G. Hinze

Reorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy

The method of stimulated echoes was used to investigate the reorientational mechanism in the selectively deuterated glass-former glycerol, C3D5(OH)3 about 15 K above its calorimetric glass temperature. The reorientation process is fully isotropic. This enables an accurate determination of the decay constant, T1Q, of the quadrupolar spin order in the regime of ultraslow motion. The knowledge of this time constant has made it possible to reliably determine the rotational correlation function. The experimentally obtained evolution time-dependent correlation functions are compared with those from a simulation procedure involving a distribution of molecular jump angles. It is found that in glycerol small angles in the 2°–3° range dominate. They are accompanied by a small, but significant, fraction of larger jump angles.

The spectral density in simple organic glass formers: Comparison of dielectric and spin-lattice relaxation

The spin-lattice relaxation time T1 of simple organic glass formers is analyzed by introducing a spectral density obtained from broadband dielectric susceptibility data χ″(ω). For this purpose χ″(ω) was measured for several glass formers, that do not exhibit a Johari-type secondary relaxation process, covering a frequency range between 10−2 Hz and 109 Hz at temperatures above and below the glass transition temperature Tg. We introduce an analytical function to fit the shape of the main relaxation (α-process) above Tg, in particular taking into account high-frequency contributions in χ″(ω) commonly known as high-frequency wing. Below Tg the latter feature appears as a power law susceptibility χ″(ω)∝ω−γ, with γ<0.1 and a characteristic temperature dependence χ″(T)∝exp(T/const.), yielding almost 1/ω behavior in the spectral density. On the base of this complete description of χ″(ω), a quantitative comparison of dielectric and NMR spectroscopy is possible, which is carried out in full detail for glycerol-d3 (...

2H nuclear magnetic resonance study of supercooled toluene: Slow and fast processes above and below the glass transition

2H‐NMR spin‐lattice relaxation times T1 of phenyl ring deuterated toluene have been measured as a function of temperature (75–290 K) at Larmor frequencies between 13.8 and 55.8 MHz. The results are interpreted by assuming a Cole–Davidson rotational correlation time distribution for the α process dominating T1 at temperatures down to about 20 K above the glass transition (Tg=117 K) and one further process at lower temperatures. The latter is analyzed below Tg using a model with temperature dependent librational angles ΔΦ<13° and mean correlation times τ≳10−9 s. Self‐diffusion coefficients are determined at temperatures down to 136 K (D=1.5×10−14 m2 s−1) using a static field gradient 1H‐NMR method. The product of D and the mean rotational correlation time shows a tendency to increase at the lowest temperatures.

New perspectives of NMR in ultrahigh static magnetic field gradients

Abstract The magnetic field gradient nuclear magnetic resonance (NMR) stimulated echo experiment measures the incoherent (or self) part of the intermediate scattering function S( Q ,t) ∼ 〈 exp [ − i Qr (O)] exp [ i Qr (t)]〉 with a ‘generalized’ scattering vector Q = γ· g ·τ (γ is the gyromagnetic ratio, g is the magnetic field gradient, τ is the evolution time). With ultrahigh static field gradients up to ≈ 180 T/m, a prototype of which has recently been installed in Mainz, Q-values up to > 10−2 A−1 become accessible. The first part of the paper focusses on details of this technical development and points out the close analogy with incoherent neutron scattering. In the second part, the enormous new possibilities of this kind of gradient NMR are demonstrated through a collection of most recent applications: the measurement of small self-diffusion coefficients down to about 10−15 m2s−1 in supercooled liquids and in molecular crystals, long chain polymer dynamics, restricted diffusion in systems of confined mesocopic geometries and anomalous diffusion on fractal structures.

A detailed test of mode-coupling theory on all time scales: Time domain studies of structural relaxation in a supercooled liquid

The dynamics of supercooled salol (phenyl salicylate) was measured in the time domain using optical Kerr effect techniques. By combining several experimental setups, data spanning more than six decades in amplitude and time (∼100 fs to ∼1 μs) were observed. The data have a complex shape, ranging from high-frequency intramolecular oscillations at short times, to nearly exponential relaxation at long times. As predicted by mode-coupling theory (MCT), the data for some ranges of time appear as power laws. The slowest power law, the von Schweidler power law, has an almost constant exponent of ∼0.59 over the entire temperature range studied (247–340 K). Above the MCT Tc (T>∼1.17 Tg, where Tg is the laboratory glass transition temperature) for t>∼1 ps, the decays are shown to be in excellent agreement with the master curve predicted by ideal MCT when higher order terms are included. However, the data do not display the plateau predicted by ideal MCT. To discuss the data at all temperatures, the intermediate tim...

