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Presents a correction to the article "Binding Energies of CO on Gold Cluster Cations Aun+ (n=1-65): A Radiative Association Kinetics Study," published in a previous issue of the "Journal of Chemical Physics."

Presents a correction to an article titled "Heterodyned Fifth-Order 2D-IR Spectroscopy of the Azide Ion in an Ionic Glass" that was previously published in an issue of the "Journal of Chemical Physics."

NMR relaxation experiments have provided a wealth of information about molecular motions in macromolecules and ordered fluids. Even though a rigorous theory of spin relaxation is available, the complexity of the investigated systems often makes the interpretation of limited datasets challenging and ambiguous. To allow physically meaningful information to be extracted from the data without commitment to detailed dynamical models, several versions of a model-free (MF) approach to data analysis have been developed. During the past 2 decades, the MF approach has been used in the vast majority of all NMR relaxation studies of internal motions in proteins and other macromolecules, and it has also played an important role in studies of colloidal systems. Although the MF approach has been almost universally adopted, substantial disagreement remains about its physical foundations and range of validity. It is our aim here to clarify these issues. To this end, we first present rigorous derivations of the three well-known MF formulas for the time correlation function relevant for isotropic solutions. These derivations are more general than the original ones, thereby substantially extending the range of validity of the MF approach. We point out several common misconceptions and explain the physical significance of the approximations involved. In particular, we discuss symmetry requirements and the dynamical decoupling approximation that plays a key role in the MF approach. We also derive a new MF formula, applicable to anisotropic fluids and solids, including microcrystalline protein samples. The so-called slowly relaxing local structure (SRLS) model has been advanced as an alternative to the MF approach that does not require dynamical decoupling of internal and global motions. To resolve the existing controversy about the relative merits of the SRLS model and the MF approach, we formulate and solve a planar version of the SRLS model. The analytical solution of this model reveals the unphysical consequences of the symmetrical two-body Smoluchowski equation as applied to protein dynamics, thus refuting the widely held belief that the SRLS model is more accurate than the MF approach. The different results obtained by analyzing data with these two approaches therefore do not indicate the importance of dynamical coupling between internal and global motions. Finally, we explore the two principal mechanisms of dynamical coupling in proteins: torque-mediated and friction-mediated coupling. We argue by way of specific analytically solvable models that torque-mediated coupling (which the SRLS model attempts to capture) is unimportant because the relatively slow internal motions that might couple to the global motion tend to be intermittent (jumplike) in character, whereas friction-mediated coupling (which neither the SRLS model nor the MF approach incorporates) may be important for proteins with unstructured parts or flexibly connected domains. [ABSTRACT FROM AUTHOR]

A study of the structural relaxation of nitromethane subsequent to shock loading normal to the (100) crystal plane performed using molecular dynamics and a nonreactive potential was reported recently [J. Chem. Phys. 131, 064503 (2009)]. Starting from initial temperatures of T0=50 and 200 K, shocks were simulated using impact velocities Up ranging from 0.5 to 3.0 km s-1; clear evidence of melting was obtained for shocks initiated with impacts of 2.0 km s-1 and higher. Here, we report the results of analyses of those simulation data using a method based on the Eckart frame normal-mode analysis that allows partitioning of the kinetic energy among the molecular degrees of freedom. A description of the energy transfer is obtained in terms of average translational and rotational kinetic energies in addition to the rates of individual vibrational mode heating. Generally, at early times postshock a large superheating of the translational and rotational degrees of freedom (corresponding to phonon modes of the crystal) is observed. The lowest frequency vibrations (gateway modes) are rapidly excited and also exhibit superheating. Excitation of the remaining vibrational modes occurs more slowly. A rapid, early excitation of the symmetric C–H stretch mode was observed for the shock conditions T0=50 K and Up=2.0 km s-1 due to a combination of favorable alignment of molecular orientation with the shock direction and frequency matching between the vibration and shock velocity. [ABSTRACT FROM AUTHOR]

Electronic relaxation dynamics subsequent to s→p excitation of the excess electron in large anionic water clusters, (H2O)n- and (D2O)n- with 25≤n≤200, were investigated using time-resolved photoelectron imaging. Experimental improvements have enabled considerably larger clusters to be probed than in previous work, and the temporal resolution of the instrument has been improved. New trends are seen in the size-dependent p-state lifetimes for clusters with n≥70, suggesting a significant change in the electron-water interaction for clusters in this size range. Extrapolating the results for these larger clusters to the infinite-size limit yields internal conversion lifetimes τIC of 60 and 160 fs for electrons dissolved in H2O and D2O, respectively. In addition, the time-evolving spectra show evidence for solvent relaxation in the excited electronic state prior to internal conversion and in the ground state subsequent to internal conversion. Relaxation in the excited state appears to occur on a time scale similar to that of internal conversion, while ground state solvent dynamics occur on a ∼1 ps time scale, in reasonable agreement with previous measurements on water cluster anions and electrons solvated in liquid water. [ABSTRACT FROM AUTHOR]

Continuum solvent models have become a standard technique in the context of electronic structure calculations, yet no implementations have been reported capable to perform molecular dynamics at solid-liquid interfaces. We propose here such a continuum approach in a density functional theory framework using plane-wave basis sets and periodic boundary conditions. Our work stems from a recent model designed for Car–Parrinello simulations of quantum solutes in a dielectric medium [D. A. Scherlis et al., J. Chem. Phys. 124, 074103 (2006)], for which the permittivity of the solvent is defined as a function of the electronic density of the solute. This strategy turns out to be inadequate for systems extended in two dimensions: the dependence of the dielectric function on the electronic density introduces a new term in the Kohn–Sham potential, which becomes unphysically large at the interfacial region, seriously affecting the convergence of the self-consistent calculations. If the dielectric medium is properly redefined as a function of the atomic coordinates, a good convergence is obtained and the constant of motion is conserved during the molecular dynamics simulations. The Poisson problem is solved using a multigrid method, and in this way Car–Parrinello molecular dynamics simulations of solid-liquid interfaces can be performed at a very moderate computational cost. This scheme is employed to investigate the acid-base equilibrium at the TiO2-water interface. The aqueous behavior of titania surfaces has stimulated a large amount of experimental research, but many open questions remain concerning the molecular mechanisms determining the chemistry of the interface. Here we make an attempt to answer some of them, putting to the test our continuum model. [ABSTRACT FROM AUTHOR]

