ABSTRACT
Photodesorption of oxygen from nickel oxide was investigated by Haber and Stone [28] who stated that the photoactivity is almost entirely confined to wavelength3 of 650 and 900 nm comprising the 3A2g -+ lEg and 3A2g -+ 3Tg transitions (Fig. 3.10), and was not active in oxygen desorption. The authors assumed that desorption occurs mainly from the (110) face, which is supposed, as already mentioned, to be the most densely covered with chemisorbed oxygen owing to the highest crystal field stabilization effect. It was also postulated that two oxygen atoms are desorbed simultaneously from one adsorption center, thus changing the coordination from octahedral to tetrahedral. In such a model the photoactivation of the nickel ion to the 1Eg or 3T1g state renders the octahedral adsorption complex unstable with respect to the tetrahedral one, as is shown in the Orgel diagram in Fig. 3.10. As a consequence the simultaneous desorption of two oxygen atoms should occur.
