**GID_sl** provides multiple options to define the Bragg diffraction geometry
as well as the possibility to simulate various scans around the Bragg peaks.

The key point in proper using those options is understanding the difference between
coplanar and non-coplanar diffraction geometries. We call the geometry coplanar where the incident wave vector **k _{0}**,
the reciprocal lattice vector

By now the most of x-ray experiments are carried out in coplanar geometries since they are simpler to implement.

The major advantage of **GID_sl** as compared to any other known to me x-ray
diffraction simulation program is that **GID_sl** can handle arbitrary
non-coplanar cases of diffraction including those
with grazing incidence/exit and the x-ray specular reflection effects.

The most frequently used example of non-coplanar Bragg diffraction is the
grazing-incidence diffraction, GID (see Fig.2). Here the scattering plane
(the plane defined by **k _{0}** and

In addition, the non-coplanar diffraction geometries offer a wide choice of
scan options. For example, as shown on Fig.2, one may rock the crystal around
the surface normal **N** in order to keep the incidence angle unchanged,
or around the reciprocal vector **h** in order to vary the incidence angle
while preserving the Bragg condition.

The table below lists all the available ways to specify diffraction geometries
to the **GID_sl** program:

No | Geometry specification by... | Geometry type | Surface specification | Asymmetry parameter | Scan axis |
---|---|---|---|---|---|

[1] | Surface orientation & incidence angle of K0 | non-coplanar^{(1)} | required | incidence angle of K0 | any |

[2] | Surface orientation & exit angle of Kh | non-coplanar^{(1)} | required | exit angle of Kh | any |

[3] | Surface orientation & condition of coplanar grazing incidence | coplanar | required | no | [K0*h] |

[4] | Surface orientation & condition of coplanar grazing exit | coplanar | required | no | [K0*h] |

[5] | Surface orientation & condition of symmetric Bragg case | non-coplanar^{(2)} | required | no | any^{(2)} |

[6] | Condition of coplanar reflection & angle of Bragg planes to surface | coplanar | no | Bragg planes angle to surface | [K0*h] |

[7] | Condition of coplanar reflection & incidence angle of K0 | coplanar | no | incidence angle of K0 | [K0*h] |

[8] | Condition of coplanar reflection & exit angle of Kh | coplanar | no | exit angle of Kh | [K0*h] |

[9] | Condition of coplanar reflection & asymmetry factor beta=g0/gh | coplanar | no | asymmetry factor g0/gh | [K0*h] |

^{(1)} |
The geometry may become coplanar if the extreme incidence and exit angles are substituted. These extreme angles are calculated in the geometry specifications [3] and [4] respectively. |

^{(2)} |
The geometry becomes coplanar if the reciprocal vector
of Bragg diffraction is perpendicular to the surface. The GID_sl detects the
coplanar case automatically and then chooses [K0*h] as the scan axis. |