The absorption line shapes of model molecular aggregates are investigated using the recently developed Liouville space hierarchical equations of motion (HEOM) method. The exact results are further exploited for the assessment of several approximation schemes, including the high temperature approximation of HEOM, the stochastic Liouville equation approach, and the perturbative time-local and time-nonlocal quantum master equations (QMEs). The calculations on dimers, larger ring-shaped aggregates, and a model of the B850 ring in the LH2 of purple bacteria show that while the other approximate methods can give reasonable absorption line shapes over a wide range of parameter regimes, the second-order time-nonlocal QME is generally inaccurate and may give spurious peaks in the absorption spectra. [ABSTRACT FROM AUTHOR]

A practical quantum theory for unifying electronic and nuclear dynamics, which were separated by the Born–Oppenheimer approximation, is proposed. The theory consists of two processes. Nonadiabatic (quantum) electron wavepacket dynamics on branching (non-Born–Oppenheimer) nuclear paths are first constructed. Since these paths are not the classical trajectories, most of the existing semiclassical theories to generate quantum wavepacket do not work. Therefore, we apply our own developed semiclassical wavepacket theory to these generated non-Born–Oppenheimer paths. This wavepacket is generated based on what we call the action decomposed function, which does not require the information of the so-called stability matrix. Thus, the motion of nuclei is also quantized, and consequently the total wave function is represented as a series of entanglement between the electronic and nuclear wavepackets. In the last half of the article, we show the practice to demonstrate how these independent theories can be unified to give electron-nuclear wavepackets in a two-state model. The wavepackets up to the phases and resultant transition probabilities are compared to the full quantum-mechanical counterparts. It turns out that the lowest level approximation to the wavepacket approach already shows a good agreement with the full quantum quantities. Thus, the present theoretical framework gives a basic method with which to study non-Born–Oppenheimer electronic and nuclear wavepacket states relevant to ultrafast chemical events. [ABSTRACT FROM AUTHOR]

Penning ionization electron spectra (PIES) of pyrene (C16H10), chrysene (C18H12), and coronene (C24H12) in the gas phase are recorded using metastable He*(2 3S) atoms. The assignments of PIES are presented based on the outer valence Green’s function calculations with the correlation consistent polarized valence triple-ζ basis sets and the exterior electron density calculations of contributing molecular orbitals. The definite positions of all of the π bands in the PIES are identified making use of the large PIES cross sections. Broad bands are observed in low-electron-energy regions for chrysene and coronene and are ascribed to ionization processes of non-Koopmans types from σ orbitals. The anisotropic interaction potential energy surfaces for the colliding systems are obtained from ab initio model potentials for the related systems with similar outer valences Li(2 2S)+C16H10, C18H12, and C24H12, respectively. The attractive well depths in the out-of-plane directions are found to be similar between these molecules, and the repulsive walls embrace the in-plane perimeters uniformly. Collision energy dependencies for partial Penning ionization cross sections and negative peak shifts in PIES for chrysene support these anisotropic interactions. Effects from thermal populations in low-frequency vibrational modes are estimated to be minor in one-electron ionization processes. [ABSTRACT FROM AUTHOR]

Exothermic chemical reaction dynamics at the gas-liquid interface have been investigated by colliding a supersonic beam of F atoms [Ecom=0.7(3) kcal/mol] with a continuously refreshed liquid hydrocarbon (squalane) surface under high vacuum conditions. Absolute HF(v,J) product densities are determined by infrared laser absorption spectroscopy, with velocity distributions along the probe axis derived from high resolution Dopplerimetry. Nascent HF(v≤=3) products are formed in a highly nonequilibrium (inverted) vibrational distribution [≤Evib≥=13.2(2) kcal/mol], reflecting insufficient time for complete thermal accommodation with the surface prior to desorption. Colder, but still non-Boltzmann, rotational state populations [≤Erot≥=1.0(1) kcal/mol] indicate that some fraction of molecules directly scatter into the gas phase without rotationally equilibrating with the surface. Nascent HF also recoils from the liquid surface with excess translational energy, resulting in Doppler broadened linewidths that increase systematically with internal HF excitation. The data are consistent with microscopic branching in HF-surface dynamics following the reactive event, with (i) a direct reactive scattering fraction of newly formed product molecules leaving the surface promptly and (ii) a trapping desorption fraction that accommodates rotationally (though still not vibrationally) with the bulk liquid. Comparison with analogous gas phase F+hydrocarbon processes reveals that the liquid acts as a partial “heat sink” for vibrational energy flow on the time scale of the chemical reaction event. [ABSTRACT FROM AUTHOR]

Freezing and melting behaviors of a fluid confined to pores of mesoporous silicon with a modulated structure have been studied using NMR techniques. The molecular self-diffusivities, measured along the freezing and melting transitions, unveiled essential differences in the configuration of the frozen domains. This suggests that freezing is dominated by a pore-blocking mechanism. Freezing kinetics is found to exhibit very slow long-time dynamics, following a ln2(t) dependence. This type of time dependence may result if the front of the frozen phase is assumed to propagate in the random potential field created by the disorder of the porous silicon channels, similar to the mechanism of Sinai diffusion. The free energy barriers calculated from the kinetic measurements and estimated using a thermodynamical model yield a consistent picture of the freezing process in the presence of quenched disorder. [ABSTRACT FROM AUTHOR]

We propose a simple and tractable method to treat quantum electron wavepacket dynamics that nonadiabatically couples with “classical” nuclear motions in mixed quantum-classical representation. The electron wavepacket is propagated inducing electronic-state mixing along our proposed paths. It has been shown in our previous studies that classical force working on nuclei in a nonadiabatic region is represented in a matrix form (called the force matrix), and the solutions of the Hamilton canonical equations of motion for nuclei based on this force matrix give rise to a cascade of infinitely many branching paths when solved simultaneously with electronic-state mixing. As a tractable approximation to these rigorous solutions, we here devise a method to provide much simpler nonadiabatic paths: (i) extract one or a few number of representative paths by taking an average over the paths in phase space (not averaging over the forces) that should be otherwise undergo the fine branching. (ii) After the nonadiabatic coupling becomes sufficiently small, let these paths naturally branch by running them with their own individual eigenforces (the eigenvalues of the force matrix). Since the eigenforces coincide with the forces of adiabatic potential energy surfaces in the limit of zero nonadiabatic coupling, these branching paths eventually run on one of possible adiabatic potential energy surfaces, converging to a classical path (Born–Oppenheimer path). The paths thus created are theoretically satisfactory in that they realize the coherent mixing of electronic states in the manner of quantum entanglement and yet eventually become consistent with the Born–Oppenheimer classical trajectories. We test the present method numerically with the use of two- and three-state systems that are extracted from ab initio calculations for the excited states of LiH molecule. [ABSTRACT FROM AUTHOR]

