We report on recent advances of laser spectroscopic schemes applied to air pollution monitoring. All systems presented are based on tunable narrowband infrared sources including a conventional line-tunable CO₂ laser, a continuously tunable high-pressure CO₂ laser, an optical-parametric- oscillator-based difference-frequency (DFG) laser and a diode-based DFG laser. These systems cover the mid-infrared wavelength range important for fundamental molecular absorptions. Gas detection is performed with a multipass transmission cell, with specially designed photoacoustic cells using resonant and nonresonant configurations with a single microphone or multi-microphone arrays or with a photothermal arrangement sensing the temporal change of the refractive index. The performance of these systems is illustrated with various examples of trace gas monitoring on the one hand and determination of molecular relaxation rate constants on the other hand with the photothermal scheme. In many cases detection limits corresponding to minimum concentrations in the ppb (10^-9, nmole/mole) or even sub-ppb level are achieved. It is demonstrated that this kind of laser spectrometers offer easiness of operation (room temperature, neither sample preparation nor pretreatment, portability), excellent sensitivity and selectivity, large dynamic range (up to 7 orders of magnitude) and good temporal resolution.

Date Published: 25 January 2000
PDF: 9 pages
Proc. SPIE 4063, 13th Symposium and School on High-Resolution Molecular Spectroscopy, (25 January 2000);

Published in SPIE Proceedings Vol. 4063:
13th Symposium and School on High-Resolution Molecular Spectroscopy
Leonid N. Sinitsa, Editor(s)