Diffractive grating spectrometer

This hydrogen band at 3 = 0.2 was also studied with a diffraction grating spectrometer, and its optical density was found to be 0.1 and its half-width to be 21 cm.- (Fig. 10). Condon (134) showed that infrared spectra were expected to be induced by high electric ffelds and that the selection rules... 

The data illustrated in Fig. 4(a) are methane absorption lines (0.02 cm-1 wide) observed with a four-pass Littrow-type diffraction grating spectrometer. For these data also, 256 points were taken. The data were obtained at low pressure, so that Doppler broadening is the major contributor to the true width of the lines. The straightforward inverse-filtered estimate with 15 (complex) coefficients retained is shown in Fig. 4(b). Figure 4(c) shows the restored function. The positions and intensities of the restored absorption... 

Figure 12. Optical schematic representation of an aberration-corrected toroidal diffraction grating spectrometer designed for use with an I PDA detector system. (Reproduced with permission from Ref. 12.

S.P. Davis Diffraction Grating Spectrometers (Holt, Rinehard, Winston, New York 1970)... 

The principle of the dispersive prism or diffractive grating spectrometer can be readily extended to the infrared and even to the far infrared. However the photographic plate for the recording and measurement of the spectrum is not available here. Early devices for sensing infrared radiation consisted of sensitive thermometric devices called bolometers. Bolometers do not form images and hence a spectrum must be recorded by sequentially displacing the bolometer to measure the intensity of radiation at different wavelengths. 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Another approach to multielemental analysis is to use a multichannel instrument that allows for the simultaneous monitoring of many analytes. A simple design for a multichannel spectrometer consists of a standard diffraction grating and 48-60 separate exit slits and detectors positioned in a semicircular array around the diffraction grating at positions corresponding to the desired wavelengths (Figure 10.50). 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

Today s commercially available chromatogram spectrometers usually employ diffraction gratings for monochromation. These possess the following advantages over prism monochromators which are still employed in the Schoeffel doublebeam spectrodensitometer SD 3000 and in the Zeiss chromatogram spectrometer ... 

Visible and ultraviolet absorption spectra are measured in an absorption spectrometer. The source gives out intense visible light or ultraviolet radiation. The wavelengths can be selected with a glass prism for visible light and with a quartz prism or a diffraction grating for ultraviolet radiation (which is absorbed by glass). A typical absorption spectrum, that of... 

Reflective coatings are found in astronomical instruments, telescopes, spectrometers, rangefinders, projectors, microfilm readers, television tubes, diffraction gratings, interferometers, beam splitters, lasers, and many other applications. 

As for miniaturisation, examples are UV/VIS spectrometers using a linearly variable filter rather than a diffraction grating as the wavelength separation device (matchbox size), and double-focusing mass sensors (postage stamp size). 

The outline of the construction of a typical plasma emission spectrometer is to be seen in Figure 8.10. The example shown has an inductively coupled plasma, excitation source, but the outline would be similar were a dc source to be fitted. Different combinations of prisms and diffraction gratings may be used in the dispersion of the emitted radiation, and in the presentation of the analytical signal. Instruments are computerized in operation and make use of automatic sample handling. Sophisticated data handling packages are employed routinely to deal with interferences, and to provide for clarity in data output. 

Sometimes the spectrometer completely obliterates the information at all Fourier frequencies co beyond some finite cutoff Q. This is specifically true of dispersive optical spectrometers, where the aperture determines 1. The cutoff Q may be extended to high Fourier frequencies by multipassing the dispersive element or employing the high orders from a diffraction grating. 

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