FTIR study on molecular contamination on surface of optical materials

The ER spectra of the molecular contaminants on surface of optical materials were measured. The optical disks used were SiO2, BK7 (SiO2 70%, B2O3 10%, K2O 8%, N2O 8%), CaF2, ZnSe and Al2O3. N-2, O-2, H2O, and CO2 were adopted as contamination gases. IR spectra of H2O (2.7kPa) on BK7 at 373K showed two absorption bands (OH stretching vibration: around 4000cm(-1)-3500cm(-1) and OH bending vibration: around 2000cm(-1)-1500cm(-1)). The absorption intensity decreased with a decrease in temperature and a new band (around 3250cm(-1)) appeared at 173K. The new band was attributed to phase transition of H2O. These phenomena were also observed on the other three discs, except for SiO2. IR spectra of SiO2 showed OH stretching band (3676cm(-1)). The absorption intensity decreased with a decrease in temperature. But two new bands (3720cm(-1) and 3620cm(-1)) appeared under an atmosphere of N-2 (66.5kPa), O-2 (66.5kPa), H2O (2.7kPa) or CO2 (0.7 or 13.3kPa). A similar phenomenon was also observed for BK7, which has OH group. These results suggested the functional group of SiOH interacted with contamination molecules.