Short-time Decay Research Articles

The influence of the collision strength on the short time behavior for orientational correlation functions of nonspherical molecules.

The first five terms are analytically calculated for the Taylor expansion of symmetric and asymmetric top orientational correlation functions (OCFs) in the Keilson-Storer (KS) model. The three stages of the orientational relaxation (OR) are established: (a) the free rotor stage, (b) the stage which is governed by the angular momentum relaxation frequency, and (c) the stage which is determined by the collision strength. It is suggested that this last stage determines not only the rate of the short time decay of OCFs, but also their long time properties. The time development of logarithmic OCFs is also studied.

Vibrational Dephasing of Carbonmonoxy Myoglobin

The autocorrelation function of vibrational frequency fluctuations of the CO ligand in carbonmonoxy myoglobin, Cδδ(t), is computed from molecular dynamics simulations. Electrostatic interactions are assumed to dominate the modulation of the CO vibrational frequency. The simulated Cδδ(t) is consistent with linear and nonlinear infrared spectroscopic measurements. The short-time decay of Cδδ is dominated by dynamics of the distal histidine, and of a water molecule that can occupy the heme pocket. Correlated protein and solvent dynamics induce spectral diffusion of the CO frequency on longer time scales.

Low-temperature kinetics of exciton–exciton annihilation of weakly localized one-dimensional Frenkel excitons

We present results of numerical simulations of the kinetics of exciton–exciton annihilation of weakly localized one-dimensional Frenkel excitons at low temperatures. We find that the kinetics is represented by two well-distinguished components: a fast short-time decay and a very slow long-time tail. The former arises from excitons that initially reside in states belonging to the same localization segment of the chain, while the slow component is caused by excitons created on different localization segments. We show that the usual bimolecular theory fails in the description of the behavior found. We also present a qualitative analytical explanation of the nonexponential behavior observed in both the short- and the long-time decay components. Finally, it is shown that our theoretical estimate for the annihilation time of the fast component is in good agreement with data obtained from transient absorption experiments on J-aggregates of pseudoisocyanine.

Determining the solvation correlation function from three-pulse photon echoes in liquids

The decay of three-pulse photon echo signals from a solute in a liquid solvent is sensitive to the solute’s transition frequency fluctuations, as characterized by its two-point time correlation function, otherwise known as the solvation correlation function. The most widely used method for determining this solvation correlation function from photon echo data involves the three-pulse photon echo peak shift (3PEPS) method. Using this method the long-time decay of the solvation correlation function can be obtained directly, but the determination of the short-time decay requires a difficult numerical fitting procedure. In this study we propose several alternative approaches to determining the solvation correlation function from echo data, the most promising and straightforward of which we call the S3PE (short-time slope of the three-pulse photon echo) method. The accuracy and efficacy of this approach is illustrated by extracting the solvation correlation function from “experimental” data obtained from classical molecular dynamics computer simulations.

The form of an internal reorientational correlation function

A non-Markovian, non-linearly coupled quantum stochastic dynamics model was used to investigate the reorientational correlation function of the internal motion of a methyl group attached to a large molecule. The values for the parameters used were determined by comparing angular momentum correlation functions calculated from the stochastic model to results of a molecular dynamics simulation. The results show that the correlation function has a rapid short time decay due to inertial effects, followed by a slower, exponential tail. Thus if there is significant inertial motion it is necessary to have at least two order parameters as well as two correlation times, one each for the overall and the internal motion, in order to accurately describe the system. The model can also explain data of Lee and McClung that cannot be explained with a Markovian model.

Efficient viscosity estimation from molecular dynamics simulation via momentum impulse relaxation

A new momentum impulse relaxation method for obtaining the shear viscosity of Newtonian fluids using molecular dynamics simulations is introduced. The method involves the resolution of a decaying coarse-grain Gaussian velocity profile in a properly thermostated simulation box. This localized velocity profile, along with a modification of the periodic boundary conditions, allows computations in a periodic box with minimal phonon feedback due to periodicity. The short-time decay of the small-amplitude velocity profile yields shear viscosities for atomic and molecular species that are in quantitative agreement with those obtained using conventional techniques, but with more than an order of magnitude reduction in computational effort.

