Photothermal Determination of Absorption Coefficients in Optically Dense Fluids: Application to Oleic Acid and Water at CO Laser Wavelengths

Abstract

Values of optical absorption coefficient β in the infrared spectral region covered by the CO laser emission were measured for homogeneous, nonscattering, and optically very dense (4 × 10⁴ m⁻¹ < β< 4 × 10⁵m⁻¹) fluids (oleic acid and water) with the use of optothermal window (OW) spectroscopy. This new approach, which combines a photothermal technique with piezoelectric detection, offers several practical advantages over conventional techniques (easy loading and cleaning, no dead volume, simple recovery of sample, etc.). A linear correlation between the optothermal signal and β exists when thermal diffusion length μ in a sample is shorter than optical penetration depth 1/β. The apparatus was calibrated with distilled water (β = 3.38 × 10⁴ m⁻¹ at 1781 cm⁻¹ wave-numbers). Experimentally found β values for oleic acid and water are consistent with those measured by the spectrophotometric method. Satisfactory measurements could be performed for (0.1 < βμ < 3); the optothermal signals were large enough and far from saturation.