bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of electrically and chemically induced swelling in polyelectrolyte gels
Authors: Grimshaw P. E. , Nussbaum J. H. , Grodzinsky A. J. , Yarmush M. L.
Published: July 2002

Controlled swelling and shrinking of polyelectrolyte gels is useful for regulating the transport of solutes into, out of, and through these materials. A macroscopic continuum model is presented to predict the kinetics of swelling in polyelectrolyte gel membranes induced by augmentation of electrostatic swelling forces arising from membrane fixed charge groups. The model accounts for ionic transport within the membrane, electrodiffusion phenomena, dissociation of membrane charge groups, intramembrane fluid flow, and mechanical deformation of the membrane matrix. Model predictions are compared with measurements of chemically and electrically induced swelling and shrinking in crosslinked polymethacrylic acid (PMAA) membranes. Large, reversible changes in PMAA membrane hydration were observed after changing the bath pH or by applying an electric field to modify the intramembrane ionic environment and fixed charge density. A relatively slow swelling process and more rapid shrinking for both chemical and electrical modulation of the intramembrane pH are observed. The model indicates that retardation of membrane swelling is dominated by diffusion-limited reaction of H+ ions with membrane charge groups, and that the more rapid shrinking is limited primarily by mechanical processes.
Quenching of O(1S) studied by a pulsed vacuum-ultraviolet-resonance lamp
Authors: Sondermann F. , Stuhl F.
Published: July 2002

Rate constants for the quenching of metastable O(1S) by a number of collision partners, in particular hydrocarbons, were determined at room temperature. Care was taken to obtain data with high precision and to disturb the system as little as possible by the pulsed vuv photolytic generation of O(1S). Comparison with data for the other O atom states and isoelectronic NH states reveals little similarity.
A reinterpretation of the observed metastability of NeH
Authors: Gellene Gregory I.
Published: July 2002

The metastability of 20NeH and 20NeD previously identified by neutralized ion beam studies in this laboratory has been confirmed by the observation of metastability for 22NeH and 22NeD in a similar study. A lifetime of τ≥10 μs is determined for the metastable states which are tentatively identified as Rydberg levels of intermediate n(n≤14) and possibly high l. The discrepancy between our previously measured dissociative kinetic energy release for 20NeD and that of Peterson et al. [J. Chem. Phys. 91, 6880 (1989)] is resolved.
Crossover from capillary wave to van der Waals regime for fluid interfaces in two dimensions very close to the critical point
Authors: Chen L.-J. , Knackstedt M. , Robert M.
Published: July 2002

An analytic expression is derived for the interfacial profile of a liquid–vapor or a liquid–liquid phase equilibrium by assuming, in accord with current ideas, that the equilibrium interface consists of an intrinsic interface of the nonclassical van der Waals type broadened by capillary wave fluctuations. It is shown that in two dimensions of space, the interfacial thickness exhibits a crossover with change of critical exponent from capillary wave behavior at low and near-critical temperatures, to van der Waals behavior at temperatures very close to the critical temperature. The location of the crossover temperature is determined by the ratio of the critical amplitudes of the root-mean squared interfacial thickness and the bulk correlation length. From available experimental data and theory, an estimate of the order of magnitude of the critical amplitude ratio is made, from which the crossover is found to occur in the temperature range 3×10−4 K<(Tc−T)<0.3 K. In the currently accessible temperature range, the capillary wave prediction prevails, in contrast to the case of three-dimensional systems.
A study of the unimolecular dissociation of SO+2(C̃,D̃,Ẽ) using the photoelectron–photoion coincidence method
Authors: Norwood K. , Ng C. Y.
Published: July 2002

Photoelectron–photoion coincidence spectra for SO+2, SO+, and S+ resulting from the photoionization of SO2 over the photon energy range of 15.5–17.2 eV (720–800 Å) have been obtained at an electron energy resolution of ≊40 meV (full width at half maximum). The breakdown diagram is presented for SO+2, SO+, and S+. For excited SO+2(C̃,D̃,Ẽ) ions formed initially with internal energies in the range of 4.18–4.87 eV, the breakdown curves for the three ions are relatively smooth. The relative yields for the formation of S+, SO+, and SO+2 via radiative stabilization are approximately 1:80:20. The lack of structure observed in the breakdown curves in this internal energy region is consistent with the conclusion that rapid internal conversion occurs prior to ionic decay. For the SO+2 internal energy range of 3.88–4.18 eV, the S+ breakdown curve is structured, suggesting that the formation of S+ follows a state-specific dissociation pathway. The results of the present experiment do not support the previous findings that the SO+2 and S+ product channels are only formed for photon energies ≤16.7 eV.
Effect of initial phase on ultraviolet/visible absorption in shocked carbon disulfide/hexane mixtures
Authors: Yoo C. S. , Gupta Y. M.
Published: July 2002

