Time-resolved two-photon induced anisotropy decay: The rotational diffusion regime

Two‐photon excitation (TPE) of randomly oriented chromophores in solution generates an anisotropic distribution. In a previous paper [Chem. Phys. 179, 513 (1994)], the polarization dependence of the TPE signal probed by a secondary spectroscopic transition (fluorescence or transient absorption) was determined. In this paper, the time dependence of anisotropic two‐photon induced fluorescence or transient absorption signals due to rotational diffusion is treated in spherical tensor formalism. The two‐photon signal in general contains isotropic (orientation independent) and anisotropic (orientation dependent) contributions. The latter decay with up to five exponential components. Four time‐dependent anisotropy parameters can be defined and measured, allowing additional information, not available in conventional one‐photon fluorescence depolarization measurements, to be determined. The special case of one‐color TPE is discussed in particular. It is shown that by measurement of the linear and circular anisotro...