Four-time rotational correlation functions

A scheme to analyze four-time rotational correlation functions of any rank is developed and details for rank L = I and 2 are given. The scheme provide s a transparent way for identifying deviations from simple Markovian dynami cs as observed, e.g., in complex liquids close to the glass transition. The method should be applicable to NMR and optical multiple-pulse techniques a s well as to photon correlation spectroscopy. Results are given for H-2-NMR multiple-pulse data in supercooled glycerol. We identify and analyze the d ynamical heterogeneity of molecular reorientation in a range of 205-215 K c lose to the glass temperature T-g = 190 K.