Saturated spectroscopy

The Doppler-free saturation spectroscopy, often short saturation spectroscopy, laser spectroscopy is a high-resolution spectroscopic method for investigation of atomic spectra, the effects of Doppler broadening can be avoided in which, by suitable experimental setup. The method allows to measure effects, such as hyper fine structure and the natural line width of the atomic spectra.

Experimental Setup

The basic structure in the saturation spectroscopy is that a tunable laser is split by a beam splitter into two sub- beams of different intensity. Both sub-beams are deflected by the mirror so that they are parallel but extend in opposite directions through the sample to be measured (e.g. a gas). This is called the stronger beam pumping or even saturation beam, the weaker test, query or sample beam. The sample beam is directed into a spectrometer, can be read on the which frequencies are absorbed in the sample.

Alternatively, it is possible to use without the use of a beam splitter, two lasers of the same frequency. However, it must also be ensured that both lasers are actually operated at the same frequency, which requires a major effort.

The use of two counter-rotating beams represents the difference from the "normal" spectroscopy in which only a single beam is directed through the sample directly into the detector.

Observation and explanation

For a description of using the examination of different speed classes of the particles of the sample. If both the pump and probe beam frequency, and is an absorption frequency of the sample, only two cases can occur:

• Both beams are due to the optical Doppler effect of oppositely moving particles, ie different speed classes, absorbed. On the detector shows the Doppler-broadened profile of the interrogating beam.

• (Resonance) Both rays are absorbed by the beams relative to the direction perpendicular to the beam stationary or moving particles, which is the same speed class. Due to the high intensity of the pump beam, there is a large number of excited states, the lower level is depopulated. The interrogating beam is therefore hardly absorbed and the absorption profile is shown by the curve originally dopplerverbreiterten a strong incision, called the Lamb dip in the shape of the natural line width.

If one forms in the case of resonance, the difference between the absorption spectra with and without saturation beam, one obtains an absorption profile without Doppler broadening. The width of the lines is now given only by the natural linewidth.