Ultrafast electron diffraction<xref rid="cpb_27_7_076102_fn1" ref-type="fn">*</xref><fn id="cpb_27_7_076102_fn1"> <label>*</label><p>Project supported by the National Natural Science Foundation of China (Grant No. 11774409), the National Basic Research Program of China (Grant No. 2013CBA01501), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB16010200 and XDB07030300).</p></fn>

Ultrafast electron diffraction* *

Project supported by the National Natural Science Foundation of China (Grant No. 11774409), the National Basic Research Program of China (Grant No. 2013CBA01501), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB16010200 and XDB07030300).

Wang Xuan^{1, †}, Li Yutong^{1, 2, 3, ‡}

(color online) (a) Bragg diffraction in real space. Two beams with identical wavelength and phase approach a crystalline solid and are scattered off two parallel planes of atoms. The lower beam traverses an extra length. Constructive interference occurs when this length is equal to an integer multiple of the wavelength of the radiation. (b) Laue condition in reciprocal space. Constructive interference occurs when the difference between the wave vectors of the original and the diffracted beams equals to a reciprocal vector.