Oxides the Case of AEO

The PL spectra were actually very sensitive to the overall surface structure and this allowed the study of the behavior of each type of luminescence center upon thermal treatment or adsorption of probe molecules. These studies have also shown that, by the way of an energy-transfer process, emission can arise from surface sites that are not necessarily those that absorbed light in the first step of the PL phenomenon. For instance, at 300 K, the energy absorbed by 5- and 4-coordinated sites is transferred to the 3-coordinated ones, whilst at 77 K, the energy-transfer process is largely suppressed and the original emission profiles of 4- and 5-coordinated centers can be observed [41]. 

From these early studies, the PL technique, which was initially aimed at confirming the observahon of surface sites, was gradually extended to explore other related aspects, such as surface structure, decay of the excited states and surface reactivity. 

In addition, the adsorption of quencher molecules, such as O2 and H2, on AEOs has allowed the clarification of the nature of the luminescence sites present at the surface [81]. In particular, in the case of H2 adsorption on SrO, a change in the excitation band shape was observed. H2 may react at different rates with species absorbing in different parts of the excitation band, producing a change in the band shape. No corresponding change in band shape was observed in the emission spectrum. By contrast, O2 adsorption did not change the shape of either the excitation or the emission bands only a decrease in intensity was observed. This 

Surface excitons require less energy in their formahon than bulk excitons, owing to the reduced Madelung constant of the coordinahvely unsaturated ions at the surface [81]. 

Furthermore, analysis of the excitation spectra of AEOs has revealed that the excitation bands are characterized by several components, which is evidence that centers with different coordinahon are present on the surface. The lower the energy, the lower the coordinahon number, ft was estabhshed that the three main components identified in the excitahon spectra were associated with three surface states with different coordinahon of the ion involved O5C on extended (001) faces, O4C on the edges and O3C on the corners. In the bulk lathee of an AEO the ions are octahedrally coordinated. 

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