We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated epitaxial graphene grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The samples of epitaxial graphene have been intercalated in the temperature range of 790 to 1250$^circ$C and compared to the reference samples of hydrogen etched SiC. We have found that although the Si-H bonds form at as low temperatures as 790$^circ$C, the well developed bond order has been reached only for epitaxial graphene intercalated at temperatures exceeding 1000$^circ$C. We also show that the hydrogen intercalation degradates on a time scale of few days when samples are stored in ambient air. The optical spectroscopy shows on a formation of vinyl and silyl functional groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process.
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