Nonstationary Rotational Diffusion in Room Temperature Liquids Measured by Femtosecond Three-Pulse Transient Anisotropy

A femtosecond three-pulse χ⁽⁵⁾ polarization anisotropy experiment is used to examine the time dependence of the rotational diffusion of coumarin 153 in polar liquids. By probing the polarization anisotropy decay at various points during the molecule's excited state lifetime, a time-dependent diffusion coefficient is found in several solvents. This anomalous behavior is consistent with the relaxation of the solvent friction to accommodate the solute's excited state charge distribution. Rotational diffusion times measured immediately after photoexcitation by two-pulse χ⁽³⁾ experiments may reflect a nonstationary bath dynamics, rather than the equilibrium friction of the solvent.