We present the layout and technical details of a trace gas monitor based on photothermal deflection. The operating principle of this instrument, i.e., the deflection of a (weak) probe laser beam by the thermal refractive index gradient induced by trace gas absorption of an intense pump laser beam, allows nonintrusive measurements with good space and time resolution. An intra-cavity CO2 laser is used as the pump beam and a red HeNe laser as the probe. The latter runs perpendicular to the pump beam to optimize spatial resolution. To increase sensitivity, the probe laser is incorporated in a multipass setup. The instrument is demonstrated by the localization of ethylene emission sites on a cherry tomato and by monitoring ammonia production due to nitrogen fixation by cyanobacteria. Both C,H, and NH, can be detected at the 1-3 ppb level, at a spatial resolution of 2 mm (along the pump laser)X0.6 mm (perpendicular to it), and a response time of 0.1 s (without background correction) or 15 s (including background correction). Sensitivity can be increased at the expense of spatial resolution, and vice versa. In principle, this instrument is applicable to all those gases possessing a characteristic ("lingerprint") spectrum in the CO2 laser range. The great advantage of the photothermal deflection technique with respect to other trace gas detection schemes lies in the nonintrusive character of the measurements. There is no need to enclose the sample in a vessel or to suck large volumes of air into the detector; measurements can be performed in open air and in real time. This should prove especially useful where sticky {polar) gases, like H,O, NH,, CHsOH, etc., are to be detected quantitatively. Main applications include air quality monitoring, especially concerning dry deposition rate measurements using the eddy correlation technique, and the study of volatile metabolite emission of biological samples. 0 1995 American Institute of Physics.

Please be advised that this information was generated on 2018-05-09 and may be subject to change. Per a.o.Inl 'rared Phys. Technol. Vol. 36, No. I, pp. 483488, 1995 Copyright '7 1995 Elsevier Science Ltd 1350 A~tract--The photothermal effect is utilized for the detection of molecular trace gases in the open air, with high spatial and temporal resolution. An IR CO 2 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 CO 2 laser beam. To improve the detection limit of the system the CO, laser has an intracavity focus near to which the HeNe laser passes 31 times in a multipass mirror setup. A detection sensitivity of I ppbv (C2H4) and 0.5 ppbv (NH3) is achieved under practical conditions.