Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS XNTERFACE 185Langmuir and Kind~n~~many years ago and has since been used intermittentlyby other workers66-68. However, Farnsworth et ~ 1 . ~ made ~ 1 the ~ technique~ 1 ~ ~completely acceptable by showing that surfaces cleaned in this way gave a lowenergy diffraction pattern which was characteristic of a clean surface. Furtherconfirmation was obtained by Hagstrum and D’AmicoZ2 who showed that tungstensurfaces produced by ion bombardment did not differ from those produced by hightemperature heating.The technique requires a combination of high temperature outgassing, ionbombardment and subsequent annealing by further heating. It is necessary torepeat the ion bombardment several times to remove contaminant which hasdiffused from the bulk during the heating. Annealing is necessary to remove defectsproduced by ion bombardment and inert gas owluded near the surface. The totalheating time may be several hundred hours at 500-1000” C in order to obtain asurface which remains substantially clean for a number of hours.A thermionically maintained discharge, with an argon pressure of torr,is used to bombard the crystal face. Discharge voltages in the range 200-600 voltsare used, giving a positive ion current density of 10-100 microamp. cm-’ on thecrystal face. An ion bombardment of a few minutes is normally sufficient to removethe surface layers but bombardment times and annealing temperatures must beadjusted to the material being studied. Low current densities are used to preventpitting of the crystal face. The apparatus must be designed to shield the crystalfrom material which might evaporate from the hot filament, or from atomssputtered from other parts of the apparat~s’~*~~*~~. This method is useful inobtaining clean surfaces on both single crystals and polycrystalline materials. Theseinclude metals, semiconductors and insulators7’. Only a relatively small area canbe produced and the electrodes necessary for the ion bombardment technique mayinterfere with the experiments to be carried out on the clean surface23i24.A circuit diagram suitabfe for combined electron bombardment heating andion bombardment is shown in Fig. 1. Electron and ion energies up to lo00 electronvolts can be obtained at current densities of 5 microamp. cm-’ to several milliamp.cm-’.cf> Field desorptionThis is a specialised technique which has been applied in field emission andfield ion microscopy (see Section 2.1 Sc). It is achieved by giving the tip a positivepotential. Tungsten can then be removed at liquid helium temperatures with anapplied field of 5.7 x lo3 V.cm-’. Perfectly regular surface structures are exposedcontaining many different lattice planes. Clean surfaces have been produced ontungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentiallyapplicable to a wide range of materials, but the area of clean surface exposed isonly about lo-’’ cm2.References pp. 270-278