We investigate the relationship between ionic structure and viscosity in room-temperature ionic liquids. We build on an earlier theoretical work and derive an ionic property we call the charge lever moment (CLM) that provides insight on ionic liquid dynamics. We use electronic structure calculations to determine the CLM for ions in typical ionic liquids and demonstrate a correlation between this property and the experimental viscosities of ionic liquids. The relationship provides insight into the role of librational motion in ionic liquids in general, and the interpretation of Kerr effect experiments is discussed. [ABSTRACT FROM AUTHOR]

This paper presents a new approach to propagating the density matrix based on a time stepping procedure arising from a Trotter factorization and combining the forward and backward incremental propagators. The sums over intermediate states of the discrete quantum subsystem are implemented by a Monte Carlo surface hopping-like procedure, while the integrals over the continuous variables are performed using a linearization in the difference between the forward and backward paths of these variables leading to classical-like equations of motion with forces determined by the quantum subsystem states. The approach is tested on several models and numerical convergence is explored. [ABSTRACT FROM AUTHOR]

Femtosecond pump-probe experiments are performed on flavin biomolecules isolated in an ion trap. Mass spectra of the photoinduced fragments show that the fragmentation pathways can be modified using two-color two-photon excitation. In particular, when an infrared probe pulse (810 nm) is added subsequent to the first excitation step (excitation of the S1 state of flavin mononucleotide at 405 nm), branching ratios between lumichrome and lumiflavin production are inverted relative to the single excitation case. [ABSTRACT FROM AUTHOR]

How useful it is to think about the potential energy landscape of a complex many-body system depends in large measure on how direct the connection is to the system’s dynamics. In this paper we show that, within what we call the potential-energy-landscape ensemble, it is possible to make direct connections between the geometry of the landscape and the long-time dynamical behaviors of systems such as supercooled liquids. We show, in particular, that the onset of slow dynamics in such systems is governed directly by the lengths of their geodesics—the shortest paths through their landscapes within the special ensemble. The more convoluted and labyrinthine these geodesics are, the slower that dynamics is. Geodesics in the landscape ensemble have sufficiently well-defined characteristics that it is straightforward to search for them numerically, a point we illustrate by computing the geodesic lengths for an ordinary atomic liquid and a binary glass-forming atomic mixture. We find that the temperature dependence of the diffusion constants of these systems, including the precipitous drop as the glass-forming system approaches its empirical mode-coupling transition, is predicted quantitatively by the growth of the geodesic path lengths. [ABSTRACT FROM AUTHOR]

Time-resolved coherent nonlinear optical experiments on small molecules in low-temperature host crystals are exposing valuable information on quantum mechanical dynamics in condensed media. We make use of generic features of these systems to frame two simple, comprehensive theories that will enable the efficient calculations of their ultrafast spectroscopic signals and support their interpretation in terms of the underlying chemical dynamics. Without resorting to a simple harmonic analysis, both treatments rely on the identification of normal coordinates to unambiguously partition the well-structured guest-host complex into a system and a bath. Both approaches expand the overall wave function as a sum of product states between fully anharmonic vibrational basis states for the system and approximate Gaussian wave packets for the bath degrees of freedom. The theories exploit the fact that ultrafast experiments typically drive large-amplitude motion in a few intermolecular degrees of freedom of higher frequency than the crystal phonons, while these intramolecular vibrations indirectly induce smaller-amplitude—but still perhaps coherent—motion among the lattice modes. The equations of motion for the time-dependent parameters of the bath wave packets are fairly compact in a fixed vibrational basis/Gaussian bath (FVB/GB) approach. An alternative adiabatic vibrational basis/Gaussian bath (AVB/GB) treatment leads to more complicated equations of motion involving adiabatic and nonadiabatic vector potentials. Computational demands for propagation of the parameter equations of motion appear quite manageable for tens or hundreds of atoms and scale similarly with system size in the two cases. Because of the time-scale separation between intermolecular and lattice vibrations, the AVB/GB theory may in some instances require fewer vibrational basis states than the FVB/GB approach. Either framework should enable practical first-principles calculations of nonlinear optical signals from molecules in cryogenic matrices and their semiclassical interpretation in terms of electronic and vibrational decoherence and vibrational population relaxation, all within a pure-state description of the macroscopic many-body complex. [ABSTRACT FROM AUTHOR]

The vibrational spectra of linear alkanes, with lengths ranging from n-propane to n-octane, were examined on a copper surface by reflection-absorption infrared spectroscopy. The appearance and frequency of the “soft mode,” a feature routinely seen in studies of saturated hydrocarbons adsorbed on metals, were examined and compared between the different adsorbates. The frequency of the mode was found to be dependent on both the number of methylene units of each alkane as well as specific aspects of the order of the monolayer phase. Studies of monolayer coverages at different temperatures provide insights into the nature of the two-dimensional (2D) melting transitions of these adlayer structures, ones that can be inferred from observed shifts in the soft vibrational modes appearing in the C–H stretching region of the infrared spectrum. These studies support recently reported hypotheses as to the origins of such soft modes: the metal-hydrogen interactions that mediate them and the dynamics that underlay their pronounced temperature dependencies. The present data strongly support a model for the 2D to one-dimensional order-order phase transition arising via a continuous rather than discrete first-order process. [ABSTRACT FROM AUTHOR]

We study the statistics of photon emission from three-level single molecule systems. The generating function method [Y. Zheng and F. L. H. Brown, Phys. Rev. Lett. 90, 238305 (2003)] is used to calculate steady state absorption line shapes and Mandel’s Q parameter as a function of excitation frequency, as well as the time dependence associated with approach to the steady state. The line shape calculations confirm known results derived via other methods, while the Q parameter results display complex frequency dependences not amenable to simple interpretation. This study confirms the applicability of the generating function formalism to multilevel quantum systems, including the proper modeling of quantum coherence effects. [ABSTRACT FROM AUTHOR]

