Volume 111, Issue 14, 08 October 1999
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


Resonance Raman spectroscopy of matrixisolated massselected and
View Description Hide DescriptionRaman spectroscopy of matrixisolated, massselected and reveal that both are Jahn–Teller distorted triangular molecules. The observed spectrum of can be accounted for adequately using an approximate Jahn–Teller potential truncated at the quadratic term [Wedum et al., J. Chem. Phys. 100, 6312 (1994)] with parameters and For the spectrum is more complex, most likely due to the fact that spin–orbit coupling plays an important role in this highspin cluster in addition to the quadratic Jahn–Teller terms. The overall pattern of the spectrum suggests that in the Jahn–Teller distortion is likely small, and a peak at 249 is tentatively assigned to its symmetric stretch.

Calorimetric indications of a cooperativity onset in the crossover region of dynamic glass transition for benzoin isobutylether
View Description Hide DescriptionHeat capacity spectroscopy (HCS), dielectric spectroscopy, and shear viscosity data are reported for liquid benzoin isobutylether (BIBE). Dielectric and viscosity peculiarities indicate the crossover region of dynamic glass transition at where the extrapolated Johari Goldstein relaxation intersects the main transition trace in an Arrhenius plot. Although HCS does not reach the crossover frequency of order the linear decrease of the square root of cooperativity as calculated from HCS data at lower frequencies indicates a cooperativity onset in the crossover temperaturefrequency range. As BIBE belongs to another dielectric crossover scenario as the substances where such an onset could previously be observed, it seems that the cooperativity onset is a general property of the crossover region.

An integral representation of isolated binary collisions in vibrational relaxation
View Description Hide DescriptionPath integral techniques are used to show that the quantum mechanics of a harmonic oscillator driven by Campbell’s noise provides an exact integral representation for a model of condensed phase vibrational relaxation in which the rate of transitions from one excited state to another is governed by independent, pairwise collisions between the excited state species and the molecules of the surrounding medium. In the limit of infrequent collisions and delta correlated noise, this exact representation of the rate can be reduced to the product of the frequency of collisions and a quantity that is related to the likelihood of a vibrational transition in a given collision. In this form, the vibrational relaxation rate has the same structure as the rate expression that can be regarded as the defining relation of the isolated binary collision model.

Ultrafast spectroscopy of dark states in solid state sexithiophene
View Description Hide DescriptionFemtosecond time resolved photoinduced transmission studies are carried out in nanocrystalline dihexylsexithiophene films with long range structural order. The results are compared with those obtained for sexithiophene in several states of aggregation. We explain the lack of radiative recombination in the solid phase with the formation of two nonradiative excitations, dark excitons and charge transfer states. The dark exciton dynamics is studied in detail in dihexylsexithiophene films; thermalization occurs within 200 fs, then decay takes place by mutual annihilation in the first ps, and by monomolecular recombination at longer times. Optical dynamics indicates that within the pump pulse a second excitation is formed, and we assign it to charge transfer states. The latter decay monomolecularly in the hundred ps time scale.

Two novel approaches to the Kramers rate problem in the spatial diffusion regime
View Description Hide DescriptionAt present, there are two general theoretical approaches to calculating the rate of thermally activated escape of a Brownian particle over a barrier out of a metastable well in the spatial diffusion regime. A direct approach involves techniques entirely based on the underlying Fokker–Planck equation, such as the Kramers flux over population method, the mean first passage time formalism, and the eigenmode expansion. An alternative consists of replacing the original onedimensional stochastic dynamics by an infinite dimensional Hamiltonian system. The rate is then calculated using reactive flux methods. Both approaches are rather efficient when treating bistable potentials with high parabolic barriers. However, complications arise if the barrier is not parabolic. In such a case, large deviations of theoretical predictions from exact numerical rates are observed in the intermediate friction region. The latter holds true even though the barrier is infinitely high, to say nothing of low barriers for which the problem of finite barrier height corrections remains effectively unresolved. Based on the expansion of the Fokker–Planck equation in reciprocal powers of the friction coefficient, two novel methods for calculating analytically the rate of escape over an arbitrarily shaped barrier are presented. These are a continued fraction expansion method and a selfsimilar renormalization technique developed recently for summation of divergent fieldtheoretical series, respectively. In this way, two different rate expressions are constructed that agree in the limiting case of high friction with the rate following from the corresponding Smoluchowski equation and reduce to the transition state theory rate at zero damping. Comparison with a known rate expression for a purely parabolic barrier and from numerical simulations for bistable potentials with cusped and smooth barriers of different heights show excellent agreement between the present theories and exact numerical results. As long as the escape dynamics is dominated by spatial diffusion across the barrier top, the maximal relative errors attained with the continued fraction method and the selfsimilar renormalization technique are less than 3% and 7%, respectively. This is in drastic contrast to known rate formulas derived by other means, whose relative errors are larger by factors and even by orders of magnitude.

