Ultrasensitive Photothermal Deflection Spectrometry Using an Analyzer Etalon

The theory and experlmentat scheme for uslng an analyzer etalon to detect photothermal deflection slgnals is developed. First, a theory for photothermal deflection spectrometry Is developed, which describes the observed signal decay In terms of the characteristic thermal decay tlme constant and which accounts for a flnlte probe laser beam walst radius. Second, a theory for the angular response for an analyzer etalon Is described. The analyzer etalon Is found to be extremely sensltlve to beam angle varlatlons and dramatlcally Increases the sensltlvlty of photothermal beam deflection measurements. A theoretical enhancement over conventional deflectlon angle detectlon schemes of 100 Is calculated. Although the experlmental enhancement Is calculated to be only 0.4 of theoretlcal, a slngte laser pulse detectlon llmlt of 0.7 ppm (v/v) of CFC-12 In argon Is obtained by uslng a carbon dloxide laser operatlng at 933 cm-1 wlth a pulse energy of 1 mJ. This constitutes a slgnificant Improvement over previously determlned detection Ilmlts. Ensemble averaglng can be used to decrease this llmlt In systems where analysls tlme Is not critical. The problems encountered In uslng this detectlon scheme are also due to the extreme angle sensltlvlty. The apparatus Is very susceptible to environmental factors such as air currents, laboratory temperature varlatlons, and vlbratlons