Search results for "Rotational diffusion"
showing 10 items of 19 documents
Simultaneous measurement of rotational and translational diffusion by forced Rayleigh scattering. Colloid spheres in suspension
1996
Abstract It is shown that the technique of forced Rayleigh scattering, traditionallyemployed to probe translational diffusion, can be employed to probe rotational diffusion as well. Thus with a single experiment both quantities are measured. The system under investigation is colloid spheres suspended in a glass-forming liquid. Furthermore, it is shown that the Stokes-Einstein and Debye-Stokes-Einstein relations for translational and rotational diffusion, respectively, are valid for spherical colloid particles in a supercooled liquid matrix in the vicinity of the glassy state.
Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering
1994
It has been shown that translational diffusion coefficients, Dt, in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T ≲ 1.2Tg and can be described by Dt ∼ η−χ with χ < 1 whereas Dr ∼ η−1 applies for the mean rotational diffusion coefficients, Dr, down to the glass transition temperature, Tg. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to Tg.
Photoinduced optical anisotropy in organic molecular films controlled by an electric field
1993
Abstract The photoinduced reorientation of dye molecules in molecular films on solid substrates has been controlled with external dc electric fields, leading to solid state structures, which are macroscopically polar, temporally stable at room temperature, and well-defined at the molecular level. A simulation based on the excitation-driven rotational diffusion of the molecules in the potential of their neighbors yielded a realistic model and indicated a collective character of the process. This shows that we are close to a quantitative comprehension of the molecular interactions within these films.
CHLOROPHYLL a AGGREGATES IN HYDROCARBON SOLUTION, A PICOSECOND SPECTROSCOPY AND MOLECULAR MODELING STUDY
1993
Chlorophyll a aggregates in 3-methylpentane solution have been studied by using picosecond absorption and fluorescence spectroscopy and molecular modeling. Chlorophyll a aggregates give rise to reversible temperature changes in the absorption and fluorescence spectra. Time-dependent anisotropies were used to estimate rotational correlation times of the aggregates. These were indicative of the sizes of the aggregates. The rotational diffusion of the monomer and the two identified aggregates was hydrodynamic over the viscosity range studied (0.29–1.8 cP). Molecular mechanics calculations were used to predict the minimum energy structures of several chlorophyll a dimers suggested earlier in th…
Rotational diffusion of colloid spheres in concentrated suspensions studied by deuteron NMR
1997
We present a study of the application of deuteron-nuclear magnetic resonance spectroscopy (NMR) to the investigation of the rotational diffusion of spherical colloidal particles. We performed NMR pulse experiments on colloidal suspensions of polystyrene latex spheres in water-glycerol mixtures in a wide range of particle volume fractions \ensuremath{\varphi} from the dilute suspension up to \ensuremath{\varphi}=0.504. We have analyzed the stimulated echo NMR signal in the time domain. The full shape of the orientational correlation function deviates from an exponential behavior in the whole \ensuremath{\varphi} range examined. We evaluate the rotational diffusion coefficient and calculate i…
Anisotropic motion of toluene above and below the glass transition studied by 2H NMR
1995
Abstract 2 H nuclear magnetic resonance spin-lattice relaxation experiments on two selectively deuterated toluene molecules have been performed over a wide temperature range, spanning liquid and glassy states, to examine anisotropic molecular dynamics. In the liquid regime, the relaxation data are analyzed by the model of anisotropic rotational diffusion. A more phenomenological ansatz is used for the whole temperature regime to obtain information about anisotropic reorientation and its temperature dependence. We find that the anisotropy is reduced in the supercooled state and the motion becomes approximately isotropic below 140 K which is interpreted as the onset of cooperative reorientati…
Rotational Correlation Functions of Single Molecules
2004
Single molecule rotational correlation functions are analyzed for several reorientation geometries. Even for the simplest model of isotropic rotational diffusion our findings predict non-exponential correlation functions to be observed by polarization sensitive single molecule fluorescence microscopy. This may have a deep impact on interpreting the results of molecular reorientation measurements in heterogeneous environments.
2 H-NMR-Stimulated Echo Study of Ultraslow Reorientational Motion in Viscous Glycerol near Its Glass Transition Temperature
1990
2H-NMR stimulated echo experiments have been performed in order to study the molecular basis of the ?-process in viscous glycerol near its glass transition temperature. Decay functions following modified Jeener-Broekaert pulse sequences were compared with predictions from different models for molecular reorientation. Rotational diffusion, rotational random jumps, rotational fixed-angle jumps and combinations of diffusive and jump motions have been tested. All data are fitted with a log-Gaussian distribution of correlation times. Thereby, small-but finite-angle reorientation processes turn out to dominate in the 10-3 s.. 100 s regime. Pure large-angle rotational jumps can be ruled out with h…
2H NMR Time Domain Analysis of Ultraslow Reorientations in Supercooled Liquids
1998
A method for evaluating 2H NMR stimulated echo experiments in the time domain is presented. It exhibits a high sensitivity to molecular reorientation mechanisms for small angles. Reorientations with jump angles below 25degrees can be resolved with a precision of approximately 1degrees so that, e.g., rotational diffusion and finite jump angle mechanisms become distinguishable. The method, applicable to isotropic reorientation models, is thought to extend the domain of 2D exchange spectroscopy where the best resolution is obtained in the large angle range of anisotropic reorientational mechanisms. Application to reorientation in the supercooled melt of ortho-terphenyl is presented. These data…
Experimental Determination of Four-Time Stimulated Echoes in Liquids, Colloidal Suspensions, and Crystals
1998
Four-time stimulated echo experiments have been used to monitor the temporal evolution of the reorientation rates of deuterated molecules or colloidal suspensions. We present extended phase cycles for this seven-pulse experiment. In order to test its performance three vastly different materials are chosen. These include a crystal in which the molecules carry out well-defined 180 degrees flips and a supercooled liquid characterized by a distribution of jump angles centered around 10 degrees. As an example for rotational diffusion, data on a concentrated suspension of polystyrene spheres in a viscous medium are presented. Copyright 1998 Academic Press.