Infrared reflection-absorption spectra of anodic oxide films on aluminum

Infrared reflection spectra of anodic aluminum oxide films prepared in boric acid and natural oxide films on aluminum show a common absorption band in the 900–1000 cm^-1 range. This band is shown to be due to the longitudinal mode of the Al-O stretching vibration. In transmission spectra this absorption band is absent; instead one observes the corresponding transverse mode centered around 650 cm^-1.