4.4.1.1.4. - Alpha Fixed (g_mode = 5)
This mode allows you to hold the value of α
constant while moving to various values of
(H,K,L).
This is useful in
experiments in which it is necessary to control the X-ray penetration
depth into a sample.
More generally, if looking at a weak signal from 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:
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. ( S.G.J. Mochrie, J. Appl. Cryst. 21 , 1-3 (1988). )
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:
1.FOURC> setmode 5
Now using Alpha-fixed mode. 2.FOURC> ALPHA=0.4126
3.FOURC> br 0 2 2
4.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. ( S.G.J. Mochrie, J. Appl. Cryst. 21 , 1-3 (1988). )