A tungsten ribbon and a grounded sample form two electrodes of a vibrating capacitor connected to a sensitive radio frequency capacitance bridge and to a source of dc and ac bias. The ac bias frequency is tuned to the fundamental resonance of the ribbon (fquest;2.5 kHz,Qgsim;500) and the work function difference between the ribbon and the sample is determined from the dc bias for which the ribbon capacitance variation at the ac driving frequency vanishes. Work function differences at different positions on a flat sample can be measured to millivolt precision. An example of the change in work function of evaporated Agndash;Au alloys with oxygen exposure is given.
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