Optoacoustic detection with near-infrared diode lasers: Trace gases and short-lived molecules

It was shown for the first time that optoacoustic detection in conjunction with near-infrared diode lasers presents a viable alternative in the spectroscopy and monitoring of molecules or radicals in trace concentrations. It was shown that if the diode laser is wavelength modulated at the resonance frequency of the optoacoustic cell, the background signal is very effectively suppressed. The system provides minimum detectable concentrations in the ppb range (8 ppb of ammonia at atmospheric pressure). This sensitivity is around three orders of magnitude better than that achievable in direct absorption in a 1 metre long single-pass cell. It was shown that the system can be used in a broad concentration range and measurements can also be performed in a continuous flow. It also allows the optoacoustic detection of short-lived molecules in a discharge. Prospective applications in the future could include direct atmospheric detection of radicals as well as trace gases.