Photoacoustic Shock Waves and Sound Velocities in Metal Vapors

Most photoacoustic experiments are concerned with the generation and detection of small amplitude sound waves after optical absorption. In this paper, we study the opposite case: generation and detection of large amplitude shock waves by a pulsed laser. The generation is achieved by taking advantage of the efficient ionization of metal vapors by resonant1 or quasi-resonant2,3 laser beams. In general, photoacoustic generation can be due to various causes; for example: collisional deexcitation resulting in thermal expansion, photodissociation, gas evolution, photoionization and optical breakdown, and so on. For the present case of optical breakdown in a vapor, the photoacoustic generation efficiency can be especially high, because ionization causes an increase in the total particle density, and also the electrons produced usually have large kinetic energies. Hence, large-amplitude shock waves can be generated by laser-induced breakdown.