Surface induced dipole moment of adsorbed atoms

2 Surface induced dipole moment of adsorbed atoms 

The dipole moment of adsorbed atoms can be derived by measuring the change in work function as a result of the presence of the adatoms. In macroscopic measurements, the states of the deposited adatoms are unknown. They may combine into clusters of different sizes and some of them may be absorbed into lattice steps. One can also use the field emission microscope for this purpose. However, similar uncertainties exist. To achieve a well characterized surface and a known number of individual adsorbed atoms on a surface, a combined experiment with field ion microscope observations and field emission Fowler-Nordheim (F-N) plots has been most successful.198,200 

The field emission current from an area A of a field electron emitter surface of work function 0O at a field F is given by 

When the same surface has deposited on it n adatoms per unit area, each with a dipole moment p and polarizability a, the work function in the presence of an applied field fiV changes to 

If the number density of adatoms is not too large and the applied field is in the field electron emission range, one can show that, in general, Aimp p0, AimafiV 0O and afiV p. The slope of the F-N plot is therefore given by 

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