Focusing on the off-critical condition, the quench depth dependence of surface-directed phase separation in the polymer binary mixture is numerically investigated by combination of the Cahn-Hilliard-Cook theory and the Flory–Huggins–de Gennes theory. Two distinct situations, i.e., for the wetting, the minority component is preferred by the surface and the majority component is preferred by the surface, are discussed in detail. The simulated results show that the formation mechanism of the wetting layer is affected by both the quench depth and the off-critical extent. Moreover, a diagram, illustrating the formation mechanisms of the wetting layer with various quench depths and compositions, is obtained on the basis of the simulated results. It is found that, when the minority component is preferred by the surface, the growth of the wetting layer can exhibit pure diffusion limited growth law, logarithmic growth law, and Lifshitz-Slyozov growth law. However, when the majority component is preferred by the surface, the wetting layer always grows logarithmically, regardless of the quench depth and the off-critical extent. It is interesting that the surface-induced nucleation can be observed in this case. The simulated results demonstrate that the surface-induced nucleation only occurs below a certain value of the quench depth, and a detailed range about it is calculated and indicated. Furthermore, the formation mechanisms of the wetting layer are theoretically analyzed in depth by the chemical potential gradient. [ABSTRACT FROM AUTHOR]

Optically heterodyne-detected optical Kerr effect (OHD-OKE) experiments are conducted to study the orientational dynamics of a discotic liquid crystal 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HPT) in the isotropic phase near the columnar-isotropic (C-I) phase transition. The OHD-OKE signal of HPT is characterized by an intermediate power law t-0.76±0.02 at short times (a few picoseconds), a von Schweidler power law t-0.26±0.01 at intermediate times (hundreds of picoseconds), and an exponential decay at long times (tens of nanoseconds). The exponential decay has Arrhenius temperature dependence. The functional form of the total time dependent decay is identical to the one observed previously for a large number of molecular supercooled liquids. The mode coupling theory schematic model based on the Sjögren [Phys. Rev. A 33, 1254 (1986)] model is able to reproduce the HPT data over a wide range of times from <1 ps to tens of nanoseconds. The studies indicate that the HPT C-I phase transition is a strong first order transition, and the dynamics in the isotropic phase display a complex time dependent profile that is common to other molecular liquids that lack mesoscopic structure. [ABSTRACT FROM AUTHOR]

Time-resolved coherent anti-Stokes Raman-scattering (CARS) measurements are carried out to study the interaction between xenon atom and iodine molecule in a solid krypton matrix. Interference between the CARS polarizations of the “free” and complexed iodine molecules is observed, while the quantum beats of the complex are not detected due to low concentration. Vibrational analysis based on the polarization beats yields accurate molecular constants for the I2–Xe complex. The harmonic frequency of the I2–Xe complex is found to be redshifted by 0.90 cm-1 when compared to the free I2, whereas the anharmonicity is approximately the same. The dephasing rate of the complex is found to be somewhat higher than that of the free iodine molecule in solid Kr, showing that the complexation affects dephasing, although not dramatically. Molecular dynamics simulations are carried out to find the conformation of the complex, and wave packet simulations are used to reproduce the CARS signal to confirm the assignments of the observed beatings as quantum and polarization beats. The results show that the polarization beats are a useful tool for investigating weak interactions in condensed phase. [ABSTRACT FROM AUTHOR]

We measured angular distributions of HD and D2 molecules desorbed via the reactions H+D/Si(100)→HD [abstraction (ABS)] and H+D/Si(100)→D2 [adsorption-induced-desorption (AID)], respectively. It was found that the angular distribution of HD molecules desorbed along ABS is broader than that of D2 molecules desorbed along AID, i.e., the former could be fit with cos2.0±0.2 θ, while the latter with cos5.0±0.5 θ. This difference of the angular distributions between the two reaction paths suggests that their dynamic mechanisms are different. The observed cos2 θ distribution for the ABS reaction was reproduced by the classical trajectory calculations over the London-Eyring-Polanyi-Sato potential-energy surfaces. The simulation suggests that the HD desorption along the ABS path takes place along the direction of Si–D bonds, but the apparent angular distribution is comprised of multiple components reflecting the different orientations of D-occupied Si dimers in the (2×1) and (1×2) double domain structures. [ABSTRACT FROM AUTHOR]

The dynamics of two nematic liquid crystals, 4-(trans-4′-n-octylcyclohexyl)isothiocyanatobenzene and 4-(4-pentyl-cyclohexyl)-benzonitrile, are investigated as a function of temperature both in the homeotropically aligned nematic phase and in the isotropic phase using optical heterodyne-detected optical Kerr effect experiments, which measures the time derivative of the polarizability–polarizability-correlation function (orientational relaxation). Data are presented over a time range of 500 fs-70 μs for the nematic phase and 500 fs to a few hundred nanoseconds for the isotropic phase. The nematic dynamics are compared with a previously studied liquid crystal in the nematic phase. All three liquid crystals have very similar dynamics in the nematic phase that are very different from the isotropic phase. On the slowest time scale (20 ns–70 μs), a temperature-independent power law, the final power law, t-f with f∼0.5, is observed. On short time scales (∼3 ps to ∼1 ns), a temperature-dependent intermediate power law is observed with an exponent that displays a linear dependence on the nematic order parameter. Between the intermediate power law and the final power law, there is a crossover region that has an inflection point. For times that are short compared to the intermediate power law (<=2 ps), the data decay much faster, and can be described as a third power law, although this functional form is not definitive. The isotopic phase data have the same features as found in previous studies of nematogens in the isotropic phase, i.e., the temperature-independent intermediate power law and von Schweidler power law at short to intermediate times, and a highly temperature-dependent long time exponential decay that is well described by the Landau-de Gennes theory. The results show that liquid-crystal dynamics in the nematic phase exhibit universal behavior. [ABSTRACT FROM AUTHOR]