The effect of initial phase on shock-induced spectral changes in carbon disulfide/hexane has been examined by monitoring the edge shift of the V-band. Experiments were carried out to a peak pressure of 10 GPa for three different concentrations of carbon disulfide. Shifts of the absorption band in solid mixtures differ markedly from those in liquid mixtures at high pressures. The rate of edge shift with pressure in solid mixtures becomes independent of the carbon disulfide concentration and approaches the rate of shift in pure carbon disulfide. Similar results are not obtained for liquid mixtures. Temperature results cannot explain the observed differences. The experimental data suggest the occurrence of phase separation in the solid mixtures and two possible mechanisms for phase separation are indicated. The present results show the importance of the initial phase in understanding the spectral differences observed for these mixtures under shock and static loading.
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o self-consistent-field approach to molecular solids under pressure. IV. MP3 and MP4 correlation corrections
Authors: Raynor S.
Published: July 2002

Our previously reported ab initio subcell approach for determining the electronic structure of molecular solids and clusters is extended to include correlation at the MP3 and MP4 SDTQ levels. The approach is demonstrated with calculations on solid H2 at four pressures (∼3×106, 1×106, 0.5×106, and 0.1×106 atm) for which contributions of correlation at the MP3 and MP4 levels are found to be small (∼1%–3% of the total calculated interaction energy per molecule). It is also demonstrated that a pair potential approach, although providing a poor model for the HF energy, provides reasonable estimates of the MP2, MP3 and MP4 correlation corrections to the interaction energy in solid H2, at all pressures studied.
A new model of positronium formation: Resonant positronium formation
Authors: Zhang Zhicheng , Ito Yasuo
Published: July 2002

‘‘Resonant model,’’ a new model of positronium (Ps) formation is proposed. The first stage of Ps formation is the inelastic collisions of e+ through formation of an intermediate state (M*.e+) by resonant energy absorption and e+ attachment. The second stage of Ps formation involves thermalized e+ and loosely bound electrons (excess electrons, excited molecules, and certain anions) through resonant electron transfer. Experimental results are reviewed from the viewpoint of the new model comparing it with the older models, the Ore model and the spur model.
Interface motion for mass redistribution at small supersaturation
Authors: Harris S.
Published: July 2002

We consider the problem of mass redistribution in a two-phase system. Solutions are obtained for the interface motion utilizing both a model and the exact microscopic Fokker–Planck equation. In each case an initial linear growth is followed by a distinctive transition to the long time t1/2 diffusive growth.
Orientation of ethylene and propylene on Ag(110) from near edge x-ray adsorption fine structure
Authors: Solomon J. L. , Madix R. J. , Stöhr J.
Published: July 2002

The π bond orientation of ethylene and propylene on the clean and atomic oxygen covered Ag(110) surfaces has been determined with near-edge x-ray absorption fine-structure (NEXAFS) measurements. The π system of ethylene is tilted 20±5° and 18±5° from the surface normal for ethylene on the clean and atomic oxygen covered surfaces, respectively. The tilt angle of the π system is 20±5° on the clean surface and 31±5° on the atomic oxygen covered surface. On both the clean and atomic oxygen covered surface, the carbon–carbon double bond in ethylene and propylene appears to form a π-donor type bond with the Ag(110) surface.
Characterization and photocurrent spectroscopy of single quantum wells
Authors: Parsons C. A. , Thacker B. R. , Szmyd D. M. , Peterson M. W. , McMahon W. E. , Nozik A. J.
Published: July 2002

GaAs/AlxGa1−xAs single quantum wells (SQWs) have been characterized by various spectroscopies and studied as electrodes in photoelectrochemical cells and in gold Schottky barrier structures. The techniques used were photoreflectance (PR), photoluminescence (PL), photocurrent (PC) spectroscopy, and impedance measurements. The energy level structure and well shape were best determined from PR data. The p doping of the buffer layer eliminated p–n junctions between the buffer layer and substrate and the inner AlxGa1−xAs barrier, thereby producing nearly ideal Mott–Schottky plots (no frequency dispersion or light dependence) so that the flatband potentials of these SQW electrodes could be well determined. Stark shifts in the PC spectra could also be seen if the buffer layer of the electrodes is doped p type. Very high quantum yields exhibited by SQW electrodes at room temperature with thick (270 Å) AlxGa1−xAs barriers are explained by thermionic emission. At low temperatures with Schottky barriers, other mechanisms, such as tunneling, dominate. Hot photoluminescence spectra were also obtained and the hot carrier behavior of SQWs is compared with multiple quantum wells.
Aluminum and boron nuclear quadrupole resonance with a direct current superconducting quantum interference device
Authors: Connor C. , Chang J. , Pines A.
Published: July 2002

We report the application of our dc SQUID (superconducting quantum interference device) spectrometer [C. Connor, J. Chang, and A. Pines, Rev. Sci. Instrum. 61, 1059(1990)] to nuclear quadrupole resonance (NQR) studies of aluminum-27, and boron-11 in crystalline and glassy solids. Our results give e2qQ/h=2.38 MHz and η=0.0 for α–Al2O3 at 4.2 K. For the natural mineral petalite (LiAlSi4O10), we obtain e2qQ/h=4.56 MHz and η=0.47. The quadrupole resonance frequency is 1467 kHz in boron nitride, and in the vicinity of 1300 kHz for various borates in the B2O3⋅xH2O system. The distribution of boron environments in a B2O3 glass gives rise to a linewidth of about 80 kHz in the SQUID detected resonance.
A novel nonlinear optical effect: Stimulated Raman–Kerr scattering in a benzene liquid-core fiber
Authors: He Guang S. , Burzynski Ryszard , Prasad Paras N.
Published: July 2002

