  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.
