bluuebunny/crossref_metadata_2025 huggingface dataset:
An investigation of the electronic and optical properties of dehydrated sodalite fully doped with Na
Authors: Blake Nick P. , Srdanov Vojislav I. , Stucky Galen D. , Metiu Horia
Published: July 2002

Prolonged exposure of colorless dry sodalite to alkali vapor causes the material to gradually turn blue, dark blue, and finally black. The blue color observed at low sodium uptake appears because the absorbed sodium atoms are spontaneously ionized. The electron produced by ionization is shared by the four sodium ions present in the sodalite cage (three initially there and the fourth originating from the absorbed atom). The color center created in this way is represented by the formula (Na+)4eF3−. Here, e stands for the electron and F3− for the negatively charged frame surrounding a zeolite cage. At the highest loading, when each cage contains an absorbed alkali atom, the color centers are arranged in a body-centered cubic lattice, allowing the electrons associated with the centers to form bands. This may explain the black color observed at high concentration. In this paper we present measurements of the absorption coefficient of the black sodalite for photon energies between 0 and 3 eV, and interpret them by performing one-electron band structure calculations for a fully loaded compound. These calculations deal only with the ‘‘solvated’’ electrons. The effect of the other electrons is taken into account through an empirical potential energy representing the interaction of a solvated electron with the zeolite frame. Because of this we study only the bands formed by the electrons of the color centers. Since the gap in the electron energy bands of the dry sodalite is over 6 eV, the color of the black sodalite is controlled by the solvated-electron bands formed in this gap. The measured spectrum has a threshold of about 0.6 eV which seems to suggest that the system has a gap in the electronic structure and is therefore a semiconductor. The calculations indicate, however, that, if the one-electron picture is valid, the fully doped black sodalite is a narrow-band metal. The threshold in the spectrum appears because the transition matrix element is zero for transitions responsible for photon absorption, and not because of a gap in the density of states. The calculated spectrum is in reasonable agreement with the measured one. Conclusions based on one-electron calculations can be altered by electron–electron interactions, which could turn a metal into an insulator. Two simple criteria, proposed by Mott and Hubbard, were used to test whether this transition might occur in our system. Unfortunately the results indicate that the system is close to the transition region which means that predictions made by these simple criteria are not reliable.
Surface phase behavior in binary polymer mixtures. I. Miscibility, phase coexistence, and interactions in polyolefin blends
Authors: Scheffold Frank , Eiser Erika , Budkowski Andrzej , Steiner Ullrich , Klein Jacob , Fetters Lewis J.
Published: July 2002

We have used composition depth profiling of polymer bilayers, based on nuclear reaction analysis, to determine miscibility, phase coexistence, and critical temperatures in mixtures of random olefinic copolymers of mean composition E1−x/EEx; here E is the ethylene group −(C4H8)−, EE is the ethylethylene group −[C2H3(C2H5)]−, and one of the copolymers is partially deuterated. The components in each binary mixture have different values x1,x2 of the EE fraction. Using a simple Flory–Huggins mixing model, our results enable us to extract an interaction parameter of the form χ(x1,x2,T)=A(x1,x2)/T, where for given x1,x2, A is a constant. Calculated binodals using this form fit our measured coexistence curves well, while allowing χ a weak composition dependence improves the fit further. Within the range of our parameters, our results suggest that in such binary polyolefin mixtures the interaction parameter increases roughly linearly with the extent of chemical mismatch expressed as the difference in degree of ethyl branching between the two components.
Calculation of dissociative attachment of electrons to diatomic molecules by the Schwinger–Lanczos approach
Authors: Horáček J. , Gemperle F. , Meyer H.-D.
Published: July 2002

Numerical studies of resonant scattering of electrons by diatomic molecules with full account of the nonlocal level shift and resonance width operators are carried out with emphasis on the various approximations of the nonlocal potentials. The Schwinger–Lanczos approach proposed recently by Meyer, Horáček and Cederbaum [Phys. Rev. A 43, 3587 (1991)] is applied and its performance is investigated. The efficiency of the method is further improved by introducing a new local complex potential. Very accurate values of the dissociative attachment cross sections for a d-wave resonance model are reported.
Virtual angular momentum: The dynamics of inverting molecules in an electrostatic hexapole field
Authors: Cho Victoria A. , Pipes Leonard C. , Kim Dae Young , Brandstater Nathan , Lester Brian W. , Torres Elva A. , Baugh Delroy A.
Published: July 2002

The semiclassical dynamics of a molecule which undergoes tunneling inversion while traversing an electrostatic hexapole field is shown to be described by the motion of a two dimensional harmonic oscillator with angular momentum Lv. Analytical expressions are provided which show explicitly the dependence of Lv on the molecular inversion frequency. A comparison between calculated and experimental focusing curves is also presented.
Temperature determination of transient species by degenerate four wave mixing: Application of the independently determined power law of the transition dipole moment and geometric factors
Authors: Lehr L. , Motzkus M. , Pichler G. , Kompa K. L. , Hering P.
Published: July 2002

