Time-resolved laser-induced photoacoustics (PA), also termed time-resolved laser-induced optoacoustic spectroscopy (LIOAS), is a high sensitivity photobaric method for non-destructive material probing, that monitors the pressure changes induced in a liquid sample after photoexcitation with a pulsed laser. The technique delivers information generally not available from optical studies and therefore complements other methods. After excitation, commonly by nanosecond pulsed lasers, the time evolution of the pressure pulse is monitored by fast piezoelectric transducers located in a plane either parallel (right-angle geometry) or perpendicular (front-face geometry) to the direction of the laser beam.  The right angle geometry has been used mainly for studies with weakly absorbing solutions, whereas front-face geometry has been generally used with strongly absorbing samples.
So far, time-resolved information can be used only in the case of pseudo first order reactions, either parallel or sequential, including thermal equilibria, since the deconvolution analysis of experimental waveforms has been developed for schemes complying with a time-dependent pressure evolution represented by a sum of single-exponential decays.
The heat evolved is used for the determination of energy levels of intermediates, provided that the quantum yields for the reactions are known.  On the other hand, if the energy level of the intermediates is known from other spectroscopic techniques, the determination of quantum yields is possible, e.g., for isomerization, charge transfer, fluorescence, intersystem crossing, and energy transfer processes.
The structural volume changes are essentially density changes and reflect movements in the chromophore such as photoinduced changes in charge distribution and in the strength of the interactions with solvent molecules. Time-resolved PA data afford also the decay rate constants of the transients, and therefore provide a further check for the assignment of the observed events to a particular intermediate.  A further assignment of the transients involved is offered by the action spectrum of the pressure evolved.
Time-resolved PA represents an interesting tool for investigating photoinduced intermediates displaying no absorption or emission.