Heterodyne technique for potential modulation differentiation

Abstract

A potential modulation differentiation technique which uses heterodyne modulation is presented. The second derivative I″(V) of the collected current I(V) is obtained by modulating the voltage V using two synchronized signals with angular frequencies Kω0 and (K − 1)ω0, K≫1, and detecting the component of the collected current with angular frequency ω0. This technique eliminates problems caused by stray capacitively coupled signals and harmonic distortions produced by the signal generator and the preamplifiers. It can be applied to techniques in which the desired signal is the second derivative of the collected current, such as retarding field Auger electron spectroscopy, appearance potential spectroscopy, and work-function measurements. Auger spectra obtained using a multigrid retarding field analyzer and the proposed modulation technique show that only three grids are needed rather than the four grids required in the conventional experimental setup. Furthermore, there is no need for resonant filters or neutralization (bridge) amplifiers, and the high dynamic range commercial lock-in amplifier can be replaced by a circuit based on a single chip.