Cavity Ring-Down Spectroscopy- The Methodology of the Experiment

Direct absorption spectroscopy of atoms and molecules in the gas phase, yielding both quantitative absolute concentrations as well as absolute frequency-dependent cross-sections, is a very powerful tool in analytical chemistry and physical chemistry. This absoluteness is the reason why sensitive absorption spectroscopy techniques have gained renewed interest, even in research fields where more sophisticated laser-based diagnostic techniques are commonly applied. Among the various direct absorption techniques, the cavity ring-down (CRD) technique has proven to be a valuable addition, since it combines a good sensitivity with a rather simple and straightforward experimental set-up.

Typical optical chain applied in CRDS experiment

In a `conventional ’ absorption experiment, one measures the amount of light that is transmitted through a sample. If the light source is monochromatic (e.g. a laser), one can record an absorption spectrum of the sample by recording the transmitted intensity as a function of the frequency. Alternatively, a broad light source can be used when the incident light or the transmitted light is spectrally dispersed. A drawback of direct absorption might be its limited sensitivity. A small attenuation in transmitted light has to be measured on top of a large background. High sensitivity is obtained by using modulation schemes and by increasing the absorption path length. Alternatively, other experimental spectroscopy techniques can be used which are based on the detection of phenomena which are induced by absorption of light, such as pressure changes in photoacoustic spectroscopy, fluorescence in laser-induced  fluorescence (LIF), or ions in resonant enhanced multiphoton ionization (REMPI). The great advantage of these techniques is that they are background free. A disadvantage is the sometimes difficult calibration procedure which is needed to make these techniques absolute (i.e. these techniques are not self-calibrating). CRD spectroscopy is a sensitive absorption technique in which the rate of absorption rather than the magnitude of the absorption of a light pulse confined in an optical cavity is measured. The sample is placed inside a high-finesse optical cavity consisting of two highly reflective mirrors. A short laser pulse is coupled into the cavity, the light is reflected back and forth inside the cavity and, every time that the light is reflected, a small fraction of this light leaks out of the cavity. Instead of measuring the total intensity of the light exiting the cavity, one determines the decay time by measuring the time dependence of the light leaking out of the cavity. In this way the rate of absorption can be obtained ; the more the sample absorbs, the shorter is the measured decay time.

                                                              The Schematics of the operating principle of Cavity Ring-Down Spectroscopy.

   There are several advantages to this approach. Since the absorption is determined from the time behaviour of the signal, it is independent of pulse-to-pulse fluctuations of the laser. Furthermore, the effective absorption path length, which depends on the reflectivity of the cavity mirrors, can be very long (up to several kilometres), while the sample volume can be kept rather small. Compared with other sensitive absorption techniques, especially those using modulation schemes, CRD spectroscopy has the additional advantage that the absorption is measured on an absolute scale. Another attractive property is its simplicity, it is rather easy to construct a CRD set-up out of a few components.