Cavity ring-down spectroscopy

The cavity ring-down spectroscopy ( CRDS ) is a spectroscopic method using optical resonators. Usually, only the English term is used, it can be translated with cavity or Resonatorabklingzeitspektroskopie. An alternative name is cavity ring -down laser absorption spectroscopy ( CRLAS ). As this alternative name suggests, laser light is used for this method of spectroscopy usually.

Operation

The method is based on the storage of electrical energy between two mirrors through the circulation initially irradiated in the resonator electromagnetic radiation. Since the mirror even at very high degrees of reflection still transmit a certain portion of the radiation and in the cavity between the mirrors or absorption occurs in the presence of a medium ( gas ), the energy decreases exponentially with time.

The decay constant is the storage or decay time of the resonator. It is

Here, the refractive index of the medium, the distance between the mirrors, the speed of light, the reflectivity of the mirrors and the absorption coefficient of the medium. stands for other losses.

Is the size of, and are known, obtained from the measurement of the absorption coefficient at the frequency of the laser with which the resonator is excited. From the evaluation of the absorption coefficients at different laser frequencies, and the comparison with the spectrum of known substances can be derived statements about the composition of the sample introduced.

You can either record the decay curve, thus enabling very fast data processing requires, or the integral under the curve, in this case one speaks of Cavity -enhanced absorption spectroscopy.