We investigated the dependence of the degenerate four wave mixing (DFWM) signal intensities on the electronic transition dipole moment of isolated lines in the A 1Σ+←X 1Σ+ transition band of NaH. We applied a new method to determine this dependence in transient species without previous knowledge of the sample temperature. By using different appropriate pairs of DFWM lines sharing common lower level, the relative population difference is eliminated. We found that this ratio is well described by a power law (μ1/μ2)x where μi is the electronic one-photon transition dipole moment. As a result of saturation the exponent x depends on the total laser energy deposited in the medium. The observations are in good agreement with a modified two-level model, which includes the effects of polarization of the laser beams used in the experimental setup by introducing geometric factors. The exponent of the integrated signal intensities varies between 3 and 8 for high and low laser intensities with a rapid decrease to high intensities. This makes it possible to compare signal intensities at different DFWM laser intensities in order to obtain relative level populations, i.e., temperature, with high sensitivity.
Preferential adsorption on porous glass near the liquid/liquid coexistence curve of a binary liquid mixture
Authors: Grüll H. , Woermann D.
Published: July 2002

A differential refractometric method is used to study the temperature and composition dependence of preferential adsorption of water on the surface of pore-controlled glass out of 2-butoxyethanol (abbreviated C4E1)/water mixtures. The experiments are carried out close to the liquid/liquid coexistence curve in the vicinity of the lower critical point of the system. When the temperature of phase separation Tp of the mixture is approached at (almost) constant bulk composition from temperatures below Tp the amount of substance of adsorbed water ñsw per unit mass of adsorbent extrapolated to Tp increases along the liquid/liquid coexistence curve with increasing values of x° (x°, mole fraction of C4E1 of the mixtures before adding the absorbent). In experiments with mixtures of composition x°≳xc the quantity ñsw is about 10 times larger than that found in experiments with mixtures of compositions x°<xc (xc, critical composition). The data are interpreted by presuming that the extent of the preferential adsorption of water and its temperature and composition dependence is primarily caused by the presence of −SiOH and −SiO− groups at the hydrophilic surface of the adsorbent [and H+(aq) counter ions] in combination with concentration fluctuations occurring in a wide range of temperatures and compositions along the binodal curve.
Solvent–solute reaction path curvature effects on energy transfer corrections to the solute reaction rate
Authors: Reese Sabine K. , Tucker Susan C.
Published: July 2002

We present a new rate theory which accounts for anharmonicities (nonlinearities) in the solute potential (force) over the complete range of solvent damping. The theory is based on a new method for calculating energy diffusion rates which incorporates anharmonicity-induced solvent–solute reaction path curvature and is thus valid throughout the intermediate to large damping regimes. This energy diffusion factor is combined with the microcanonical variational transition state theory spatial diffusion correction factor. The new theory is applied to the case of a cubic solute potential coupled to a long time scale bath and shown to be significantly more reliable than the turnover theory of Pollak, Grabert, and Hänggi [J. Chem. Phys. 91, 4073 (1989)] in the Kramers turnover regime.
Wide-band and narrow-band approximations for donor–bridge–acceptor systems
Authors: Amos A. T. , Burrows B. L. , Davison S. G.
Published: July 2002

The techniques of wide-band and narrow-band approximations, which have been used in solid state theory, are applied to the theory of donor–bridge–acceptor models which are commonly used to represent molecular switches or wires. The approximations lead to analytical solutions which closely match the numerical calculations. These approximations are based on one of the simplest one-electron models where the interaction is time independent and the bridge band is completely initially empty. The form of the analytical solutions and the stability of the numerical calculations are discussed.
A quantitative theory of linear chain polymer dynamics in the melt. III. Dependence of quantities on bead location along the chain contour
Authors: Lorenz K. Thomas , Herman Michael F.
Published: July 2002

The dependence of the bead mean squared displacement on the position of the bead along the chain backbone is considered within the lateral motion model for the dynamics of linear chain polymer melts. The bead position dependence has been ignored in the previous development of this theory. In the lateral motion model, the effective bead friction coefficient increases as the bead mean squared displacement increases, due to the greater interchain correlations that result because of the noncrossability of the chain backbones. In this work, a position dependent model is considered for this bead friction coefficient. The resulting equations of motion for the chain have the form of a generalized Rouse model with a position and time dependent bead friction coefficient. These equations are solved numerically. It is found that the time dependence of the center bead mean squared displacement has the same form as predicted by the simpler theory, in which the dependence of quantities on the position of the bead along the chain backbone is ignored. The scaling of the terminal time and the center of mass diffusion constant on chain length are also found to be unchanged by the inclusions of the bead position dependence of the friction coefficient. The mean squared displacement, averaged over all beads in the chain, shows a stronger time dependence than the same quantity for the center bead. The predictions are in excellent agreement with the results from previous numerical simulations.
Infrared profile of single molecules adsorbed on ionic substrates: Relaxational mechanism for the homogeneous linewidth
Authors: Lakhlifi A. , Girardet C.
Published: July 2002

