Optical and Other Physicochemical Changes by Adsorption

A potential curve of an endothermically chemisorbed atom or molecule represents an excited state with respect to the normal state of the physically adsorbed atom or molecule. When cesium atoms are adsorbed on salt layers or on cesium oxide, they are adsorbed as atoms and not, as they would be on metal surfaces, as ions. Ionization can be brought about by absorption of light 172) or by thermal excitation (173). 

The potential curves of the adsorption of cesium on a CaF2 surface are given in Fig. 21, which shows that the curve for the ion represents an endothermic chemisorption. By the absorption of light of suitable wave length the system is transferred from minimum B to a point P of the upper curve and an electron is freed and may be drawn off as a photoelectron. The phenomenon of the selective photoelectric effect could be fully explained by this photoionization process (174). By thermal excitation the transfer can be effected at point S and this mechanism may serve to explain the electron emission of oxide cathodes. Point S is reached by taking up an amount of energy, which may be called the work function of the oxide cathode in this case but which is completely comparable with the energy of activation in chemisorption discussed in Sec. V,9 and subsequently. We shall not discuss these phenomena in this article but refer to a book of the author where these subjects are dealt with in detail (174)  

A similar relationship may be found in the adsorption of other atoms or molecules. Many organic substances with peripheric dipoles when adsorbed on salt layers or on the surfaces of metallic oxides show absorption spectra which are shifted appreciably to the red side of the spectrum. Thus p-nitrophenol, having a maximum of light absorption at 316 m. when adsorbed on CaF2, has its absorption spectrum shifted to the red side and is yellow instead of colorless (175), its absorption maximum being at 365 m i. (176). Adsorbed on BaF2 it shows an absorption maxi- 

Similar color changes were reported later by Weitz and his collaborators (179), apparently without knowing the older work of the author (180). They describe the bright-red coloration resulting from the physical adsorption of phenolphthalein. 

In all these cases we are concerned with physically adsorbed molecules, pointing with their peripheric dipoles to the negative ions of the surface. The excited states of these molecules are far more polar in character light absorption causes an electron shift in the molecule in a direction away from the surface, resulting in a far stronger bond with the negative ion on which the molecule is adsorbed (181). We may illustrate 

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