Photoacoustic technique for simultaneous measurements of thermal effusivity and absorptivity of pigments in liquid solution

A photoacoustic (PA) methodology, in the transmission configuration, for simultaneous measurements of thermal effusivity and molar absorption coefficient (absorptivity) for pigments in liquid solution is introduced. The analytical treatment involves a self-normalization procedure for the PA signal, as a function of the modulation frequency, for a strong absorbing material in the thermally thin regime, when the light travels across the sample under study. Two fitted parameters are obtained from the analysis of the self-normalized PA amplitude and phase, one of them proportional to the samples optical absorption coefficient and from which, taking it for a series of samples at different concentrations, the pigments absorptivity in liquid solution can be measured, the other one yields the samples thermal effusivity. Methylene blues absorptivity in distilled water was measured with this methodology at 658 nm, finding good agreement with the corresponding one reported in the literature.