Laser-induced-impurity colloid formation and dissociation in MgO single crystals

Abstract

Optical-absorption measurements were used to monitor impurity precipitation and dissociation of precipitates in both laser-irradiated and furnace-annealed MgO single crystals doped with Li, Co, or Ni impurities. In as-grown MgO:Li crystals, both ${\mathrm{Li}}_{2}$O precipitates and isolated substitutional ${\mathrm{Li}}^{+}$ ions are present; oxidation above 1100 K disperses lithium ions from the precipitates and forms stable $[\mathrm{Li}{]}^{0}$ centers (substitutional ${\mathrm{Li}}^{+}$ ions, each with a trapped hole) around the precipitates. Laser-induced experiments showed that both lithium precipitation and dispersion were enhanced by the presence of cracks in the samples. A cross-section value of 0.2 bs was obtained for Li displacement, indicating that the diffusion of lithium occurs via a thermal process rather than a radiation-induced diffusion, as in the case of ionizing radiation. In MgO:Co crystals, metallic colloids were produced by thermochemical reduction at high temperatures that resulted in an extinction band at 3.7 eV due to Mie scattering from the precipitates. Dissociation of cobalt precipitates was produced under laser irradiation. An activation energy of 3.0 eV and a cross section of 0.01 b s was obtained for displacement of cobalt ions from the precipitates. Laser-induced desorption and ablation was observed when the laser was focused on the crystal. In nickel-doped MgO crystals, an activation energy of 1.9 eV was determined for displacement of Ni ions from the metallic nickel precipitates. In such crystals, different patterns can be inscribed with the laser.