The homogeneous line shape of the infrared spectrum of a single molecule physisorbed on ionic substrates is determined on the basis of a renormalization technique of the total system Hamiltonian. The absorption profile can be written in terms of a limited cumulant series expansion of the dynamical couplings between the optical system formed by the vibration–orientation modes of the molecule, and the bath modes characterized by molecular translations and substrate phonons. The various causes of homogeneous broadening of the lines are studied, implying phase and population relaxations through intramolecular transfers. In this latter case, two situations are considered depending on whether the internal vibration modes of the admolecule are localized or resonant (quasi-resonant) with the substrate phonon modes. These situations are usual for small polyatomic molecules adsorbed on substrates with extended optical phonon bands. An application to the calculation of the Q*, P* and R* branch widths for CO, CO2 and NH3 molecules adsorbed on NaCl(001) and/or MgO(001) is extensively discussed. We show that, at low temperature T≲ 20 K, the homogeneous width is quite negligible and inhomogeneity must be invoked to interpret the measured width. By contrast, for increasing T values (T≳ 50 K), homogeneous broadening increases and contributes significantly (about to 50%) to the measured width for CO and CO2 molecules adsorbed on clean substrates.
Behavior of longitudinal spin orders in NMR measurements of self-diffusion coefficients using radiofrequency field gradients
Authors: Mutzenhardt Pierre , Canet Daniel
Published: July 2002

Weakly coupled spin 1/2 systems of the type AX and AX2 can be easily brought into states of longitudinal order represented (in terms of operator products) by 2IzAIzX and 4IzAIzXIzX′, respectively. It proves possible to follow the evolution of these states under translational diffusion processes through a simple experiment involving two identical radiofrequency field gradient pulses separated by an interval allowing diffusion to take place. Relevant theory and experimental verifications are provided. The main conclusion is that, for a part of the decay, the effective gradient is multiplied by a factor of two in the case of a two spin order (2IzAIzX) and a factor of three in the case of a three spin order (4IzAIzXIzX′). These properties can be employed for accessing to self-diffusion coefficients, smaller by factors of four and nine respectively, as compared with those measured by conventional methods.
Nuclear magnetic resonance field-cycling proton relaxation study of polymer dispersed liquid crystals
Authors: Schwarze-Haller D. , Noack F. , Vilfan M. , Crawford G. P.
Published: July 2002

The frequency and temperature dependence of the longitudinal proton relaxation time T1 has been studied in liquid crystal droplets embedded in a solid polymer matrix in the nematic and isotropic phase over a broad Larmor frequency range (500 Hz≤νL≤84 MHz) employing the fast-field-cycling technique. The comparison of the droplet data, bulk 5CB data, and the pure polymer data show that T1 is dominated by the cross relaxation at the liquid crystal–polymer interface in the entire frequency range. In the low frequency range (νL≤1 MHz), an additional relaxation process determines T1 in both phases, namely reorientations mediated by translational replacements in the nematic phase and the exchange relaxation in the isotropic phase. The analysis of the cross relaxation rate k reveals that the simplified model of Vilfan is only applicable in the nematic phase and leads to an anchoring time τAS of the molecules at the surface which is ≊1.3×10−4 s. This model, however, cannot be applied in the isotropic phase, but must be extended by the spin diffusion time τP, which in this case determines the cross relaxation rate. The anchoring time τAS, which in the isotropic phase corresponds to the correlation time of the additional process, is much shorter than in the nematic phase, namely τAS≊10−6 s.
Molecular dynamics simulation for the cluster formation process of Lennard-Jones particles: Magic numbers and characteristic features
Authors: Ikeshoji Tamio , Hafskjold Bjo/rn , Hashi Yuichi , Kawazoe Yoshiyuki
Published: July 2002

Cluster formation of Lennard-Jones particles (65 536 atoms in a unit cell with an overall number density equal to 0.0149) was simulated by molecular dynamics. The temperature was set to decrease linearly with time by various thermostats, starting from a gas state temperature and ending at zero temperature. With the Nosé–Hoover thermostat, it was found that the translational temperature of the clusters suddenly decreased almost to zero when the cluster formation drastically increased around a reduced temperature (T*) of 0.5, while the internal temperature decreased linearly. Using the Andersen thermostat, which could simulate the aggregation of particles in an inert gas, both the internal and translational temperatures decreased almost linearly with time. When these thermostats were used, cluster–cluster and cluster–atom collisions did not give any magic number peaks in the size distribution up to 250 atoms/cluster at any temperature. Careful tracing of the cluster growth of 13-atom clusters showed no difference in reactivity between icosahedral and nonicosahedral clusters. To simulate cooling in a supersonic jet, a thermostat which controlled only the translational temperature was introduced. After the clusters were formed by cooling the system with this thermostat, their internal temperature stayed at T*≊0.5, while the translational temperature decreased linearly to zero with time as it was controlled. A long-time evaporation from these high-temperature clusters gave peaks at 13 and 19 (and less significantly at 23 and 26) which are magic number sizes corresponding to single, double, triple, and quadruple icosahedra, respectively. The internal temperatures of 13- and 19-atom clusters were higher than those of other size clusters. Higher evaporation energy was observed for the clusters of 13, 19, 23, and 26 atoms than for other size clusters after the long-time evaporation, but only the 13-atom clusters had the higher evaporation energy after cooling by the Andersen thermostat. These results suggest that magic number clusters were formed by evaporation to be trapped at the magic number sizes, and not by either cluster–atom or cluster–cluster collisions. Analyses of the radial distribution functions and the overall shapes of the generated solidlike clusters consisting of many isomers revealed the following characteristic features: The clusters around 13 and 26 atoms were close to being spherical, and the clusters around 19 atoms were oblate. Clusters around 13 atoms had an icosahedron-based structure. The clusters around 55 atoms formed by the Nosé–Hoover and the Andersen thermostats were close to spherical and had an ordered structure. Clusters from 30 to 50 atoms had a disordered structure or a mixture of the different series of structures.
Quantum chemical study of carbon monoxide adsorption at the MgO(100) surface
Authors: Jug Karl , Geudtner Gerald
Published: July 2002

