Eigenmodes of Photoacoustic T-Cells Bernd Baumann 1 , Bernd Kost 1 , Hinrich Groninga 2 , Marcus Wolff 1,2 1 University of Applied Sciences Hamburg, Mechanical and Production Engineering, baumann@rzbt.haw-hamburg.de, 2 PAS-Tech GmbH, Zarrentin, groninga@pas-tech.com 1 Introduction The photoacoustic effect is based on resonant absorption of light by a sample and the transfer of the excitation energy into thermal energy via inelastic collisions of gas molecules. A modulated irradiation of the sample causes periodic pressure variations that can be detected by a microphone and measured using lock-in technique (Demtröder 2002). Photoacoustic spectroscopy finds many applications in the field of concentration measurements of gaseous compounds. The signal detection sensitivity of photoacoustic sensors strongly depends on the geometry of the photoacoustic cell. It can be considerably improved by taking advantage of acoustical cell resonances, i.e., the radiation is modulated at a frequency equivalent to an acoustical eigenmode of the measuring chamber. In order to optimize a photoacoustic system, it is key to precisely understand the distribution of pressure in the sample cell. Only then, it is possible to optimize the coupling of optical excitation, sound wave generation and microphone detection (Michaelian 2003). Therefore we have analysed the eigenmode structure of photoacoustic cells of different geometries using the finite element tool FEMLAB. Additionally, we have compared the resulting eigenfrequencies to experimentally determined eigenfrequencies. 2 Photoacoustic Sensors absorption cylinder: diameter A 26 mm D = length A 82 mm L = resonance cylinder: diameter R 11 mm D = length adjustable diminution: diameter D 8.9 mm D = length D 2 mm L = Fig. 1. Experimental set-up and dimensions of the photoacoustic sensor with T-cell. All dimensions are measured inside the cell. The most frequently used type of photoacoustic sensors is based on a cylinder shaped container (Michaelian 2003). In recent years the interest in photoacoustic cells with unconventional shape is increasing. A cell-type consisting of two intersecting cylinders has been proposed in (PAS-Tech 2004). The two cylinders form a T-geometry, consisting of an optical absorption cylinder and, centrically perpendicular to that, an acoustical resonance cylinder. This design allows independent optimization of the key parameters affecting the signal strength. At the end of the resonance cylinder the microphone is mounted. Figure 1 shows the experimental set-up and the dimensions of the photoacoustic sensor investigated in this paper. Measurements were performed on n -butane ( 4 10 CH ) at atmospheric