced–dipole absorption bands associated with the Ba 6s2 1S0–6s7s 1S0 forbidden transition for all rare-gas perturbers.
Highly asymmetric electrolyte: Triplet correlation functions from simulation in one- and two-component model systems
Authors: Linse Per
Published: July 2002

The colloid triplet correlation function of a two-component asymmetric (charge asymmetry 1:20) electrolyte has been investigated by means of molecular dynamics simulation. The departure from Kirkwood’s superposition approximation is qualitatively the same as that found in hard-sphere and LJ model systems. The simpler one-component model with an effective pair potential underestimates the probability of finding triplets at small separations. In this region, the polarization of the small ions exerted by the triplets becomes nonadditive, that is, the polarization cannot be described as a sum of the polarization of three separate colloidal pairs. Thus, at these separations an effective pair potential can not fully describe the effective colloidal interaction.
Orbiting in high energy scattering
Authors: Sisak Michael , Tuzun Robert E. , Secrest Don
Published: July 2002

The behavior of an atom-rigid rotor scattering system is investigated for the case when much energy is in internal states and only a little is in translational motion. The l-average coupled and recoupled states approximations are shown to fail completely for inelastic collisions in this regime. These approximations are also shown to be very inaccurate for transitions between highly excited states and slightly excited states and vice versa. L-labeled coupled states cross sections for a transition are proven to be directly proportional to those for the reverse transition, a symmetry not shown by exact cross sections.
Product rotational state distribution in collinear-favored atom–diatom chemical reactions
Authors: Nakamura Masato
Published: July 2002

A simple formula for the product rotational state distribution in atom–diatom chemical reactions is proposed. The formula is derived from the simplified distorted-wave reactive cross section, previously obtained, by making an additional approximation that the reaction takes place predominantly in the collinear configuration. The expression is applicable when the collision energy is not far above the threshold energy and when the potential anisotropy is so weak that the effects of the rotationally inelastic half-collisions in both initial and final channels are relatively small. Test computations are made for some reactions and the usefulness of the method is illustrated. A simple functional form representing the profile of the product rotational distribution for the rotationally ground-state reactant, is found. Furthermore, there exists an approximate geometrical relation between the cross sections for the rotationally ground-state reactant and those for rotationally excited-state reactants.
Ionizing and nonionizing decays of superexcited acetylene molecules in the extreme-ultraviolet region
Authors: Ukai Masatoshi , Kameta Kosei , Chiba Ryo , Nagano Kazunori , Kouchi Noriyuki , Shinsaka Kyoji , Hatano Yoshihiko , Umemoto Hironobu , Ito Yoshiro , Tanaka Kenichiro
Published: July 2002

Absolute measurements of the photoionization and photodissociation cross sections of C2H2 have been made using continuum monochromatized synchrotron radiation in the 53–93 nm region. The absolute photoabsorption cross section and photoionization quantum yield of C2H2 have also been measured. The excitation spectra of C2(d 3Πg→a 3Πu), C2(C 1Πg→A 1Πu), CH(A 2Δ→X 2Π), and H(Lyman-α) fluorescence have also been obtained. The obtained results have been compared with theoretical calculations. An unresolved problem of the spectral interpretation concentrated on the σ * and π* shape resonances has been clarified by the straightforward demonstration of the photoionization quantum yield. The ionizing and nonionizing decay processes of the superexcited C2H2 molecules have been discussed in view of the strong competition of autoionization and neutral dissociation. An overlapping nature of Rydberg states with the shape resonance is found to be important.
A
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t
i
o self-consistent-field molecular orbital calculations on defects associated with radiation damage in alpha quartz
Authors: Sim Fiona , Catlow C. R. A. , Dupuis M. , Watts J. D.
Published: July 2002

A series of ab initio self-consistent-field molecular orbital (SCF-MO) calculations, on defect centers involved in radiation damage of α quartz, is reported. Calculations were performed on a 21 atom cluster (Si5O12−16), using a 3–21 G basis set. The long-range Coulomb interactions of α quartz were included, by surrounding the 21 atom cluster with 956 point charges, so that on geometry optimization, the characteristic features of α quartz were maintained. Various defect centers containing Al3+ substitutional ions, and their compensating electron-hole and proton defects were studied. The results obtained for the [AlO4]0 center are in close agreement with electron spin resonance (ESR) data. Calculations were also performed on the oxygen vacancy and related defects, such as the E′1 and E4 defects, where qualitative agreement with ESR results was obtained. An assessment is made, of the effect of including long-range Coulomb interactions, by comparison with other SCF-MO cluster models of these systems, which do not include these electrostatic effects.
Monte Carlo dynamics study of motions in c
i
s-unsaturated hydrocarbon chains
Authors: Levine Y. K. , Kolinski A. , Skolnick J.
Published: July 2002

