Electron Transitions in Chemisorption

Electron transitions can take place between the valence band and the conduction band, as well as between the energy bands and local levels depicted in Fig. 10. In the case of a semiconductor and not too low temperature, these transitions are of thermal origin. Because of them an electron may pass to the conduction band or to an acceptor level. A, or it may be removed from a donor level D. 

The appearance of an electron on a level A denotes the transition of the chemisorbed particle C from a state of weak to a state of strong acceptor bonding with the surface. This may be effected in two ways, as is seen from Fig. 10 by a free electron of the conduction band falling onto level A or by an electron of the valence band being thrown onto level A. The removal of an electron from the level D denotes the transition of the chemisorbed particle C from a state of weak to a state of strong donor bonding. This may also be effected in two ways by the recombination of an electron of level D with a free hole wandering in the valence band or by the ejection of this electron from level D into the conduction band. 

These electron transitions, depicted in Fig. 10 by vertical arrows in bold type (transitions 1, 2, 3, 4, and 6) may be written down in the notation of Sec. I,A as follows  

We see that the participation of the electrons and holes of the semiconductor in the chemisorption processes may be described in the language of the energy band scheme, which thus represents yet another (energy) aspect of the electron mechanism of chemisorption. 

The electron transitions depicted in Fig. 10 correspond to transitions of the system between states characterized by different adsorption curves. Such adsorption curves which represent the energy of the system E as function of the distance r between the particle C and the adsorbent surface for the case when particle C is a monovalent atom are schematically depicted in Fig. 11 (3, 4)- The curve I represents adsorption on an unexcited crystal, i.e., on a crystal that does not contain free electrons and holes. Curve I represents curve I shifted a distance u upwards parallel to itseff that is, it corresponds to adsorption on an excited crystal containing a free electron (in the conduction band) and a free hole (in the valence band). Curves p and n represent the adsorption curves for, respectively, strong donor, and strong acceptor chemisorption (curve n can lie either below or above curve p). The minima of curves I, n, p, I correspond to the states OL, CbL d- pL, CpL eL, CL cL -1- pL. 

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