Long Path Length Samples in Thermal Lens Calorimetry

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Abstract
Laser-induced thermal lens calorimetry as applied to small absorbance determinations has traditionally relied upon the approximation that the sample behaves optically as a thin lens. The choice of a cell length which is not ideal for theory may be necessary for increased sensitivity. Two models for treating long samples of variable absorbance and enhancement are presented. The simpler of these is based on an approximation that such samples will behave according to the integrated position dependence of a thin lens. A more complex model, requiring numerical evaluation, includes the effects of sample refractive index and lens element coupling. The numerical model is verified experimentally and used to establish the limits of accuracy of the thin lens and integrated position dependence models. The consequences of using long path length samples in analytical thermal lens measurements are discussed in light of the numerical model results.