Production of the photoacoustic effect and transient gratings by molecular volume changes

Changes in molecular volume arising from photochemical reactions can produce acoustic waves in a manner analogous to heat release. Here a derivation of two coupled differential equations for the state variables is given for such molecular volume changes based on the linearized hydrodynamic equations for fluids. The results of the calculation are given in terms of a chemical expansion coefficient and a chemical potential, 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 chemical species to another. The coupled equations show that chemical changes act analogously to temperature changes in the launching of acoustic waves. Wave equations are derived from the coupled equations. Application of the theory to the problems of generation of sound wave generation by photodissociation of a diatomic molecule in the gas phase, production of transient gratings in solution through exponential change in the concentration of a chemical species, and production of the photoacoustic effect with a Gaussian laser beam is given. (C) 1996 American Institute of Physics.