Negative ion formation by field ionization—studied by field ion microscopy and appearance energy measurements

Abstract

The formation of negative ions by field ionization (NFI) is difficult even for molecules of high electron affinity because of a small difference in threshold field strength between ion formation by NFI and the onset of electron emission. For tetracyanoethylene and dichlorodicyanobenzoquinone we have studied the negative ion formation on a field cathode tip by field ion microscopy using the negative ions for imaging. The experiments reveal the growth of a conducting polymer layer under NFI conditions and the decisive role of this layer in the prevention of electron emission. The polymer layer has no significant effect on the field ion appearance energies of the molecular ions because the electron affinity obtained from measurements of the appearance energy of a number of molecules agrees quite well with those reported in the literature. The appearance energies of iodide and iodine cluster ions are also determined and interpreted by ion-forming field reactions.