Charge distribution and electronic density of states

FIM can also be used to study properties, such as the surface induced dipole moment and the effective polarizability of some surface atoms, kink site atoms and adsorbed atoms etc. The charge distribution of a surface atom is obviously completely different from that of a free atom because of its interaction with the surface and in addition surface atoms are partially shielded by itinerant charges of the surface. The charge distribution of a surface atom can be described by the magnitudes of the electric multipoles of the atom. 

The amount of the charge transfer can be changed by an applied field as can be seen in Fig. 4.39(6). When an electric field is applied to the metal surface, the atomic band will be shifted upward by eFd. The dipole moment of the atom is now field dependent, and is given by 

The charge transfer induced polarizability of a surface atom is closely related to the electronic density of states of the atom as can be seen by 

This is a surprisingly simple result where the electronic charge density of states of a surface atom at eFd below the Fermi level is related to the polarizability of the atom measured at that field. Therefore it is possible to investigate the adatom electronic band by measuring the dipole moment and the polarizability of the adatom as a function of the electric field. 

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