FTIR STUDY ON MOLECULAR CONTAMINATION ON SURFACE OF OPTICAL MATERIALS

The IR 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. N₂, O₂, 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 N₂ (66.5kPa), O₂ (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.