Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

216 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSslices of germanium and silicon. The surface conductance is measured by a potentialprobe. The field effect mobility is measured at the same time; this is definedas the change in surface conductivity produced by a field applied normal to thesurface on the sample. The applied field induces a charge on the semiconductorsurface. If the charge is positive the conductivity of a p-type surface increases,whereas that of an n-type surface decreases. By comparison with Hall-effectmeasurement^'^^ on the bulk material, it is possible to decide if the surface andbulk show the same type of semiconductivity.2.1.8 Electron diffractionThe technique of Low Energy Electron Diffraction (LED) is well suited to theobservation of surface monolayers and the structure of clean surfaces. Farnsworthet ~ 1.~'~ 31960p69*70 have done a great deal of work in this field, obtaining measurementson single crystals cleaned in ultra high vacuum by ion bombardment. Onedisadvantage of the method used is that the diffracted electrons are collected by aFaraday cage which is rotated about the axis perpendicular to the incident beam.Even with automatic recording this is rather laborious although it gives a relativelyaccurate picture of the intensity distributions in the diffraction pattern.More recently Germer and co-worker~~~~-~~~ha ve developed a different typeof apparatus which allows the entire diffraction pattern to be displayed on afluorescent screen. Both variations of the LEED technique are rather difficult toapply to the measurement of adsorption kinetics, which in effect requires a measurementof the change in the intensity of the diffracted beam(s) with time. Thisintensity change is not necessarily proportional to the rate of adsorption; however,detailed data on the adsorption of oxygen and carbon monoxide on nickel7'have been obtained by a combination of LEED and work function measurements.2.1.9 Spectroscopic measurements'60- 16'Optical spectroscopy has been applied with a good deal of success'to the identificationof chemisorbed species and of the nature of the surface bond. Infrared spectrahave been most useful in studies of simple molecules, such as carbon monoxideadsorbed on platinum or nickel, and ultraviolet spectra for the characterisation ofmore complex interqediates, such as carbonium ions and ion radicals. The frequencyof the adsorption band (or bands) often serves to identify the adsorbedspecies by comparison with spectra of known compounds. Quantitative informationmay then in principle be obtained by measuring the area under the adsorp-