  The separation of solids in adhesive contact, or the fracture of solid
bodies, often results in the emission of high energy photons, e.g., visible
light and X-rays. This is believed to be related to charge separation. We
propose that the emission of high energy photons involves surface roughness and
surface diffusion of ions or electrons, resulting in the concentration of
charge at the tips of high asperities, and to electric field enhancement, which
facilitate the discharging process which result in the high energy photons. If
the surface diffusion is too fast, or the separation of the solid surfaces too
slow, discharging start at small interfacial separation resulting in low energy
photons.
