Fast stochastic librations and slow small-angle rotations of molecules in glasses observed on nitroxide spin probes by stimulated electron spin echo spectroscopy

Electron spin echo (ESE) spectroscopy as applied to nitroxide spin probes in glasses is sensitive to molecular motions of two types. The first type of motion is fast stochastic librations, with correlation times on the scale of nanoseconds. In this work, these librations were found in glassy glycerol above ∼190 K and in glassy o-terphenyl above ∼250 K (glass transition temperatures for these two solvents are Tg = 185 K and Tg = 243 K, respectively). Most likely, these librations are of the same origin as anharmonic atomic vibrations, as seen in glasses by neutron scattering above a so-called dynamical transition temperature. The second type of motion is slow millisecond inertial rotations, which are observed in the experimentally accessible microsecond time scale as developing within an angular range of ∼0.1–2°. Over the temperature range studied, the square of the characteristic rotation rate was found to be proportional to the dielectric α-relaxation rate that is known for glycerol and o-terphenyl from literature. This empirical fact probably means that small-angle inertial rotations of spin probes and dielectric α-relaxation in the solvent are interrelated phenomena.