Charging of MgO

MgO has been well known as a material with high secondary electron yield. The escape depth of electrons for MgO with the ordered structure is relatively large, therefore the electrons are usually re-emitted from bulk MgO. In addition, when the electrons are injected deep into a perfect MgO crystal, polarons are formed, because the electrons have polarized its surroundings. When the density of the polaron sites is high enough, a narrow energy band is established. In the space charge physics, such a band in perfect crystal MgO has been predicted to have a very low barrier between adjacent sites, thus the polarons in the crystal are usually free to move or hop. Any excess electrons will eventually escape to the surface and later they will be neutralized by positive ions or removed by air molecules. Therefore, the material is normally positively charged under electron beam bombardment. In this report, we will verify this proposal in the space charge physics by the well-known mirror method experiment.