Polarized fluorescence in tryptophan excited by two-photon femtosecond laser pulses

The decay of polarized fluorescence of tryptophan in water-propylene glycol solution two-photon excited by femtosecond laser pulses was explored. Linearly and circularly polarized light in the wavelength band of 385–510 nm was used for excitation. Processing of the experimental fluorescence signals as functions of time and light polarization provided an opportunity to identify two characteristic lifetimes characterizing the fluorescence intensity decay and the rotational diffusion time $\tau_{\mathrm{rot}}$. The spectral dependences of the anisotropy parameter $\Omega$ and anisotropy parameters $r$ for linearly and circularly polarized excitation have been determined. It is shown that the fluorescence temporal dependence is well described by the sum of two exponents with amplitudes $a_1$ and $a_2$ and characteristic times $\tau_ {f 1}$ and $\tau_{f 2}$ at all wavelengths in the studied spectral range. The experimental dependences of parameters $\Omega$, $r$, $\tau_{f 1}$, and $a_2/a_1$ are nontrivial, which indicates the complexity of the excitation relaxation processes. The obtained results can be used for interpretation of the experiments and prove of the theoretical models describing the intensity and polarization of fluorescence of tryptophan molecules in solutions and organic compounds under optical excitation.