A Monte Carlo dynamics study of the motions of hydrocarbon chains containing cis double bonds is presented. The simulations utilize the high-coordination {2 1 0} lattice for the simultaneous representation of the tetrahedrally bonded carbon atoms and the planar unsaturated segment. Results on single chains undergoing free motion in space and tethered to an impenetrable planar interface are reported. The introduction of a cis double bond into a hydrocarbon chain induces a slowdown in the dynamics. The simulations show this to be a universal result independent of the representation of the chain on the lattice. In contrast, polyunsaturated chains are found to be more mobile than saturated ones.
Mode-selective bond fission: Comparison between the photodissociation of HOD (0,0,1) and HOD (1,0,0)
Authors: Bar I. , Cohen Y. , David D. , Arusi-Parpar T. , Rosenwaks S. , Valentini James J.
Published: July 2002

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H or O–D stretching vibrational energy is described. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) (O–H stretch) or (1,0,0) (O–D stretch) vibrationally excited HOD. The OD and OH fragments are detected by laser-induced fluorescence. In the photodissociation of HOD (0,0,1), the yield of both fragments is enhanced [relative to HOD (0,0,0)], but the yield of OD is increased 2.5±0.5 times more than that of OH. In the photodissociation of HOD (1,0,0), no enhancement of the yield of the fragments is obtained. Our results show that even the very lowest possible level of vibrational excitation can be ‘‘leveraged’’ to effect selective bond breaking. Also, these results demonstrate that bond cleavage does not necessarily occur on the weakened bond and they agree with theoretical calculations indicating that the yield of OD and OH fragments depends on the Franck–Condon overlap of the vibrational wave function with the repulsive surface of the upper state.
External noise effects on quasimarginal relaxations in a bistable chemical system
Authors: Careta A. , Sagués F.
Published: July 2002

Relaxational processes close to a hysteresis limit in the bistable iodate–arsenous chemical system are analyzed under the effect of external fluctuations affecting the flow rate parameter. A dynamical characterization is proposed based on the use of the first-passage–time-distribution (FPTD) technique. Exact numerical results are presented showing the different relaxational behavior above and below marginality. Supplementary, we analytically obtain characteristic scaling laws for the first two moments of the FPTD in terms of a unified description valid on both sides of the hysteresis limit.
Flux analysis of electron and spin transfer processes. II
Authors: Ohta Katsuhisa
Published: July 2002

A flux analysis of electron transfer is carried out for a many-electron system to see the pathway of the transfer. Projection operators which contract variables in wave functions are introduced to separate the dominant dynamics of the transfer from its chemical background effects. A numerical example is given for allyl radical. Dynamic polarization and quantum whirlpool of σ electrons are induced by the transfer of π electrons.
Measurements and calculations of oscillations and phase relations in the driven gas-phase combustion of acetaldehyde
Authors: Tsujimoto Kim K. , Hjelmfelt Allen , Ross John
Published: July 2002

Oscillations in light emission and species concentrations, are measured as periodic perturbations are simultaneously applied to the input rates of acetaldehyde and oxygen in the gas-phase combustion of acetaldehyde in a continuous-flow stirred tank reactor for conditions where the autonomous reaction itself is oscillatory. The experimental results are compared with the predictions of a five-variable thermokinetic model. We measure periodic responses in the fundamental entrainment band (ratio of frequency of perturbation to frequency of response equal to unity) for four different values of phase shift between the acetaldehyde and oxygen perturbation wave forms as we vary the frequency and amplitude of the external periodic perturbations. Outside of the entrainment bands we find quasiperiodic response. We determine the phases of the light emission and six species concentrations, as measured with a mass spectrometer, with respect to the periodic perturbation, the variation of these phases across the fundamental entrainment band for different values of reactant phase shift and for different amplitudes of perturbation, and the effects of the phase shift between the two input perturbations on the light emission response of the system for different frequencies of perturbation. Both the experiments and calculations predict a widening of the entrainment band with an increase in perturbation amplitude, and the same variation in bandwidths for the four values of reactant phase shift studied. The experiments and calculations also predict the same general trends in light phase and species phases (difference between the light emission and species concentrations with respect to the perturbing wave form) as the band is traversed for different amplitudes of perturbation and for different values of reactant phase shift. Some discrepancies in the species phases exist; however, the similarities which exist between the experiments and the calculations provide further substantiation for the model. We find that changing the phase shift between reactant inflows from 0° to 360° produces a sharp change in light phase near 180°, in both the experiments and calculations.

