OPTOACOUSTIC DETECTION WITH NEAR-INFRARED DIODE-LASERS - TRACE GASES AND SHORT-LIVED MOLECULES

It was shown for the first time that optoacoustic detection in conjunc tion with near-infrared diode lasers presents a viable alternative in the spectroscopy and monitoring of molecules or radicals in trace conc entrations. It was shown that if the diode laser is wavelength modulat ed at the resonance frequency of the optoacoustic cell, the background signal is very effectively suppressed. The system provides minimum de tectable concentrations in the ppb range (8 ppb of ammonia at atmosphe ric pressure). This sensitivity is around three orders of magnitude be tter than that achievable in direct absorption in a 1 metre long singl e-pass cell. It was shown that the system can be used in a broad conce ntration range and measurements can also be performed in a continuous flow. It also allows the optoacoustic detection of short-lived molecul es in a discharge. Prospective applications in the future could includ e direct atmospheric detection of radicals as well as trace gases.