Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 21 1For a free electron g = 2, but various interactions cause some deviations.A commonly used EPR spectrometer is shown diagrammatically in Fig. 17. Itconsists of a four arm microwave bridge, which acts in a manner analogous to aWheatstone bridge. On arm (1) is the Klystron power supply which acts as asource of microwaves. The microwave cavity containing the sample is connectedto arm 4. Arm 3 contains a dummy load which absorbs all the microwave powerincident on it. Arm 2 is connected to a silicon-tungsten detector crystal. Themicrowave bridge has the property that when arms 3 and 4 are matched to thebridge, i.e. when all power incident on the cavity and the dummy load is absorbed,no power is detected in arm 2. The microwave cavity can only be matched atcertain frequencies and the sample is placed in that part of the cavity where themicrowave magnetic field is maximal and the microwave electric field is zero.As in NMR, the microwave magnetic field is at right angles to the steady fieldproduced by the electromagnet. Coupling of the microwave cavity to the bridgeis accomplished by a small hole or iris, which is adjusted in size to give a zeroreflection coefficient, i.e. a 'matched' cavity. Alteration of the slide screw tunerwill then slightly unbalance the bridge, the EPR absorption is observed and can bedisplayed on a cathode ray oscilloscope or a graphic recorder.2.1.7 Measurement of changes in electrical conducti~ity'~~*'~~(a) MetalsWhen a gas is adsorbed on a metal, electrons may be abstracted from theconduction band or added to it. In either case, the conductivity will change.However, since conductivity is a bulk property, the change will only be measurableif the surface-to-volume ratio is high. More precise interpretation can be givento measurements on single crystals, but in practice this is difficult and thin evaporatedfilms are more often used. The metal film can be evaporated by any of theusual methods (see Section 2.1.1) onto the walls of a vessel containing two contactsand the film resistance between the contacts measured122. It is often necessaryto heat the film above the adsorption temperature to anneal out defects, otherwisethe resistance may change slowly with time. Unless the films are prepared takingextreme precautions to avoid contamination,. variable results may be obtained.This point has been well demonstrated by Sa~htler'~~, who found that the adsorptionof hydrogen on nickel caused the conductivity to change in opposite directionsfor 'clean' and 'moderately clean' films. It must be assumed that the film is uniformand that grain boundary resistance is negligible. Since metal films can be veryporous, this may not be true. However, the recent advances in technique whichhave enabled the formation of single crystal metal filmss2 should remove some ofthese objections. The second assumption is that the only effect of chemisorptionis to change the number of the conduction electrons and it ignores the effect on theReferences pp. 270-278