Dynamic heterogeneity in the glass-like monoclinic phases of CBr(n)Cl(4-n), n = 0,1,2.

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately.

Spin-Echo Mapping Spectroscopy Applied to NQR.

A theoretical description of nuclear spin-echo Fourier-transform mapping spectroscopy (NSEFTMS) for broad NQR lines is derived from the time-domain spin-echo theory for a spin I = 32 system. The expression found for the effective broadband RF excitation profile is similar to that in NMR spectroscopy. The main difference is the angular dependence of the nutation frequency, but this does not change the overall behavior.