It is a surprisingly simple instrument consisting of a needle-sharp tip which is placed in a vacuum chamber and pointed towards a fluorescent screen. A small amount of an imaging gas, typically hydrogen or helium, is released into the chamber until the pressure reaches about 5 mTorr. A high electric field around the tip is created by applying a high positive voltage (5-20 kV) to the tip. As the image gas atoms approach the tip they are ionized and accelerated towards the fluorescent screen, where they form an image representative of the surface of the tip.
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