Eigenmodes of Photoacoustic T-cells

Baumann, B.1, Kost, B.1, Groninga, H.2, Wolff, M.1, 2
1 University of Applied Sciences Hamburg, Mechanical and Production Engineering
2 PAS-Tech GmbH, Zarrentin

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. 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.