We present a molecular-dynamics simulation study of the bulk and liquid-vapor interfacial properties of ethanol using a polarizable force field based on the fluctuating charge (FQ) formalism, as well as the nonpolarizable CHARMM22 force field. Both models are competitive with respect to the prediction of ambient liquid properties such as liquid density, enthalpy of vaporization, dielectric constant, and self-diffusion constants. The polarizable model predicts an average condensed-phase dipole moment of 2.2 D associated with an induced liquid-phase dipole moment of 0.6 D; though qualitatively in agreement with earlier nonadditive models as well as recent Car-Parinello calculations, the current FQ model underestimates the condensed-phase dipole moment. In terms of liquid structure, both models are in agreement with recent neutron-diffraction results of liquid ethanol structure, although the polarizable model predicts the hydroxyl-hydrogen–hydroxyl-hydrogen structure factor in closer agreement with the experimental data. In terms of interfacial properties, both models predict ambient surface tension to within 4% of the experimental value of 22.8 dyn/cm, while overestimating the surface excess entropy by almost a factor of 2. Both models display the characteristic preferential orientation of interfacial molecules. The polarizable model allows for a monotonic variation of the average molecular dipole moment from the bulk value to that of the vapor phase. Consequently, there is a dramatic difference in the surface potential predicted by the polarizable and nonpolarizable models. The polarizable model estimates a surface potential of -209±3 mV, while the nonpolarizable model yields a value of -944±10 mV. Finally, based on the vapor-liquid equilibrium simulation data from several temperatures, we estimate the critical properties of both models. As observed with other FQ models for associating fluids (such as water and methanol), and counter to what one would anticipate by modeling more physically the electrostatic response to local environment, the current FQ model underestimates the critical temperature and overestimates the critical density of ethanol; moreover, the FQ model is, in this respect, equivalent to the underlying fixed-charge model. These results further suggest the need to revisit polarizable models in terms of quantitative vapor-liquid equilibrium prediction. [ABSTRACT FROM AUTHOR]

We report a study of control of the HCN→CNH isomerization in a liquid Ar solution. We show, using molecular dynamics simulations, nearly complete conversion from HCN to CNH can be achieved in solution on the subnanosecond time scale without requiring laser pulse shaping or molecular alignment. The mechanism of the isomerization reaction involves multiphoton rovibrational excitation on the ground electronic state potential energy surface coupled with rapid rovibrational relaxation in solution. The results demonstrate the important role of rotation-vibration coupling in multiphoton excitation of small molecules and constitute the first realistic computational demonstration of fast, robust, and high-yield laser field manipulation of solution-phase molecular processes. [ABSTRACT FROM AUTHOR]

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j=1←0 and j=2←1 transitions of HC14N at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2←1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He–HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy. [ABSTRACT FROM AUTHOR]

We illustrate how the preparation and probing of rotational Raman wave packets in O2 detected by time-dependent degenerate four-wave mixing (TD-DFWM) can be manipulated by an additional time-delayed control pulse. By controlling the time delay of this field, we are able to induce varying amounts of additional Rabi cycling among multiple rotational states within the system. The additional Rabi cycling is manifested as a change in the signal detection from homodyne detected to heterodyne detected, depending on the degree of rotational alignment induced. At the highest laser intensities, Rabi cycling among multiple rotational states cannot account for the almost complete transformation to a heterodyne-detected signal, suggesting a second mechanism involving ionization. The analysis we present for these effects, involving the formation of static alignment by Rabi cycling at moderate laser intensities and possibly ion gratings at the highest intensities, appears to be consistent with the experimental findings and may offer viable explanations for the switching from homodyne to heterodyne detection observed in similar DFWM experiments at high laser field intensities (>1013 W/cm2). [ABSTRACT FROM AUTHOR]

Centroid molecular dynamics (CMD) is applied to the study of collective and single-particle dynamics in liquid para-hydrogen at two state points and liquid ortho-deuterium at one state point. The CMD results are compared with the results of classical molecular dynamics, quantum mode coupling theory, a maximum entropy analytic continuation approach, pair-product forward- backward semiclassical dynamics, and available experimental results. The self-diffusion constants are in excellent agreement with the experimental measurements for all systems studied. Furthermore, it is shown that the method is able to adequately describe both the single-particle and collective dynamics of quantum liquids. [ABSTRACT FROM AUTHOR]

A linearized optimal control method in combination with mixed quantum/classical molecular dynamics simulation is used for numerically investigating the possibility of controlling photodissociation wave packets of I2- in water. Optimal pulses are designed using an ensemble of photodissociation samples, aiming at the creation of localized dissociation wave packets. Numerical results clearly show the effectiveness of the control although the control achievement is reduced with an increase in the internuclear distance associated with a target region. We introduce effective optimal pulses that are designed using a statistically averaged effective dissociation potential, and show that they semiquantitatively reproduce the control achievements calculated by using optimal pulses. The control mechanisms are interpreted from the time- and frequency-resolved spectra of the effective optimal pulses. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photodissociation of diiodomethane (CH2I2) in acetonitrile solution has been studied with ab initio molecular dynamics simulations, which show how the iso-diiodomethane photoproduct (CH2I-I) can be formed. The first excited state, described by the “restricted open-shell Kohn-Sham” density functional method, is dissociative and photoexcitation of diiodomethane induces a breaking of one of the C-I bonds. In the simulations, we observe that energy dissipation to the surrounding solvent is essential in the formation of a stable iso-diiodomethane molecule. The caging effect of the solvent results in a recombination of the CH2I and I fragments into iso-diiodomethane on a picosecond time scale. The molecular dynamics simulations enable us to study the cage effect as well as the relaxation of intermediates and the distribution of energy. The CH2I fragment is formed vibrationally excited along the C-I stretching mode. After recombination of the CH2I and I fragments, iso-diiodomethane shows a strong vibration excitation in the CH2 group, which could be used as a fingerprint of the proposed mechanism. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultrafast photoinduced dynamics of electronic excitation in molecular dimers is drastically affected by the dynamic reorganization of inter- and intramolecular nuclear configuration modeled by a quantized nuclear degree of freedom [Cina et al., J. Chem Phys. 118, 46 (2003)]. The dynamics of the electronic population and nuclear coherence is analyzed by solving the chain of coupled differential equations for population inversion, electron-vibrational correlation, etc. [Prezhdo, Pereverzev, J. Chem. Phys. 113, 6557 (2000)]. Intriguing results are obtained in the approximation of a small change of the nuclear equilibrium upon photoexcitation. In the limiting case of resonance between the electronic energy gap and the frequency of the nuclear mode these results are justified by comparison to the exactly solvable Jaynes—Cummings model. It is found that the photoinduced processes in the model dimer are arranged according to their time scales: (i) Fast scale of nuclear motion, (ii) intermediate scale of dynamical redistribution of electronic population between excited states as well as growth and dynamics of electron-nuclear correlation, (iii) slow scale of electronic population approach to the quasi-equilibrium distribution, decay of electron-nuclear correlation, and decrease of the amplitude of mean coordinate oscillation. The latter processes are accompanied by a noticeable growth of the nuclear coordinate dispersion associated with the overall nuclear wave packet width. The demonstrated quantum relaxation features of the photoinduced vibronic dynamics in molecular dimers are obtained by a simple method, applicable to systems with many degrees of freedom. [ABSTRACT FROM AUTHOR]

