Gratings, diffraction echelle

The cross section of an echelle diffraction grating is given by ... 

The theoretical resolution of grating (R) of an echelle is derived as for conventional diffraction gratings as follows ... 

Harrison, G.R. (1949) The production of diffraction gratings II. The design of echelle grating and spectrographs, Journal of the Optical Society of America, 39, pp522-528. 

Figure 7.6 Schematic diagram of an emission spectrometer with an electrical discharge excitation source. The prism is meant only to illustrate a dispersive device a diffraction grating is used in all modem spectrometers with a single dispersive device. Echelle spectrometers use two dispersive devices, either a prism and a grating or two gratings.

Sequential and simultaneous ICP systems of many types are available from numerous instrument manufacturers. Most are available in radial or axial view, or both. Most spectrometers can be purged to reach 160 nm. Examples of some of the companies who make ICPs and DCPs are given. Jobin Yvon, Inc. makes a line of PMT-based high-resolution sequential systems, simultaneous systems, and combination sequential-simultaneous systems. PerkinElmer Inc. makes a scanning ICP with CCD detection, and a line of simultaneous CCD-based Echelle systems. Leeman Laboratories, Inc. makes an Echelle ICP spectrometer with an aperture grid available as a sequential system with rapid scanning PMT detection, a simultaneous system with a PMT array, and a combination system. Varian, Inc. makes a sequential ICP with PMT detection and a simultaneous system with CCD detection. ThermoElemental offers ICP Echelle spectrometers with CID detection and a sequential diffraction grating instmment with PMT detection. 

Figure 121 illustrates a sequential echelle spectrometer. Light from the entrance slit is directed by the collimating mirror to the echelle grating, and the diffracted beam is then reflected to the prism lense (order-sorter) and focusing lense. A two-dimensional spectrum is formed in the focal plane of the system. An aperture plate, which contains preset apertures, is mounted at the focal plane. Slits and mirrors are placed at appropriate positions on a... 

As described in Chapter 2, spectral resolution determines the amount of detail that can be seen in the spectrum. If the resolution is too low, it will be impossible to distinguish between spectra of closely related compounds if the resolution is too high, noise increases without any increase in useful information. Spectral resolution is determined by the diffraction grating and by the optical design of the spectrometer. With a fixed detector size, there is a resolution beyond which not all of the Raman wavelengths fall on the detector in one exposure. Ideally, gratings should be matched specifically to each laser used. A dispersive Raman echelle spectrometer from PerkinElmer Instruments covers the spectral range 3500-230 cm with a resolution better than 4 cm . ... 

---