Recent progress in the techniques of manufacture of spectroscopic diffraction gratings is described. Three main topics are discussed in detail: (i) the mechanical ruling of gratings, and the way in which the mechanical aspects of the ruling process affect the spectroscopic performance of the grating; (ii) the development of interference (or 'holographic') techniques for manufacturing gratings, and the relationship between ruled gratings and those made interferometrically; (iii) x-ray gratings for use at grazing incidence, and the different criteria that arise through the use of gratings in this way. 2.7. Electromagnetic theory of gratings 2.8. Wood's anomalies . 2.9. Surface plasmons . 3.1. Introduction . 3.2. The ruling tool . 3.3. Films for ruled gratings 3.4. Recent studies of the ruling process 3.5. Contemporary ruled gratings . 3.6. Ruling engines . 3.7. Replication. 4.1. Introduction . 4.2. The spectroscopic performance of plane interference gratings 4.3. Blazed interference gratings . 4.4. Focusing and aberration-corrected gratings 4.5. Summary . 5 . X-ray gratings .
This paper describes an X-ray diffraction appraisal of an iron catalyst extracted after use in the Fischer-Tropsch process for the synthesis of hydrocarbons from carbon monoxide and hydrogen. The substance, in which the iron atoms form an approximately hexagonal dose-packed array, is monoclinic with cell dimensions a=b=2.794 /~, c=4.360 A and y=120.92 °. It is asmuned to be a carbide, with small amounts of associated oxygen. It is not possible, with X-ray powder diffraction, to identify the structure completely, but on the basis of the formula Fe~C the possibilities are discussed.
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