Evidence for a wide continuum of polymorphs in a-SiO2.

The irradiations of vitreous amorphous silica with subband-gap laser radiation at 248 nm induces reversible-irreversible compaction. The former is consistent with the creation of a wide continuum of densified amorphous silica (a-${\mathrm{SiO}}_{2}$) polymorphs whilst the second stage corresponds to the ablation through laser radiation of the damaged a-${\mathrm{SiO}}_{2}$. These observations are discussed in terms of conformational structure modifications (i.e., changes in the distribution of planar rings) and stable superficial defect creation.