Using Contact Resistance to Measure Adsorption of Gases on Metals

Measurement of the contact resistance between metal electrodes can be used to study surface films of nonconducting substances such as adsorbed gases. In comparison with other techniques it has the following advantages: 1. It can be used in the presence of the gas. 2. It can be used over a wide temperature range. 3. It does not depend primarily on the change in work function, which might be small for some adsorbates. * T h r o u g h o u t this p a p e r t h e unit of pressure will be t h e " t o r r " , a unit widely used in E u r o p e to designate one millimeter of m e r c u r y of pressure, i.e., 1.332 X 3 10 d y n e s / c m 2 . (See, for example, W. H. W e s t p h a l , Physihalisches Worterbuch, Springer Verlag, Berlin, 1952). T h i s avoids the awkward " m i l l i m e t e r s of m e r c u r v of gas X." 925 92G T H E BELL SYSTEM TECHNICAL J O U R N A L , JULY 1 9 5 8 4. Under m a n y conditions it does not destroy the surface film. 5. Adsorption can be studied on the bulk metal, which can be degassed by vigorous heating; i.e., dispersed conditions such as powders or porous films are not required. In general, the contact resistance arises from two effects, the constriction of the lines of current flow into a relatively small region of contact and the resistance offered by a surface film. T h e pioneering paper in this field is due to Holm and Meissner, 1,2 who resorted to low temperatures to minimize the constriction resistance and showed t h a t , for reasonably clean metals, there remained a resistance due to the surface film of the order of 5 X 1(T9 ohm -cm 2 .