The influence of rotational diffusion on transient anisotropy in ultrafast experiments.

An extensive analysis of transient anisotropy is presented including an ansatz to describe the temporal evolution of anisotropy in multiphoton experiments in the limit of Brownian motion. For the general case, this evolution is described by means of a step model interpolating between collision-free reorientation and Brownian diffusion for different geometries. The presented ansatz is able to calculate the time dependence of the anisotropy for symmetric top molecules. This dependence is shown to be in third order with respect to the solvent-solute interaction irrespective of the molecular geometry. Differences to former models are worked out and an extension to rotational coherence effects is given. Finally, the influence of collisions on the anisotropy decay is modeled by Monte-Carlo simulations allowing for a variation of angular correlation and energy transfer.