MEASUREMENT OF THE THERMAL-DIFFUSIVITY AND SPEED OF SOUND OF HYDROTHERMAL SOLUTIONS VIA THE LASER-INDUCED GRATING TECHNIQUE

We use the laser-induced grating technique to measure the thermal diff usivity and speed of sound of hydrothermal solutions. In this noninvas ive optical technique, a transient grating is produced in the hydrothe rmal solution by optical absorption from two crossed, time-coincident nanosecond laser pulses. The grating is probed by measuring the diffra ction efficiency of a third laser beam. The grating relaxes via therma l diffusion, and the thermal diffusivity is determined by measuring th e decay of the grating diffraction efficiency as a Function of the pum p-probe delay time. In addition, intense pump pulses produce counterpr opagating acoustic waves that appear as large undulations in the trans ient grating decay spectrum. The speed ol sound in the sample is simpl y the gating fringe spacing divided by the undulation period. The cell is made From a commercial high-pressure fitting and is equipped with two diamond windows for optical access. Results are presented for dilu te dye/water solutions with T = 400 degrees C and pressures between 20 and 70 MPa.