By using a long interaction length (250 cm) provided by a liquid (benzene)-core multimode hollow fiber, we have observed a novel superbroadening stimulated scattering added to the first- and second-order Stokes stimulated Raman scattering lines of liquid benzene. To explain this new effect, a photon-scattering model of Raman-induced optical Kerr effect, named as Raman–Kerr scattering, is proposed.
Mid- and far-infrared absorption of alkali borate glasses and the limit of superionic conductivity
Authors: Liu Changle , Angell C. A.
Published: July 2002

Transmission spectra of alkali borate glass films of known thickness have been studied as a function of film thickness in the frequency range 50–2000 cm−1, and the composition range 0–70 mol % Na2O. The apparent absorption coefficient of the Na+ ‘‘rattling’’ mode at ≊200 cm−1 becomes almost independent of film thickness for d>5 μ, implying that the absorption coefficient α of the bulk glass can be determined with reasonable accuracy. α proves to be a linear function of alkali oxide content. We use the value of α to estimate the ionic conductivity at infrared frequencies, and argue that this value establishes the theoretical maximum for superionic conductivity—it lies some 10 orders of magnitude above the ambient temperature conductivity of the high alkali glass. It is suggested that the peak value of the IR conductivity is connected to the ac conductivity of glass by a log σ(f) vs log f regime of unit slope [specifically σ/(S cm−1)=10−12.3f ] up to 100 GHz, followed by a short f2 regime. The mid-IR spectra indicate the presence of four coordinated borons with maximum concentration at 40 mol % Na2O, and confirm that B2O4−5 and BO3−3 moities dominate the structure at the high alkali extreme.
Theory of branched polymers on fractal lattices
Authors: Roy A. K. , Blumen A.
Published: July 2002

A phenomenological approach, which takes into account the basic geometry and the topology of fractal lattices and of branched polymers, is used to derive a new expression for the Flory exponent describing the average radius of gyration of branched polymers on fractals. The mean-field version of this formula nicely reproduces the values of the Flory exponent, calculated using real-space renormalization group methods on several fractal lattices. This technique also allows the determination of the scaling exponent of the radius of gyration of branched polymers without excluded-volume interactions on fractal lattices. As an application, the findings are exemplified by analyzing the direct, incoherent energy transfer (via multipolar interactions and exchange) from excited donors to acceptors, which are attached to branched polymers.
High energy inelastic neutron scattering from physisorbed hydrogen in cesium intercalated graphite
Authors: Kellogg G. J. , White J. W. , Herwig K. W. , Sokol P. E.
Published: July 2002

The inelastic neutron scattering from H2 in C24Cs(H2)x has been measured at momentum transfers of 8<Q<22 Å−1 as a function of filling, 0.5<x<2.0, and temperature from 20 to 150 K. The observed scattering approaches the limiting behavior predicted by the Impulse Approximation (IA) for molecular hydrogen for Q’s greater than 15 Å−1 and is insensitive to the filling factor and temperature up to kT/M≊0.1, where M is the enthalpy of hydrogen sorption. The results are analyzed using a model for the IA scattering from molecular hydrogen which takes into account the excitation of rotational transitions and the presence of a local orientational potential at the hydrogen adsorption sites. The average translational kinetic energy per molecule, which is determined by the effective local potential for the molecule, can be extracted from the measured scattering. For a powder sample, such as used in this measurement, determination of the kinetic energy requires independent knowledge of the orientational potential of the H2 molecules. Using the orientational potential inferred from previous tunneling measurements we extract an average translation kinetic energy of 320 K for the H2 molecule in the intercalate.
Multiconfigurational electronic wave functions without a reference configuration: Analysis of a simulated annealing strategy
Authors: Estrin Dario A. , Tsoo Chiachin , Singer Sherwin J.
Published: July 2002

Multiconfigurational floating Gaussian electronic wave functions are obtained for a variety of 2-electron atoms and for H2. These correlated wave functions are unusual in that they are constructed without taking excitations from a reference configuration. The description of electron correlation, the degree to which the proper ground state symmetry is attained, and the behavior of wave functions obtained by simulated annealing as a function of nuclear position are analyzed for two types of basis functions, product Gaussians and a second type of basis function that explicitly includes electron correlation. This work is aimed toward applications in mixed quantum/classical condensed phase simulations. Several recent studies have used simulated annealing to build floating Gaussian electronic wave functions in condensed phase or cluster simulations. We analyze the ability of the simulated annealing method to follow the optimum, or near optimum wave functions as nuclear positions move.
Comparison of light scattering of colloidal dispersions with direct force measurements between analogous macroscopic surfaces
Authors: Gee Michelle L. , Tong Penger , Israelachvili Jacob N. , Witten Thomas A.
Published: July 2002

The equilibrium force laws (force vs distance) between two macroscopic surfactant-coated mica surfaces separated by organic liquids have been determined. The aim was to ascertain whether directly measured forces are able to be employed in the prediction of second virial coefficients as obtained from light scattering experiments on analogous colloidal suspensions (i.e., spherical particles coated with the same surfactant and dispersed in the same medium). The quantitative agreement between the two independent measurements was excellent. This suggests that the interaction energy per unit area, which governs the physical properties of a bulk colloidal system, is indeed, the same as the equilibrium interaction energy per unit area of two macroscopic surfaces, for the systems investigated here. Such a correlation has been tacitly assumed in the past, and it is the first time that this assumption has been confirmed experimentally for any system.
Magnetic optical rotation in H2 and D2
Authors: Bishop David M. , Cybulski Sl/awomir M.
Published: July 2002