Semiempirical molecular orbital calculations are performed to study the physisorption of carbon monoxide molecules at the MgO (100) surface. This surface is simulated by clusters of different surface and layer sizes. Besides the submonolayer coverage four different types of monolayers are investigated. The stability and structure of the adsorbate system are determined. An adsorption energy per unit cell of the substrate is defined for a comparison of the stability of different monolayers with varying coverages. Agreement is found between the theoretical and experimental overlayer structure.
A multidimensional semiclassical method for treating tunneling in molecular collisions
Authors: Guo Yin , Thompson Donald L.
Published: July 2002

A semiclassical method for treating tunneling in molecular collisions is presented. The procedure incorporates tunneling into standard classical trajectory simulations. The tunneling rate is computed by calculating the JWKB tunneling probabilities along predefined tunneling paths at the classical turning points which are determined by classical trajectory calculations. The method is illustrated for the collinear H+H2 atom-exchange reaction on the Porter–Karplus surface. The calculated reaction probabilities are in good agreement with the quantum-mechanical values, and are in fact as good as the results obtained from more sophisticated semiclassical treatments such as the S-matrix theory and the instanton model. The method can readily be extended to higher dimensions.
Control of transition state spectra: Enhancement of diffuse structure in the photodissociation spectrum of CO2
Authors: Sadeghi R. , Skodje Rex T.
Published: July 2002

The photodissociation spectrum for collinear CO2 is assigned using the spectral quantization method. The diffuse structure in the spectrum is dramatically enhanced through the use of a newly developed control algorithm. An initial state is variationally designed to suppress the broad direct background, which usually dominates the spectrum, while it simultaneously enhances the intensity of the resonance peaks. All the diffuse structure can be attributed to resonance states trapped near the saddle point. Using time-independent wave functions, which are extracted by Fourier transforming the wave packet at the frequencies of the spectral peaks, the spectrum is assigned to an assortment of Feshbach and barrier resonances built on the asymmetric and symmetric stretch modes. The influence of periodic orbits on the diffuse structure is also assessed.
Recovery of broad and weak electron spin echo envelope modulation (ESEEM) lines in disordered systems with mixing-frequency ESEEM method
Authors: Song R.
Published: July 2002

The mixing-frequency electron spin echo envelope modulation (MIF-ESEEM) spectroscopy method has been used to recover broad and weak lines in disordered systems by means of choice of an appropriate increment ratio γ. A γ-irradiated powder alanine sample was used to demonstrate the advantage of this new scheme, in which two new ESEEM lines were observed.
Dissociation dynamics of Na+n in collision with rare-gas atoms
Authors: Nonose Shinji , Tanaka Hideki , Mizuno Tomoyuki , Kim Nam Jun , Someda Kiyohiko , Kondow Tamotsu
Published: July 2002

Dissociation dynamics of a sodium cluster ion, Na+n (n=2–9 and 11), in collision with a rare gas atom (He or Ne) was investigated by measuring the absolute cross sections for the production of fragmented ions by using a tandem mass-spectrometer equipped with several octapole ion guides. The mass spectra of the fragmented ions show that release of Na and/or Na2 from Na+n occurs dominantly. The absolute total cross section for the dissociation of Na+n and the absolute partial cross sections for the Na and/or the Na2 release were determined at different collision energies and cluster sizes. The absolute total dissociation cross sections were calculated by a scheme that collisionally excited Na+n dissociates with leaving Na and Na2 unimolecularly. On the other hand, the partial cross sections for the Na and the Na2 release were successfully explained by the orbital correlation diagram for the dissociation system; the dissociation channel involving an adiabatic transition was found to be influenced significantly by the collision energy and the cluster size.
Representing molecular shape and interactions: A reduced intermolecular potential for copper phthalocyanine
Authors: Liu Da-Jiang , Blumberg Selinger Robin L. , Weeks John D.
Published: July 2002

