Polarized fluorescence in indole under two-photon excitation by femtosecond laser pulses

We have analyzed the decay of fluorescence intensity in indole dissolved in propylene glycol under two-photon excitation by linearly and circularly polarized femtosecond laser pulses in the wavelength range 475–510 nm. The dependences of fluorescence intensity $I_0$, anisotropy $r$, and parameter $\Omega$ on the excitation energy have been determined and analyzed. In particular, a nonmonotonic behavior of $I_0$ indicating the increase of indole excited state density in the excitation energy range above 5.1 eV and the anisotropy sign reversal due to the variation of the symmetry of indole vibronic states have been observed and interpreted theoretically.