PHOTOTHERMAL DETERMINATION OF ABSORPTION-COEFFICIENTS IN OPTICALLY DENSE FLUIDS - APPLICATION TO OLEIC-ACID AND WATER AT CO LASER WAVELENGTHS

Values of optical absorption coefficient beta in the infrared spectral region covered by the CO laser emission were measured for homogeneous , nonscattering, and optically very dense (4 x 10(4) m(-1) < beta < 4 x 10(5) m(-1)) fluids (oleic acid and water) with the use of optotherm al window (OW) spectroscopy. This new approach, which combines a photo thermal technique with piezoelectric detection, offers several practic al advantages over conventional techniques (easy loading and cleaning, no dead volume, simple recovery of sample, etc.). A linear correlatio n between the optothermal signal and beta exists when thermal diffusio n length mu in a sample is shorter than optical penetration depth 1/be ta. The apparatus was calibrated with distilled water (beta = 3.38 x 1 0(4) m(-1) at 1781 cm(-1) wave-numbers). Experimentally found beta val ues for oleic acid and water are consistent with those measured by the spectrophotometric method. Satisfactory measurements could be perform ed for (0.1 < beta mu < 3); the optothermal signals were large enough and far from saturation.