Direct Comparison of the Kelvin and the Electron-Beam Methods of Contact-Potential-Difference Measurement

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Both the Kelvin and the retarding potential, or electron beam, techniques have been used to measure the contact potential differences between (110) and (100) oriented tungsten single-crystal surfaces clean and covered with vapor deposited barium films. The evaporation of barium in an ultra-high vacuum is difficult and it was necessary to double evaporate the metal before the final evaporation could be made without a pressure increase inside the experimental tube above, at best, 3×10−10 Torr. Contrary to the result obtained from a previous similar experiment, the contact-potential-differences measured by the two methods differed by 130 mV for the (100) crystal face and by 150 mV for the (110) crystal face. Differences of this magnitude can be explained either in terms of a two-patch model, provided there is a work function difference of up to 1.0 eV between the patches, or by a change in the reflection coefficient for electrons. To account for the difference in terms of changes in the reflection coefficient only, a change of from zero for the clean crystal to about 0.6 for the barium-covered surface would be required.