Alkaline-earth oxides absorption bands

In the previous section we summarized the chemical evidence that oxide ions in a state of low coordination can act as electron donors. At the same time, spectroscopic evidence has been accumulated which shows that highly dispersed alkaline-earth oxides have optical absorption bands that are not present in the pure single crystal. This is surprising at first because the energy required for electronic excitation of bulk MgO corresponds to a frequency in the vacuum ultraviolet. In order to understand this we must look at the absorption process more closely. 

Absorption, Excitation, and Emission Bands of the Alkaline-Earth Oxide Powders... 

Two separate experimental approaches, diffuse reflectance and photoluminescence spectroscopy, were then taken both led to similar results. The latter technique is the more sensitive, and well-resolved spectra can often be observed, but only when a radiative decay of the excited state occurs. The diffuse reflectance spectra are broader in scope but the absorption bands appear as shoulders. The reflectance spectra of alkaline earth oxides were examined by Zecchina et al. (77, 78), Garrone et al. (79), and Zecchina and Scarano (80), but an overpressure of a quenching gas (usually oxygen) had to be used to suppress the fluorescence and to allow observ ation of the reflectance absorption bands (Fig. 10). In addition to usual bands in the U V region due to bulk excitations (bulk cxcitons), new absorption bands which correspond to excitations localized on the surface ions are present. 

The formation of oxide surfaces occurs during the decomposition of the precursor hydroxide surface in vacuo (81). For MgO (82). two emissions with peaks at about 400 nm (3.1 cV) can be observed when the oxide is excited at 230 nm (5.4 eV) and 274 nm (4.5 cV), and the intensities of these emissions reach a maximum at temperatures between 1073 and 1273 K, being associated with the Olc ions. These photoluminescence spectra are immediately quenched by oxygen, and the emitting sites are destroyed by CO2. The absorption bands measured in the reflectance spectra and the corresponding bands in the excitation spectra show a good coincidence for all the alkaline earth oxides (Table 1). 

From comparisons of the absorption and excitation spectra for the oxides, as shown in Table I (66) it appears that the energy decreases with an increase in the cation size from Mg to Ba in the alkaline earth metal cation series. This pattern has been satisfactorily explained by using the approach of Levine and Mark (84), whereby ions located on an ideal surface are considered to be equivalent to the bulk ions, except for their reduced Madelung constants. A more detailed analysis has been carried out by Garrone et al. (60, 79), who reinterpreted earlier reflectance spectra and suggested that there is evidence of three absorption bands corresponding to ions in live, four, and three coordination—aU three for MgO, CaO, and SrO. 

A much mure troublesome problem occurs when Ihe source of absorption or scattering originates in Ihe sample matrix. In this instance, the power of the transmitted beam P is reduced by the matrix components, but the incident beam power Pq is not a positive error in absorbance and thus concentration results. An example of a potential matrix interference because of absorption occurs in the determination of barium in alkaline-earth mixtures. As shown by the solid line in Figure 8-8, the wavelength of Ihe barium line used for atomic absorption analysis appears in Ihe center of a broad absorption band for CaOH. We therefore anticipate that calcium will interfere in barium determinations, but the effect is easily eliminated by substituting nitrous oxide for air as Ihe oxidant. The higher temperature of the nitrous oxide flame decomposes the CaOH and eliminates Ihe absorption hand. 

A more common type of spectral interference in either emission or absorption measurements arises from the occurrence of band emission-spectra due to molecular species in the flame. (In fact, many elements can be measured by means of the band spectra of the molecules they form in certain flames.) Calcium and strontium, for example, exist partially as molecular hydroxides and oxides in a flame and emit bands in the vicinity of both the sodium and lithium resonance lines. When the alkaline-earth/alkali-metal ratio is high, the interference can become serious, unless a high-resolution monochromator is used. 

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