In general, we also find that the light emission remains entrained near the total flux minimum of the perturbing wave form, and we attribute this correlation to the net heat loss which arises from the reactant flux through the reactor.
Scattering and recoiling analysis of oxygen adsorption site on the Ir{110}-c(2×2)-O surface
Authors: Bu H. , Shi M. , Boyd K. , Rabalais J. W.
Published: July 2002

The oxygen adsorption site on the Ir{110}-c(2×2)-O surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) using 4 keV Ne+ for backscattering and Ar+ for recoiling. The oxygen site was analyzed from scans of (i) backscattering intensity versus incident angle, (ii) oxygen recoil intensity versus incident and azimuthal angle, and (iii) oxygen recoil energy versus azimuthal angle. Calibrated shadow cones and trajectory simulations were used to obtain the site coordinates. This TOF-SARS data is contrasted with that of Ni{110}-p(2×1)-O, in which it is well established that the adsorption site is in the long-bridge position along the 〈001〉 rows. Adsorption of oxygen in the short-bridge sites above the 〈11̄0〉 Ir rows is the only model consistent with all of the experimental data and simulations. The O–Ir bond length is estimated to be ≊1.8 Å.
Photoinjection into electrolyte solutions. The role of thermalization
Authors: Rips Ilya , Urbakh M. I.
Published: July 2002

Photoinjection of low-energy electrons from a metal electrode into electrolyte solution is analyzed. Screening of the image potential during the emission and thermalization stages is neglected. Observed deviations from the Fowler law are associated with the energy dependence of the electron thermalization distance. Thermalization of injected electrons is treated assuming nonlocal energy loss to librations and phonons (the Fröhlich–Platzman model) and diffusive dynamics. The value of the exponent in the photoinjection law is predicted to be α=2.75 for Debye solvents. The measured photocurrent scales linearly with the Debye relaxation time of the solution. The theory describes isotope effect on photoinjection into aqueous electrolyte solutions.
Chain stretching in ordered microphases of diblock copolymers
Authors: Tang Hai , Freed Karl F.
Published: July 2002

The mean field theory of diblock copolymers is modified by partially lifting the Gaussian chain assumption. In the weak segregation regime, this new approach produces the experimentally observed significant chain stretching of microphase separated diblock copolymers [Almdal et al., Phys. Rev. Lett. 65, 1112 (1990)].
Rotational isomerism of molecules in condensed phases
Authors: Sakka Tetsuo , Iwasaki Matae , Ogata Yukio
Published: July 2002

A statistical mechanical model is developed for the description of the conformational distribution of organic molecules in the liquid and solid phases. In the model, they are assumed to have one internal freedom of rotation. The molecules are fixed to lattice sites and have two types of ordering, conformational and distributional. The latter is supposed to represent an ordering typical of solid state. The model is compared with the experimental results of the rotational-isomeric ratio of 1,2-dichloro-1,1-difluoroethane, in the temperature range from 77 to 300 K. It explains successfully the experimental results, especially the behavior near the melting point. From the point of view of melting, the present model is an extension of the Lennard-Jones and Devonshire model, because, when the distinctions between the two conformers are neglected, the parameter representing the distributional ordering of the molecules results in the same equation as that derived from the Lennard-Jones and Devonshire model.
A small-angle neutron scattering study of 2-butoxyethanol/D2O solutions
Authors: D’Arrigo G. , Teixeira J. , Giordano R. , Mallamace F.
Published: July 2002

Small-angle neutron scattering (SANS) measurements have been performed in heavy water solutions of 2-butoxyethanol as a function of concentration and temperature. The results indicate that there exist some alcohol aggregation above a narrow concentration range (x=0.015–0.02, where x is the alcohol molar fraction) and the size of the aggregates increases with temperature taking a maximum value at a composition close to the critical concentration. The analysis of the size and of the absolute scattered intensity at q=0(I0) in terms of a microscopic model of aggregation, as well as the indications from other experiments, suggest that these aggregates are micelle-like structures.
Submillimeter wave spectroscopy of XeH+ and XeD+
Authors: Peterson Kirk A. , Petrmichl Rudolph H. , McClain Robert L. , Woods R. Claude
Published: July 2002

