動力学的回折

dynamical diffraction

[目次：理論(電子の散乱／回折／結像)]

結晶性の試料に入射した電子はブラッグ条件を満たす格子面で反射(回折)する。試料が厚いと何回も反射が起こり入射波は減少し、ついには反射波の強度が入射波の強度を上回る。すると今度は反射波が入射波のほうに反射される。このように回折波と入射波の相互作用を考慮する回折現象のことをいう。入射波と一つの回折波のみを考える二波近似動力学回折理論によれば、回折強度は結晶構造因子(散乱振幅)の二乗でなく一乗に比例する。試料が薄い場合(＜3 nm)、反射が1回しか起こらないと仮定して扱う回折現象を運動学的回折という。

An electron that passes through a crystalline specimen is reflected (diffracted) by lattice planes satisfying a Bragg condition. As a specimen is thicker, the reflection occurs many times. At a certain depth, the amplitude of the reflected wave becomes greater than that of the incident wave. Then, the reflected wave is reflected in the direction of the incident wave. Dynamical diffraction takes accounts of such interactions between the incident and diffracted waves. Based on the two-beam dynamical theory, the diffraction intensity is proportional to the crystal structure factor (scattering amplitude) of the reflection. If a specimen is very thin (<3 nm), kinematical diffraction is applied, in which the Bragg reflection is assumed to occur only one time.

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