ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5pPAa3. Transient grating studies of nonlinear photoacoustic effects in particulate solutions.

G. J. Diebol

H. X. Chen

Y. N. Cao

M. B. Zimmt

Dept. of Chem., Brown Univ., Providence, RI 02912

Experiments where transient grating signals are recorded from irradiation or particulate matter are reported. A relatively low power, picosecond laser produces transient grating signals in reverse micellar solutions that have a unique time dependence, and which can be explained by heat diffusion from the interior of the micelle to a surrounding organic solvent. A 16-ns-long, high-power pulse from a Q-switched ND:YAG laser irradiating a gold sol produces a diffracted light signal that is explainable in terms of a thermally generated photoacoustic effect. Particulate carbon, on the other hand, gives transient grating signals that appear to possess an overtone of the usual thermally generated fundamental frequency. The effect is nonlinear in its dependence on the laser intensity, and is dependent on the time of irradiation of the solution. A second characteristic of the effect is that a pronounced noise is generated by the solution on firing the laser. It is believed that the effects can be attributed to endothermic chemical reactions at the surface of the carbon that act to unbalance the thermal and acoustic modes producing an acoustic density that oscillates about the ambient. Since the diffracted light signal is proportional to the square of the acoustic density, what appears to the eye as an overtone is seen in the recorded signal.