The potential energy between molecules is often represented as a sum of pairwise additive potentials for all atom pairs in both molecules. Such atomistic potentials encode much physical and chemical information, and in particular give an accurate representation of the molecular shape. However, the number of terms in the sum for a pair of molecules goes as N2 where N is the number of atoms in a molecule, and thus grows rapidly inefficient for large N. Starting with an atomistic pairwise additive potential for Copper Phthalocyanine (CuPc), a planar tile-shaped molecule with 57 atoms, we determine a simpler reduced intermolecular potential consisting of a sum of effective pair interactions between 13 appropriately chosen ‘‘interaction sites’’ on each molecule. This potential reproduces many qualitative features of the full atomistic potential model for CuPc including the very anisotropic molecular shape, but is much easier to evaluate numerically, requiring only 1% as much computation time as the full atomistic potential. Crystal structures of CuPc using both the atomistic and reduced potentials are determined and compared, and a discussion of diffusion barriers is given. Some of the general issues and physical considerations that arise when attempting this reduction are discussed along with other possible applications of these ideas.
Density functional studies of aluminum phosphide cluster structures
Authors: Tomasulo Antonietta , Ramakrishna Mushti V.
Published: July 2002

We have carried out a systematic search for the lowest energy structures of (AlP)N (N⩽6) clusters using density functional theory within the local density approximation. We employ a novel search strategy based on genetic algorithm to find the global minima of these clusters. This study reveals that (AlP)N alloy cluster geometries are significantly different from the Si2N cluster structures, even though AlP and Si2 are isoelectronic and the corresponding bulk materials have similar lattice and band structures. We also observed that phosphorous–phosphorous bonding is favored in small clusters (N⩽3), while in larger clusters phosphorous atoms cap the triangular faces consisting of aluminum atoms. The ground state structures are determined primarily by the need to minimize the electrostatic repulsion between diffuse lone pair electrons on phosphorous atoms. Finally, we observed that large clusters undergo buckling type relaxation dynamics similar to the way the ideal (110) plane of bulk AlP surface reconstructs. The cluster structures are also similar to the reconstructed (110) surface of bulk AlP. These findings lead to the conclusion that AlP clusters with N⩾4 are nanoscale fragments of the bulk surface.
Internal rotation of methyl group in o- and m-toluidine cations as studied by pulsed field ionization–zero kinetic energy spectroscopy
Authors: Ikoma Hideyuki , Takazawa Ken , Emura Yuji , Ikeda Shigeru , Abe Haruo , Hayashi Hisaharu , Fujii Masaaki
Published: July 2002

Internal rotational levels of the methyl group in o- and m-toluidine cations have been observed by pulsed field ionization–zero kinetic energy photoelectron spectroscopy. Level energies and transition intensities were reproduced by a one-dimensional rotor model with a free-rotor basis set, and the potential curves of the internal rotation in the cations have been determined. Analysis for m-toluidine shows a drastic increase of the barrier height for internal rotational motion from the neutral to the corresponding cation. On the other hand, the barrier in o-toluidine slightly decreases by ionization. The mechanism of the change in barrier height will be discussed in terms of geometrical and electrical change by ionization. It is suggested that the drastic change of internal rotational motion is mainly determined by the electronic structure.
A three-dimensional quantum mechanical study of the NH+NO reactions
Authors: Szichman Henrik , Baer Michael
Published: July 2002

In this article is described a three-dimensional quantum mechanical study within the nonreactive infinite order sudden approximation (IOSA) of the title system. The study was performed using a recently introduced global potential energy surface [J. Chem. Phys. 102, 6696 (1995)]. Integral total cross sections for the two separate products, namely, N2O+H and N2+OH, were calculated as a function of kinetic energy in the range 0.05–0.50 eV. Our main findings are (a) the overall cross sections and the cross sections for N2O+H are only mildly dependent on the energy; (b) the cross sections for N2+OH, in conrast to those for N2O+H, depend on the energy and increase as the energy increases; (c) the yield of N2O+H is about 80–90 % of the total yield, in accordance with experiment; (d) the overall cross sections are about 1 to 3 times smaller than the quasiclassical-trajectory ones and about 5 to 15 times smaller than the experimental ones.
Self-diffusion of potassium on ultra-thin epitaxial potassium layers
Authors: Fuhrmann D. , Hulpke E.
Published: July 2002

The self-diffusion of potassium on potassium layers supported by a Ni(001) substrate was studied using quasielastic helium atom scattering. From the broadening of the quasielastic peak in the energy transfer spectra as a function of surface temperature the activation energy for surface diffusion was determined to be 63 ±15 meV. This value is of the same order as that found in the diffusion study of Na on Cu(001) using the same technique. The way in which the energy broadening depends on the transfer of parallel momentum suggests a jump diffusion mechanism.
Optimized cluster theory of polymer blends: General formulation and application to isotopic polyethylene mixtures
Authors: Melenkevitz J. , Curro J. G.
Published: July 2002

