Laser Photoacoustic Spectroscopy

Fig. 7 — Principle of the photoacoustic effect. Photoacoustic spectroscopy (PAS), also known as optoacoustic spectro­ scopy, was pioneered by Alexander Graham Bell more than a century ago. The advent of the laser strongly stimu­ lated research and applications of this technique and the interest is such that international topical meetings on photo­ acoustic and photothermal phenomena are now held biennially. The photoacoustic (PA) effect is es­ sentially an energy-conversion process. When a sample (solid, liquid or gas) is irradiated by a laser beam or some other radiation source, part of the absorbed energy is converted into translation energy of the molecules by radiation­ less transitions. It is this de-excitation channel which is responsible for heat production within the sample although secondary reactions may also play a role (see Fig. 1). If either the incident radiation or the absorption by the sam­ ple is modulated, the periodic heating finally results in a pressure modulation. The PA signal thus originates from the thermal response of the sample to the absorbed energy detected as pressure fluctuations acoustically. Owing to ther­ mal diffusion processes the magnitude of this signal depends not only on the thermal and optical characteristics of the sample but also on the nature of the coupling mechanism between sample and detector. Consequently the cou­ pling medium plays an important role and one can differentiate between the light-to-heat conversion and the heatto-sound conversion efficiencies.