Local Reorientation Dynamics of Semiflexible Polymers in the Melt

The reorientation dynamics of local tangent vectors of chains in isotropic amorphous melts containing semiflexible model polymers was studied by molecular dynamics simulations. The reorientation is strongly influenced both by the local chain stiffness and by the overall chain length. It takes place by two different subsequent processes: A short-time non-exponential decay and a long-time exponential reorientation arising from the relaxation of medium-size chain segments. Both processes depend on stiffness and chain length. The strong influence of the chain length on the chain dynamics is in marked contrast to its negligible effect on the static structure of the melt. The local structure shows only a small dependence on the stiffness, and is independent of chain length. Calculated correlation functions related to double-quantum NMR experiments are in qualitative agreement with experiments on entangled melts. A plateau is observed in the dependence of segment reorientation on the mean-squared displacement of the corresponding chain segments. This plateau confirms, on one hand, the existence of reptation dynamics. On the other hand, it shows how the reptation picture has to be adapted if, instead of fully flexible chains, semirigid chains are considered.

Prereactive evolution of monoalkenes excited in the 6 eV region

The time evolution of the first excited states of ethylene, and alkyl substituted ethylenes, isomers with formula C6H12, has been studied by the femtosecond pump probe method, using mass spectrometric detection, in the region of 6 eV (200 nm). Two cyclic alkenes of the formula C6H10 have also been studied. These systems exhibit a multi-exponential decay characterized by a very short time decay, ranging from 20 fs(ethylene) to 100 fs (trans hex-2-ene) and a longer decay, in the picosecond range follows for most of the alkyl isomers. The short time evolution is characteristic of wave packet motion on a steep potential surface. The initial motion has been identified as the torsion about the CC double bond resulting from excitation of the valence state. The evolution of the valence excited state of excited state ethylene [first studied by the group of Radloff, Chem. Phys. Lett. 288, 2044 (1997)] has been taken as a reference. The extremely rapid evolution, 20 fs, without any longer temporal component is explained by the disappearance of the wave packet from the Franck–Condon region into a conical intersection leading to the ground state surface by reference to the theoretical calculations of Ohmine [J. Chem. Phys. 83, 2348 (1985)]. This motion is essentially multidimensional to reach the funnel to the ground state; it combines the torsion about the CC double bond with a pyramidalization about one of the carbon atoms and/or H atom migration from one carbon to the other. Cyclic alkenes exhibit a similar behavior as ethylene with a single ultrashort decay that arises from this same mechanism. Also in the other substituted alkenes the short decay has been assigned to the wave packet motion away from the Franck–Condon region under the influence of the torsion about the double bond. The final longer decay could also be captured in the case of tetramethylethylene by a 800 nm probe as the internal conversion to the ground state via a funnel more difficult to reach. These measurements emphasize the role of conical intersections which could not be brought into evidence without time dependent methods.

Interpreting transient photocurrents as a signature of the density of states distribution: the profound importance of the short-time decay

We present a study exploring the fidelity of recovery of the density of states from transient photocurrents for different distributions of traps based on computer simulation of transient photocurrent decay (TPC). The most important result of the work is that much of the information on the form of the density of states (DOS) over an energy range of up to 0.5 eV depth, is contained in the detailed form of the initial decay of the photocurrent on a timescale <10 −10 s in which we might expect only the shallowest traps to have an affect. To illustrate this point, we demonstrate that substantially different DOS distributions (exponential, Gaussian, rectangular) can be made to give apparently identical TPC decays over several orders of magnitude of time after an initial trapping phase in which there are only small differences in the shape of the response. Recombination effects may also be included. Nevertheless, using a Fourier transform TPC analytical method we can make a distinction between such DOS distributions, provided the short-time initial decay is included.

Room-temperature persistent spectral hole burning in SrFCl:Sm2+ films: Temporal and spatial response

Persistent spectral hole burning was performed on the 7F0–5D1 transition of Sm2+ in thin films of SrFCl. Depending on the substrate and the growth conditions, a total hole depth between 47% and 70% was reached. The holes were Lorentzians of width 4(±0.3) cm−1. The time evolution of the hole depth was studied. It is described by two exponentials: a short time decay (t1=0.37 days) and a long time decay (t2=20.4 days) with a 20% infinite time limit. One- and two-photon burning mechanisms act.

Ion solvation dynamics in supercritical water

A theoretical study of ionic solvation dynamics in supercritical water is presented. The short-time decay of the calculated equilibrium solvation energy time correlation function ( S E ( t)) is found to be in good agreement with the recent computer simulation results. The simulated long-time decay is, however, somewhat slower than that which is predicted by the present theory. Two plausible molecular explanations for the slow long-time decay of S E ( t) are offered.

