OPTICAL-TRANSITIONS AND EPR PROPERTIES OF 2-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-coordinate d Si, Ge, and Sn defect centers and the magnetic interactions of the c orresponding [=Si-H](.), [=Ge-H](.), and [=Sn-H](.) structures, the H( I), H(II), and H(III) centers, respectively, have been studied by mean s of ab initio quantum-chemical methods. Using cluster models and conf iguration interaction wave functions, we have determined the transitio n energies to the lowest singlet and triplet states and the correspond ing emissions and lifetimes. The computed optical properties are in cl ose agreement with those proposed for the two-coordinated centers in p ure and doped silica. The hyperfine interactions of the paramagnetic H (I), H(II), and H(III) centers have been studied using unrestricted Ha rtree-Fock and density-functional theory approaches. Also, in this cas e, 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 absorp tion band in amorphous silica.