Direct observation of electron self-trapping inPbCl2crystals

Direct experimental evidence is reported for the self-trapping of electrons produced at 80 K by x-ray irradiation or by UV illumination above the band gap in pure, orthorhombic (${\mathit{D}}_{2\mathit{h}}^{16}$) ${\mathrm{PbCl}}_2$ crystals. The ESR analysis demonstrates that the electron is trapped at a pair of nearest-neighbor substitutional ${\mathrm{Pb}}^{2+}$ ions along the a direction, resulting in a paramagnetic ${\mathrm{Pb}}_2^{3+}$ molecular ion with electronic configuration complementary to that of the (halogen${)}_2^{\mathrm{-}}$ self-trapped hole center in the alkali halides. The results of this paper suggest that the self-trapped exciton in ${\mathrm{PbCl}}_2$ consists of a hole trapped in an excited orbital around the ${\mathrm{Pb}}_2^{3+}$ center.