A crystal consists of a regular arrangement of atoms in space, also called crystal lattice, which is held together by the mutual electrostatic attraction of the electron clouds of neighboring atoms. Most of the electrons are strongly bound to an individual, positively charged nucleus. The outermost electrons of an atom are called valence electrons and build up the bond to neighboring atoms. These bonds determine the atomic spacing in the crystal as well as essential properties, such as its electrical conductivity or mechanical stability.
The atoms in a crystal lattice are not at rest, but vibrate about their respective equilibrium position. The spatial displacement of the movement of the atomic nuclei together with their electrons in the inner shells is typically only one percent of the distance between the atoms. How the outer valence electrons behave during this lattice vibration was not yet clear and the magnitude of their displacement was completely unknown. Direct measurement of this movement in real time is very important for a basic understanding of the static and dynamic electrical properties of the crystal.