Results are reported of the first definitive calculation of the Verdet constant for H2 and D2. This constant governs the Faraday effect. A new and compact formalism is introduced and applied with the aid of explicitly electron correlated wave functions. After ro-vibrational and thermal averaging (factors which affect the results by about 10%), our values are in good agreement with the experimental ones, which, at best, are probably only accurate to 1%. Approximations and an appropriate dispersion formula are also discussed. Our results show that for H2 and D2 the exact constant is almost exactly proportional to the so-called normal Verdet constant for the experimentally accessible frequencies. The recommended dispersion formula for H2 is V≂2.0701 (ℏω/Eh)2/[0.2435−(ℏω/Eh)2]2×10−7 rad e a0 ℏ−1.
Potential energy surfaces of eight low-lying electronic states of Rh3
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Complete active space multiconfiguration self-consistent-field (CASSCF) followed by multireference singles plus doubles configuration-interaction (MRSDCI) calculations which include 27 active electrons are carried out on eight low-lying electronic states of Rh3. The MRSDCI calculations included up to 2.3 million configurations. The spin-orbit effects are included by using the relativistic configuration-interaction (RCI) method. All the low-lying states considered here lie within 0.20 eV. All the doublet states are Jahn–Teller components of the doubly degenerate 2E′ and 2E″ states in equilateral triangular geometry (D3h), while the quartet states arise from the Jahn–Teller components of 4E′ and 4E″ states. The splittings between the two Jahn–Teller components of both the 2E′ and 2E″ states, which yield barriers to pseudorotation, are 3.9 kcal/mol. The lowest-lying 2A2 and 2A1 states are separated only by 0.03 eV. Thus, low-lying electronic states of Rh3 are best described using a dynamic Jahn–Teller model. The Mulliken population analyses of the MRSDCI natural orbitals reveal the larger s population of the apex atom of the isosceles triangle in comparison to the base atoms. The base atoms have larger d populations for all electronic states. The present calculations also reveal a considerable mixing among the 4d85s1, 4d9, and 4d85p1 configurations of the rhodium atom. The atomization energy of Rh3 is calculated as 188 kcal/mol. The trimer (Rh3) is predicted to be considerably more stable than the dimer (Rh2).
Vibrational properties of (R)-methylthiirane from Mo/ller–Plesset perturbation theory
Authors: Amos Roger D. , Handy Nicholas C. , Palmieri Paolo
Published: July 2002

The infrared, Raman frequencies and intensities and the vibrational dichroism of the molecule have been computed using Hartree–Fock and Mo/ller–Plesset perturbation theories. In general, the frequencies and intensities computed by perturbation theory improve the Hartree–Fock description of the spectra. For one of the spectral regions, only the values corrected by perturbation theory are entirely compatible with the measured intensities and signs.
Nonlocal ionization in encounter theory
Authors: Zharikov A. A. , Burshtein A. I.
Published: July 2002

A nonadiabatic theory of electron transfer in solutions is proposed in which the spatial dispersion of both intercenter interaction and activation energy is taken into account as well as the mutual diffusion of particles and diffusion along the reaction coordinate to the intersection point of potential energy terms. When spatial diffusion is considered to be the slowest step, the transition to the conventional theory is demonstrated and the generalized local probability of the transition is introduced to describe correctly the activationless electron transfer. It has been found that in the opposite case when diffusion along the reaction coordinate is hampered, the theory is actually nonlocal but allows the calculation of the rate constant which may be kinetic, pseudodiffusive or superdiffusive depending on the reaction heat.
Hund’s coupling case sequences in resonant multiphoton transitions
Authors: Maïnos C. , Castex M. C. , Nkwawo H.
Published: July 2002

Different Hund’s coupling case sequences are considered for the n1+n2 near resonant multiphoton rovibronic process in electric dipole allowed transitions of any spin multiplicity. The transitional path interferences strength tensor is introduced. This tensor involves a polarization and rotational dependence as well as a transitional path dependence which couples the electronic vibrational motion with the rotational structure. The intensity of a rotational line may decompose in terms of the matrix element of this tensor and a pure electronic vibrational tensor. The specificity of the coupling case sequence is found condensed in the rotational line factors which are explicitly determined for all the coupling case sequences obtained from the case (a) and case (b) coupling.
Chemical reactions from the point of view of statistical thermodynamics far from equilibrium
Authors: Kurzyński Michal/
Published: July 2002

A formal derivation of equations of chemical kinetics in statistical terms is presented. In the case of reactions proceeding according to activated mechanisms, it is shown that equations of conventional kinetics, as well as time correlation function formulas for the rate constants, are valid arbitrarily far from equilibrium. Contrary to activated reactions, the time course of reactions controlled by processes of internal equilibration depends on the distance from equilibrium. A general theory requires nonlinear corrections to the equations of conventional chemical kinetics. Close to equilibrium, the kinetics of controlled reactions differs from the kinetics of activated ones in that direct transitions between any pair of species are generally allowed without the equilibration of transient species. Open problems for the controlled reactions are the necessity of satisfying the detailed balance condition and the possibility of inertial effects being manifested on a thermodynamic scale.
Time-dependent calculations of molecular photodissociation resonances
Authors: Kulander K. C. , Cerjan C. , Orel A. E.
Published: July 2002

