Negative ion images generated by tetracyanoethylene (TCNE) under inverted field conditions in a field ion microscope have been investigated in more detail. The experiments confirm previous findings / I / and reveal further unique ionization phenomena such as the discontinuous development of ring structures in the ion image, the oscillation of rings in size and switching between different ion emitting surface structures. The observation of these phenomena critically depends on maintaining a tip

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... emperature lower than the ambient gas temperature. The phenomena are attributed to the formation of an electrically conducting polymer layer of TCNE and to an elastic response of the layer surface to the field stress. Thus field enhancing and ion emitting protrusions are formed.