The interaction of H 2 with the defect sites of the SiO 2 surface has been studied by means of gradient-corrected density functional theory calculations on cluster models. The mechanism of hydrogen dissociation, the energy of reactants and products, and the corresponding activation energies and transition states have been determined for the following defect sites: Si singly occupied sp 3 dangling bonds (E′ centers), tSi • ; nonbridging oxygen centers (NBO), tSi-O • ; divalent Si, dSi:; and neutral oxygen vacancies, tSi-Sit. H 2 cracking on the NBO sites is exothermic by ∼0.4 eV and has an energy barrier of ∼0.1 eV (or less considering nonadiabatic effects) which suggest the occurrence of the process even at low temperature. On Si dangling bonds the formation of tSi-H and neutral H atom is endothermic and occurs with an activation energy of less than 0.5 eV; the reaction can occur at room temperature. The interaction of molecular hydrogen with the diamagnetic oxygen deficient centers, dSi: and tSi-Sit, leads to the formation of stable tSi-H groups with exothermic processes and relatively high activation energies of about 2 eV. Thus, H 2 cracking is predicted to occur at room temperature on paramagnetic defects and only at high temperatures on the diamagnetic centers.

Electron paramagnetic resonance and IR spectral properties of hydrogen-associated centers in silica have been studied by means of first-principles Hartree-Fock and density-functional-theory calculations. The geometric and electronic structure of the paramagnetic centers ͑wSi-O͒ 3 Si • , ͑wSi-O͒ 2 Si • ͑OH͒ and ͑wSi-O͒ 2 Si • ͑H͒, and of the corresponding diamagnetic centers formally derived from the addition of one H atom, ͑wSi-O͒ 3 Si͑H͒, ͑wSi-O͒ 2 Si͑OH͒͑H͒, and ͑wSi-O͒ 2 Si͑H͒͑H͒, have been determined using cluster models. A substantial agreement is found between the computed hyperfine splittings and those measured for bulk paramagnetic centers in high-purity amorphous silica. The different experimental hyperfine splittings obtained for ͑wSi-O͒ 3 Si • centers at the surface of mechanically or thermally activated silica are explained in terms of small structural differences from the bulk counterparts. An almost quantitative agreement with the experiment is found also for the vibrational properties of the Si-H groups of the silylhydride centers. The calculations allow a firm assignment of the spectral features to specific point defects in silica. ͓S0163-1829͑98͒08136-3͔

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