Dependence of Photoacoustic Signal on Optical Absorption Coefficient in Optically Dense Liquids

A quantitative investigation of the photoacoustic (PA) signal amplitude on the exponential optical absorption coefficient, β(=absorbance X (In 10)/cm), In thermally thick liquid samples Is presented. Experimental results agree with a practical approximation to the Rosencwalg-Gersho theory of the PA effect. The experimental results fit the theoretical predictions from values of β of about 0.005 X (thermal diffusion length)”1 upward. The effect of the direct acoustic coupling probably limits the usefulness of the Rosencwalg- Gersho theory at low values of β. theoretically, below values of about 0.03 tor fairly opaque samples (having absorbancies of about 1 or more). A normalized PA signal (ratio between signal and the maximal, saturation, value) Is used for comparison with β. Values for β are measured directly, rather than estimated or extrapolated by using Beerfs law. A straight-forward method to extract optical absorption data In absolute units from the PA signal amplitude only is presented and applied to concentrated samples.