Apparatus with nanosecond field transition times for field reversal studies of surface processes at high temperatures

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Abstract
A new type of field‐reversal apparatus is described. It is intended for desorption and surface reaction studies in systems with surface ionization. The problems of mechanically modulating the molecular beam are circumvented by instead reversing the electric field outside the surface and thus perturbing the steady‐state desorption conditions. The short field reversal time of ⩽ 10 ns with a 90 V voltage step is reached by using VMOS transistors with optoisolated driving circuits. The ions from the ionizing surface are converted to electrons on a Cu–Be dynode. The electrons give photons in a scintillator, and the photons are detected by a photomultiplier outside the vacuum wall. The apparatus dependent signal rise time is around 40 ns, which means that surface processes with very short characteristic time constants can be measurable.