Heterogeneity at the glass transition: what do we know?

We critically discuss the information that can be obtained from experiments with respect to the existence, the life time, and the length scale of dynamical heterogeneity in glass-forming liquids. The ability to select a dynamically distinguishable subensemble and observe its return to the full equilibrium ensemble is illustrated by examples from multi-dimensional NMR. We also discuss non-resonant hole burning spectroscopy as an example for which two separate time scales are involved.

Intramolecular electronic excitation energy transfer in donor/acceptor dyads studied by time and frequency resolved single molecule spectroscopy

Electronic excitation energy transfer has been studied by single molecule spectroscopy in donor/acceptor dyads composed of a perylenediimide donor and a terrylenediimide acceptor linked by oligo(phenylene) bridges of two different lengths. For the shorter bridge (three phenylene units) energy is transferred almost quantitatively from the donor to the acceptor, while for the longer bridge (seven phenylene units) energy transfer is less efficient as indicated by the occurrence of donor and acceptor emission. To determine energy transfer rates and efficiencies at the single molecule level, several methods have been employed. These comprise time-correlated single photon counting techniques at room temperature and optical linewidth measurements at low temperature (1.4 K). For both types of measurement we obtain broad distributions of the rate constants of energy transfer. These distributions are simulated in the framework of Forster theory by properly taking into account static disorder and the flexibility of the dyads, as both effects can substantially contribute to the distributions of energy transfer times. The rate constants of energy transfer obtained from the calculated distributions are smaller on average than those extracted from the experimental distributions, whereby the discrepancy is larger for the shorter bridge. Furthermore, by plotting the experimentally determined transfer rates against the individual spectral overlaps, approximately linear dependencies are found being indicative of a Forster-type contribution to the energy transfer. For a given single molecule such a linear dependence could be followed by spectral diffusion induced fluctuations of the spectral overlap. The discrepancies between measured energy transfer rates and rates calculated by Forster theory are briefly discussed in light of recent results of quantum chemical calculations, which indicate that a bridge-mediated contribution is mainly responsible for the deviations from Forster theory. The availability of the inhomogeneous distributions of donor and acceptor electronic transition frequencies allows for comparing the energy transfer process at liquid helium and room temperature for the same set of molecules via simple simulations. It is found that on average the energy transfer is by a factor of approximately 3 faster at room temperature, which is due to an increase of spectral overlap.

Dynamics in supercooled polyalcohols: Primary and secondary relaxation

We have studied details of the molecular dynamics in a series of pure polyalcohols by means of dielectric spectroscopy and 2H nuclear magnetic resonance (NMR). From glycerol to threitol, xylitol and sorbitol a systematic change in the dynamics of the primary and secondary relaxation is found. With increasing molecular weight and fragility an increase in the width of the α-peak is observed. Details of the molecular reorientation process responsible for the α-relaxation were exploited by two-dimensional NMR experiments. It is found that in the same sequence of polyalcohols the appearance of the secondary relaxation changes gradually from a wing type scenario to a pronounced β-peak. From NMR experiments using selectively deuterated samples the molecular origin of the secondary relaxation could be elucidated in more detail.

A nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl

Using deuteron NMR techniques two-, effective three-, and various four-time correlation functions were recorded for supercooled ortho-terphenyl at 10–15 K above the calorimetric glass transition in order to characterize the heterogeneous nature of its primary response. The experimental results could successfully be described within various energy landscape models as well as via continuous time random walk simulations. These theoretical considerations provide a suitable basis for a definition of the term dynamic heterogeneity. We discuss the power but also some limitations of the present multidimensional NMR techniques when applied to amorphous materials.

Dielectric relaxation in the fragile viscous liquid state of toluene

Dielectric measurements were carried out on viscous toluene covering a frequency range from 0.1 Hz to 1 MHz. In order to suppress the pronounced crystallization tendency of this supercooled liquid it was contained in thin walled capillaries with outer diameters of 300 μm. From the temperature dependence of the characteristic dielectric relaxation times it was found that toluene is one of the most fragile low molecular weight glass-forming liquids, with a fragility index m=105. By comparison with time constants available from other experimental techniques it appears that near the glass transition the dielectric relaxation mode is not the slowest one.