4.4.1.1.4. - Alpha Fixed (g_mode = 5)
This mode allows you to hold the value of 
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 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:
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  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).
)
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