The Seya-Namioka Spectrometer

If it is necessary to observe wavelengths below approximately 1850-1900 A, air must be removed from the spectrometer. Displacing air with nitrogen extends the low-wavelength region to about 1750 A. However, if a vacuum system is used, the minimum wavelength is further extended. 

FIGURE 4-19. Schematic diagram of the Seya-Namioka concave grating spectrometer. [Courtesy McPherson Instrument Corp.] 

Almost any spectrometer can be used in the vacuum ultraviolet (about 1100-2000 A) by properly designing the enclosure and utilizing a vacuum pumping system. Vacuum pumping systems usually include a mechanical pump with an oil diffusion pump. A cold trap provides protection of the grating and the internal optics from long-term oil contamination. A vacuum as low as 10 Torr frequently is used. 

Vacuum spectrometers require windows over the entrance and exit slits to maintain the vacuum. They are usually made of lithium fluoride or calcium fluoride. If photographic plates are used to record spectra, special arrangements are required to either place the plate inside the monochromator case or provide a system to remove the plate and replace it without breaking the vacuum. Both arrangements are used commercially. 

---