ULTRASONIC ABSORPTION SPECTROSCOPY IN FLUIDS BY OPTICAL DETECTION OF PHOTOACOUSTIC PULSE PROFILES.

Abstract

The authors have demonstrated a new optical, noncontact method for ultrasonic absorption spectroscopy. A pulsed laser is used to produce a short-duration photoacoustic pulse in a sample, and the acoustic pulse shapes can be monitored at different propagation distances by a focused continuous probe beam. This pulse-shape monitoring is possible since the time-dependent deflection of the probe beam is proportional to the transient refractive-index gradient associated with the acoustic pulse. Fourier decompositions of these probe deflection signals provide the ultrasonic absorption spectrum. It is shown that this provides a novel technique for ultrasonic absorption spectroscopy in fluids. The arrival time of the photoacoustic pulse at the probe beams which depends on its displacement from the excitation beam, provides accurate values for the ultrasonic propagation speeds in the samples studied.