High-Sensitivity Methods of Absorption Spectroscopy

This would affect spectroscopy with lasers in the same way as that with incoherent sources. In most cases it is negligible. 

This brief overview of some of the advantages of lasers in spectroscopy will be outlined in more detail in the following chapters and several examples wiU illustrate their relevance. 

The general method for measuring absorption spectra is based on the determination of the absorption coefficient a( u) from the spectral power density 

However, the spectroscopist often has only the choice to maximize L and Po and to minimize NEP. 

The question is now, how the noise can be reduced and the absorption path length increased. For small values of aL the direct measurement of the quantity AP = Po - Pt ns the small difference of two large quantities cannot be very accurate since already small fluctuations of Pq or Pj may result in a large relative change of AP. Therefore, other techniques have been developed leading to an increase in sensitivity and accuracy of absorption measurements by several orders of magnitude compared to direct absorption measurements. These sensitive detection methods represent remarkable progress, since their sensitivity limits have been pushed from 

With the reference intensity /r = /q, as produced, for example, by a 50% beam splitter with the reflectivity / = 0.5 (Fig. 6.1b), one can measure the absorption coefficient 

The absorption coefficient aikico) of the transition /) k) with an absorption cross section aik is determined by the density N( of absorbing molecules (see Sect. 5.1.2) 

If the population is small compared to Ni, we obtain from (6.6) for the minimum detectable density Ni over the absorption path length x L  

The minimum, still detectable concentration Nj of absorbing molecules is determined by the absorption path length L, the absorption cross section ajjj, and the minimum detectable, relative intensity change AI/Iq caused by absorption. 

In order to reach a high detection sensitivity for absorbing molecules, Loii should be large and the minimum detectable value of A/// as small as possible. 

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High-Sensitivity Methods of Absorption Spectroscopy the value where = y. This gives with (6.13) go-y... 

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