The molecular photodissociation dynamics of a model, collinear CO2 system is investigated using the time-dependent wave packet method. Resonance structures in the absorption line shape, found previously in time-independent studies, are correlated to particular oscillatory motions in the dissociating wave packet dynamics. Dramatic changes in the absorption line shape are predicted for this system for short pulse excitation due to the removal of the effects of one class of resonances. Three different methods of solving the time-dependent Schrödinger equation were tested and the optimal scheme was used in these calculations.
Radiative and nonradiative decay of the NH(ND) A 3Π electronic state: Predissociation induced by the 5Σ− state
Authors: Patel-Misra Dipti , Parlant Gérard , Sauder Deborah G. , Yarkony David R. , Dagdigian Paul J.
Published: July 2002

Lifetimes for excited ND A 3Πv′=1–3 and NH v′=2 in specified rotation/fine-structure levels were determined by excitation on isolated rotational lines in the A 3Π–X 3Σ−Δv =+1 sequence, using molecules prepared in a pulsed supersonic beam. The observed lifetimes for NH v′=2 levels were found to be significantly shorter than those expected for purely radiative decay, indicative of additional removal of excited-state population by predissociation. The observed fine-structure dependence of the removal rates is consistent with a mechanism in which the A 3Π state is predissociated by spin–orbit coupling to the repulsive 1 5Σ− state which correlates with the ground-state asymptote, N(4S)+H(2S). This mechanism is also expected to be responsible for the previously observed predissociation of high N′ levels in NH v′=0 and 1. By contrast, no significant evidence for predissociation was found for the decay of excited ND v′=1–3 for the low J′ levels investigated. These observations were confirmed with a combination of ab initio electronic structure, and coupled electronic state dynamics, calculations. Using an extended contracted Gaussian basis and large configuration state function expansions(160 000–380 000 terms) potential energy curves for, and spin–orbit induced coupling (using the full microscopic Breit–Pauli interaction) between, the A 3ΠΩ and 1 5Σ−Ω states were determined. These electronic structure data were used as the basis for the determination of the radiationless decay rates using a semiclassical coupled state model. These results were in turn combined with radiative decay rates for the A 3Π→X 3Σ− transition to determine the total decay rates which were found to be in excellent accord with the available experimental observations, thereby confirming the mechanism of the predissociation.
Kinetic-energy dependence of the reactions of C+(2P)+N2 from threshold to 28 eV CM
Authors: Burley J. D. , Sunderlin L. S. , Armentrout P. B.
Published: July 2002

Guided-ion-beam mass spectrometry is used to examine the kinetic-energy dependence of the reaction of ground-state atomic carbon ion with molecular nitrogen. Integral reaction cross sections are measured for the three possible product channels: N2+ + C, CN+ + N, and N+ + CN, all strongly endothermic. Thresholds for all three channels are found to be consistent with literature thermochemistry, although formation of N2+ is inefficient near threshold. The observed behavior suggests that the Π3 state of CN+ is either the ground state or within 0.14 eV of the ground state, and that N2+ is formed primarily in its A 2Πu first excited state rather than the X 2Σg+ ground state. The thermochemistry measured here finds ΔfH00(CN)=102±3 kcal/mol, D00(CN)=7.82±0.14 eV, and IE(CN)=14.02±0.14 eV. The reaction mechanism for this system is explored by examination of the electronic state correlations and by phase-space calculations.
Transient chaos in a closed chemical system
Authors: Scott Stephen K. , Peng Bo , Tomlin Alison S. , Showalter Kenneth
Published: July 2002

Complex oscillations and even aperiodicity can exist as transient phenomena in closed chemical systems. These effects are illustrated through the analysis of a simple, isothermal chemical model based on mass action kinetics for autocatalytic feedback, involving the conversion of a reactant to a final product via three intermediate species. The use of the so-called pool chemical approximation and of pseudo-steady-state analyses for such systems is indicated and discussed, particularly with relevance to ‘‘real’’ chemical situations where small perturbations due to extraneous noise are inevitably present.
Solvent relaxation effects on the kinetics of photoinduced electron transfer reactions
Authors: Najbar J. , Dorfman R. C. , Fayer M. D.
Published: July 2002

The three-potential surface problem of electron transfer in solution is analyzed using Zusman-type kinetic equations. The model describes ultrafast formation and recombination of radical–ion pairs limited by solvent dielectric relaxation. The problem begins with a donor on an electronic excited state surface. The system evolves with crossing to the radical–ion pair surface (with the possibility of recrossing to the excited donor surface included). Solvent relaxation moves the system to lower energy on the radical–ion pair surface where crossing to the ground state neutral surface occurs (with the possibility of recrossing to the radical–ion surface included). Model calculations of the transient radical–ion pair populations are presented. The time dependent results that are presented show a dramatic dependence on the relative free energy differences (ΔG’s) among the three potential surfaces. Comparisons to other formalisms and to less detailed approximations are made. The mean populations of the transient species for a system of a donor and many acceptors in the absence of spatial diffusion are also derived.
The mobility of electrons in simple insulating fluids as a percolation problem
Authors: Simon Steven H. , Dobrosavljević Vladimir , Stratt Richard M.
Published: July 2002

