The widespread assumption that primary and secondary relaxations in glass-forming materials are independent processes is scrutinized using spin-lattice relaxation weighted stimulated-echo spectroscopy. This nuclear magnetic resonance (NMR) technique is simultaneously sensitive to the dynamics on well-separated time scales. For the deeply supercooled liquid sorbitol, which exhibits a strong secondary relaxation, the primary relaxation (that is observable using NMR) can be modified by suppressing the contributions of those subensembles which are characterized by relatively slow secondary relaxations. This is clear evidence for a correlation between primary and secondary relaxation times. In the disordered crystal orthocarborane high-frequency processes are absent and consequently no such modifications could be achieved.

Using spin-lattice relaxation weighted stimulated-echo spectroscopy, we report evidence for a correlation of the primary and secondary relaxation times. The experiments are performed using deuteron nuclear magnetic resonance somewhat above the calorimetric glass-transition of ortho-terphenyl, D-sorbitol, and cresolphthalein-dimethylether. The data analysis is based on the procedure outlined in the accompanying theoretical paper [B. Geil, G. Diezemann, and R. Böhmer, Phys. Rev. E 74, 041504 (2006)]. Direct experimental evidence for a modified spin-lattice relaxation is obtained from measurements on a methyl deuterated acetyl salicylic acid glass. The limitations of the present experimental method are discussed.

Acetyl salicylic acid, deuterated at the methyl group, was investigated using 2 H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin-lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.

The guest dynamics in tetrahydrofuran ͑THF͒ clathrate hydrate ͑THF· 17H 2 O͒ was studied using several deuteron nuclear magnetic resonance ͑NMR͒ techniques. At low temperatures the magnetization recovery proceeds in two steps. The weight of the faster contribution decreases with decreasing temperatures. This behavior is the signature of a dynamical effect. The two contributions cannot be ascribed separately to the metadeuteron and to the paradeuteron of the THF molecule. The thermal evolution of the NMR spectra was described semiquantitatively using a distorted octahedral reorientational jump model. Pseudorotation has no significant impact on the spectral width. The motional correlation times, measured using two-time stimulatedecho and spin-relaxation techniques, cover a dynamic range of nine decades. Four-time stimulated-echo measurements reveal a statically heterogeneous rotational motion of the guest molecules in the 16-faced polyhedral cages of the structure II lattice.

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