FIM phenomena observed with benzene as image gas

Properties of benzene as the image gas in field ion microscopy (FIM) and its ionization behavior have been investigated. In FIM of PtIr tips prepared by field evaporation large bright spots and dynamic phenomena with the appearance of ring structures are observed. These phenomena, which are not observed with cyclohexane as the image gas, are attributed to field ionization of molecules at the end of charged polymer chains grown by field induced cationic polymerization of benzene on the anode surface. Thermochemistry reveals that the cationic polymerization of benzene is only exothermic in a high electric field and depends on the formation of carbocations on the tip surface.