Temperature dependence of amplitudes of libration motion of guest spin-probe molecules in organic glasses

Electron paramagnetic resonance (EPR) lineshapes of nitroxide spin probes i n a variety of organic supercooled liquids and glasses were analyzed within the framework of the fast librational motion model. Computer simulation co nfirmed that the EPR lineshape may be fitted well by the librational model. For all systems studied, a mean-squared amplitude of this motion, [alpha(2 )], was found to depend linearly on temperature in the low-temperature regi on, which is typical of harmonic solids. The slope of the linear dependence varies from glass to glass and does not change for different nitroxides in the same glass. This slope seems to correlate with the strength of the int ermolecular bonds and with a degree of the fragility of the glass. Steep ri ses in [alpha(2)] were observed at high temperature. (C) 1999 American Inst itute of Physics. [S0021-9606(99)70814-7].