Gri"sa Mo"cnik, M. Franko, Z. Bozoki, D. Bicanic, H. Jalink:

On simultaneous effect of thermal lensing and beam deflection

Spectroscopy in theory and practice, Bled, April, 1997

Abstract

Photothermal measurements in highly absorbing samples are recently attracting more and more attention. Examples of such measurements are inverse TLS, where an absorbing compound added to the HPLC eluent enables detection of nonabsorbing compounds; photothermal investigation of liquid samples in the IR, where the the solvent itself absorbs highly and contributes to the strong effects; investigation of samples with high background absorption in general. These applications offer new possibilities for exploiting the potentials of photothermal techniques.

However, the classical theory of photothermal effects in liquids is based on assumption of low absorbances and is no longer valid for highly absorbing samples. To avoid erroneous interpretation of experimental results a general model that accounts for combined effects of thermal lensing and photothermal beam deflection was derived. The model enables estimation of mutual contributions of both effects to the measured photothermal signal. Signals in the case of ``normal'' and lock-in detection were calculated for a thermal lens and a beam deflection experiment. Crucial experimental parameters: pump beam modulation frequency, its diameter and the distance of the detector from the sample were varied in the calculation of the signal for different values of the power absorbed by the sample. The values of experimental parameters which minimise the unwanted effects of pump and probe beam misalignment or pointing instabilities can be used to optimise an experiment and improve the accuracy of the thermal lens or beam deflection spectrometry.

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Last update made on Apr. 17, 1997.