Showing papers by "Gerald Hinze published in 1999"

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

Abstract: 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-NMR), yielding almost identical spectral densities at the Larmor frequency in both cases. In particular the temperature dependence of the high-frequency wing reappears in that of the spin lattice relaxation rate. In addition a semiquantitative analysis is given for trinaphthyl benzene (1H-NMR) and tricresyl phosphate (31P-NMR).

Translational-rotational coupling in supercooled liquids : heterodyne detected density induced molecular alignment

Abstract: We present a new time domain technique for studying molecular orientational relaxation in viscous liquids. A molecular velocity gradient (acoustic disturbance) associated with a density change induced by weak absorption of a 1.06 μm excitation pulse, causes molecular alignment through translational–rotational coupling. Using an optical heterodyne detection method, molecular orientational relaxation is monitored. An eightfold experimental cycle, analogous to phase cycles in NMR, is used to separate the DIHARD signal (density induced heterodyne amplified rotational dynamics) from optical Kerr effect (OKE) contributions and thermal lensing effects. Calculations combining the Navier–Stokes equation with translational–rotational coupling are presented that describe the nature of the method. The method is analyzed theoretically and demonstrated with experiments on supercooled salol (phenyl salicylate). DIHARD experiments on salol combined with heterodyne detected OKE experiments are used to examine long time sc...

87 Rb NMR spectra of hydrogen-bonded Rb 3 D ( S O 4 ) 2 and Rb 3 H ( S O 4 ) 2 crystals

Abstract: The paraelectric and antiferroelectric phases of ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ and ${\mathrm{Rb}}_{3}{\mathrm{H}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ were investigated using rubidium NMR. The angle dependence of the second order quadrupole shifted central ${}^{87}\mathrm{Rb}$ transitions was recorded at 300 K. It was found to be in full agreement with the symmetry elements of a monoclinic $A2/a$ structure. The temperature dependence of the magnitude and asymmetry of the electrical field gradient (EFG) tensors at the Rb sites was determined using powder samples and revealed no differences between protonated and deuterated specimens. The antiferroelectric transition of ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ ${(T}_{N}=82\mathrm{K})$ is associated with a quadrupling of the unit cell. Below ${T}_{N}$ the magnitudes of the EFG at the Rb sites remain constant and smoothly tilt away from their high temperature orientations. The temperature dependence of the NMR line splittings associated with this tilt follows a power law implying an order parameter critical exponent of 0.21.