4.4.1.1.3. - Azimuth Fixed (g_mode = 4)
This mode fixes the value of
the rotation angle 
of a reference vector about
the scattering
vector
Q .
Azimuth-fixed mode provides a degree of
control that is particularly useful in a surface-diffraction experiment.
If the reference vector is chosen to be the surface normal,
setting to 90° rotates the surface normal
into the plane defined by the scattering vector and the
diffractometer  -2 axis.
This means that
the incidence angle
 will equal the exit angle  .
The reference vector is defined by the geometry
parameters
g_haz, g_kaz, and
g_laz. Use the macro
setaz to specify the
(H,K,L)
of the
reference vector.
For example,
to set the reference vector to (0,0,2), use
1.FOURC> setaz 0 0 2
2.FOURC>
Azimuth-fixed mode will
fail if you try to make measurements with
Q parallel to the reference
vector, since
there is then no way to define a rotation
about
Q . The remedy is either to switch to another mode
(usually the best choice)
or to switch to another reference
vector.
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