Survival Probability for the Yamaguchi Potential

Different decay behaviours of the survival probability are examined. Technically, the survival probability of an arbitrary stateψis determined by theψmatrix element of the resolvent of the full Hamiltonian. The analytical properties of this matrix element can be analyzed in terms of the properties of the kernel of the Lippmann–Schwinger equation. For the Yamaguchi potential and several initial states, all functions are explicitly calculated. This approach allows the decomposition of the survival amplitude into a sum of decaying exponential terms andw-functions associated with the pole positions of the resolvent matrix element in the complex momentum plane. Novel decay behaviours are found for the decay of states associated with the resolvent poles and the decay of a state which is dominated by the form of the wave function rather than by the resolvent poles. Certain anomalous short time decay behaviour is also exemplified.

The self-consistent Mori approach to transport in exciton - phonon systems

In earlier calculations of exciton transport in exciton - phonon systems diffusivity in general has been achieved by means of semi-phenomenological elements. The present investigation aims at giving an ab initio derivation of the diffusion function, involving only intrinsic characteristics of the model. Using a projection operator technique we have studied a one-dimensional molecular crystal model with site-diagonal coupling to a phonon bath of acoustic type. The relevant memory kernels in the transport equations have been evaluated in a consistent perturbative treatment up to second order in the exciton - phonon interaction for two different kinds of local excitation. The evolution of the second moment of the exciton probability density helps us to discuss coherent and diffusive transport. The time dependence of these memory functions displays an oscillatory short-time decay in the case of a broad exciton band, which results in diffusive transport. In the high-temperature limit the diffusion constant is found to decrease with increasing temperature with the power law . In the opposite case of a small excitonic band the decay remains incomplete and the time dependence of the second moment is governed by the -law typical of coherent transport processes. Since this is a rigorous second-order result, the vanishing of the second-order term may serve as a check for any non-perturbative calculation of the diffusion constant in this case.

Characterization of the cerebral distribution of general anesthetics in vivo by two-dimensional 19F chemical shift imaging.

The distribution of two fluorinated anesthetics, halothane and isoflurane, in rabbit brain was mapped during the course of their uptake using 19F NMR chemical shift imaging techniques. Use of a short phase-encoding time and free induction decay acquisition minimized T2-related signal losses in two-dimensional chemical shift imaging. Localization of the brain and exclusion of surrounding tissues was achieved by the use of a surface coil. The spatial distribution of halothane and isoflurane in the brain was nonuniform and both anesthetics were detected predominantly in the cerebral cortex. During the early stages of uptake, anesthetic concentration in the brain was extremely low, but it continued to increase for several hours with sustained anesthetic delivery. Time-dependent variations in anesthetic concentration were observed in different brain regions.

Short-Time Molecular Orientational Dynamics of PCH5

Abstract We present results of a molecular dynamics simulation of the liquid crystalline substance p-n-pentyl-(p'- cyanophenyl)-cyclohexane (PCH5) within a united atom force field approach and analyze in detail the behavior of the orientational autocorrelation functions (CF's). Orientational CF's are calculated for different sections within the molecule in the frame of the nematic director and in the internal frame. Comparison with autocorrelation functions of the torque acting on the molecules and cross-correlation factors have led us to the conclusion that the strong non-exponentiality, observed in the short-time decay of experimentally determined CF's for some mesogens originates from molecule flexibility. Some proposals for model treatments of the internal rotation of mesogen molecules are also specified.

Molecular orientational motion in PCH5: computer simulations and models

The tumbling molecular motion in mesomorphic systems in discussed, based on a molecular dynamics computer simulation of a typical mesogen PCH5 within the united atom approach. It is shown that conformational molecular flexibility plays an important role not only in the orientational distribution of the molecules in the mesophase, but also in their dynamics. Being subjected to strongly fluctuating intermolecular forces, the relative position of the atoms in the molecule exhibit relaxation in the same time-scale as the intermolecular torque, which is much shorter than expected from the corresponding molecular moments of inertia. This results in a very fast short-time decay of the reorientational autocorrelation functions of the molecule fragments. In spite of this rather complicated picture, cage models can be used to fit the reorientational autocorrelation functions. For a more precise fit to the experimental functions one has to account for the real distribution function of the cage potential curvature.

