Cavity ring down spectroscopy

The cavity ring-down spectroscopy ( CRDS ) is a spectroscopy method by means of optical resonators . Usually only the English term is used, it can be translated as cavity or resonator decay time spectroscopy. An alternative name is Cavity Ring-down Laser Absorption Spectroscopy (CRLAS). As this alternative name suggests, laser light is usually used for this method of spectroscopy .

functionality

The method is based on the storage of electrical energy between two mirrors through the circulation of electromagnetic radiation initially radiated into the resonator . Since the mirrors still transmit a certain proportion of the rays even with very high degrees of reflection and absorption still occurs in the presence of a medium (gas) in the cavity between the mirrors , this energy drops exponentially over time.

{\ displaystyle I (t) = I_ {0} \ exp \ left (-t / \ tau \ right)}

The decay constant is the storage or decay time of the resonator. It applies ${\ displaystyle \ tau}$

{\ displaystyle \ tau = {\ frac {n} {c}} \, {\ frac {l} {1-R + X + \ alpha l}}}

here is the refractive index of the medium, the distance between the mirrors, the speed of light , the degree of reflection of the mirrors and the absorption coefficient of the medium. stands for other losses. ${\ displaystyle n}$ ${\ displaystyle l}$ ${\ displaystyle c}$ ${\ displaystyle R}$ ${\ displaystyle \ alpha}$ ${\ displaystyle X}$

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 absorption coefficients at different laser frequencies and the comparison with the spectrum of known substances, statements about the composition of the filled sample can be derived. ${\ displaystyle n}$ ${\ displaystyle R}$ ${\ displaystyle l}$ ${\ displaystyle X}$ ${\ displaystyle \ tau}$ ${\ displaystyle \ alpha}$

You can either record the decay curve, which requires very fast data processing, or the integral under the curve, in which case one also speaks of cavity-enhanced absorption spectroscopy .

Web links

Extensive information and links (English)