Ions in glass-forming glycerol: close correlation of primary and fast β relaxation

We provide broadband dielectric loss spectra of glass-forming glycerol with varying additions of LiCl. The measurements covering frequencies up to 10 THz extend well into the region of the fast β process, commonly ascribed to caged molecular dynamics. Aside from the known variation of the structural α relaxation time and a modification of the excess wing with ion content, we also find a clear influence on the shallow loss minimum arising from the fast β relaxation. Within the framework of mode-coupling theory, the detected significant broadening of this minimum is in reasonable accord with the found variation of the α-relaxation dynamics. A correlation between α-relaxation rate and minimum position holds for all ion concentrations and temperatures, even below the critical temperature defined by mode-coupling theory.