The J=0–1 rotational transition of XeH+ and the J=1–2 transition of XeD+, which both occur near 390 GHz, have been studied by microwave absorption spectroscopy. For each of these all nine naturally occurring stable isotopes of xenon were detected. The magnetic hyperfine structure for the 129Xe forms and the electric quadrupole–magnetic hyperfine structure of both 131Xe forms were fully resolved and analyzed to determine the CI and eQq0 constants. The scaled spin–rotation parameter CI/(gIB) is found to be practically identical in XeH+ and HI, as it is also in the isoelectronic pair KrD+ –DBr/HBr. The available magnetic field was too small to resolve the rotational Zeeman effect, but the rotational g factor of XeH+ was estimated from Zeeman broadening. The mass-independent Dunham parameters U01, ΔH01, and ΔXe01 were determined from the very accurate microwave frequencies combined with higher order Dunham coefficients from published Fourier-transform infrared (FTIR) spectroscopy of XeH+ . Although it was not practical to observe XeH+ or XeD+ in excited vibrational states, we were able to detect the J=0–1 (v=1) transition of ArD+ .
Temperature dependence of the kinetics of the reaction BO+H2→HBO+H
Authors: Garland Nancy L. , Stanton C. T. , Nelson H. H. , Page Michael
Published: July 2002

We have measured the rate constant for the reaction of BO with H2 between 690 and 1030 K. The results are compared to transition-state-theory calculations consistent with a revised value for the heat of formation of the product HBO. By scaling the frequency of the doubly degenerate bend of the HHBO transition state in analogy with the isoelectronic HHCN transition state and lowering the calculated barrier from 9.5 to 8.5 kcal/mol, we are able to bring the theoretical and experimental rate constants into excellent agreement. Over the temperature range 300–3000 K, the theoretical rate constant is well represented by k(T)=7.49×10−23 cm3 s−1 T3.53 exp(−1590/T).
Conclusive evidence for site-specific C–H bond cleavage resulting from 248 nm photolysis of the ethyl radical
Authors: Brum Jeffrey L. , Deshmukh Subhash , Koplitz Brent
Published: July 2002

Experiments involving two photolysis lasers and one probe laser demonstrate that 248 nm excimer laser radiation will induce C–H bond cleavage preferentially at the β position in the ethyl radical. To facilitate carbon site labeling, selectively deuterated chloroethanes (ClCH2CD3 and ClCD2CH3) are used as precursor compounds. Two-photon ionization via resonance with the Lyman-α transition is used to detect H (or D) atoms. An initial 193 nm photolysis pulse serves to cleave the C–Cl bond in ClCH2CH3, while a second pulse at 248 nm dramatically enhances H-atom production. Experiments on ClCH2CD3 and ClCD2CH3 clearly show that this enhancement occurs preferentially through carbon–hydrogen bond cleavage at the β carbon site. It is apparent that 248 nm photon absorption by the ethyl radical is an important step in the overall mechanism.
A molecular-dynamics study of the velocity distribution for the reagent and product of thermally activated chemical reaction
Authors: Gorecki J.
Published: July 2002

A molecular-dynamics technique for hard spheres, generalized to model a chemical reaction, is applied to study the velocity distribution for a system with thermally activated reaction. The first four moments of the distribution of speed for reagent and for products have been calculated for the activation energies of the reaction in the range between 0 and 3kB T. The influence of observed effects on the rate of chemical reaction is discussed.
A simple analysis of the classical hard sphere one component plasma. I. Hole corrected Debye–Hückel theory
Authors: Penfold Robert , Nordholm Sture , Jönsson Bo , Woodward C. E.
Published: July 2002

The principal simplifying assumptions in the description of Coulomb fluids are the neglect of correlations within the ionic atmosphere and the disregard of ion size effects. In order to account for short range repulsions, a straightforward heuristic approach is considered where a Debye–Hückel charge density is augmented with a central hard sphere and the possibility of an electrostatic exclusion zone is included to prevent an unphysical negative contact density. This approach leads to an analysis describing the relative competition between hard sphere and electrostatic mechanisms, which undergoes a well-defined interchange in the (γ,η) parameter space. While the structural information is, by construction, limited, thermodynamic properties agree remarkably well with both the rescaled mean spherical approximation and Monte Carlo simulations, up to moderate densities.
Monte Carlo description of A
f homopolymerization: Diffusional effects
Authors: Gupta Anshu M. , Hendrickson Robert C. , Macosko Christopher W.
Published: July 2002

