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-res olved measurements of relaxation processes. This work discusses the va lidity of theoretical equations derived from the diffraction theory an d used to model the response of the photothermal method. It is shown t hat both the time-dependent signal and the maximum signal are satisfac torily related to the absorbance, the excitation energy: the thermo-op tical 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 an d geometrical requirements are fulfilled, experimental data are accura tely described by theoretical predictions. The confocal distance and t he relative size of the excitation and probe beams are important param eters that must be considered. Depending on the application, the best arrangement should be a compromise between the need for an optimum sen sitivity and/or an appropriate time resolution and the limitations imp osed by optical constraints. (C) 1998 Elsevier Science B.V.