Fluorescence anisotropy of indole molecules under two-photon excitation in the spectral range of 485–510 nm

Abstract

Decay of polarized fluorescence in indole dissolved in propylene glycol under two-photon excitation by femtosecond laser pulses in the wavelength range of 485–510 nm has been studied. It is shown that under the experimental conditions used the fluorescence decay signal can be well described by a single excited state lifetime τ_f and a single rotation diffusion time τ_rot. By processing the data obtained, the times τ_f and τ_rot as well as anisotropy parameter r₀ characterizing the symmetry of two-photon excitation of indole molecules have been determined. Decreasing of the anisotropy parameter r₀ down to zero under two-photon excitation energy higher than 5.1 eV has been observed. Interpretation of the obtained results have been done on the basis of ab initio quantum-mechanical computations. A model of energy relaxation under the condition of twophoton excitation of indole in a polar solvent has been discussed.