The mobility of electrons injected into insulating fluids is extraordinarily sensitive to the density of the fluid. However, the process of understanding this fact is complicated by the availability of a large number of potentially important factors capable of influencing the electron; everything from single atom–electron scattering considerations to Anderson localization to polaron formation could all be relevant in principle. We show in this paper that the behavior of the mobility edge (the minimum electron energy needed for conduction) can be calculated within a reasonable accuracy, for the noble gas fluids, without invoking any mechanism beyond a kind of classical percolation. The model proposed is actually a semiclassical one that takes into account the local zero-point energy of the electron in the definition of an effective potential surface, but the mobility edge itself is simply identified as the height above the potential surface that permits a connected wave function to span the system—a purely geometrical calculation. The results so obtained are in good agreement with both electron-mobility experiments in He and with photoinjection experiments in Ar, Kr, and Xe. During the course of the development, we also arrive at some conclusions about the usefulness of the so-called Zallen filling fraction as a criterion for percolation in liquids.
Symmetry-invariant reaction-path potentials
Authors: Ischtwan Josef , Collins Michael A.
Published: July 2002

A general method is presented for constructing a molecular potential-energy surface, based on a reaction path, which is invariant to operations of the complete nuclear permutation inversion group. The method is illustrated with a number of examples, including an approximate potential for NH+3+D2 which allows both abstraction and exchange reactions.
An inelastic incoherent neutron scattering study of ice II, IX, V, and VI—in the range from 2 to 140 meV
Authors: Li J.-C. , Londono J. D. , Ross D. K. , Finney J. L. , Tomkinson J. , Sherman W. F.
Published: July 2002

Inelastic incoherent neutron scattering (IINS) cross sections have been measured for the recovered high-pressure phases of ice, ices II, V, VI, and IX (H2O) and ices II and IX (D2O) in the energy range from 2 to 150 meV at 20 K using the TFXA (time-focused crystal analyzer) neutron spectrometer on the ISIS pulsed neutron source at the Rutherford-Appleton Laboratory. The excellent resolution of the TFXA (less than 2%) covers the whole range of translational and librational modes for the ices investigated and enables us to identify the fine structures in their spectra. Hence a direct comparison between the amplitude-weighted phonon density of states derived from the experimental spectrum and from a lattice dynamic calculation is possible [E. Whalley and J. E. Bertie, J. Chem. Phys. 46, 1271 (1967); J. E. Bertie and E. Whalley, ibid. 40, 1646 (1964)]. The spectra from these polymorphic ices show many new features in the translational region, which are not visible using infared and Raman techniques. In the librational band region, the general features vary remarkably between phases. Thus, for instance, ice II, in which the protons are ordered, shows more structure in this region and has a sharper low-energy cutoff than do the other phases examined. The most significant observation is that the low-energy cutoff is characteristic of the phase. Relative to ice Ih, there is a shift of 4.3 meV for ice IX, of 8.8 meV for ice II, 8.6 meV for ice V, and 12.2 meV for ice VI.
Weak infrared bands in solid SiH4 and GeH4
Authors: Calvani P. , Lupi S. , Ciotti C.
Published: July 2002

The infrared side bands associated with the vibrational modes of SiH4 and GeH4 have been studied in the solid phase. At low T, all of the observed bands exhibit librational transitions. The tunneling splitting EA–T has been extracted from the librational energies. It turns out to be 0.2 μeV in SiH4 II, in excellent agreement with previous nuclear magnetic resonance determinations, and about 5×10−2 μeV in GeH4 (IV). A broad but distinguishable librational absorption is still present in the plastic phases at high T. The observation of overtone and combination bands is also reported.
Partial cross sections in the photoionization of open-shell atoms: Photoelectron spectroscopy of Te
Authors: Goodman G. L. , Berkowitz J.
Published: July 2002

A general expression is derived for the relative partial cross sections for formation of fine structure states in the photoionization of open-shell atoms. The expression is particularized to the cases of chalcogen and pnicogen atoms. By utilizing spectroscopic parameters, branching ratios from photoionization of the ground state, or other components of the lowest configuration, are predicted. New experimental data on atomic tellurium are reported, and compared with predicted branching ratios. The good agreement provides support for the calculational method, since this element is best characterized by intermediate coupling, and the intensities depart significantly from L–S based statistical weights. The comparative study of the pnicogen atoms is less revealing. Most of the intensity is concentrated in the triad 3P0,1,2, where experimental data suffer from limited resolution. A weak 1D2 peak is predicted for bismuth, where it has been observed, and for antimony, for which the relevant data are not yet available.
Rotational dielectric friction on a generalized charge distribution
Authors: Alavi D. S. , Waldeck D. H.
Published: July 2002

The dielectric friction on a solute molecule reorienting in a liquid solution is computed by modeling the solute as a rigid collection of point charges rotating within a spherical cavity in a dielectric continuum. Such a calculation on an extended charge distribution is a logical progression from existing theories, which treat only single point charges or point dipoles. It is shown how a more realistic charge distribution can change the calculated friction coefficient by several orders of magnitude, and the generalized theory is applied to rotational diffusion data for three phenoxazine dyes in dimethylsulfoxide.
Calculation of the effective mass of excess electrons in fluid argon, krypton, and xenon
Authors: Plenkiewicz B. , Frongillo Y. , Plenkiewicz P. , Jay-Gerin J.-P.
Published: July 2002

