PRODUCTION OF THE PHOTOACOUSTIC EFFECT AND TRANSIENT GRATINGS BY MOLECULAR VOLUME CHANGES

Changes in molecular volume arising from photochemical reactions can p roduce acoustic waves in a manner analogous to heat release. Here a de rivation of two coupled differential equations for the state variables is given for such molecular volume changes based on the linearized hy drodynamic equations for fluids. The results of the calculation are gi ven in terms of a chemical expansion coefficient and a chemical potent ial, the former describing the change in fluid density and the latter the change in the energy per mass of the fluid on conversion of one ch emical species to another. The coupled equations show that chemical ch anges act analogously to temperature changes in the launching of acous tic waves. Wave equations are derived from the coupled equations. Appl ication of the theory to the problems of generation of sound wave gene ration by photodissociation of a diatomic molecule in the gas phase, p roduction of transient gratings in solution through exponential change in the concentration of a chemical species, and production of the pho toacoustic effect with a Gaussian laser beam is given. (C) 1996 Americ an Institute of Physics.