Pulsed-laser mode-mismatched dual-beam thermal lens spectrometry: comparison of the time-dependent and maximum signals with theoretical predictions

Thermal lens spectrometry with pulsed-laser excitation is well-suited for the analysis of small volume samples and is necessary for time-resolved measurements of relaxation processes. This work discusses 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: the 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. When optical and geometrical requirements are fulfilled, experimental data are accurately described by theoretical predictions. The confocal distance and the relative size of the excitation and probe beams are important parameters that must be considered. Depending on the application, the best arrangement should be a compromise between the need for an optimum sensitivity and/or an appropriate time resolution and the limitations imposed by optical constraints. (C) 1998 Elsevier Science B.V.