Optical transitions and EPR properties of two-coordinated Si, Ge, Sn and related H(I), H(II), and H(III) centers in pure and doped silica from ab initio calculations

The photoabsorption and photoluminescence properties of two-coordinated Si, Ge, and Sn defect centers and the magnetic interactions of the corresponding [=Si-H](.), [=Ge-H](.), and [=Sn-H](.) structures, the H(I), H(II), and H(III) centers, respectively, have been studied by means of ab initio quantum-chemical methods. Using cluster models and configuration interaction wave functions, we have determined the transition energies to the lowest singlet and triplet states and the corresponding emissions and lifetimes. The computed optical properties are in close agreement with those proposed for the two-coordinated centers in pure and doped silica. The hyperfine interactions of the paramagnetic H(I), H(II), and H(III) centers have been studied using unrestricted Hartree-Fock and density-functional theory approaches. Also, in this case, the proposed assignments are consistent with the calculations. The results provide strong theoretical support to the proposal of the two-coordinated Si as the origin of one component of the 5.0 eV B-2 absorption band in amorphous silica.