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Photoacoustic-pulse generation and propagation in a metal vapor.

Photoacoustic-pulse generation by breakdown is achieved in dense cesium metal vapors of vapor pressures ranging from 2 to 130 Torr by using a dye laser pulse of energy variable from 10(-6) to 10(-3) J, tuned to the Cs transition at 6010 A. The acoustic-pulse propagation is detected by the transient photorefractive deflection of a cw probe laser beam that is displaced from but parallel to the pulsed laser beam. The temperature-dependent velocity of infinitesimal ultrasonic waves in a corrosive metal vapor is measured for the first time. The supersonic propagation of finite amplitude acoustic pulses (blast waves) obtained with a higher pulse energy is also studied. Our data, with Mach numbers ranging from 2.1 down to below 1.01, agree surprisingly well with the prediction of Vlases and Jones for cylindrical blast waves. This provides a new experimental support for their theoretical trajectory formula for blast waves in the extremely weak amplitude limit.