Pulsed thermal lens spectroscopy: calibration, signal time resolution, non-linear absorption, examples of application

Thermal lens spectrometry is based on the measurement of thermal energy released by relaxation of excited species after absorption of radiation. Pulsed-laser excitation allows time-resolved measurements of relaxation process. In the first part of this work, we discuss the validity of theoretical equations derived from the diffraction theory and used to model the response of the photothermal method. It is shown that both the time-dependent signal and the maximum signal are satisfactorily related to the absorbance, the excitation energy, thermo-optical properties of the sample, the position of the sample cell along the beam axis, the radius of the excitation beam waist and the degree of mode-mismatching of the excitation and probe beams. All experimental data obtained in this work are accurately described by the proposed theoretical model. In the second part of this work, we have studied the effects of non-linear absorption arising in pulsed-laser thermal lens spectroscopy of dye solutions. Experiments have been carried out with rhodamine 6G which is a fluorescent dye, quenched rhodamine and two non-fluorescent compounds, pseudoisocyanine and erythrosine, which deactivate mainly by internal conversion and intersystem crossing to a triplet state, respectively. Non-linear absorption is shown to arise even at very low excitation energies, which can deteriorate quantitative measurements such as the determination of fluorescence quantum yield and absorption coefficients. Transient absorption of the probe beam by the erythrosine triplet has been investigated to determine the triplet lifetime, depending on the solvent and oxygen and dye concentrations. In nitrogen-saturated solutions, the decay rate depends on the solvent and dye concentrations. The quenching rate constants obtained in homogeneous solvents such as water and ethylene glycol are typical of diffusion-controlled reactions and depend on the viscosity of the solvent. In aqueous micellar solution of Brij 35, self-quenching is less operative due to the protecting effect of micelles and the triplet state deactivates mainly by unimolecular decay. The last part of this work is devoted to the study of europium-thenoyltrifluoroacetone-trioctylphosphine oxide-Triton X-100 chelate by absorption fluorescence and photothermal spectroscopies. We have shown determine that chelate formation is mainly controlled by the relative concentrations of each constituents. The fluorescence of the chelate is mainly dominated by its actual concentration into the solution rather than by its composition. The overall fluorescence quantum yield of the chelate, measured by the photothermal method, is found to be 0.22 when both ligand and co-ligand are in excess. On the basis of the time-resolved experiments, the fluorescence quantum yield of the D state is expected to be = 0.8 and the energy transfer efficiency = 0.28. (author)