The Photoacoustic Effect at Phase Transitions

Abstract

The pressure response of the gas to the absorbed light is determined essentially by the thermal properties of the sample. At a first order phase transition, the latent heat influences strongly the pressure signal. Recently, first experimental observations of the PA at phase transitions in various substances have been published. It has been found that the amplitude of the pressure of the gas as a function of temperature runs through a minimum in the transition region, whereas the phase angle shows different patterns. For a formal description of the PAE in the transition region a “model of oscillating interface” has been proposed. It is based on the assumption that the periodic illumination of the sample creates a steady-state temperature gradient on which an oscillation of temperature is superimposed. Therefore, there is a temperature region around the transition temperature where the interface between both the thermodynamic phases oscillates around an average position. It was found theoretically that the amplitude of the acoustic signal for samples of arbitrary thickness becomes minimal when the mean temperature of the sample surface is equal to the transition temperature. However, the phase-angle of the signal as a function of the temperature depends strongly on the thermal thickness of the sample. Experimental results of measurements on Ga, In, H₂O K₂SnC1₆, BaTiO₃, VO₂, the metal hydrogen interstitial alloys Ta H_0.5, Nb H_0.8, V H_0.517 and on DPPC vesicles with chlorophyll a are presented. They are qualitatively in fairly good agreement with theory.