Double beam interferometry

Thus, any successful kinetic analysis requires an optimal choice of the spectroscopic detection method, whereby special methods (e.g. pre-separation by chromatography) or modem approaches (e.g. interferometry) must also be considered. Therefore in the following sections different approaches in spectroscopy are presented, some combined irradiation and measurement devices introduced, and different set-ups classified with respect to their principle of operation (sequential, multiplex, single and double beam equipment). In addition some special devices are presented which allow an automated examination even of complex photochemical reactions (in part superimposed by thermal reactions) at a highly sophisticated level using various combinations of modem equipment and supplying data for multicomponent analysis. 

The exact measurement of the optical path in cells of low thickness is made by interferometry (interference pattern method). The transmittance of the empty cell is measured for an interval between two wavenumbers rq and (in cm ). Figure 10.22 shows that the beam S2 has undergone a double reflection from the internal walls of the cell, thus for a normal incidence, there would be, if 21 = kX, addition of both light intensities (the two beams Sj and S2 are in phase). As a function of the wavelength a modulation of the main beam Sj of a few percent is observed. After calculation, if N represents the number of interference fringes counted between rq and PJ (in cm ), then ... 

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