Tian Hanjing, Wachs Israel E, Briand Laura E
Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA.
J Phys Chem B. 2005 Dec 15;109(49):23491-9. doi: 10.1021/jp053879j.
The visible (532 and 442 nm) and UV (325 nm) Raman spectra of bulk mixed metal oxides (metal molybdates and metal vanadates) were compared on the same spectrometer, for the first time, to allow examination of how varying the excitation energy from visible to UV affects the resulting Raman spectra. The quality of the Raman spectra was found to be a strong function of the absorption properties of the bulk mixed oxide. For bulk mixed metal oxides that absorb weakly in the visible and UV regions, both the visible and UV Raman spectra were of high quality and exhibit identical vibrational bands, but with slightly different relative intensities. For bulk mixed metal oxides that absorb strongly in the UV and visible regions and/or strongly in the UV and weakly in the visible regions, the visible Raman spectra are much richer in structural information and of higher resolution than the corresponding UV Raman spectra. This is a consequence of the strong UV absorption that significantly reduces the sampling volume and number of scatterers giving rise to the Raman signal. The shallower escape depth of UV Raman, however, was not sufficient to detect vibrations from the surface metal oxide species that are present on the outermost surface layer of these crystalline mixed metal oxide phases as previously suggested. It was also demonstrated that there is no sample damage by the more energetic UV excitation when very low laser powers and fast detectors are employed, thus avoiding the need of complicated fluidized bed sample arrangements sometimes used for UV Raman investigations. The current comparative Raman investigation carefully documents, for the first time, the advantages and disadvantages of applying different excitation energies in collecting Raman spectra of bulk mixed metal oxide materials.
首次在同一台光谱仪上比较了块状混合金属氧化物（金属钼酸盐和金属钒酸盐）的可见（532和442 nm）和紫外（325 nm）拉曼光谱，以研究从可见光到紫外光改变激发能量如何影响所得的拉曼光谱。发现拉曼光谱的质量强烈依赖于块状混合氧化物的吸收特性。对于在可见光和紫外区域吸收较弱的块状混合金属氧化物，可见光和紫外拉曼光谱质量都很高，且呈现相同的振动带，但相对强度略有不同。对于在紫外和可见光区域吸收较强和/或在紫外区域吸收强而在可见光区域吸收弱的块状混合金属氧化物，可见拉曼光谱比相应的紫外拉曼光谱在结构信息方面更丰富且分辨率更高。这是由于强烈的紫外吸收显著减小了产生拉曼信号的采样体积和散射体数量。然而，如之前所建议的，紫外拉曼较浅的逸出深度不足以检测这些结晶混合金属氧化物相最外层表面上存在的表面金属氧化物物种的振动。研究还表明，当使用非常低的激光功率和快速探测器时，更高能量的紫外激发不会对样品造成损伤，因此无需有时用于紫外拉曼研究的复杂流化床样品装置。当前的比较拉曼研究首次详细记录了在收集块状混合金属氧化物材料的拉曼光谱时应用不同激发能量的优缺点。
