excitation error

"Excitation error", denoted as s_g, means a deviation from a Bragg condition for a certain reflection g. The excitation error is defined as the distance from the reciprocal lattice point g to a point on the Ewald sphere measured along the propagation direction of the electron beam (usually perpendicular to the upper surface of the specimen).
The diffraction intensity varies with the excitation error. The intensity change curve is called the rocking curve. The rocking curve can be directly observed in a convergent-beam electron diffraction pattern.
When the Bragg condition is accurately satisfied, s_g = 0. In the figure, s_g is defined to be positive in the downward direction. Thus, when the reciprocal lattice point is located inside the Ewald sphere, s_g> 0, whereas the reciprocal lattice point is located outside the Ewald sphere, s_g< 0. s_g is an observable quantity with units of inverse length.

The dimensionless parameter (tilt parameter) ω = s_g×ξ_g,  ξ_g being the extinction distance, is used instead of s_g as a useful parameter in the theoretical treatment. However, it should be noted that ω is not experimentally measurable.

Fig. Schematic of the definition of positive and negative signs of excitation error.
s_g＝0 when the Ewald sphere passes through the reciprocal lattice point g, s_g>0 when the reciprocal lattice point is inside the Ewald sphere, s_g<0 when it is outside the Ewald sphere. In this figure, reflection 5g exactly satisfies the Bragg reflection and s_g＝0 .

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