In order to probe the local dynamics of lipid bilayers in the gel phase, we measured the rotational time trajectories of a membrane probe, diI(3), in supported bilayers of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) using single molecule fluorescence polarization imaging. diI(3) has two hydrocarbon tails that mimic phospholipid tails and has its transition dipole moment lying mostly on the plane of the membrane; hence it is an excellent probe for rotational dynamics in membranes. Above the transition temperature, the probes are laterally mobile and do not display polarized emission. In the gel phase below the transition temperature, lateral mobility is severely reduced and the emission becomes polarized with its polarization direction changing in the milliseconds time scale. Molecule by molecule analysis of the rotational time scales revealed significant heterogeneities among molecules, much larger than would be due to statistical noise. Control experiments using small unilamellar vesicles suggest that the heterogeneities are not caused by surface interactions and are intrinsic to the gel phase membrane. The rotational dynamics is strongly temperature dependent and the thermally activated state for the rotational motion has a large entropic barrier (>30k[sub B]), indicating that relatively large local disorder is required for the rotational motion to occur. Rotational hopping between discrete angles has been observed at the lowest temperatures (∼10 °C). Our results suggest that the gel phase membrane is not uniform at the microscopic level but is highly dynamic with the rigidity of local environments constantly changing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the detailed experimental study on the production of electron-spin-polarized Sr[sup +] ions through one-photon resonant two-photon ionization via laser-excited 5s5p [sup 3]P[sub 1] (M[sub J]=+1) of Sr atoms produced by laser-ablation. We have experimentally confirmed that the use of laser-ablation for the production of Sr atoms prior to photoionization does not affect the electron-spin polarization. We have found that the degree of electron-spin polarization is 64±9%, which is in good agreement with our recent theoretical prediction. As we discuss in detail, we infer, from a simple analysis, that photoelectrons, being the counterpart of electron-spin-polarized Sr[sup +] ions, have approximately the same degree of electron-spin polarization. Our experimental results demonstrate that the combined use of laser-ablation technique and pulsed lasers for photoionization would be a compact and effective way to realize a pulsed source for spin-polarized ions and electrons for the studies of various spin-dependent dynamics in chemical physics. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved coherent anti-Stokes Raman scattering, with a resolution of 20 fs, is used to prepare a broadband vibrational superposition on the ground electronic state of I[sub 2] isolated in solid Kr. The coherent evolution of a packet consisting of ν=1–6 is monitored for as many as 1000 periods, allowing a precise analysis of the material response and radiation coherence. The molecular vibrations are characterized by ω[sub e]=211.330(2) cm[sup -1], ω[sub e]x[sub e]=0.6523(6) cm[sup -1], ω[sub e]y[sub e]=2.9(1)×10[sup -3] cm[sup -1]; the dephasing rates at 32 K range from 110 ps for ν=1 to 34 ps for ν=6, with ν dependence: γ(ν)=8.5×10[sup -3]+4.9×10[sup -4]ν[sup 2]+2.1×10[sup -6]ν[sup 4] ps[sup -1]. The signal amplitude is also modulated at ω[sub q]=41.56(3) cm[sup -1]; which can be interpreted as coupling between the molecule and a local mode. The surprising implication is that this resonant local mode is decoupled from the lattice phonons, a finding that cannot be rationalized based on a normal-mode analysis. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this article, we employ a recently discovered criterion for selecting important contributions to the semiclassical coherent state propagator [T. Van Voorhis and E. J. Heller, Phys. Rev. A 66, 050501 (2002)] to study the dynamics of many dimensional problems. We show that the dynamics are governed by a similarity transformed version of the standard classical Hamiltonian. In this light, our selection criterion amounts to using trajectories generated with the untransformed Hamiltonian as approximate initial conditions for the transformed boundary value problem. We apply the new selection scheme to some multidimensional Henon–Heiles problems and compare our results to those obtained with the more sophisticated Herman–Kluk approach. We find that the present technique gives near-quantitative agreement with the the standard results, but that the amount of computational effort is less than Herman–Kluk requires even when sophisticated integral smoothing techniques are employed in the latter. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The prospect of employing the stimulated Raman adiabatic Passage (STIRAP) technique under the influence of pure dephasing is explored. A general analysis of how decoherence influences the performance of STIRAP is provided. Starting from a general and fully quantum-mechanical system–bath Hamiltonian, we derive a quantum master equation (QME) that describes the reduced dynamics of a dissipative STIRAP system. The derivation is based on the standard assumptions of (1) weak system–bath coupling; (2) Markovity, in the sense that the relaxation times are long in comparison to the bath correlation time, τ[sub c]; and (3) weak field–matter interaction, in the sense that the Rabi period of the driving laser fields, Ω[sup -1], is longer than τ[sub c]. The dissipative term in this QME is the same as it would have been in the absence of the driving fields, because of the assumption of weak field–matter interaction. This type of uncontrollable dephasing is seen to diminish the efficiency of STIRAP, although the actual loss strongly depends on the specific dephasing mechanism. We also derive a more general QME, which is applicable to driving fields of arbitrary intensity. The dissipative term in the new QME is explicitly dependent on the driving fields, and therefore controllable. Intense fields are shown to effectively slow down the dephasing when Ωτ[sub c]>1, which suggests that it may be possible to use STIRAP in order to transfer population between the quantum states of a solute molecule embedded in a solvent. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We introduce an efficient configuration interaction (CI) method for the calculation of mixed quantum and classical nonadiabatic molecular dynamics for multiple electrons. For any given realization of the classical degrees of freedom (e.g., a solvent), the method uses a novel real-space quadrature to efficiently compute the Coulomb and exchange interactions between electrons. We also introduce an approximation whereby the classical molecular dynamics is propagated for several time steps on electronic potential energy surfaces generated using only a particularly important subset of the CI basis states. By only updating the important-states subset periodically, we achieve significant reductions in the computational cost of solving the multielectron quantum problem. We test the real-space quadrature for the cases of two electrons confined in a cubic box with infinitely repulsive walls and two electrons dissolved in liquid water that occupy a single cavity, so-called hydrated dielectrons. We then demonstrate how to perform mixed quantum and classical nonadiabatic dynamics by combining these computational techniques with the mean-field with surface hopping algorithm of Prezhdo and Rossky [J. Chem. Phys. 107, 825 (1997)]. Finally, we illustrate the practicality of the approach to multielectron nonadiabatic dynamics by examining the nonadiabatic relaxation dynamics of both spin singlet and spin triplet hydrated dielectrons following excitation from the ground to the first excited state. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of the O(³P) + HCl and Ar-HCl reactions is investigated using a multiple configuration quantum/classical approach. In this work the dynamics of the hydrogen atom is propagated quantum mechanically in the three Cartesian coordinates of the atom, while the dynamics of the other atoms is propagated classically, in a center-of-mass frame. It is found that the introduction of the argon atom affects the reaction probability through two mechanisms. For nearly collinear O+Ar-HCl collisions, the argon atom blocks the transition state for the O+HCl reaction and inhibits the reaction. On the other hand when the collision geometry is such that the oxygen atom does not collide with the argon atom, the reaction probability is increased. These results are analyzed in terms of the shape of the ground state Ar-HCl wave function. The energy transfer dynamics from the oxygen atom and to the argon atom is also investigated. [ABSTRACT FROM AUTHOR]