We have developed a new algorithm that simulates the structure buildup process during homopolymerization of Af monomers. The algorithm is an off lattice percolation solution on a cube with periodic boundary conditions. All monomers are treated as point particles. After choosing any two nodes at random, the probability of their reaction is computed and compared with a random number. We present results where the probability rule is a step function over the interunitary distance between the chosen pair. We reason that this simulates, in a simplistic but effective fashion, the importance of diffusion vs reaction times in the problem. Thus, the case where units react in immediate neighborhoods corresponds to a diffusion limited growth and where they react with equal probability with all other units to the mean field or kinetic limited growth. We present results for five step reactive potentials with careful analysis of finite size effects. We find that the gel conversion is delayed as a consequence of diffusion limitations. We find that cyclization is only partially responsible for this effect. We also find that small mobilities, such as might be available from rotational and translational degrees of freedom, give results realistically close to the mean-field results, thus explaining the experimental success of the mean-field theory. We find that the (effective) scaling exponent γ depends on the reactive radius and thus on diffusion. It is not entirely clear to us if this suggests nonuniversal behavior or is simply a crossover problem. We conclude that the dependence of the critical exponent on diffusion over a wide range of conversions suggests that some of the ‘‘nonuniversal’’ experimental observations might be due to diffusional effects.
Interchain dispersion and second hyperpolarizability of conjugated polymers
Authors: McWilliams P. C. M. , Soos Z. G.
Published: July 2002

π-electron dispersion forces between parallel conjugated strands are shown to alter χ(3) coefficients, even in the limit of nonoverlapping strands, through dipole processes involving virtual excitation and deexcitation of adjacent strands. The position and relative intensity of two- and three-photon resonances in the third-harmonic-generation (THG) spectrum depend both on the strength of interchain dispersion forces and on the lattice coordination z. The admixture of even-parity states above the one-photon gap Eg with biexcitons on adjacent strands shifts the two-photon resonance to lower energy and reduces its amplitude, while the amplitude of the three-photon resonance at Eg/3 increases with z due to new biexcitonic pathways. THG spectra based on exact solutions to Pariser–Parr–Pople (PPP) models of polyenes are reproduced by a dimer model for each strand. Interchain dispersion in the lattice of dimers leads to an exciton problem whose solution explicitly gives the dependence on interchain dispersion, intrachain correlations, and lattice coordination. Interchain dispersion in polyacetylene, with z=6 neighboring strands in van der Waals contact, accounts qualitatively for its THG spectrum, while two-photon processes in conjugated polymers whose backbones are separated by bulky side groups are in accord with single-strand PPP results.
Direct Monte Carlo simulation of chemical reaction systems: Simple bimolecular reactions
Authors: Piersall Shannon D. , Anderson James B.
Published: July 2002

In applications to several simple reaction systems we have explored a ‘‘direct simulation’’ method for predicting and understanding the behavior of gas phase chemical reaction systems. This Monte Carlo method, originated by Bird, has been found remarkably successful in treating a number of difficult problems in rarefied dynamics. Extension to chemical reactions offers a powerful tool for treating reaction systems with nonthermal distributions, with coupled gas-dynamic and reaction effects, with emission and adsorption of radiation, and with many other effects difficult to treat in any other way. The usual differential equations of chemical kinetics are eliminated. For a bimolecular reaction of the type A+B→C+D with a rate sufficiently low to allow a continued thermal equilibrium of reactants we find that direct simulation reproduces the expected second order kinetics. Simulations for a range of temperatures yield the activation energies expected for the reaction models specified. For faster reactions under conditions leading to a depletion of energetic reactant species, the expected slowing of reaction rates and departures from equilibrium distributions are observed. The minimum sample sizes required for adequate simulations are as low as 1000 molecules for these cases. The calculations are found to be simple and straightforward for the homogeneous systems considered. Although computation requirements may be excessively high for very slow reactions, they are reasonably low for fast reactions, for which nonequilibrium effects are most important.
Absolute measurements of photoabsorption cross sections, photoionization cross sections, and photoionization quantum yields of silane in the 13–40 eV region
Authors: Kameta Kosei , Ukai Masatoshi , Chiba Ryo , Nagano Kazunori , Kouchi Noriyuki , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