Intermolecular energy transfer in liquid water and its contribution to heat conduction: A molecular dynamics study
View Description Hide DescriptionIntermolecular energy transfer (IET) is a dominant factor in heat conduction in liquid. The IET in liquid water and its contribution to macroscopic heat conduction under a temperature gradient were analyzed by a molecular dynamics simulation utilizing the extended simple point charge (SPC/E) potential model. Intermolecular energy exchange rates (IEERs) for both the translational and rotational motion of molecules were defined and their characteristics examined. The IEER of hydrogenbonded molecules and nonbonded molecules have different characteristics. The IEER oscillates with a high amplitude and its time average, which is much smaller than the magnitude of the IEER, gives the effective rate of the IET that contributes to macroscopic heat conduction. In the present study, the effective rate of the IET was assumed to be proportional to the magnitude of the IEER. Based on the characteristics of the IEER and the above supposition, contributions of the translational and rotational IET between hydrogenbonded molecules and nonbonded molecules to macroscopic heat conduction were evaluated. The evaluated results were compared with the results of a molecular dynamics (MD) simulation of heat conduction under a constant temperature gradient, and good qualitative agreement between the predicted value and the simulated result was found. The rotational IET was found to be dominant as compared with the translational IET, and the contribution of hydrogenbonded molecules to heat conduction was found to be relatively small. The possibility of a mechanism that cancels the IET between distant molecules and the development of a precise model for this mechanism were also discussed.

Multiple spin echo generation by gradients of the radio frequency amplitude: Twodimensional nutation spectroscopy and multiple rotary echoes
View Description Hide DescriptionNMR(nuclear magnetic resonance) nutation is treated with respect to demagnetizingfield effects on the evolution of spin coherences. A twodimensional NMR nutation spectroscopy scheme is suggested consisting of a single radio frequency(RF) pulse and a freeevolution period. The RF pulse amplitude as well as the external magnetic field are assumed to be subject to gradients in the same but otherwise arbitrary direction. Cross peaks are predicted as frequency domain counterparts of multiple echoes. It is suggested to analyze the cross peak shape in terms of distributions of internal gradients arising from magnetic susceptibility inhomogeneities in heterogeneous samples. Furthermore, a pulse scheme solely based on gradients of the RF amplitude is treated resulting in the prediction of multiple rotary echoes as counterparts to the conventional rotary echo. The origin again is evolution in the presence of spatially modulated longitudinal magnetization in the tilted rotating frame.

Quantum beats and ultrafast solvation dynamics
View Description Hide DescriptionThe beats during solvation dynamics have been studied theoretically and experimentally, using the heterodyne optical Kerr effect and pump–probe spectroscopy. We showed that the solvation process increases the beat contrast due to the holeburning effect. Owing to the dipole activity of the intramolecular vibration responsible for the beats, a dipole–dipole coupling between the intramolecular vibration and the solvation coordinate is available. We have calculated a correlation function of the perturbation of nuclear motion during electronic transition, when such a coupling exists. Taking into account the linkage between the intramolecular vibration and the solvation coordinate yields better agreement between theory and the experimental data. We have evaluated the value of the linkage. The evaluation conforms with the values obtained by the computer fit of the experimental signals.

Homogeneous nucleation rates of supercooled water measured in single levitated microdroplets
View Description Hide DescriptionHomogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paultrap. The size of a single droplet is continuously measured by analyzing the angleresolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleationtheory in order to obtain the size and the formation energy of the critical nucleus.

Numerical simulations of Coulomb systems: A comparison between hyperspherical and periodic boundary conditions
View Description Hide DescriptionNumerical simulations of Coulomb systems can be performed in various geometries, for instance in a cube with periodic boundary conditions or on the surface of a hypersphere We make a detailed comparison between electrostatics in these two geometries with a special emphasis on the problem of properly defining the zero of energy of a system of charges. This analysis enables the derivation of the correct configurational energies of important models of Coulombic fluids or plasmas in and in a unified way. The cases of the one component plasma and the restricted primitive model of electrolytes are considered in detail.

Thermodynamic limit of the excess internal energy of the fluid phase of a onecomponent plasma: A Monte Carlo study
View Description Hide DescriptionThe thermodynamic limit of the excess internal energy per particle of the fluid phase of the threedimensional onecomponent plasma is investigated by means of Monte Carlo simulations. The simulations are performed in the canonical ensemble within hyperspherical boundary conditions. is computed for 31 values of the coupling parameter Γ in the range For each value of Γ the thermodynamic limit of is obtained by studying the scaling law which governs the behavior of with the number of particles.

Spin echoes of nuclear magnetization diffusing in a constant magnetic field gradient and in a restricted geometry
View Description Hide DescriptionWe study the influence of restriction on Carr–Purcell–Meiboom–Gill spin echoes response of magnetization of spins diffusing in a bounded region in the presence of a constant magnetic field gradient. Depending on three main length scales: pore size, dephasing length and diffusion length during halfecho time, three main regimes of decay have been identified: free, localization and motionally averaging regime. In localization regime, the decay exponent depends on a fractional power of the gradient, denoting a strong breakdown of the second cumulant or the Gaussian phase approximation (GPA). In the other two regimes, the exponent depends on the gradient squared, and the GPA holds. We find that the transition from the localization to the motionally averaging regime happens when the magnetic field gradients approach special values, corresponding to branch points of the eigenvalues.Transition from one regime to another as a function of echo number for a certain range of parameters is discussed. In this transition region, the signal shows large oscillations with echo number. For large asymptotic behavior sets in as a function of for the decay exponent per echo. This is true for all values of the parameters and