The effective mass (m■) of excess electrons in fluid argon, krypton, and xenon is calculated as a function of fluid density (n). The calculations are carried out within the framework of the Wigner–Seitz model for nonpolar fluids, using high-precision pseudopotentials to represent the electron–rare-gas atom interactions. The calculated electron effective masses are compared to available m■(n) data estimated from experiment.
Matrix isolation spectroscopy of metal atoms generated by laser ablation. I. The Li/Ar, Li/Kr, and Li/Xe systems
Authors: Fajardo Mario E. , Carrick P. G. , Kenney John W.
Published: July 2002

Results of experiments on lithium doped rare gas solids (Ar, Kr, and Xe) prepared by laser ablation of solid lithium are presented, including near UV–visible absorption spectra, and photobleaching and annealing studies of the matrices, as well as visible emission spectra of the laser ablated Li plume. The UV–visible absorption spectra of Li/Ar and Li/Kr matrices generated by cocondensing the rare gas and laser ablated Li atoms at 12 K are dominated by a ‘‘blue-shifted triplet’’ absorption not observed in previously published studies. Control experiments on Li/Ar and Li/Kr matrices generated using a Knudsen oven as the Li atom source showed exclusively an ‘‘unshifted triplet’’ absorption, in agreement with previous studies. The new absorption features are attributed to Li atoms trapped in novel sites in the Ar and Kr solids; sites not accessible to Li atoms generated by the conventional Knudsen effusion technique. Spectra of Li/Xe samples prepared by the laser ablation method showed exclusively the unshifted triplet absorption pattern, in agreement with previous Knudsen effusion studies. All of these observations are explained by a simple model which compares the sizes of the various trapping sites in the rare gas solids to the collision diameters obtained from the Li atom–rare gas atom pair potentials.
Generalized Brownian dynamics. III. Vibrational relaxation of heteronuclear diatomics in rare gases
Authors: Zhang J. Z. , Harris C. B.
Published: July 2002

A simple, classical stochastic model has been developed to study the vibrational energy relaxation of heteronuclear diatomic molecules in simple monatomic liquids. This work is an extension of an earlier study on homonuclear diatomic molecules [D. E. Smith and C. B. Harris, J. Chem. Phys. 92, 1312 (1990)]. The model is based on the generalized Langevin equation for generalized Brownian dynamics. The memory function of each atom in the diatomic is determined directly from classical molecular dynamics simulations in the solvent of interest and then scaled by a screening factor. The memory function is modeled using an autoregressive technique, which provides efficient evaluation of the friction integral. The effect of screening is accounted using a simple model based on the equilibrium structure of the diatomic in the Lennard-Jones solvent, which is shown to provide very reasonable results. The model developed is applied to simulate the vibrational relaxation of iodine chloride, ICl, in its ground electronic state in liquid xenon. Good agreement is found between the stochastic and molecular dynamics simulations. This is encouraging considering the fact that the stochastic technique is computationally more efficient and has been generalized from homonuclear to heteronuclear diatomics. It also indicates that the underlying assumptions, such as the linear response approximation, are valid in the present stochastic model.
Shapes and sizes of molecular anions via topographical analysis of electrostatic potential
Authors: Gadre Shridhar R. , Shrivastava Indira H.
Published: July 2002

The theorem proposed by Pathak and Gadre [J. Chem. Phys. 93, 1770 (1990)], that the electrostatic potential (ESP) of negative ions must exhibit a directional negative valued minimum along any arbitrary direction has been verified for some small negative molecular ions, viz., OH−, CN−, N−3, NO−3, and NH−2. Also, as predicted by Gadre and Pathak [Proc. Ind. Acad. Sci. (Chem. Sci.) 102, 18 (1989)], the molecular ESP (MESP) maps are found to be devoid of local maxima. As a consequence, these maps reveal rich topographical details in the form of several saddle points as well as point minima. From the location of these critical points, estimates of the sizes and shapes of the negatively charged molecular ions are obtained. For anions, there exists a surface on which ■V⋅dS=0 and which passes through all the negative valued critical points (∇V=0). The ionic size estimates from the location of the critical points of the MESP are found to be in good agreement with the corresponding (spherically averaged) literature values.
Analytic reference interaction site model-mean spherical approximation theory of flexible polymer blends: Effects of spatial and fractal dimensions
Authors: Schweizer Kenneth S. , Curro John G.
Published: July 2002

