Reorientational relaxation of a linear probe molecule in a simple glassy liquid

Within the mode-coupling theory (MCT) for the evolution of structural relax ation in glass-forming liquids, correlation functions and susceptibility sp ectra are calculated characterizing the rotational dynamics of a top-down s ymmetric dumbbell molecule, consisting of two fused hard spheres immersed i n a hard-sphere system. It is found that for sufficiently large dumbbell el ongations, the dynamics of the probe molecule follows the same universal gl ass-transition scenario as known from the MCT results of simple liquids. Th e cu-relaxation process of the angular-index j = 1 response is stronger, sl ower, and less stretched than the one for j = 2, in qualitative agreement w ith results found by dielectric-loss and depolarized-light-scattering spect roscopy for some supercooled liquids. For sufficiently small elongations, t he reorientational relaxation occurs via large-angle flips, and the standar d scenario for the glass-transition dynamics is modified for odd-j response s due to precursor phenomena of a nearby type-A MCT transition. In this cas e, a major part of the relaxation outside the transient regime is described qualitatively by the beta-relaxation scaling laws, while the cu-relaxation scaling law is strongly disturbed.