FIELD ION MICROSCOPY OF GALLIUM (Ga) ON MOLYBDENUM (Mo) THE CHARGE DISTRIBUTION AT SURFACE

The double layer due to the inhomogeneous charge distribution at the surface is represented by a two-dimensional array of the surface atom dipoles. The work function increment caused by the coverage of the pseudomorphic Ga on Mo is explained as the results of the different dipole of adatom and molybdenum surface atoms. The field-ion images of the adsorbates in the pseudomorphic layer are found to he significantly brighter and sharper than those of the substrate molybdenum on the {114} areas of molybdenum. The dipole of an adatom, derived from the proposed simple model of the charge distribution, produces an additional field to the applied external field at the position of the field adsorbed helium. The resultant field is high enough to prolong the dwelling lime of the field-adsorbed helium atom and to promote field-ionization of the successively incoming helium atoms, thereby increasing the brightness and sharpness ofthe images.