Infrared molar absorption coefficient of H2O stretching modes in SiO2

Infrared absorption of the H2O stretching modes around 3400 cm− 1 is commonly used to detect the presence of H2O inside SiO2. Obtaining the exact concentration of water remains difficult since the molar absorption coefficient (ε) is not known. In this work ε is determined for H2O absorbed in sub-atmospheric chemical vapor deposited SiO2 and compared to liquid water. By integration over the whole absorption band width, ε is found to be 7.64 ± 1.08 × 104 M− 1 cm− 2. When considering peak absorption at the absorption band maximum only, the value is ε = 139.5 ± 11.8 M− 1 cm− 1 which is significantly lower than the literature value of liquid water for which ε = 231.66 ± 0.05 M− 1 cm− 1. This difference is attributed to the absence of hydrogen bridges between H2O molecules, evidenced by the featureless absorption spectrum of H2O inside SiO2.