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Download X-Ray Diffraction Software USER MANUAL and TUTORIALS
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a surface, keeping α small will keep the background small with no restriction (in principle) on the momentum transfer normal to the surface. Suppose you are studying a sample of copper and you want to fix the incidence angle to be equal to the critical angle for total external reflection. Set the reference vector to the surface normal of the crystal and then type the following commands: 6.FOURC> setmode 5 Now using Alpha-fixed mode. 7.FOURC> ALPHA=0.4126 8.FOURC> br 0 2 2 9.FOURC> To implement alpha-fixed mode, fourc calculates the value of ψ needed to fix the angle between the incident wave vector and the reference vector.6 Beta Fixed (g_mode = 6) This is the same as alpha-fixed mode except that β, rather than α, may be fixed. 6 S. G. J. Mochrie, J. Appl. Cryst. 21, 1-3 (1988). FOUR-CIRCLE REFERENCE 171