The ultrafast dissociation dynamics of O[sub 6, sup -]·X (X = O[sub 2], N[sub 2], Xe, or N[sub 2]O) was investigated by femtosecond photoelectron spectroscopy. The transients, monitoring nascent O[sub 2, sup -], exhibit biexponential rises with two distinct time constants—the fast component (τ[sub 1] ∼ 200 fs) corresponds to the joint rate constant for electron recombination and direct dissociation of the O[sub 4, sup -] core perturbed by solvent molecules, whereas the slow component (τ[sub 2] = 2.0-7.7 ps, depending on the solvent) corresponds to the process for the liberation of O[sub 2, sup -], which is governed by vibrational predissociation and intramolecular vibrational-energy redistribution. These observations are consistent with the mechanism proposed in the earlier communication of this work [Paik et al., J. Chem. Phys. 115, 612 (2001)]. The wave packet bifurcates via two separate dissociation pathways: electron transfer followed by electron recombination, and electron transfer followed by vibrational predissociation. Unlike all other solvents, the anomalous behavior observed for O[sub 6, sup -]·N[sub 2]O—a threefold increase in τ[sub 2] value, compared to the other solvents, and a factor of 10 increase for τ[sub 2], compared to that of O[sub 6, sup -]—reflects the more effective energy dissipation via solute-solvent vibration-to-vibration and rotational couplings. Moreover, for all solvents, the ratio of the slow-rise contribution to the total signal can be correlated with the degree of cooling, supporting the concept of bifurcation in the two channels. [ABSTRACT FROM AUTHOR]

Alkyl chain dynamics in monolayer protected metal cluster systems has been studied by the quasielastic neutron scattering technique, using two different instruments having very different energy windows. Long chain thiolate protected clusters such as Au-ODT (Au-SC[SUB18]H[SUB37]) are rotationally frozen at room temperature (RT) in a wide time scale of 10[SUP-9] to 10[SUP-12] s. The rotator phase is absent at RT even in much smaller chain length systems, Au-OT (Au-SC[SUB8]H[SUB17]) and Au-HT (Au-SC[SUB6]H[SUB13]). Dynamics was found to evolve upon increase in temperature. Alkyl chains in a metal cluster superlattice such as Ag-ODT (Ag-SC[SUB18]H[SUB37]) are also dynamically frozen at RT in the whole time scale range. Evolution of dynamics with temperature is found to be different in the superlattice and isolated cluster systems. In the former, the chains participating in the inter-cluster interaction were found to be dynamically different from those without. On heating just above the chain melting temperature (T[SUBcm]), chains not involved in the inter-cluster interaction become dynamic first. Other chains start softening subsequently and, on increasing temperature near to the superlattice melting, all the chains eventually become dynamic. The uniaxial diffusion motion about the chain axis consistently described the dynamics of the monolayers, both in isolated cluster and in superlattice systems. Direct information on the dynamics of cluster monolayers is obtained for the first time. [ABSTRACT FROM AUTHOR]

The N[sub 2]O[sub 4]→2 NO[sub 2] dissociation reaction was investigated at a high level of theory using the couple cluster with all single and double excitations and connected triples [CCSD(T)] and complete active space self-consistent field approaches, and the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. Only at the coupled cluster level a first-order saddle point was found connecting reactant and products. Collectively, structural, vibrational, and thermodynamic data for the three stationary points represent the best theoretical description of this reaction system to date, and are in good agreement with available experimental results. Unimolecular transition state theory rate constants (k[sub ∞]) were also evaluated at 250, 298.15, and 350 K. At the CCSD(T)/cc-pVTZ level of calculation these results are 0.62×10[sup 1], 1.90×10[sup 3], and 1.66×10[sup 5] s[sup -1], respectively. Known experimental results at 298 K vary from 1.7×10[sup 5] to 1.0×10[sup 6] s[sup -1]. Including an estimate for basis set superposition error, we predict ΔH[sub 298][sup 0] for the dissociation reaction to be 12.76 kcal/mol (Expt. 13.1–13.7 kcal/mol). © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a time-dependent semiclassical method based on quantum trajectories. Quantum-mechanical effects are described via the quantum potential computed from the wave function density approximated as a linear combination of Gaussian fitting functions. The number of the fitting functions determines the accuracy of the approximate quantum potential (AQP). One Gaussian fit reproduces time-evolution of a Gaussian wave packet in a parabolic potential. The limit of the large number of fitting Gaussians and trajectories gives the full quantum-mechanical result. The method is systematically improvable from classical to fully quantum. The fitting procedure is implemented as a gradient minimization. We also compare AQP method to the widely used semiclassical propagator of Herman and Kluk by computing energy-resolved transmission probabilities for the Eckart barrier from the wave packet time-correlation functions. We find the results obtained with the Herman-Kluk propagator to be essentially equivalent to those of AQP method with a one-Gaussian density fit for several barrier widths. [ABSTRACT FROM AUTHOR]

