Franz Fujara

The molecular dynamics around the glass transition and in the glassy state of molecular organic systems: A 2H−nuclear magnetic resonance study

2H‐nuclear magnetic resonance(NMR)‐spin–lattice relaxation experiments have been performed for studying the crossover from viscous (α process) to secondary (β processes) dynamics in the van der Waals liquid orthoterphenyl and the H‐bridged network glycerol. The essential and general features, observed in both systems, are the following: (a) a dominating α process in the liquid and viscous regime; (b) a change from exponential to nonexponential spin–lattice relaxation as the temperature is lowered below a characteristic temperature above Tg; (c) the existence of a slow (>10−9 s) secondary reorientational process in the highly viscous regime; and (d) the existence of a fast (∼10−12 s) local process in the glassy state. Whereas the slower process is shown to be the one known from dielectric studies, we attribute the fast mode to a β process found in quasielastic neutron scattering.

Kinetics of the high- to low-density amorphous water transition

In situ neutron diffraction experiments have been carried out to study the kinetics of the transformation of high-density amorphous (HDA) water into its low-density amorphous state at temperatures 87 K ≤ T ≤ 110 K. It is found that three different stages are comprised in this transformation, namely an annealing process of the high-density matrix followed by a first-order-like transition into a low-density state, which can be further annealed at higher temperatures T ≤ 127 K. The annealing kinetics of the HDA state follows the logarithm of time as found in other systems showing polyamorphism. According to the theory of transformation by nucleation and growth the apparent first-order transition follows an Avrami–Kolmogorov behaviour. An energy barrier ΔE ≈ 33 k Jmol−1 is estimated from the temperature dependence of this transition.

Formation of ice XII at different conditions

The structural versatility of solid water has been reinforced by the identification of a metastable, crystalline phase called ice XII (ref. 1). It was obtained by cooling liquid water to 260 K at a pressure of 0.55 GPa. We have found ice XII in a completely different region of waters phase diagram and show that it can be formed under different conditions from those previously reported.

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.

FAST RELAXATION IN A FRAGILE LIQUID UNDER PRESSURE

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron time-of-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The alpha-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of beta relaxation. The beta-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure P_c(T). alpha and beta relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Gamma ~ n*T**{-1/4}.

Ice XII in Its Second Regime of Metastability

We present neutron powder diffraction results which give unambiguous evidence for the formation of the recently identified new crystalline ice phase [2], labeled ice XII, in completely different conditions. Ice XII is produced here by compressing hexagonal ice I(h) at T = 77, 100, 140, and 160 K up to 1.8 GPa. It can be maintained at ambient pressure in the temperature range 1.5<T<135 K. Accurate structural properties are obtained from high-resolution diffraction carried out at T = 1.5 K. At T = 140 and 160 K additionally ice III/IX is formed. The increasing amount of ice III/IX with increasing temperature gives an upper limit of T approximately 150 K for the successful formation of ice XII with the presented procedure.

Solid State Field-Cycling NMR Relaxometry: Instrumental Improvements and New Applications

The paper reviews recent progress in field cycling (FC) NMR instrumentation and its application to solid state physics. Special emphasis is put on our own work during the last 15years on instrumentation, theory and applications. As far as instrumentation is concerned we report on our development of two types of electronical FC relaxometers, a mechanical FC relaxometer and a combination of FC and one-dimensional microimaging. Progress has been achieved with respect to several parameters such as the accessible field and temperature range as well as the incorporation of sample spinning. Since an appropriate analysis of FC data requires a careful consideration of relaxation theory, we include a theory section discussing the most relevant aspects of relaxation in solids which are related to residual dipolar and quadrupolar interactions. The most important limitations of relaxation theory are also discussed. With improved instrumentation and with the help of relaxation theory we get access to interesting new applications such as ionic motion in solid electrolytes, structure determination in molecular crystals, ultraslow polymer dynamics and rotational resonance phenomena.

Reorientational and translational dynamics of benzene in zeolite NaY as studied by one- and two-dimensional exchange spectroscopy and static-field-gradient nuclear magnetic resonance

One- and two-dimensional 2H- and 13C-NMR (nuclear magnetic resonance) echo spectroscopy and 1H static field gradient NMR self-diffusion experiments have been used to study the reorientational and translational dynamics of benzene molecules adsorbed on zeolite NaY as a function of loading. Comparison of the data with model calculations establish that the elementary motional process of the guest molecules is consistently identified as a jump process among well defined adsorption sites inside a supercage and/or a jump between nearby supercages. In cases where the zeolite cavities contain high loadings of guest molecules, each molecular jump is accompanied by concomitant relaxation of the local environment. Molecular jump events between adsorption sites correspond to the elementary processes from which long range translational diffusion evolves.

Reorientation of benzene in its crystalline state: A model case for the analogy between nuclear magnetic resonance spin alignment and quasielastic incoherent neutron scattering

The close analogy between 2H‐NMR spin alignment and 1H quasielastic incoherent neutron scattering [J. Chem. Phys. 84, 4579 (1986)] in determining the geometry and time scale of molecular reorientation is illustrated by an experimental example. Analysis of the final states of both methods show consistently that benzene in its (poly)crystalline state reorients by rotational jumps about the molecular sixfold symmetry axis. Emphasis is put on the quasielastic structure factor of incoherent neutron scattering, which excludes random jumps among the six orientations as the reorientation mechanism, allowing only single rotational jumps.

1H-14N cross-relaxation in trinitrotoluene - a step toward improved landmine detection

Motivated by the lack of efficient detection techniques for metal-free trinitrotoluene (TNT) containing landmines, 1 H– 14 N cross-relaxation experiments on TNT have been performed using field-cycling spectroscopy in an electronically switchable volume coil. Using 1 H NMR detection (indirect method), the 14 N quadrupole spectra in several small (about 0.5 g) TNT samples of different producers are determined. The experiment is considered as a step toward a double resonance TNT landmine detection scheme using surface coils.