Improvements of an acoustic window cell

Abstract

A new type of photoacoustic cell has been developed. It consists of a sapphire-window and a piezoelectric crystal mounted tightly together in a metal frame (Figure 1). The sapphire-window is sensing temperature variations in the sample due to absorption of pulsed radiation. The sapphire is then exhibiting a periodical length expansion which is measured with the piezoelectric crystal. The new cell is in some senses similar to the acoustically closed cell using a gas volume and a microphone to measure the variation in the sample temperature. It has for instance the same chopper frequency dependence. The methods for depth discrimination2 and for absolute determination of absorption coefficient suggested for the closed cell3-6 can therefore be applied to this new cell. The main difference to the usual cell is that this new cell is open and is well suited for measurements on liquids. Measurements on liquids have been done by Patel et al7 using a hydrophone. An open photoacoustic cell gives the experimentator additional freedom. The sample can be affected during the measurement for instance by addition of substances or by exposure to a controlled atmosphere. The cell can also be modified to allow the sample to flow through the cell which gives the possibility to make on line measurements in a system with flowing liquids. The new cell gives the additional freedom to measure from the bottom or from one side as well as from the top of a sample volume.