We develop a wave packet interferometry description of multidimensional ultrafast electronic spectroscopy for energy-transfer systems. After deriving a general perturbation-theory-based expression for the interference signal quadrilinear in the electric field amplitude of four phase-locked pulses, we analyze its form in terms of the underlying energy-transfer wave packet dynamics in a simplified oriented model complex. We show that a combination of optical-phase cycling and polarization techniques will enable the experimental isolation of complex-valued overlaps between a "target" vibrational wave packet of first order in the energy-transfer coupling J, characterizing the one-pass probability amplitude for electronic energy transfer, and a collection of variable "reference" wave packets prepared independently of the energy-transfer process. With the help of quasiclassical phase-space arguments and analytic expressions for local signal variations, the location and form of peaks in the two-dimensional interferogram are interpreted in terms of the wave packet surface-crossing dynamics accompanying and giving rise to electronic energy transfer. [ABSTRACT FROM AUTHOR]

The conditions under which an open quantum-mechanical system may be described by mixed quantum-classical dynamics are investigated. Decoherence is studied using influence functional methods in a model composite quantum system comprising two coupled systems, A and C, interacting with a harmonic bath with Ohmic and super-Ohmic spectral densities. Subsystem A is directly coupled to subsystem C, while C is coupled directly to the bath. Calculations are presented for a model where subsystem A is taken to be a two-level system which is bilinearly coupled to a single harmonic oscillator C subsystem. The loss of quantum coherence in each subsystem is discussed in the extreme nonadiabatic regime where the intrinsic dynamics of subsystem A is essentially frozen. Subsystem C is shown to lose its coherence rapidly, while subsystem A maintains coherence for longer time periods since C modulates the influence of the bath on A. Thus, one may identify situations where the coupled A C system evolution effectively obeys mixed quantum-classical dynamics. [ABSTRACT FROM AUTHOR]

Nonlinear time-resolved vibrational spectroscopy is used to compare spectral broadening of the amide I band of the small peptide trialanine with that of N-methylacetamide, a commonly used model system for the peptide bond. In contrast to N-methylacetamide, the amide I band of trialanine is significantly inhomogeneously broadened. Employing classical molecular-dynamics simulations combined with density-functional-theory calculations, the origin of the spectral inhomogeneity is investigated. While both systems exhibit similar hydrogen-bonding dynamics, it is found that the conformational dynamics of trialanine causes a significant additional spectral broadening. In particular, transitions between the poly(Gly)II and the α[SUBR] conformations are identified as the main source of the additional spectral inhomogeneity of trialanine. The experimental and computational results suggest that trialanine adopts essentially two conformations: poly(Gly)II (80%) and α[SUBR] (20%). The potential of the joint experimental and computational approach to explore conformational dynamics of peptides is discussed. [ABSTRACT FROM AUTHOR]

Electron transfer processes in Debye solvents are studied using a spectral analysis method recently proposed. Spectral structure of a nonadiabatic two-state diffusion equation is investigated to reveal various kinetic regimes characterized by a broad range of physical parameters; electronic coupling, energy bias, reorganization energy, and solvent relaxation rate. Within this unified framework, several kinetic behaviors of the electron transfer kinetics, including adiabatic Rabi oscillation, crossover from the nonadiabatic to adiabatic limits, transition from the incoherent to coherent kinetic limits, and dynamic bath effect, are demonstrated and compared with results from previous theoretical models. Dynamics of the electron transfer system is also calculated with the spectral analysis method. It is pointed out that in the large reorganization energy case the nonadiabatic diffusion equation exhibits a nonphysical behavior, yielding a negative eigenvalue. [ABSTRACT FROM AUTHOR]

Recent experiments have investigated the dissociative ionization of NaI and CsI, each aggregated with a molecule of acetonitrile CH[sub 3]CN, using two femtosecond laser pulses. The first, the pump pulse, excites the NaI or CsI diatom to a dissociative state. The second, the probe pulse, then ionizes the sodium or cesium atom after a variable delay time, and the resulting ion is detected. In the case of NaI, the ion signal is characterized by a single maximum. For CsI, however, an oscillatory signal is observed. By performing two-dimensional wave packet propagations, we are able to reproduce this behavior, which can be explained using simple physical arguments. [ABSTRACT FROM AUTHOR]

In a bid to explore how the presence of electrolyte ions influence the friction experienced by hydrogen bonding and nonhydrogen bonding solute molecules, rotational dynamics of two structurally similar nondipolar probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in dimethylsulfoxide (DMSO) at several concentrations of LiNO[sub 3]. The reorientation times of DMDPP, which does not strongly interact with the solvent, follow solution viscosity and dielectric parameters as the electrolyte concentration is increased. However, for DPP, which forms hydrogen bonds with DMSO, there is a 30% decrease in the viscosity-normalized reorientation times upon the addition of 2M LiNO[sub 3] due to the presence of electrolyte ions that shield the hydrogen-bonding interactions between the solute and the solvent. However, the reorientation times correlate well with the solution dielectric parameters with an increase in the electrolyte concentration as in the case of DMDPP. An attempt has been made to model the specific interactions between DPP and DMSO as dielectric friction using the extended charge distribution model of Alavi and Waldeck since both are electrostatic in nature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper reports measurements of both dissociative adsorption on and associative desorption from CH[sub 4] on Ru(0001). We consider the former a view of dissociation from the front side of the barrier, while the latter is considered as a view of dissociation from the back side of the barrier. A combination of both previous and new molecular beam measurements of dissociative adsorption shows that SO depends on all experimental variables (E, T[sub n], T[sub s], and isotope) in a manner similar to other close-packed transition metals. The interpretation of this behavior in terms of a theoretical description of the dissociation is discussed critically, with special emphasis on insights from new theoretical studies. The energy-resolved desorption flux D[sub f](E,T[sub s]) is obtained in associative desorption experiments using the technique of laser assisted associative desorption (LAAD). Measurements at several T[sub s], allow both a direct determination of the adiabatic barrier V[sup *](0) and considerable insight into the dynamics of dissociation. The V[sup *](0) obtained from D[sub f](E,T[sub s]) is in excellent agreement with density functional theory (DFT) calculations and with the value indirectly inferred from molecular beam experiments. The chief dynamic conclusion from an analysis of D[sub f](E,T[sub s]) is that both bending and stretching coordinates must be produced in associative desorption, although they are not populated statistically. The absence of an isotope effect in the shape of D[sub f](E, T[sub s]) argues against the importance of tunneling in the desorption/adsorption. When reactive fluxes are compared via detailed balance, both the molecular beam experiment and the LAAD experiment are in good agreement. [ABSTRACT FROM AUTHOR]