Application of Evanescent Wave Scattering to the Study of the Motion of Liposomes near a Solid Surface

This report presents measurements and calculations of dynamic light scattering of evanescent waves from negatively and positively charged liposomes diffusing near a glass-solution interface. Scattering from the glass interface provides the reference beam for heterodyne detection. The measured short time decay of the autocorrelation functions for negatively charged liposomes is consistent with the response calculated using a model with unrestricted diffusion in half space with reflecting boundary conditions at the glass interface. In contrast, positively charged liposomes show significant deviations from the model predictions. In both cases, the short time response is significantly slower than the bulk value. The diffusion coefficient of negatively charged liposomes is nearly independent of the scattering angle. The technique gives information on the effects of "association" with the interface on the diffusive motion near an interface.

A kinetic analysis of the primary charge separation in bacterial photosynthesis. Energy gaps and static heterogeneity

Abstract We consider the energetics, the mechanism and the implications of static heterogeneity for the primary electron transfer (ET) from the electronically excited singlet state of the bacteriochlorophyll dimer (1O∗) in the bacterial photosynthetic reaction center (RC) and some of its mutants. The energetics of the primary ET was inferred from an analysis of the experimental free energy relation (at T = 300 K) between the short-time decay rates of 1P∗ and the oxidation potentials of the dimer (P) for a series of single site “good” mutants, for which geometrical changes are minimized and perturbations of the prosthetic groups of the accessory bacteriochlorophyll (B) and of the bacteriopheophytin (H) by the mutants are minor. This analysis resulted in the reasonable value of λ1 = 800 ± 250 cm−1 for the (mutant invariant) medium reorganization energy and ΔG10(N) = −480 ± 180 cm−1 for the energy gap for the native (N) RC. The low value of ΔG10(N) implies that the dominant room temperature ET mechanism for the native RC involves sequential ET. Next, we have explored the effects of heterogeneity on the primary ET by model calculations for the parallel sequential-superexchange mechanism, which is subjected to Gaussian energy distributions of the energies of the P+B−H and P+BH− ion pair states (with a width parameter of σ = 400 cm−1). The modelling of the heterogeneous kinetics by varying the (mean) energy gap ΔG1 between P+B−H and 1P∗ was performed to elucidate the temporal decay of 1P∗ and the ET quantum yield in “good” mutants, to explore the gross feature of primary ET in a triple hydrogen bonded mutant and to characterize some of the temperature dependence of the primary ET. The most pronounced manifestations of heterogeneity within the native RC and its single site mutants (ΔG1 = −900 to 300 cm−1) are the nonexponential temporal decay probabilities for 1P∗, which exhibit long-time tails, with heterogeneity effects being marked (in the classical limit) when σ(ΔG1 + λ1) > λ1kBT. When ΔG1 ⪢ σ (i.e., ΔG1 ⩾ 1000 cm−1), the relaxation rate of 1P∗ is slow, being dominated by the dimer internal conversion rate, with the effects of heterogeneity being less marked, as is the case for the triple hydrogen bond mutant. Regarding mechanistic issues, our kinetic modelling implies that at room temperature, primary ET in the native RC and its single site mutants is dominated by the sequential route and only the triple mutant exhibits a marked contribution of the superexchange route. At low temperature (T = 20 K), ET in the native RC is still dominated by the sequential route (with a small (i.e., ∼ 10%) superexchange contribution being manifested in its long-time decay), for single site mutants there is an interplay between sequential and superexchange routes, while superexchange dominates ET in the triple mutant. The heterogeneous parallel sequential-superexchange mechanism is of intrinsic significance to insure the stability of primary photosynthetic ET for different native and mutagenetically modified RCs over a broad temperature domain.

Some consequences of flux avalanches in type-II superconductors.

We derive an explicit equation for the time dependence of the current density in dirty type-II supeconductors, based on scaling ideas for flux motion developed by Tang [Physica A 194, 315 (1993)]. We explore some consequences of this model, focusing mostly on the short-time decay of the Bean state, where the total potential felt by flux lines is small.

Rheology of wormlike Micelles: Two universality classes.

We identify two classes of rheology in solutions of wormlike micellar chains which break reversibly. In ``mean-field'' (MF) systems, new ends created by breaks recombine with other chains and stress relaxes exponentially [M. E. Cates, Macromolecules 20, 2289 (1987)]. By contrast, in ``diffusion-controlled'' (DC) cases, self-recombination dominates and the relaxation process is a L\'evy walk, leading to algebraic short time decay. The long time decay remains exponential, but with a different time scale. Our estimates suggest that current experimental systems include both MF and DC types.