Gamma-ray induced 2 eV band appears at different wavelengths depending on the OH-group content. It is found that 2 eV band formed in low-OH content fibers, formerly ascribed to another precursor, is caused by non-bridging oxygen hole centers (NBOHC, Si-Oƃ), as is the one formed in high-OH content fibers. The differences in peak wevelength and in oscillator strength are responsible for hydrogen bonding between NBOHC and a nearby OH-group. The NBOHC with a hydrogen bond in the shape of Si-OƃƃƃƃH-O-Si formed more in high-OH content fibers absorbs light around 600 nm, while it absorbs around 630 nm when it exists without forming a hydrogen bond (Si-Oƃ).
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