TIME-RESOLVED ANISOTROPIC 2-PHOTON SPECTROSCOPY

Two-photon excitation (TPE) in a randomly oriented liquid sample gener ates an anisotropic distribution of excitations that can be probed by a secondary spectroscopic transition, e.g. fluorescence or transient a bsorption. The orientation dependence of the secondary transition dipo le (and therefore of the fluorescence or transient-absorption signal) can be exploited to generate information on molecular symmetry and ori entation that is not available in two-photon absorption alone. A theor etical formalism is developed here for the orientationally averaged tw o-photon absorptivity taking into account the orientation of the secon dary transition dipole. Spherical tenser formalism is employed to dist inguish isotropic and anisotropic components of the two-photon absorpt ivity, and several two-photon anisotropy parameters are defined. The t heory describes time-resolved detection of TPE by transient-absorption or fluorescence decay measurements. Time-resolved measurements with t ime-resolution that is fast relative to the rotational correlation tim es are shown to provide new information, including the two-photon anis otropies. Applications to TPE fluorescence anisotropy measurements and TPE induced anisotropic transient absorption are described, and illus trated by experimental measurements on bacteriorhodopsin.