A theory for the thermodynamic properties of binary polymer blends was developed and applied to predict the compositional dependence of the effective chi parameter χS in model isotopic polyethylene mixtures. The theory is a generalization of the optimized cluster theory for single component molecular fluids. Starting from exact cluster expansions for the Helmholtz free energy and pair-correlation functions in a molecular mixture developed by Chandler et al., a series of transformations was performed to yield approximate expressions for these quantities. The resulting theory was demonstrated to be equivalent to a set of diagrammatically proper integral equations. For model isotopic polyethylene blends, we calculated the various partial structure factors in the mixture and used them to predict the compositional dependence of χS. The calculated values of χS were found to be in reasonable agreement with recent neutron-scattering measurements but the theory underestimates the compositional dependence relative to the experiment. Deviations from random mixing that extend well beyond monomeric length scales were predicted in this blend system.
Experimental determination of a longitudinal phonon dispersion curve in a quasi-two-dimensional system
Authors: Graham A. P. , Bertino M. F. , Hofmann F. , Toennies J. P. , Wöll Ch.
Published: July 2002

The structural and dynamical properties of a monolayer of Xe atoms adsorbed on a Cu(001) surface have been studied with high-resolution He-atom scattering. Angular distributions reveal a phase-transition from the high-temperature in commensurate phase to a new, more densely packed, phase upon cooling below 65 K. For both phases the strongly dispersive longitudinal phonon mode could be detected. A lattice-dynamical analysis yields a substrate mediated softening of the Xe–Xe radial force constant down to 25% of that obtained from gas-phase potentials and a fit of Xe bulk phonon data. This decrease is much larger than assumed in previous work for noble gas atoms adsorbed on metals.
Stochastic simulation of chemically activated unimolecular reactions
Authors: Vereecken Luc , Huyberechts Guido , Peeters Jozef
Published: July 2002

The kinetic master equation for the title processes can be formulated as a traditional deterministic set of coupled differential reaction-rate equations, or, alternatively, as a stochastic process in which each reaction is a random–walk transition in energy–species space. This stochastic description is the basis for three methods we describe here to numerically solve the kinetic master equation for chemically activated unimolecular reactions. The first method allows the calculation of the complete time evolution within a given mechanism, and is based on Gillespie’s exact stochastic method (ESM). It is essentially a Monte Carlo simulation of the stochastic reaction processes. The second method allows for the direct calculation of the steady-state product distribution (DCPD). It describes the random walk within the framework of a discrete time Markov chain, and reduces the calculation of the steady-state product distribution to a fairly simple matrix algebra problem. The third method calculates the steady-state population of the intermediates (CSSPI), reformulating the solution of the master equation as an eigenvector problem generated by the description as a continuous time Markov chain. To our knowledge, the DCPD method has not been described before. Also, this is the first time that a CSSPI model is used explicitly in this type of calculation. The three methods are illustrated using the simple H+HNCO reaction, important in the RAPRENOx mechanism for NOx removal from flue gases.
F
19
spin-rotation constants and shielding tensor of sulphur difluoride from its microwave spectrum
Authors: Gatehouse Bethany , Müller Holger S. P. , Gerry Michael C. L.
Published: July 2002

The microwave spectrum of the unstable molecule sulphur difluoride, SF2, prepared via a pulsed electric discharge in an SF6/OCS mixture, has been measured using a pulsed jet cavity Fourier transform spectrometer. F19 hyperfine structure has been resolved, and unusually small, negative values have been determined for two of the spin-rotation coupling constants. These constants have been used to calculate the F19 principal inertial axis nuclear shielding components; the determined paramagnetic shieldings are also unusual and produce a large positive average shielding, σav. The shieldings have been compared with the corresponding constants obtained for OF2, SiF2, and GeF2. The results for SF2 are consistent with values determined from earlier NMR experiments and ab initio calculations. The origin of the unusual shieldings has been found, and is discussed, using a semi-quantitative model, in terms of the geometry and the electronic energy level structure of the molecule.
Matrix-theoretical analysis in the Laplace domain for the time lags and mean first passage times for reaction-diffusion transport
Authors: Chen Jenn-Shing , Chang Wen-Yih
Published: July 2002

Siegel’s matrix analysis of membrane transport in the Laplace domain [J. Phys. Chem. 95, 2556 (1991)], which is restricted to zero initial distribution, has been extended to including the case of nonzero initial distribution. This extension leads to a more general transport equation with Siegel’s results as a special case. The new transport equation allows us to formulate the mean-first-passage time t̄ for various boundary conditions, if the initial distribution is stipulated to be of the Dirac delta-function type; and the steady-state permeability P and time lag tL, if zero initial distribution is employed. Based on this matrix analysis we also propose an algorithm for quick and effective numerical computations of P, tL, and t̄. Examples are given to demonstrate the application of this algorithm, and the numerical results are compared with the theoretical ones. The validity of the transport equation is also checked by a Green’s function.
Density functional study of polypropylene and its submolecules
Authors: Borrmann A. , Montanari B. , Jones R. O.
Published: July 2002

Previous density functional (DF) calculations of the structures of small hydrocarbon molecules (CH, CH2, C2H2, C2H4, C3H6, C3H8) have been extended to larger systems. The structure of a single chain of isotactic polypropylene (it-PP) has been optimized, and calculations have been performed for smaller molecules with closely related structures (propane C3H8, several conformers of isopentane C5H12, and 2,4,6-trimethyl heptane C10H22) using both local spin density (LSD) and non-local (gradient-corrected) energy functionals. The pronounced transferability of the local structural parameters between C5H12, C10H22 and it-PP suggests that the energy surfaces calculated for molecules with five carbon atoms should provide a very good representation of the local energy variations in much larger systems.
Mode excitation dynamics in the fragmentation of Ar3+: An helicity decoupling study
Authors: Buonomo E. , Gianturco F. A. , de Lara-Castells M. P. , Delgado-Barrio G. , Miret-Artés S. , Villarreal P.
Published: July 2002

