The photoabsorption and photoluminescence of divalent defects in silicate and germanosilicate glasses have been studied by first-principles quantum-chemical techniques. Divalent Si and divalent Ge defects have very similar excitations. They have singlet-to-singlet excitations at 5.2 eV ͑Si͒ and 5.1 eV ͑Ge͒, and singlet-to-triplet excitations at 3.1 eV ͑Si͒ and 3.4 eV ͑Ge͒. The excited-state geometries have been relaxed to obtain the corresponding photoluminescence energies. Singlet-to-singlet luminescence transitions occur at 4.5 eV ͑Si͒ and 4.1 eV ͑Ge͒, and triplet-to-singlet transitions occur at 2.5 eV ͑Si͒ and 2.7 eV ͑Ge͒. Excellent agreement with the corresponding experimental values suggests that divalent Si and Ge defects contribute to the 5-eV absorption band and subsequent photoemissions in silicate and germanosilicate glasses.

We have investigated the microscopic mechanisms for the thermal reactions of H2 with germanosilicate optical fibers by means of first-principles quantum chemical techniques. As a first reaction step, it has recently been shown that H2 reacts with a Ge -0 bond to form Ge -H and SiO -H. In this work, we have carefully considered the activation energies for two additional reaction channels at the Ge -H site which lead to divalent Ge defects. A channel involving hydrogen migration from Ge to a neighboring 0 is found to be energetically favored significantly and is likely to be the dominant pathway for the formation of divalent Ge.The discovery of refractive index changes in germaniurndoped silicate optical fibers upon UV irradiation is an exciting recent development that has led to considerable research efforts in this area. Diffraction phase gratings have been imprinted directly into the core of optical fibers, and this technology is likely to play an important role for future fiber devices such as lasers, mode couplers, and sensors. ' More recently, it has been observed that the refractive index changes can be greatly enhanced by loading the germanosilicate fibers with molecular hydrogen. ' In addition, recent experiments have shown that similar large refractive index changes can also be achieved by rapid thermal heating. ' Mechanistic understanding of the underlying chemical reactions is just beginning to emerge based on contributions from both experiments" and theory. Experimental studies by Lemaire et al. with several different fiber samples found that the Ge content correlates directly with the amount of OH formed. Later Raman spectral data by Greene et aI. showed clear signatures of SiO -H and Ge -H bond-stretching modes. The experimental data suggest that a chemical reaction occurs preferentially at a Ge site and that H2 breaks the Ge -0 bond of a =Ge-0 -Si= unit to form Ge -H and Si0 -H. Recently, we have performed first-principles quantum chemical calculations to investigate the thermal reactions of H2with Ge -0 and Si -0 bonds and found that the energy barrier for breaking a Si -0 bond is about 0.5 eV higher than that of breaking a Ge -0 bond. Assuming similar preexponential factors, the reaction probability at a Ge site should be the dominant mechanism even at temperatures as high as 1000 C.Our previous theoretical study considered only the initial step in the chemical reactions of Hz. There are several other features seen experimentally which require a detailed consideration of additional steps. For example, Lemaire et al. determined that a strong 242-nm band grew in proportion to the OH increases in Game-heated fibers. The 242-nm band has been proposed to correspond to divalent Ge defects. Also, under photoinitiated conditions, the H2 reaction leads to the formation of the dihydride =GeH2 as evidenced by Raman-scattering measurements where a peak at 2135 crn has been observed. Greene et al. assigned this peak to be the GeHz stretch mode based on the frequency shift of about 50 cm ' from the position (2...