A new circuit for measuring photoemission energy distribution curves (EDC's) by the ac retarding potential method is described which offers greatly improved noise performance over earlier circuits. General techniques for EDC noise reduction are outlined and the noise performance expected from different electrometer detectors is compared. A method for quantitatively evaluating the noise performance of an EDC measurement system from an experimental EDC is presented and applied to EDC's obtained with this new measurement system. Equivalent noise currents in energy distribution curves obtained with this system are found to be as low as 2 to 3×10−15 A rms. This low noise allows the measurement of EDC's with total dc photocurrents of less than 10−13 A.

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