Interpretation of Raman Spectra of Oxide Materials: The Relevance of Absorption Effects

When using Raman spectra for structural characterization of solid powder materials such as catalysts, the interpretation needs to take into account the possible absorption of radiation. For example, the reduction of oxide materials may result in new UV–vis absorption bands and therefore affect the Raman intensity. In resonance Raman spectroscopy, the absorption correction of Raman intensity based on the Kubelka–Munk theory is widely established. In contrast, in Raman spectroscopy, typically no absorption correction or normalization by a phonon mode is applied. Using a combined approach of Raman and UV–vis spectroscopy, this study examines the effect of absorption on Raman spectra as well as the usefulness of different absorption corrections for Raman spectroscopy. The results show that a change in absorption of about 10% may result in a decrease in Raman intensity of up to one-third. Corrections based on the Kubelka–Munk theory predict large intensity changes for small changes in the low-absorption region, resulting in the largest deviations among different absorption corrections. On the other hand, the commonly used normalization by a phonon mode shows increasing deviations with increasing spectral distance of the investigated feature from the position of the phonon mode, since it is not wavelength dependent. To reduce these discrepancies, we have developed a new wavelength-dependent absorption correction. As illustrated for ceria materials, the corrected Raman intensity may vary significantly from the raw data intensity. Consequently, for a profound interpretation of Raman spectra of solid powder materials, the combined use of Raman and UV–vis spectroscopy is recommended, allowing one to investigate and quantify the effect of absorption on Raman spectra, which may be of relevance even for Raman spectroscopy.