The absolute values of the photoabsorption cross section of silane have been measured in the energy region between 13 and 40 eV. The photoionization quantum yields have also been measured in the 13–22 eV region. Using these values, we have evaluated the absolute photoionization cross sections and the total cross sections for nonionizing processes, i.e., neutral fragmentations. The photoabsorption cross sections show a broad peak at around 14.6 eV and a vibrational structure between 16 and 18 eV. These structures are superimposed on the spectra of direct transitions to the ionization continuum. The decaying features of the superexcited states corresponding to these two bands are shown to be very different from each other. Corresponding to the broad absorption peak located at around 14.6 eV, considerable deviations from unity in the photoionization quantum yields are observed. This means that the superexcited states at this energy decay through fast neutral fragmentation rather than autoionization. On the other hand, the superexcited states, which appear as the vibrational structure between 16 and 18 eV, decay predominantly through autoionization over the neutral fragmentation.
Infrared spectrum of t-butyl hydroperoxide excited to the 4νOH vibrational overtone level
Authors: Fleming P. R. , Rizzo T. R.
Published: July 2002

The infrared predissociation spectrum of t-butyl hydroperoxide excited to the 4νOH level reveals extensive mixing between the zeroth-order OH stretch state and nearly isoenergetic zeroth-order dark states. Because most of these dark states have an OH stretch quantum number of zero, the predissociation spectrum strongly resembles the infrared spectrum of an unexcited molecule. The observed intensity distribution in the predissociation spectrum is what one would expect if the eigenstates prepared by 4νOH vibrational overtone excitation were statistical mixtures of all the nearly isoenergetic zeroth-order states.
Penning ionization of (CH3)4C and (CH3)3CCl by collision with He*(2 3S) metastable atoms
Authors: Takami Tomohide , Mitsuke Koichiro , Ohno Koichi
Published: July 2002

Penning ionization of (CH3)4C and (CH3)3CCl upon collision with He*(2 3S) metastable atoms were studied by angular and collision-energy resolved Penning ionization electron spectroscopy. The observed Penning ionization electron spectra (PIES) revealed peculiar features for the totally symmetric ionized state of the target molecules which can be ascribed to ionization from a molecular orbital having carbon 2s atomic orbital characters; (1) the relative PIES intensity is extremely enhanced, (2) the intensity distribution of the vibrational fine structure in PIES is clearly different from that observed for ultraviolet photoelectron spectra, (3) the angular distribution of Penning electrons is highly isotropic, and (4) the collision energy dependence of the partial Penning ionization cross section is slightly increasing and rather flattened with respect to the increase of the collision energy. These features were found to be due to the formation of an excimer-like state involving C2s hole characters in the target molecule which induces Auger-like transitions selectively from orbitals having the C2s characters. Effects of attractive interaction potentials were observed for (CH3)3CCl on the ionized state ascribed to the Cl nonbonding orbital.
Temperature dependencies of ternary ion–molecule association reactions yielding N+3, N+4, and (CO)+2
Authors: Guthrie J. A. , Chaney R. C. , Cunningham A. J.
Published: July 2002

The temperature dependencies of four ternary positive ion–molecule association reactions yielding the products N+3, N+4, and (CO)+2 have been studied using a high pressure pulsed source. The pressure range 0.5 to 4.0 Torr and the temperature range 220 to 500 K were examined. When the experiments were conducted in a pure gas, the third-order rate coefficients were found to follow an overall T−n dependence with 2.1≤n≤2.4. For the case of N+4 formation in a mixture of helium and nitrogen, a power law dependence was not found. The measured pressure and temperature dependencies indicate that the low-pressure limit for all of the reactions extends to neutral number densities as high as 1017 cm−3 throughout the experimental temperature range. The measurements support the existence of a temperature dependence in the stabilization of the intermediate complex for both molecular and atomic third bodies. The results are discussed in light of current models of the ternary association process.
Orbital exponent optimization with the analytical gradient method for molecular self-consistent-field wave functions
Authors: Hashimoto Kenro , Osamura Yoshihiro
Published: July 2002