A full quantum study of the fragmentation dynamics for argon trimer ions is carried out using a previously computed potential energy surface (PES). The initial and final internal states which are being considered are the bending–stretching states of the trimer and the rotovibrational states of the residual dimer ion. The treatment employs the helicity decoupling approximation over a broad range of total angular momentum values (J) and analyses the final distribution of rotational–vibrational states as a function of the metastable states of the J values and of the “tumbling” (helicity) quantum number |Ω|. It is found that at least two different effects, one mainly dynamically related to the centrifugal barrier and another to the orientational nature of the coupling potential field, can be discerned to explain the fragmentation results.
Electric field and ion concentration effects on the production of zero-kinetic energy states of benzene: A unified mechanism
Authors: Held Andrew , Baranov Leonid Ya. , Selzle Heinrich L. , Schlag Edward W.
Published: July 2002

Zero-kinetic energy states are highly stabilized Rydberg states that reside below each ionic eigenstate of a molecular system. The high resolution detection of these states constitutes a high resolution spectroscopy for molecular ions. Zero-kinetic energy states are produced from optically pumped Rydberg states. Such Rydberg states, once produced, are found to either decay in the presence of fields or be converted to zero-kinetic energy states in the presence of ions. A simple mechanism is proposed based on these experimental results for the range of fields and ion concentrations investigated. The experiments were performed with pulsed and static electric fields of different magnitudes at various ion concentrations. The results reconcile many previously reported observations and suggestions concerning the differences between molecular and atomic (and/or small molecule) zero-kinetic energy states and lead to a unified picture for the production of zero-kinetic energy states.
On modified Brownian motion and polymers in external fields
Authors: Sebastian K. L.
Published: July 2002

We consider the modification of Brownian motion recently introduced by Cherayil and Biswas [J. Chem. Phys. 99, 9230 (1993)] and investigated further by Washington [J. Chem. Phys. 105, 9324 (1996)]. We evaluate the path integrals for motion under the external influences exactly, obtaining fully analytical expressions.
Surfactant induced Marangoni motion of a droplet into an external liquid medium
Authors: Haidara Hamidou , Vonna Laurent , Schultz Jacques
Published: July 2002

We report in this paper a particular case of the Marangoni effect—the migration of a squalane droplet in the external water medium, driven by the asymmetric adsorption of the surfactant at one side of the droplet. The specific features of this experiment compared to similar reports on drop motion lies in both the presence of the external liquid and the existence of two “discrete” profile and velocity regimes during the migration of the drop. The amazing dynamic profile of the droplet (cometlike) observed in the earlier stage of the motion was related to the strong transient pinning of the rear side of the drop where adsorption sets in. This strong retention preceding the drop motion was related to the external hydrostatic pressure of the environmental fluid which, under the particular geometry of nonsymmetric diffusion, results in a higher contact angle hysteresis (compared to that in air), before the drop can move. As shown by these results, the whole macroscopic behavior of this drop migration, including the pinning strength, the magnitude and velocity of the motion is governed by the viscosity, the external hydrostatic pressure and the pressure within the thin wetting film on the rear side of the drop. Using a simplified hydrodynamic approach, an estimated average velocity of ∼1.5 mm/s was found, which compares well to measured ones, especially in the earlier stage of the motion where the drop retains the comet like profile. In addition, some preliminary results are briefly presented, which cleary show how both the velocity and amplitude of the migration, as well as the characteristics of the thin film on the rear side depend on the drop size and viscosity.
Surface reactivity of MgO oxygen vacancies: electrostatic mechanisms in the formation of O2− and CO− species
Authors: Ferrari Anna Maria , Pacchioni Gianfranco
Published: July 2002

The interaction of O2 and CO gas-phase molecules with oxygen vacancies on the MgO (100) surface has been studied by means of cluster models and ab initio wave functions. It is found that the surface oxygen vacancies, or Fs centers, exhibit a high reactivity toward O2 and CO at variance with the regular MgO surface. The reaction proceeds through the formation of radical anions, O2− and CO−, via the transfer of one electron trapped in the surface cavity to the empty levels of the adsorbed molecule. The resulting surface complexes, X−/Fs+ or X−/Fs2+ (X=O2 or CO), are bound by electrostatic forces. Although the mechanism of the interaction is the same for the two molecules, the details of the energetics are different. O2 spontaneously removes the electrons trapped in the MgO oxygen vacancies to form the stable O2− superoxide anion. On the contrary, CO− forms only at finite temperatures and is a metastable species. The different behavior can be rationalized in terms of electron affinities of the two molecules. The calculations are useful also for the spectroscopic characterization of the radical anions at the surface. The calculations of electron paramagnetic resonance (EPR) hyperfine coupling constants and, for CO, of the vibrational frequencies indicate that the experimental spectra are consistent with the existence of O2− and CO− surface species. The analysis of the vibrational shifts shows that the coordination mode of CO is C-down and not O-down.
Cs
+
reactive scattering from a Si(111) surface adsorbed with water
Authors: Yang M. C. , Hwang C. H. , Kang H.
Published: July 2002

