Colour centres ionisation

Figures 5.15 and 5.16 show absorption changes caused by adsorption of donor (H2) and acceptor (O2) molecules on pre-irradiated samples corresponding to the absorption spectra of surface-active (colour) centres. These are long-lived because they are deep traps, and consequently the probabihty of their thermal ionisation is very low, i.e. — 0. After irradiation, the probabilities of photoionisation ph and of...

Surface-trapped electrons can be formed at the surface of alkali-earth oxides by different methods, among these methods the UV irradiation of the solid in the presence of hydrogen has been found the most reliable and reproducible and leads to the formation of a particular type of surface colour centre named Fj (H) centre . The mechanism leading to the formation of these centres implies the heterolytic chemisorption of hydrogen at the surface of activated MgO and the formation of and H ions stabilised onto a couple of low-coordinated O -Mg ions. Upon UV irradiation a fraction of the hydride H ions are ionised and the released electrons stabilised into suitable surface anion vacancies close to the OH formed by reaction of H with surface O ions. At the end of this process the sample develops a blue colour and exhibits an EPR signal with g values slightly lower than the free spin value... 

Subsequently, it was found that F-centres can also be produced by heating a crystal in the vapour of an alkali metal this gives a clue to the nature of these defects. The excess alkali metal atoms diffuse into the crystal and settle on cation sites at the same time, an equivalent number of anion site vacancies are created, and ionisation gives an alkali metal cation with an electron trapped at the anion vacancy (Figure 5.24). In fact, it does not even matter which alkali-metal is used if NaCl is heated with potassium, the colour of the F-centre does not change because... 

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