Measurement of Photoemitted Electron Energy Distributions by an ac Method

By means of the ac retarding potential method, the energy distribution of photoemitted electrons can be measured much more rapidly and conveniently than is possible by conventional dc methods. In the ac method, a small (about 20 mV) ac voltage (24 cps) is added to a slowly varying (about 1 V/min) retarding potential and the in-phase ac component of the photoemission signal is measured. For the conventional dc techniques, it is necessary to impose very stringent stability conditions on the light source and measuring equipment. Since, in the ac method, the derivative of the I-V characteristic is taken electronically (to give the energy distribution) and is averaged over many cycles, the stability requirements are relaxed by a large amount and data may be obtained much more quickly and conveniently. With the complete system described, resolutions of about 0.1 eV have been attained and small signal conductances (ΔI/ΔV) smaller than 10−11 mhos have been measured.