NUCLEAR MAGNETIC RESONANCE PDF version 5.1 Random Field and Cluster Dynamics in the Deuteron Glass DRADP50Th. König and R. Kind
The glass phase state in DRADP50 is characterized by short range ordered antiferroelectric clusters of nanometric size. While the hydrogen bonds within the clusters are strongly biased, they are less biased in the disordered intercluster regions. This leads to a strongly heterogeneous dynamics in these systems. For the investigation of the dynamical behavior one needs a microscopic method having the proper frequency window. While the frequency window of 1DNMR extends from 10^{12} to about 10^{3} Hz, 2DNMR methods cover a region between 10^{4} and 1 Hz or even lower depending on the spinlattice relaxation time T_{1}. The following mechanisms in DRADP50 can lead to ^{87}RbNMRfrequency fluctuations and thus to offdiagonal intensities in a 2DNMRexchange experiment: Intrabond and interbond deuteron jumps, randomfield or local polarization fluctuations, and spin diffusion. ^{87}Rb 2Dexchange difference NMR measurements in DRADP50 performed with a mixing time of t_{mix1} = 50 ms revealed a crosspeak intensity at room temperature which was first assigned to spin diffusion. [1] The 2D difference spectrum drawn in Fig. 1 shows the diagonalpeak and the two crosspeaks. The characteristic time constant of this mechanism was found to be 8.3 ms. This is much too long for intrabond hopping and much too short for spin diffusion. Additional measurements are needed to rule out one of the two remaining possibilities.
5.2 Quadrupolar Glasses Containing HF_{2}^{} as Reorientable UnitsC. Jeitziner and R. Kind in cooperation with A. Fuith, University of Vienna
Rb(HF_{2})_{1x}Cl_{x} is a model system for studying the properties of quadrupolar glasses. The aim of the measurements is to investigate the type of ordering of the reorientable HF_{2}^{} molecules as a function of Cl content and temperature. We expect that for low temperatures, the mixed crystal will be in a glass state, i.e. the HF_{2}^{} molecules exhibit a short range correlated freeze out. We have measured ^{1}H and ^{19}F NMR spectra in various powder samples (x = 0.05 and 0.1) of Rb(HF_{2})_{1x}Cl_{x} as a function of temperature. The observed line shape transition at 430 K indicates the transition of the reorientable HF_{2}^{} units from the slow motion regime to the fast motion regime (Fig.1). It coincides with the phase transition from a tetragonal to a cubic structure. The powder samples have the drawback that they are chemically not very stable at temperatures above 400 K and set possibly HF free. For x = 0.05 proton spinlattice relaxation time (T_{1}) measurements between 265 and 374 K have been performed. A frequencyresolved experiment revealed that the central peak of the proton spectrum relaxes faster than the rest, which may be assigned to freely rotating HF_{2}^{} ions. T_{1} is for 260 K in the order of 3 s, reaches its minimum of 0.6 s at room temperature and grows again above room temperatures to values of about 1 s. The ^{19}F spinlattice relaxation time at room temperature was found to be in the order of 10 s. First ^{1}H and ^{19}F measurements on a single crystal, grown from a solution of 50% RbCl and 50% RbHF_{2}, have been performed between room temperature and 77 K. The spectra show that the quality of the crystals is probably not sufficient for our investigations.
^{19}F spectra of Rb(HF_{2})_{0.9}Cl_{0.1} at 297 K and 440 K, respectively.
5.3 Dynamics in Polymer GlassesP. Cereghetti and R. Kind in cooperation with J. Higgins, Imperial College, London
An interesting feature in polymer glasses is the heterogeneity of the relaxation in these systems. The basic question concerns the probability distribution function of the single particle autocorrelation time which is believed to be characteristic for the correlation length of the local order. While T_{1}measurements yield information about fast fluctuations, the 2DNMR measurements cover the low frequency spectrum down to 1/T_{1}. In the framework of her diploma thesis Paola Cereghetti has performed extended deuteron 2DNMRexchange measurements on partially deuterated PMMA using fourpulse sequence methods. The aim was to observe the CD_{3} rotational jumps around the methyl group symmetry axis directly. The measurements were mainly performed at 40 K. The experiments showed some offdiagonal intensity in the 2Dspectrum which, however, could not be assigned to the motion under investigation. Several improvements of the probehead have brought a gain in signal intensity and a wider irradiation spectrum. Moreover, the phase corrections for a pure 2Dabsorption spectrum were performed in a more sophisticated way. We could improve the signal detection substituting the formerly used fourpulse sequence by a fivepulse sequence. In this way the 2Dtime domain signal was unambiguously collected and we obtained a well defined 2Dspectrum after Fourier transformation. Further problems arose because of the long spin lattice relaxation time of the sample, particularly at 40 K, where it is in the order of 18 s. It turned out that to obtain an acceptable signaltonoise ratio prohibitively long measuring times. where needed. Exchangedifference measurements were also tried, but the limiting parameter was also here the measuring time. From former T_{1}measurements we know that the probability distribution of t_{c} has a width of more than 4 orders of magnitude at 40 K. Though part of this distribution falls into the observation window of the five pulseexperiment the corresponding offdiagonal intensity in the 2Dspectrum is probably too small to be detected unanimously. At present we investigate an isotactic modification of the sample. We
are interested whether the ester methyl dynamics is affected by the change
in the polymer conformation from syndiotactic to isotactic. We started with
spin alignment relaxation time measurements using a broad band pulse sequence
in order to equally excite the whole ^{2}Hspectrum. 5.4 Magnetic Field Dependence of the VaporCell Zeeman Optical TrapT. MartyCold atoms are an interesting starting point for different fields of physics, such as precision microwave spectroscopy or collisional studies of ultracold atoms with each other. While the magnetooptical trap is widely used to produce samples of cold atoms, a threedimensional description of the capture process is far too complicated to be solved analytically. Even for numeric computations the problem has to be considerably simplified. In this work the magnetic field dependence of the vaporcell Zeeman optical trap has been investigated for cesium atoms. The measured dependence of the number of trapped atoms on laser detuning and magnetic field gradient is compared by contour plots with a theoretical onedimensional model of the capture process that includes the spatially changing magnetic field.

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