Advanced Topic The Fourier Transform Spectrometer

We have seen in the previous section that Raman spectra are complementary to infrared spectra. Both spectroscopies provide quite useful information on the phonon structure of solids. However, infrared spectra correspond to a range from about 100 cm to about 5000 cm that is, far away from the optical range. Thus, infrared absorption spectra are generally measured by so-called Fourier Transform InfraRed (FTIR) spectrometers. These spectrometers work in a quite different way to the absorption spectrophotometers discussed in Section 1.3. 

FTIR spectrometers generally provide transmittance spectra, T (v). Three steps are realized before the final spectrum of a sample is obtained  

FTIR spectrometers have a number of advantages over conventional dispersive systems  

Among the disadvantages of these spectrometers, we can find their usually high prices and the requirement of computational work to determine the spectra from the interferograms, which can introduce some antifacts. 

TheNd + ion shows several absorption lines of different widths in crystals. One of the absorption lines of Nd + ion in yttrium vanadate (YVO4) peaks at 809 nm and presents a natural broadening at 2 K with a full width at half maximum of (A v)nat =18 GHz. Estimate (a) The natural broadeiung in the wavelengths units (nm) and (b) the lifetime of the excited state. 

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