Reactive scattering of hyperthermal Cs ion is examined from a Si(111) surface adsorbed with water. Collision of Cs+ beams with the Si surface at the energy of 10–100 eV produces Cs+-bound cluster ions as scattering products, including CsOH+, CsOH2+, CsSi+, CsSiHn+(n=1,2), and CsSiO+. The yields for these clusters are examined as a function of Cs+ beam energy and water exposure. Kinetic energy distributions for the clusters are measured. The reactive scattering process is explained in terms of collision-induced desorption of adsorbate, followed by ion–molecule association between the scattered Cs+ and the desorbed molecule. The probability that Cs+ undergoes reactive scattering is 5×10−4–2×10−3 for 50 eV collision energy. The corresponding probability for the Cs+–molecule association reaction is in the order of 5×10−3–2×10−2 or slightly lower. It is proposed that CsOH+ and CsOH2+ are formed from OH and H2O adsorbates, respectively, via direct collisional desorption. CsSiO+ formation can be related to desorption of surface oxide species or, at high energy, to collisional dissociation of adsorbates. Several aspects of using Cs+ reactive scattering for surface adsorbate detection are discussed.
Mean field theory of interpenetrating polymer network gels
Authors: Schulz Michael , Frisch Harry L.
Published: July 2002

Some important unique phenomena are encountered in gels of interpenetrating polymer networks (IPN) which arise from the coupling between phase transition, elasticity, and mutual entanglements and interaction of the underlying subnetworks. The relevant order parameters of IPN–gels are obtained as the local composition ratio fluctuations, the characteristic length of microphase separation and the volume fraction of the liquid. On the basis of a preaveraged Landau–Ginzburg theory, both microphase separation and volume phase transition are considered. There exists a common critical point at which the IPN–gel becomes critical for both types of phase transitions.
Vibrational structure in atomic emission spectra: Hg atoms in molecular matrices
Authors: Crépin C. , Tramer A.
Published: July 2002

Mercury atoms excited to the P13 state in rare gas and molecular (CH4, CD4, CF4, and SF6) matrices relax by a direct P13→S01 fluorescence with a ca. 60 ns lifetime or by the P13⇝P03 radiationless transition followed by the P03→S01 long-lived (τ in the 10 to 500 ms range) emission. The main difference between atomic and molecular hosts is the appearance, in the latter case, of a well resolved vibrational structure in the spectrum of the P03→S01 transition involving the frequencies of nontotally symmetric modes of the host molecules. This structure, related to a shortening of the lifetime of the P03→S01 emission is explained by the intensity borrowing from the allowed P13→S01 transition, i.e., by the P13↔P03 mixing induced by nontotally symmetric vibrations of neighbor molecules (analog of the Herzberg–Teller effect). Relative intensities of bands corresponding to different vibrational modes may be deduced from estimation of electric fields induced in the Hg atom by displacements of electric charges of the vibrating molecule
Ultrafast vibrational relaxation of diatomic chalcogen hydrides in alkali halides
Authors: Mungan C. E. , Happek U. , McWhirter J. T. , Sievers A. J.
Published: July 2002

The energy relaxation times of the fundamental stretching modes in the electronic ground state of OH−, OD−, SH−, and TeH− in a variety of alkali halides are measured by incoherent laser saturation and found to vary from 0.3 to 3 ns at 1.7 K. These vibrational lifetimes are between 4 and 8 orders of magnitude smaller than those of other heteronuclear diatomics diluted in crystals, including the ionic systems of CN− in salts and the neutral deuterides, DCl and ND, and oxides, CO and NO, in rare-gas matrices. Unlike these other systems, the chalcogen-hydride-doped alkali halides have a librational mode at frequencies well above the top of the host phonon band. This makes the librational decay channel a lower order process than relaxation into phonons. An energy gap law can be fit to the data, in which the relaxation times vary exponentially with the number of accepting reorientational modes. This model can explain the fact that OH− and OD− in KCl have nearly the same lifetimes, since the vibrational and librational frequencies both have the same isotope shift. Furthermore, previous persistent spectral hole burning measurements of SH− in mixed crystals are consistent with a picture in which the defects reorient during vibrational de-excitation. It is found that the reorientational decay rates are much faster than the equal-decay-order relaxation of CN− into translational modes in the silver and sodium halides. This could be explained by a factor of ∼3 enhancement in the vibrational coupling constant to reorientations as compared to translations, but the relative strengths of the appropriate sidebands do not appear to support such an enhancement. The relaxation times of the diatomic hydrides are also found to be much smaller in ionic than in van der Waals hosts, even for equal order reorientational relaxation, suggesting that Coulombic forces significantly increase the V–R coupling strength.