Measurements of the absorption edge in fused silica

The absorption coefficients of four commercial silicas (Spectrosil WF, Spectrosil A, Suprasil W2, Suprasil 2) have been measured over the energy range 6.2–8.1 eV. The absorption in the neighborhood of the edge varies markedly with water content. A peak or a shoulder is observed at 7.6 eV (an energy below the absorption edge) in the water‐free samples. In samples containing significant OH, no structure at energies lower than the fundamental edge is observed. The absorption edge for the dry silica occurs at an energy 0.2 eV higher than for the wet silica.