Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 203regions of low work function, and dark areas, regions of high work function. Thisis the principIe of the field emission microscope which was invented by Miiller123and has formed the subject of many publications by G~mer'~~~'~~and others.Work in this area was recently reviewed by Ehrliche8.The electron omission from the tip is described approximately by the Fowler-Nordheim equation'26, which can be expressed in the formi/v2 = a exp (-B~#'v)where i is the total current, Y is the applied voltage and a and B are constants.A plot of In (i/V2) against 1/V gives a slope &I*. B is not normally known, butif the work function of the clean surface is known, the change in work function onadsorption can be calculated from the change in slope on adsorption. The Fowler-Nordheim equation does not accurately describe the emission process. Workfunctions obtained by this method are in general slightly smaller than the truevalues. A more accurate evaluation is discussed by Ehrlioh**.Fig. 12. Field emission microscope for adsorption studies. A, gas bottle; €3, break-off seal; C,inverted ionisation gauge (also serves as a selective getter); D, GranvillePhilllps valve; E,ionisation gauge; F, grounding rings; G, double Dewar; H, emitter assembly (tip mounted onhairpin support wire, equipped with potential leads for measuring resistance); I, anode terminal;J, Willemite screen settled onto tin oxide conductive coating; K, ground glass port; L, trap.From ref. 88.References pp. 270-278