Effective polarizability of surface atoms

The effective polarizability of surface atoms can be determined with different methods. In Section 2.2.4(a) a method was described on a measurement of the field evaporation rate as a function of field of kink site atoms and adsorbed atoms. The polarizability is derived from the coefficient of F2 term in the rate vs. field curve. From the rate measurements, polarizabilities of kink site W atoms and W adatoms on the W (110) surface are determined to be 4.6 0.6 and 6.8 1.0 A3, respectively. The dipole moment and polarizability of an adatom can also be measured from a field dependence of random walk diffusion under the influence of a chemical potential gradient, usually referred as a directional walk, produced by the applied electric field gradient, as reported by Tsong et a/.150,198,203 This study is a good example of random walk under the influence of a chemical potential gradient and will therefore be discussed in some detail. 

One can obtain values of both pQ and a from the intercept and the slope of a plot of (2A T/0.867/J7) sinh (/(p) V2(p2 0) against Fc, for brevity referred as a r-plot. All the parameters in the equation can be measured field gradient from desorption voltages of adatoms at different locations on the plane, (p2)0 from a random walk diffusion experiment, and p)F from a directional walk experiment. For W adatoms on the W (110) 

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