We have applied the energy gradient technique to optimize the orbital exponents of primitive Gaussian-type functions for some simple molecules including first-row elements, starting from the exponent values of the Huzinaga–Dunning basis functions. It is found that the change of the exponents clearly shows the better description of chemical bonds compared to the atomic exponents, while the energy gain due to the exponent optimization is very small. We can, however, confirm that the values of the orbital exponents optimized in atoms give extremely good description of the molecular wave functions. The scaling factor for a hydrogen atom in a molecular environment and the effect of the polarization functions for a hydrogen atom are also discussed.
Application of the shifted large-N method to the rovibrational spectroscopy of diatomic molecules
Authors: Bag M. , Panja M. M. , Dutt R. , Varshni Y. P.
Published: July 2002

Computations of energy levels, expectation values of various powers of internuclear separation, and Franck–Condon factors for molecular potentials have been carried out within the framework of shifted large-N technique. In contrast to other perturbative as well as semiclassical calculations, we obtain compact analytic expressions for these quantities. For illustration, our predicted results for perturbed Kratzer potential are compared with those derived by numerical integration. The method gives quite accurate results for a wide range of rovibrational quantum numbers. Further possible applications, in particular, to Morse oscillators are finally discussed.
Phase equilibria in simple fluid mixtures
Authors: Malescio G.
Published: July 2002

The phase stability of a Lennard-Jones binary mixture is investigated near to the critical point of the less volatile component by making use of the hypernetted-chain integral equation. This makes possible to determine the occurrence of different types of phase equilibria depending on the potential parameters value.
N2 chemisorption on Ni(111). An infrared investigation under steady-state conditions
Authors: Yoshinobu Jun , Zenobi Renato , Xu Jiazhan , Xu Zhi , Yates John T.
Published: July 2002

The adsorption of N2 on Ni(111) at 89–115 K under steady-state (quasiequilibrium) conditions has been studied using Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS) and low-energy electron diffraction (LEED). At very low coverage, a single N 3/4 N absorption band is observed at 2218 cm−1 which is assigned to a singleton N2 adspecies. With increasing N2 pressure, two other bands develop at 2212–2208 and 2204–2203 cm−1, and a faint ( 7/8 × 7/8 )R30° LEED pattern is observed. At even higher pressure, a sharp ( 7/8 × 7/8 )R30° LEED pattern develops together with a relatively broad single band at 2194 cm−1. The enthalpy of adsorption of N2 on Ni(111) is estimated to be −8.4 kcal/mol at zero coverage from isothermal IRAS measurements. Slight attractive interactions between N2 adspecies are detected. The origins of the vibrational features which are related to island formation will be discussed in detail.
The metal-ligand binding energies for Sr(H2O)+n
Authors: Sodupe M. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

Based on accurate ab initio calculations, we estimate the successive binding energies of one and two waters to Sr+ to be 25±3 and 23±3 kcal/mol. These are about 10 kcal/mol smaller than the experimental values. Since both experiment and theory are expected to be highly accurate, an alternative determination would be desirable.
Vaporization of thallium (III) oxide and thallium activities in thallium superconductors
Authors: Wahlbeck P. G. , Richards R. R. , Myers D. L.
Published: July 2002

Vaporization reactions for both Tl2O3(s) and a Tl superconductor sample, primarily Tl2Ba2Ca2Cu3Oy(s) (Tl-2223), were determined. The vapor species were examined by Knudsen effusion mass spectrometry at temperatures in the range 657 to 773 K for Tl2O3(s) and 672 to 836 K for Tl-2223. Additional experiments with thermogravimetric analysis (TGA) equipment were performed. In both cases of vaporization of Tl2O3 and Tl-2223, the vapor species were concluded to be Tl2O(g) and O2(g). Vapor pressures were measured. For Tl2O3(s), ΔvapH0 (298.15 K) was measured as 320.4±6 kJ/mol. The activity of Tl2O3 in the Tl-2223 superconductor was measured as 0.0044±0.001 at 750 K. A measure of the stabilization of Tl2O3 in the Tl-2223 superconductor is given by μ(Tl2O3,sc)−μ(Tl2O3,pure)=−33.84±3.0 kJ/mol.
Calculation of the Si–H bond energies for the monohydride phase of Si(100)
Authors: Nachtigall Petr , Jordan Kenneth D. , Janda Kenneth C.
Published: July 2002

