Photoinduced electrification of solids: I. Plausible mechanisms
  Experiments carried out at our laboratories have led to observing a new type of electrification, now called surface photocharging. For a consistent description, we are weighing carefully the experimental evidence against the common knowledge of frictional electrification and analyze it on the premise that photocharging may be the electric face of the structural damage produced at the surface by photodesorption or laser sputtering. We find the emerging picture well consistent and elaborate photocharging mechanisms based on familiar photodesorption steps. The first mechanism based on negative-U operates in the visible and near infrared. It involves photoelectron capture by negative-U metal sites at surface dangling bonds and photohole capture by negative-U sites at active bonds. Dihole formation leading to bond breaking is the likely prerequisite of metal ion desorption. Alternatively, unihole capture at trapped electron sites can give rise to self-trapped excitons (STE) which decay leading to surface defects formation, the prerequisite of metalloid ion desorption. As a result of the coupling to normally polarized modes, surface metal or metalloid ions are desorbed while complementary ionic charges appear in the subsurface region. Alternative photoelectrification mechanisms, such as the photostimulated migration or reorientation of surface molecule ions operating in the far infrared are also discussed. Implications for the practical use of the phenomenon are also mentioned. 
O. Ivanov, R. Dyulgerova and M. Georgiev