Negative Ions Observed in Electron Transmission and Electron Attachment Spectroscopy

Abstract

So far, we have studied those temporary negative parent ions formed on resonant electron attachment, which can be observed by mass spectrometry either directly or indirectly through thermodynamically stabel ionic decomposition products. As was pointed out in Chapter 1, this is not possible for a large number of molecules such as nitrogen, ethylene or benzene. Temporary negative anions of these compounds are observable in ETS. We will now show how ETS can be used for ethylene and several fluoroethylenes in order to study the effect of fluorination on the attachment energy. Experimental techniques such as photoelectron and photoion mass spectroscopy provide information about the energy of occupied MOs. Electron transmission, on the other hand, gives information about the energy of the normally unoccupied MOs. Since ethylene possesses a negative electron affinity, the LUMO is directly accessible on electron impact. Various scattering experiments strongly support the picture of electron attachment into the ${b_{{2_g}}}$ (π*) MO [1–3]. Whereas C₂H4 ⁻₄ ions can only be observed in electron transmission, negative ion formation in numerous ethylene derivatives can be observed in EAS also. In the case of difluoroethylene, for example, the dissociated channel

$$ {C_2}{H_2}F_2^ - {(^2}II) \to {F^ - } + {C_2}{H_2}F $$

(3.1)

is now accessible from the energy of TNI, thus allowing the observation of electron attachment in both transmission and attachment spectroscopy.