Scans in the scattering plane: a flight in q space
The transverse, longitudinal and detector scan are shown below in the Ewald construction.
The sample, a thin film, is in the center of the Ewald sphere that has a radius of 1/λ.
The red arrows Kin are representations of the incident wavevector on the sample and
the yellow arrow Kout represents the outgoing wavevector.
The green rod is the q vector Kin - Kout in reciprocal space with the origin at the end of Kin.
The magnetic satellite peak is shown as a purple rod with the long axis along the normal of the samples surface.
As the real space third dimension is very small (the sample is a thin film), the satellite peak is extended in this direction in reciprocal space .
This is a common phenomenon in X-ray diffraction of two dimensional systems and the diffraction spots are usually called crystal truncation rods (CTR).
The size of the rod is largely exaggerated for clarity. As the sample rotates in real space,
the rod will rotate with it around the origin in reciprocal space.
As soon as the rod intersects the Ewald sphere it scatters and can be detected.
The intersected area of the Ewald sphere and the rod has the shape of an ellipse and is indicated in magenta.
Longitudinal scan (specular scan) |
Transverse scan (rocking scan) |
Detector scan |
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| In a specular scan the incident angle and outgoing angle are the same.
To do this you should rotate the sample by the angle theta and detector angle with two times theta. In this way, information on the out-of-plane component of the film is obtained. As can be seen in the movie, the peak is scanned through the long axis of the rod. |
In a transverse scan, the size of the q vector is fixed. The sample is rotated and the detector is kept in place. Information on the in-plane component of the film is obtained when the angle is not too far from specular(<10 degrees). |
In a detector scan, the detector is scanned over the magnetic sattelite peak
while the sample is fixed at the position of the magnetic satellite peak. As can be observed in the movie, the peak is detected in a way that is not parallel to any crystallographic axis. So the detector scan gives a combination of in-plane and out-of-plane components. With a 2D detector like a CCD camera, the peak is detected as an ellipse (the intersection of the rod and the Ewald sphere). |
Ewald sphere |
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| On a CCD camera the magnetic peak is detected as an ellipse, just like the rod is intersected by the Ewald sphere (here shown in magenta). |
Q space on the CCD detector |
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| Here the q space is displayed as it is imaged by the CCD detector. |