Limitations Arising from Optical Saturation in Fluorescence and Thermal Lens Spectrometries Using Pulsed Laser Excitation: Application to the Determination of the Fluorescence Quantum Yield of Rhodamine 6G

The high incident irradiances available with pulsed lasers can lead to a significant depletion of the ground-state population of the chromophore and to optical saturation effects. As a result, the optical absorption coefficient decreases as a function of the excitation energy and, because the amount of energy released by radiative and nonradiative relaxation processes depends on the amount of energy absorbed, nonlinear energy-dependent signals are obtained. Therefore, large errors can be introduced when fluorescence and photothermal data are used to determine fluorescence quantum yields. This work provides experimental results describing the effects of optical saturation on fluorescence and thermal lens measurements for rhodamine 6G in various media and over a wide energy range. It is shown that, when optical saturation is avoided, the photothermal method gives accurate absolute values of Φf ranging from 0.93 to 0.95, depending on the solvent. On the contrary, fluorescence measurements seem to be sensitive to additional experimental artifacts that are more difficult to characterize and to eliminate. Index Headings: Optical saturation; Thermal lens spectrometry; Fluorescence; Rhodamine 6G; Fluorescence quantum yield.