Cavity ring-down spectroscopy: Experimental schemes and applications

Cavity ring-down (CRD) spectroscopy is a direct absorption technique, which can be performed with pulsed or continuous light sources and has a signifi cantly higher sensitivity than obtainable in conventional absorption spectr oscopy. The CRD technique is based upon the measurement of the rate of abso rption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity with a high Q factor. The advantage over normal ab sorption spectroscopy results from, firstly, the intrinsic insensitivity to light source intensity fluctuations and, secondly, the extremely long effe ctive path lengths (many kilometres) that can be realized in stable optical cavities. In the last decade, it has been shown that the CRD technique is especially powerful in gas-phase spectroscopy for measurements of either st rong absorptions of species present in trace amounts or weak absorptions of abundant species. In this review, we emphasize the various experimental sc hemes of CRD spectroscopy, and we show how these schemes can be used to obt ain spectroscopic information on atoms, molecules, ions and clusters in man y environments such as open air, static gas cells, supersonic expansions, f lames and discharges.