TIME-RESOLVED PHOTOACOUSTIC-SPECTROSCOPY - NEW DEVELOPMENTS OF AN OLDIDEA

Acoustic waves generated by heat emission in radiationless transitions from photoexcited molecules can be detected by suitable transducers. Their study allows the investigation of thermal relaxations, thus prov iding thermodynamic and kinetic data on short-lived species produced b y the absorption of pulses of light. In this field of research the bes t technique has proved to be the so-called pulsed-laser, time-resolved photoacoustic spectroscopy, which is based on piezoelectric detection of pressure waves in the time domain. Deconvolution processing of the transient signals gives both the lifetimes of excited states and the energy content of the transitions, provided that decay times are in th e range 5 ns-5 mu s Moreover, when compared with proper theoretical mo dels emphasizing the energy balance, the photoacoustic results can hel p to build a complete picture of the deactivation pathways, including photochemical events. The biophysical applications, although numerous and widespread both in basic and applied research, offer the real poss ibility of giving information on photobiological processes in conditio ns very close to the living state. Among the more significant contribu tions obtained in this area, the results on photosynthesis and photose nsitivity of plants and photosynthetic micro-organisms, structural and functional dynamics of respiratory proteins, photocycles of rhodopsin and bacteriorhodopsin and photophysical properties of several natural pigments are particularly relevant, together with some medical and bi otechnological applications. Another promising field of application of photoacoustics concerns photoactive drugs and,the photophysics of flu orescent probes for conformational studies of proteins, nucleic acids and membranes. In general terms, time-resolved photoacoustic spectrosc opy promises to become one of the most powerful techniques in photobio physics, provided that some limitations in data analysis and time reso lution are removed by technical improvements.