Chaozhi Wan

Time-resolved anisotropic two-photon spectroscopy

Abstract Two-photon excitation (TPE) in a randomly oriented liquid sample generates an anisotropic distribution of excitations that can be probed by a secondary spectroscopic transition, e.g. fluorescence or transient absorption. The orientation dependence of the secondary transition dipole (and therefore of the fluorescence or transient-absorption signal) can be exploited to generate information on molecular symmetry and orientation that is not available in two-photon absorption alone. A theoretical formalism is developed here for the orientationally averaged two-photon absorptivity taking into account the orientation of the secondary transition dipole. Spherical tensor formalism is employed to distinguish isotropic and anisotropic components of the two-photon absorptivity, and several two-photon anisotropy parameters are defined. The theory describes time-resolved detection of TPE by transient-absorption or fluorescence decay measurements. Time-resolved measurements with time-resolution that is fast relative to the rotational correlation times are shown to provide new information, including the two-photon anisotropies. Applications to TPE fluorescence anisotropy measurements and TPE induced anisotropic transient absorption are described, and illustrated by experimental measurements on bacteriorhodopsin.

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...

Two-photon-induced anisotropic transient absorption in bacteriorhodopsin

Abstract Two-photon excitation (TPE) of the photocycle of bacteriorhodopsin (BR) with 1064 nm pulses was observed, and the subsequent transient absorption and linear dichroism changes measured at 568 nm. An anisotropy of 0.55 was induced in the ground-state transition of BR. A theoretical formalism is described to treat the isotropic and anisotropic compoment of the two-photon absorptivity measured by a secondary transition dipole that is induced by TPE and probed by polarized fluorescence or transient-absorption methods. The two-photon tensor for BR is found to contain components that are not parallel to the one-photon transition dipole.

Step-scan near-infrared Fourier-transform Raman spectroscopy with 100 picosecond laser pulses

Abstract Time-resolved Fourier-transform Raman scattering with picosecond excitation is reported for the first time. The resonance Raman spectrum of 9,10-diphenylanthracene in its excited single state was obtained by Raman excitation at 1064 nm with 100 ps pulses, following photoexcitation at 355 nm. The implementation and characterization of time-resolved FT-Raman spectroscopy with a step-scan interferometer is discussed. FT-Raman spectra generated with continuous and mode-locked beams in the continuous-scan mode of the interferometer are compared with step-scan FT-Raman spectra generated with 2 kHz pulses.

Picosecond Time-Resolved Fourier Transform Raman Spectroscopy of 9,10-Diphenylanthracene in the Excited Singlet State

Time-resolved Fourier transform Raman spectroscopy of the highly fluorescent chromophore 9,10-diphenylanthracene (DPA) in cyclohexane and ethanol is described. Raman spectra of the first excited singlet state of DPA were obtained with 100-ps resolution at several time delays between pump pulses at 355 nm and probe pulses at 1064 nm. The near-infrared excited-state Raman scattering is enhanced by resonance with an excited-state transition of DPA. The excited-state Raman bands decay in about 5–6 ns. Evidence for interaction of the solvent with the DPA excited state is observed in the cyclohexane C-H stretching bands.

Time-resolved resonance Raman anisotropy. Rotational motion studied by Raman spectroscopy

Abstract Time-resolved resonance Raman anisotropy (TRRRA) — a new approach to rotational motion — is described. The expressions of the time-dependent Raman susceptibility which are vibration-dependent have been derived and three Raman anisotropies analogous to fluorescence anisotropy have been obtained. The TRRRA can in principle provide more rotational information than fluorescence depolarization and linear dichroism.