Recently developed methods for obtaining exact and approximate analytical solutions of the reference interaction site model-mean spherical approximation (RISM-MSA) integral equations for liquid mixtures composed of long, flexible polymers are applied to study the critical temperature Tc for phase separation of symmetric isotopic binary blends as a function of degree of polymerization N, spatial dimension D, and fractal dimension df of the individual macromolecules. For ideal random walk coils, the theory predicts a nonclassical behavior given by Tc∝N(D−2)/2 in two and three dimensions, and the classical Flory–Huggins mean field Tc∝N law is recovered in four and higher dimensions. For arbitrary interpenetrating polymeric fractals, the theory predicts Tc∝N(D−df)/df for spatial dimensions below 2df and Flory–Huggins behavior for D>2df. These novel scaling laws for isotopic mixtures are a consequence of a consistent treatment of chain connectivity on all length scales, intermolecular excluded volume, and a short range unfavorable interaction between hydrogenated and deuterated polymers. A general, closure-independent physical argument based on a renormalization of the bare chi parameter by relatively long range correlated fluctuations in the blend is proposed which reproduces all the qualitative predictions of the RISM-MSA integral equation theory. Analogies with nonclassical critical fluctuation effects are established. Application of the analytical approach to purely athermal blends is also presented. The magnitude and composition dependence of the effective chi parameter is found to be a sensitive function of both spatial and fractal dimensions, and also local nonuniversal features. The various theoretical predictions are favorably compared with recent small angle neutron scattering measurements on binary polymer alloys.
The microwave spectrum of the ND2 radical
Authors: Kanada Mitsuka , Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational spectrum of the ND2 radical in the X̃2B1 ground state was observed in the 265– 531 GHz region by using a frequency modulation spectrometer. The ND2 radical was generated in a free space cell by a dc glow discharge in ND3, or in a mixture of N2 and D2. One hundred twenty fine and hyperfine components of 7 rotational transitions were observed, and were analyzed by least-squares methods; this yielded the rotational and centrifugal distortion constants, as well as the spin–rotation and hyperfine coupling constants for both the nitrogen and deuterium nuclei. The nitrogen nuclear spin–rotation constant Caa(N) has the anomalously large value of 0.269±0.027(3σ) MHz in this analysis, but such a value is necessary to achieve the standard deviation of the fit comparable to the experimental errors. The value is mainly caused by the relatively low-lying Ã2A1 electronic state.
Two-dimensional product probability distributions from trajectory calculations for the H+H2 reaction
Authors: Riederer Donald E. , Jorgensen Andrew D. , Gislason Eric A.
Published: July 2002

A quasiclassical trajectory study of the H+H2 reaction at Erel=0.65 eV has been carried out. A total of 9450 reactive trajectories were run. Two-dimensional Fourier series were used to compute very accurate product probability density functions, and a number of two-dimensional contour maps are presented. A simple model can explain the major features of the results.
A study of K+–He and K+–Kr atomic interaction potentials based on transverse diffusion measurements
Authors: Hogan M. J. , Ong P. P.
Published: July 2002

The ratio of the transverse diffusion coefficient to the mobility, DT/K, has been measured for K+ ions drifting in helium and krypton in the presence of a uniform electric field. Results at 298 K are reported at E/N values ranging from 10 to 220 Td for K+–He and 5 to 420 Td for K+–Kr. Values of DT/K for these two systems were also calculated with a Monte Carlo simulation method which used a number of proposed potentials as input. The accuracy of these potentials has been evaluated by comparing the results to the experimentally determined values. For K+–He none of the available potentials seems to be completely accurate, but for K+ –krypton the potential of Koutselos, Mason, and Viehland appears to be a very good representation of the actual potential. Within the ranges studied, the estimated accuracy of the experimental measurements is ±2.5% and the accuracies for the Monte Carlo calculations are ±1% for K+–He and ±0.5% for K+–Kr.
Tunneling in a linear B2H6–HCl dimer
Authors: Chuang Carl , Klots T. D. , Ruoff R. S. , Emilsson Tryggvi , Gutowsky H. S.
Published: July 2002

Rotational spectra have been observed for eight isotopic species of the diborane–HCl complex with a Balle–Flygare, pulsed nozzle, Fourier transform microwave spectrometer. The dimer has a linear, or at most slightly bent B–B⋅⋅⋅H/DCl equilibrium structure with the H/D end of the HCl attracted symmetrically to a terminal BH2 group of the diborane. Three B2H6–HCl species homonuclear in the boron were observed to tunnel while those with 10B11BH6 or DCl did not. The tunneling splits each rotational transition into two components of comparable intensity, separated by several MHz depending on J and K. The a–dipole transitions are characteristic of a prolate, very near symmetric top; only K=0 and ±1 transitions were found. Rotational constants are reported for all species. The B̄, DJ, H, (B-C), and DJK constants determined for 11B2H6–H35Cl are for the A1 tunneling state 1273.364(1) MHz, 5.56(5) kHz, 1.0(8) Hz, 5.3(2) MHz, and −2.1(3) MHz; for the A2 tunneling state 1273.856(1) MHz, 11.64(7) kHz, 33.1(9) Hz, 5.70(4) MHz, and −3.21(5) MHz. The chlorine hyperfine structure gives the average torsional displacement of the H/DCl from the a axis to be 26.3° for the HCl complexes and 22.5° for the DCl. The torsional displacement of the B2H6 was found by an inertial analysis of the complexes with HCl. It is very anisotropic, being close to 0° in the ethylene plane and about 9° in the BH2B bridging plane. It is suggested that in the tunneling the B2H6 reorients by 180° in the bridging plane, coupled with a gear-like counter rotation of the HCl by 360°. The B⋅⋅⋅H distance is 2.693 Å in the dimers with HCl and the B⋅⋅⋅D distance is slightly longer, 2.702 Å.