Time-resolved fluorescence polarization and four-wave mixing from an ensemble of free symmetric rotors

A theoretical treatment of time-resolved fluorescence polarization and two time-resolved four-wave mixing processes (polarization spectroscopy and transient gratings) from a thermal ensemble of free symmetric top rotors is presented. Time-dependent signals are calculated for both parallel and perpendicular transitions at low and high temperatures. At high J, both the fluorescence anisotropies and the nonlinear signals exhibit transients near time zero which level out to essentially constant values within several average rotational periods. These transients, which represent rigid-body rotational motion in a classical picture, arise in a quantum mechanical treatment from interferences between different intermediate rotational states that are coupled to the same pair of initial and final states. The nonlinear signals are sensitive not only to rotational dynamics but also to the total dephasing rates between the final pairs of coupled levels and to effects of selective phase matching. The relevance of these short-time effects for designing and interpreting experiments to probe rotational dynamics of large molecules is discussed.