King, S.-J.; (2009) Studies of the dissociation and energetics of gaseous ions. Doctoral thesis , UCL (University College London).

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Abstract

In this thesis the electron ionization of a number of small gas phase molecules is investigated using pulsed time-of-flight mass spectrometry coupled with a 2-D ion coincidence technique. The simultaneous acquisition of conventional TOF mass spectra and ion coincidence mass spectra, enables fragment ions formed by dissociative single, double, and triple ionization, to be distinguished and quantified. Relative partial ionization cross sections (PICS) and precursor specific relative PICS are derived for the formation of positively charged fragment ions, following electron ionization of C_2F_6, SiCl_4, C_2H_2, CO_2 and H_2O, in the ionizing electron energy range 30-200 eV. Such information is of considerable importance for the accurate modelling and understanding of the chemical processes occurring in highly energised media, such as industrial plasmas and planetary atmospheres. The relative PICS quantify the overall yield for each fragment ion, while the precursor specific relative PICS quantify the contributions to the fragment ion yield from single, double and triple ionization, respectively. Relative PICS for the formation of ion pairs via dicationic dissociation are also presented. Comparisons of the relative PICS data to existing measurements of the PICS for these molecules reveals, in many cases, considerable discrepancies for the formation of the low-mass ionic fragments, for which the present cross section measurements are considerably larger. In general, these differences are rationalised by the inefficient collection of energetic ions in the previous determinations of the PICS, in particular for those ionic fragments formed via dissociative multiple ionization, which are efficiently collected in the present investigations. The precursor specific relative PICS data provides a more detailed chemical description of each individual partial ionization cross section, and can be interpreted to explain the various energy-dependent features of the PICS curves observed in previous studies. For example, for the fragment ions formed by electron ionization of SiCl_4, the low energy maxima of the PICS curves are due to contributions from single ionization involving mainly indirect ionization processes, while the higher energy maxima are due to dissociative double ionization. For the electron ionization of small gas phase molecules, the general trend appears to be that the major contributions to the yield of small fragment ions comprising only a few atoms, are from dissociative double ionization, above 100 eV. The shapes of the peaks recorded in the 2-D ion coincidence spectra are interpreted to provide additional information on the fragmentation dynamics and energetics of the charge-separating dissociations of C_{2}F_{6}\:^{2+}, SiCl_{4}\:^{2+}, C_{2}H_{2}\:^{2+}, CO_{2}\:^{2+} and H_2O^{2+}. Estimates of the dication precursor state energies for forming ion pairs are derived, and are shown to be in good agreement with existing experimental and theoretical data. The measurements presented in this thesis include the first estimates of the electronic state energies of the C_{2}F_{6}\:^{2+} and SiCl_{4}\:^{2+} dications. A breakdown scheme for C_{2}H_{2}\:^{2+} is derived, which in combination with the relative PICS for ion pair formation, yields branching ratios for primary two-body charge separating dissociation. Comparison of this scheme with recent theoretical calculations suggests that below 75 eV, C_{2}H_{2}\:^{2+} dissociates predominantly on the ground triplet potential energy surface ({^3}\sum\,_{g}\,^-). By contrast, the dissociations of the C_{2}F_{6}\:^{2+} dication are postulated to be predominantly impulsive in nature, for which fastsequential decay mechanisms are proposed for forming ion pairs.

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