Etude par Spectrométrie de Masse de la formation d'Ions Négatifs par Attachement Electronique Dissociatif et par le processus de Paire d'Ions.

The aim of this work is to investigate the appearance of negative ions by both the mechanism of Dissociative Electron Attachment (DEA) and Ion Pair processes (IPP) by electron impact.

Using the appropriate instrumentation (a conventional Nier type mass spectrometer) the best conditions are chosen to ensure ion-molecule collision free ion production (sample pressure, impinging electron beam current), the linearity of the electron energy scale between 2 eV and 100 eV and to minimize mass (and translational kinetic energy) discrimination. The used ionizing electron beam allowed us a resolution of 0.5 eV.

An ionization model for both processes is proposed to provide us a method for the measurement of the threshold energy of DEA and IPP. Were necessary the numerical derivation of the ion current (for IPP) and the deconvolution of the ionization efficiency curves (for both DEA and IPP) are applied to increase the accuracy of the threshold energy determination and the resolution of the successive ion producing processes to be observed.

DEA and IPP’s are observed and threshold energies are measured and assigned in:
(i) diatomic molecules, i.e. H2, CO, NO and O2,
(ii) triatomic molecules, i.e. CO2 and CS2 along the reaction paths providing O- and S- and CS-. No CO- ion has been detected.
(iii) polyatomic molecules, i.e. CH4, C2H2, C2H4, C2H3F and C2H3Cl. Beside the monoatomic negative ions (H-, C-), diatomic and polyatomic negative ions are detected.

For several of these species the electron affinity (EA) of the radical is not known, e.g. CH, C2 and C2H. From the appearance of the corresponding negative ions and threshold measurements this fundamental quantity has been deduced. In the particular case of C2-, the production of the negative ion in an electronically excited state has been put forward for the first time. Dissociation energies of the same diatomic and triatomic negative ions have been deduced and compared with this same magnitude in neutral and positive ionic species.