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.