Fig. 1.

Laser-based photoacoustic trace gas detection. (A) Generation of photoacoustic sound, (B) laser frequency scan over the absorption line of a molecule and (C) schematic view of an experimental set-up. When a gas molecule absorbs laser radiation, the molecule is excited to a higher quantum state. De-excitation of that level under atmospheric pressure by collisions causes the release of energy in the form of heat. In a closed volume this causes a pressure increase. When the light beam is modulated with a chopper at an audio frequency (C, e.g. 1 kHz), a pressure wave will be generated: sound will appear at a frequency of 1 kHz. The strength of the sound will be equal to the absorption strength (gas concentration), while the laser frequency gives the selectivity to distinguish the gas in a mixture of gases.