This paper presents methods for measuring and modeling optical bleaching that occurs, by using continuous laser sources for solution-phase organic dye molecule spectrometry. Photothermal lens experiments are used to measure the nonlinear optical absorption coefficients of eosin Y, erythrosin B, and pseudo-isocyanine iodide dyes in ethanol as a function of excitation irradiance. Excitation irradiance-dependent photothermal lens data are subsequently interpreted in terms of the photophysics and excited-state relaxation dynamics of the condensed-phase dye molecules under study. The model uses first-order kinetics for excitation and subsequent metastable-state relaxation back to the ground state and accounts for both ground- and metastable-state absorption. The measurement model accounts for thermal lens formation and apparatus configuration. Nonlinear absorption measurements are fit to the model with nonlinear regression. The resulting parameters allow calculation of triplet-state absorption coefficients and triplet to ground singlet state-relaxation rate constants.
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