alpha - and beta -relaxation for single-particle motion near the glass transition

The susceptibility spectrum for single-particle motion near the liquid-glass transitions is discussed within a schematic model for the mode-coupling theory. In the supercooled liquid the spectrum has in certain cases two low-frequency maxima identified as the alpha - and beta -relaxation peaks. The latter process can also be observed in the glassy state. It is also demonstrated that the Kohlrausch exponent for the alpha -peak is not a universal number, but differs for the tagged-particle motion and the density relaxation process.