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Surface X-ray Diffraction (SXRD) : ウィキペディア英語版
X-ray crystal truncation rod
X-ray crystal truncation rod scattering is a powerful method in surface science, based on analysis of surface X-ray diffraction (SXRD) patterns from a crystalline surface.
For an infinite crystal, the diffracted pattern is concentrated in Dirac delta function like Bragg peaks. Presence of crystalline surfaces results in additional structure along so-called truncation rods (linear regions in momentum space normal to the surface). Crystal Truncation Rod (CTR) measurements allow detailed determination of atomic structure at the surface, especially useful in cases of oxidation, epitaxial growth, and adsorption studies on crystalline surfaces.
==Theory==

A particle incident on a crystalline surface with momentum K_0 will undergo scattering through a momentum change of \mathbf. If x and y represent directions in the plane of the surface and z is perpendicular to the surface, then the scattered intensity as a function of all possible values of \mathbf is given by
I(\mathbf)=\fracN_xQ_xa_x \right )}Q_xa_x\right )}\ \fracN_yQ_ya_y \right )}Q_ya_y \right )}\ \frac \cos \left (N_zQ_zc \right )}

Where \alpha is the penetration coefficient, defined as the ratio of x-ray amplitudes scattered from successive planes of atoms in the crystal, and a_x, a_y, and c are the lattice spacings in the x, y, and z directions, respectively. 〔E. Conrad (1996). "Diffraction Methods". In W. N. Unertl (Ed.), ''Physical Structure'', pp. 279-302. Amsterdam: Elsevier Science.〕
In the case of perfect adsorption, \alpha = 0, and the intensity becomes independent of Q_z, with a maximum for any \mathbf parallel to the crystal surface) that satisfies the 2D Laue condition in reciprocal space
\mathbf_\parallel = \mathbf_ = h\mathbf^
*_x + k\mathbf^
*_y

for integers h and k. This condition results in rods of intensity in reciprocal space, oriented perpendicular to the surface and passing through the reciprocal lattice points of the surface, as in Fig. 1. These rods are known as diffraction rods, or crystal truncation rods.
When \alpha is allowed to vary from 0, the intensity along the rods varies according to Fig. 2. Note that in the limit as \alpha approaches unity, the x-rays are fully penetrating, and the scattered intensity approaches a periodic delta function, as in bulk diffraction.
This calculation has been done according to the kinematic (single-scattering) approximation. This has been shown to be accurate to within a factor of 10^ of the peak intensity. Adding dynamical (multiple-scattering) considerations to the model can result in even more accurate predictions of CTR intensity. 〔V. M. Kaganer (2007). Crystal truncation rods in kinematical and dynamical x-ray diffraction theories. ''Phys. Rev. B'' 75 245425.〕

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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