IR LASER PHOTOTHERMAL TRACE GAS-DETECTION APPLIED TO ENVIRONMENTAL AND BIOLOGICAL PROBLEMS

The photothermal effect is utilized for the detection of molecular trace gases in the open air, with high spatial and temporal resolution. An IR CO2 laser is used to excite specific molecules to higher vibrational levels. The subsequent change in temperature and refractive index is monitored by the deflection of a HeNe laser beam which crosses the CO2 laser beam. To improve the detection limit of the system the CO2 laser has an intracavity focus near to which the HeNe laser passes 31 times in a multipass mirror setup. A detection sensitivity of 1 ppbv (C2H4) and 0.5 ppbv (NH3) is achieved under practical conditions.