High sensitivity detection of transient infrared absorption using tunable diode lasers

A novel technique is described for monitoring transient infrared absorptions with a tunable diode laser (TDL). The technique involves modulating the TDL at frequencies as high as 250 kHz and using both analog and digital subtraction of background signals. Transient absorptions as small as 0.001% can be detected with a response time of a few microseconds. The technique provides excellent discrimination between transient absorption lines and spurious effects such as beam deflection. Transient detection of the CF(2) radical in a photolysis cell is used to illustrate the technique, and a comparison is made with more conventional TDL detection methods. The modulation technique is clearly a very powerful tool for transient infrared spectroscopy.