  Poly(ethylene terephthalate) (PET), as most dielectric materials, is able to
retain space charge in traps. This allows the material to attain an
almost-permanent polarization when space charge is displaced by an external
electric field or it is injected from an electrode. We have studied the
influence of UV irradiation on the charge trapping capability of PET in samples
exposed for different periods of time, up to 10 weeks. The pulsed
electro-acoustic technique (PEA) has been used to determine the charge profile.
The injected charge that the material is able to retain on the irradiated
surface increases with irradiation time. This indicates the formation of new
traps. An extensive characterization of these localized states has been
performed by thermally stimulated depolarization currents (TSDC) technique.
Parameters of charge relaxation kinetics have been obtained fitting spectra of
the $\rho_c$ peak, related to injected charge, to the general order kinetics
model. A relaxation map analysis shows that relaxation times become more
distributed and the activation energy decreases as irradiation time is
increased. The activation energy decreases approximately by 10% after 10 weeks
of exposition. These results show that UV irradiation creates additional traps
on the treated surface, which agrees with PEA results, and that these traps are
shallower and their energy depth distribution is wider than in the case of
pre-existing traps.
