The spectral dependences of the nonlinear light absorption by organic dyes have been measured for the first time. To analyze the results quantitatively, we propose to redetermine the nonlinearity parameter I$-s$/ (the critical bleaching intensity) from a three-energy-level model of the dye. The resulting nonlinear absorption spectra indicate a monotonic and significant decrease in the region of the 0 - 0 transition compared with the short-wavelength part of the spectrum. A more than tenfold decrease in I$-s$/ is recorded in the anti-Stokes region in a polymeric matrix, which is associated with inhomogeneous distribution of the absorption centers in the microheterogeneous medium, and with a decrease in the rate constant for rearranging centers in the ground state.
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