The Modification of Surface Properties by Adsorbates

Adsorbates modify the inherent dipole structure of a metal surface and hence change the work function and other surface properties. This effect may be evaluated in terms of the dipole moment produced by the adsorbed layer on the metal surface. 

Since it has been shown that the deposition of dipoles on the surface of a conductor changes the work function, it is apparent that the C.P.D. between two metals A and B of surface potentials and b, respectively, will be modified by the presence of adsorbate on one of the metal surfaces. Thus, if surface A (representing surface A plus adsorbate) is substituted for surface A in Equation (2), the new C.P.D. is given by 4 b — 4 a It is possible, therefore, to determine the value of I a — t A by difference. Then if xa and xa are the corresponding surface potentials related to the work function by = — xa — n/e, etc., we have 

The evaluation of the dipole moment from the observed change in work function on adsorption demands a knowledge of the nature of the adsorption 

Van der Waals Adsorption. Referring to Fig. 9a, it is seen that the adatoms or adsorbed molecules are polarized by the surface field. Alternatively, a small amount of electron transfer may take place. In either case the resulting dipole moment is small and 

Covalent Adsorption with Ionic Character). If it is assumed that metallic conduction starts at the metal surface, the effective dipole length is diiJkl I , where duA is equal to the distance between the adsorbate and metal atoms as shown in Fig. 9c. Thus, for a dipole moment M = eduA,  

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