Thermal and photo-initiated reactions of H-2 with germanosilicate optical fibers - Comment

Reactions of H-2 with Ge doped amorphous SiO2 have been presented in a recent paper by Greene et al. [J. Non-Cryst. Solids 168 (1994) 195] based on Raman spectroscopy. They observed that 3675 cm(-1) and 2185 cm(-1) bands were generated in a heated, H-2 loaded sample. On the other hand, only the 2140 cm(-1) band was generated in an ultraviolet (UV) exposed, H-2 loaded sample. They assigned the 2185 cm(-1) and 2140 cm(-1) bands to the Ge monohydride (GeH) and dihydride (GeH2), respectively. The major conclusion of their work is that the thermally initiated reaction results in GeH and SiOH production, =Ge-O-Si= + H-2 --> =Ge-H + HO-Si=. Second, a UV-light-initiated reaction, =Ge: + H-2 --> =GeH2, is proposed. I plotted the molecular vibration frequencies as a function of the electronegativity. As a result, the vibration frequency of GeH is lower than that of GeH2 in a GeO2 matrix. I postulate that the correct assignments of the 2185 cm(-1) and 2140 cm(-1) bands are the GeH2 and GeH, respectively. I examined the thermal and photo-initiated reactions of H-2 in a Ge doped SiO2 with infrared spectroscopy. Even if Greene et al.'s assignment were true, the Greene et al. models would be inconsistent from the viewpoint of concentration. The SiOH concentration is larger than the GeH concentration by a factor of 30.