Abstract
Molecular oxygen is detected at low concentrations using photoacoustic spectroscopy despite its unfavorable photoacoustic properties. The system consists of a seed laser diode, a tapered amplifier and a quartz tuning fork based spectrophone, thus employing quartz enhanced photoacoustic spectroscopy (QEPAS). With this system a detection limit of 13 ppm is reached with a compact and long term stable setup. Further improvement of the detection limit is possible by adding suitable gases to the sample gas that promote the radiationless de-excitation of the oxygen molecules.
Highlights
An accurate determination of the concentration of molecular oxygen is important for the investigation of corrosion processes and in basic materials science research
Optical amplifiers are operated most effectively in the regime of gain saturation, i.e., the intensity of the seed laser is chosen high enough to remove the entire population inversion within the active medium. This implies that the output power is not linearly dependent on the seed power, but the gain decreases with increasing power
It enhances the photoacoustic signal according to Equation (1), i.e., the quartz enhanced photoacoustic spectroscopy (QEPAS) signal increases linearly with the incident optical power
Summary
An accurate determination of the concentration of molecular oxygen is important for the investigation of corrosion processes and in basic materials science research. The main advantages of laser based optical gas sensors are the selectivity and the low detection limits that can be reached, in combination with the capability of on-line and in situ measurements without the need for sample preparation. The X 3 g => b1 g absorption band around 762 nm is often used for absorption based optical sensing as it is the strongest optical transition of oxygen in the visible and infrared range. This optical transition is dipole forbidden, leading to a comparatively low maximum line strength of. Different spectroscopic techniques have been applied to achieve a low detection limit even with this small line strength, including multi-path cells (35 ppm) [2]
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