Orientational dynamics on glassformer 2 [Ca (NO)]⋅ 3 [KNO]: A study by transient optical Kerr effect

The fast orientational dynamics of molten 2[Ca(NO3)2].3[KNO3] is investigated by time-resolved optical Kerr effect (OKE) with subpicosecond resolution in the temperature range 380-620 K. Apart from the instantaneous electronic response and a very fast decay, the OKE intensity profiles show clearly the presence of a nonexponential decay. The simplest function which fits the experimental data is a biexponential with time constants ranging from 120 to 15 and 10 to 3 ps, respectively. The results are discussed in terms of an integrated correlation time tau(c) that does not show a diverging behavior approaching the glass transition. At high temperatures tau(c) follows a typical Arrhenius law with activation energy of 1.5 kcal/M. Different possible interpretations of the observed behavior (alpha-beta bifurcation, domain size effect) are discussed. The main conclusion is that rotational motions are decoupled from translational ones even for temperatures well above the liquid-glass transition.