BROAD BACKGROUND BANDS IN RAMAN-SPECTRA OF ALUMINA STUDIED BY BOTH VISIBLE AND NEAR-INFRARED EXCITATION

Raman spectra were obtained for two different samples of alumina. One is usually used as a support for hydro-treating catalysts in heterogen eous catalysis and the other was synthesized in the laboratory. Differ ent excitation wavelengths were used: 457.9, 488.0, 514.5, 647.1 and 1 064 nm. The commercially available alumina showed an intense broad bac kground with excitation in the near-infrared region, whereas no signif icant background was observed with visible excitation. The synthetic a lumina showed a broad background band with laser excitation in the vis ible region. It is shown that the appearance of these backgrounds is n ot due to fluorescent impurities (e.g. iron, chromium or organic compo unds). Tentative explanations of the spectral features are provided in terms of density of states (alumina A) or bulk hydroxyl groups (alumi na B).