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.The relative motion between two contacting solids can produce light, called triboluminescence [1,2]. For example, opening an envelope in a dark room usually result in flashes of blue light from the (pressure sensitive rubber) adhesive interface. Recent experiment have shown that photons with energies up to ∼ 100 keV are produced during the pealing of adhesive tape in 10 −3 Torr vacuum. The X-ray pulses were of nano-second duration, produced ∼ 100 mW, and were correlated with stick-slip peeling events.The origin of triboluminescence is believed to be related to charge separation. In order for charge separation to generate high energy photons, the discharging process must not occur until the solid walls have been separated by a relative large distance. If the surface charge density is denoted by ±σ = ±n 0 e (where e is the electron charge and n 0 the ion number density), and if the charge is uniformly distributed, the voltage drop between the two separating surfaces is given by V = Ed = 4πσd where d is the surface separation. The highest energy photons (energyhω max ) emitted during the discharging is likely to behω max = eV = 4πn 0 e 2 d, and will have an energy proportional to the surface separation at the moment of the discharge. In a typical experiment the average surface charge density n 0 ≈ 10 14 m −2 so that if the discharging would occur at the separation d ≈ 1 mm one would expect photons with energy up to a few keV. In the experiment reported on in Ref.[3] photons with energies up to 100 keV were observed, indicating even higher local charge concentration.In vacuum the initiation of the discharging must be related to field assisted emission. That is, such a strong electric field must be set up at the surface of at least one of the solids that electrons or ions are pulled away from the surface. This may involve tunneling (mainly for electrons) or thermally induced charge transfer (or a combination of both) across or over the barrier towards desorption. The charged particle is then accelerated by the electric field, and when it hits into the surface of the opposite solid it may generate photons (bremsstrahlung) and produce more charged particles, some of which (of opposite sign as the impacting particle) may accelerate towards the opposite solid and in this way generate a cascade of charged particles and photons. This m...