Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 20 1where A is a universal constant (120 amp. cm-2. OKd2),.5 is the average reflectioncoefficient for electrons at the surface, T is the absolute temperature, and k is theBoltzmann c0nstant.j can be measured as a function of temperature and a plot ofIn (j/T2) against 1/T should give a straight line of slope -e#/k. The method islimited, because the high temperatures needed to obtain suitable emission currentscause the complete desorption of most adsorbed layers. One exception is that ofadsorbed layers of alkali metals on tungsten’ ”, where the low work function makesemission possible at quite low temperatures (- 150” C). A suitable electrodeassembly has been described by Becker” O.(b) Photoelectric emissionWhen a surface is irradiated by light of variable frequency v, electrons will beemitted if the incident photons impart sufficient energy to enable them to overcomethe potential barrier at the surface. At the threshold frequency v,,hv, = e#However, at normal temperatures the threshold is blurred due to the emission ofthermally excited electrons with energies above the Fermi level. To surmountthis difficulty, it is necessary to use the theory developed by Fowler’21. Thistheory shows that the quantum yield, Z (number of electrons liberated per incidentphoton), is given bywhere A is a constant, T is the absolute temperature and the expressions for thefunction F[(hv - hv,)/kT] are as given by Fowler. A series of curves of the functionagainst hvJkT can be plotted and the experimental plot of ZITz against hvlkTfitted to them. The horizontal shift required to superimpose the calculated andexperimental curves is hv,,/kT and the work function can be obtained. If the curvesdo not fit well, some features ofthe experiment are usually suspect.There are two practical difficulties. One is the measurement of the very smallphotocurrent, which may be as low as amp, requiring the use of a vibratingreed electrometer or similar instrument. The second is that, for work functionsabove 5 eV, v, lies in the far ultraviolet. This makes the study of some adsorptionsvery difficult and 6 eV is about the practical limit of such measurements. A suitablelight source is the quartz mercury arc. The energy of the incident beam can bemeasured with a calibrated photocell, a vacuum thermopile or a radiometer. Asuitable cell for adsorption studieslzZ is shown in Fig. 11. The sample beingstudied forms the cathode B. It can be a metal foil or a film formed by evaporationfrom the filament E. A wire C is fused through the glass to make contact with theReferences pp. 270-278