Tunnelling molecular motion in glassy glycerol at very low temperatures as studied by 1H SQUID nuclear magnetic resonance

The H-1 nuclear spin-lattice relaxation process in glycerol has been studied at temperatures from 3.5 K to 300 K over a very wide range of Larmor frequency between 236 kHz (0.00554 T) and 21.0 MHz (0.4932 T). A superconducting quantum interference device (SQUID) was used to detect the longitudinal component of magnetization of the proton at very low frequencies below 1.62 MHz. At sufficiently low temperatures the nuclear spin-lattice relaxation rate obeys a relation 1/T-1 proportional to (T-2/omega(beta))integral(0)(6/T) [(x dx)/sinh x], (with beta around 0.9 below 25 K), implying that the relaxation rare is governed by an excitation of low-frequency disordered modes inherent to the glassy state of glycerol and becomes asymptotically 1/T-1 proportional to T-2 below T = 3 K and 1/T-1 proportional to T above T = 3 K. The relaxation phenomena can be interpreted as the nuclear spin dipping associated with a Raman process which is induced by a coupling of thermally activated low-frequency disordered modes or low-frequency excitation (LFE) with a phonon bath. The LFE originates from a quantum-mechanical two-level system (TLS) reflecting an asymmetric-double-well (ASDW) potential which is formed by the hydrogen bonding configuration in the glassy stare of glycerol. The maximum characteristic asymmetry of the double-well potential was found to be (3 +/- 1) K. This quantum-mechanical molecular motion dominates the other relaxation mechanisms at low temperatures, such as the dipolar relaxation due to molecular classical reorientation with distributed correlation times.