Determining the Gruneisen Coefficient for Liquids Using the PAZ-Scan Technique

The photoacoustic (PA) effect is a process by which matter absorbs incident energy and releases all or some part of that energy via vibrational degrees of freedom. In general, generated photoacoustic signal is linearly proportion to the product of absorption of the sample (µa), Grüneisen coefficient (Γ) and the optical fluence. Thus Grüneisen coefficient is one of two parameters that describes the efficiency of a molecule in transforming optical energy into sound, other being the absorption cross-section. Γ is a thermal mechanical constant and is related to the amount of light absorbed by a medium to the difference in pressure that will arise at the location of absorption. It is independent of wavelength as well as absorption coefficient. Measurement of this parameter is important in developing efficient molecular photoacoustic contrast agents. By varying either the absorption or the optical fluence, while keeping the other constant, the Grüneisen parameter can be obtained. In this thesis work, using the PAZ-scan technique the Grüneisen coefficient was obtained for various samples, wherein the optical absorption of the sample is kept constant. In a PAZ-scan, the sample is translated through the path of a focused laser beam in small steps while the generated PA signal is recorded. The incident intensity is optimum at the focal point and decreases gradually on either side of the focus. As such, the absorption and the PA signal varies according to the sample properties. Therefore at positions away from the focal point, the incident intensities are weak and the PA signal varies linearly with intensity. Utilizing the linear regime data, a plot of the PA signal versus the intensity is created and used to obtain the Grüneisen coefficient. Using this technique, the Grüneisen coefficients for crystal violet in two different solvents, food coloring dyes that are dissolved in water were determined. Results show that the linear part of the PAZ-scan can be used to determine the Grüneisen coefficient for liquids. The obtained values are compared with standard techniques in which case the optical fluence is kept constant