Ab initio calculations are carried out on Si9H12, Si9H13, and Si9H14 clusters, chosen to model the Si(100)–(2×1) reconstructed surface and its hydrides. A value of 56 kcal/mol is obtained for the energy of the recombinative hydrogen desorption. The energies required to remove the first and second H atoms from a doubly-occupied site are 81 and 76 kcal/mol, respectively.
Raman study on molecular motion in tetramethylammonium hydroxide pentahydrate
Authors: Kanesaka Isao , Iwaki Makoto , Uezawa Chikayo , Kuriyama Nobuhiro , Sakai Tetsuo , Miyamura Hiroshi , Ishikawa Hiroshi
Published: July 2002

The Raman spectrum as well as conductivity and differential-thermal-analysis (DTA) thermogram of tetramethylammonium hydroxide pentahydrate was observed at various temperatures in order to investigate molecular motion in relation to conductivity and a phase transition. It was clarified that different stepwise protonation processes become effective for relaxation in the O–H stretching of hydroxide ions as temperature is increased. The activation energy was 6.1 and 16.0 kJ/mol in 130≤T<210 and 210≤T≤250 K, respectively, whereas the activation energy in conductivity was 41.1 kJ/mol below 250 K. This discrepancy was discussed according to a reorientational motion of water molecules. The Raman band shape of the NM4 asymmetric stretching of tetramethylammonium ions was analyzed on the basis of the reorientational motion about the C3 and C2 axes on the ion, and the activation energies were 8.7 kJ/mol (about the C3 axis) and 14.5 kJ/mol (about the C2 axis).
Lattice dynamics of a pair-potential model of α-quartz
Authors: Cowley E. R. , Gross J.
Published: July 2002

A pair potential for SiO2 proposed by Tsuneyuki, Tsukada, Aoki, and Matsui [Phys. Rev. Lett. 61, 869 (1988)] is used to calculate a range of lattice dynamical properties of α-quartz. The potential energy of the structure is minimized, and the phonon frequencies are calculated for the structural parameters corresponding to the energy minimum. Frequencies calculated throughout the Brillouin zone are used to construct a frequency distribution function and the harmonic contributions to the thermodynamic functions are calculated. The coefficients of thermal expansion perpendicular and parallel to the c axis are calculated in a perturbative approximation. The expansion coefficient parallel to the c axis is found to be negative at low temperatures, in agreement with experiment. The elastic, dielectric, and piezoelectric constants are also calculated.
Mathematical analysis of phase rule for systems with electrostatic energy
Authors: Lampinen Markku J. , Vuorisalo Jari , Pursiheimo Ulla
Published: July 2002

The aim of the research has been a mathematical analysis of the phase rule and its role in solving the problem of minimizing the total free energy of the system. A special equilibrium problem is studied, where, in addition to the normal potential functions of the phases, the electrostatic energy of the system is taken into account. The electrostatic energy function is not homogeneous of the first degree with respect to the variables and therefore the phase rule becomes slightly modified. An equation for the maximum number of coexisting phases for the equilibrium problem of this kind is derived and it is illustrated by simple examples.
Complicated and dissociative dynamics for the weakly forced and weakly damped Morse oscillator
Authors: Permann Delmar , Hamilton Ian
Published: July 2002

We examine the dynamics of a Morse oscillator which is weakly forced and weakly damped with initial conditions close to the dissociation energy. We calculate higher derivatives of the phase of the Morse oscillator with respect to time and use these to construct higher-order phase plots. We use the higher-order phase plots to visualize the higher harmonics of the phase which are essential for complicated and dissociative dynamics. We focus on topological changes in the higher-order phase-space structures and show that the higher-order phase plots provide information regarding the dissociation process that is not easily obtained from the usual phase plot.
Semiclassical model for electronic-to-vibrational energy transfer in collisions of Hg(63P2) with H2, D2, and HD
Authors: El Afghani A. , Bras N.
Published: July 2002

A semiclassical treatment of the 3P1←3P2 fine-structure transition induced by collisions with diatomic molecules Q≡H2, HD, and D2, is developed, based on the earlier work of Hickman. The model illustrates how the increase of the fine-structure energy splitting provides a new mechanism during collision. This mechanism is due to an additional region of nonadiabatic coupling caused by the anisotropy of the molecule due to its vibration. Actual calculations are reported for collisions of Hg(63P2) with Q in the thermal energy range, using the available HgH2 energy surfaces of this system. The cross sections calculated are in good agreement with the experimental ones in the H2 and HD cases. The agreement is not so good for D2, which shows that the model has to be improved, probably, by including the molecular rotation.