The slope o f the peak corresponding to the ion pair has not been determ ined as the area o f the peak observed in the pairs spectrum is small and therefore the slope o f this section w ould be biased and not representative of the entire peak. In addition, unfortunately it is not possible to determ ine the KER upon form ation of this ion pair or to determ ine an accurate value of Fk e r d, due to
the m ajority o f this peak being obscured by the dead time. T herefore, no inform ation concerning the fragm entation m echanism for this m inor dissociation reaction can be obtained from the pairs spectrum. H ow ever, as discussed above, for three-body dissociation reactions, a true Coulom b explosion is rare^^’^^ and thus perhaps the dissociation m echanism involved is an initial two-body charge separation to + CIO^ follow ed by further dissociation to form the observed ion pair [Reaction (6 .IX)].
OCIO^^ - ^ 0 ^ + CIO^
i (6 .VIX)
4- + Cl
In order to derive some inform ation concerning the dissociation o f OCIO^^ to form + O^, it is necessary to decrease the dead time of the CFD. As a result, an increased area of the + O* peak would be observed in the pairs spectrum , enabling a reasonable m easurem ent of the peak slope and estim ation of reliable KER and Ekerd values from the peak w idth to be made.
Ratio o f Double-to-Single Ionization Cross Sections
The value of the ratio of has been determ ined fo r OCIO at each electron energy and are listed in Table 6.10 and plotted in Fig. 6.10. As can be seen in the figure, there is an initial increase in as the electron energy is raised and there is an increased likelihood of double ionization, reaching a m axim um at 100 eV and then decreasing as the electron energy is raised further. Betw een 250 and 320 eV, the ratio of increases once more, perhaps due to the onset o f triple ionization at high electron energies, resulting in an increase o f Pairs of singly charged ions from dissociative m ultiple ionization form ed at these high electron energies may contribute to the ion pair intensities in the pairs spectra thus increasing the so-called ratio of
At 100 eV, the ratio o f for OCIO is 15 %. This value is relatively high, as multiple ionization is conventionally a m inor reaction channel in com parison w ith single ionization.^5 Thus for OCIO, m ultiple ionization is a significant proportion of the total ionization of OCIO and the contribution o f double ionization to ion signal intensities in m ass spectra m ust be considered.
6.4.3 Conclusion
The second part of this chapter discusses the 2D investigation o f the single and double ionization of OCIO. Tim e-of-flight mass spectrom etry is used to investigate the single ionization of OCIO. Stable parent ions OCIO^ and fragm ent ions O^, 0%^, CC and CIO^ and their relevant isotopes were observed in the mass spectra, the parent ion being the m ost abundant. From these m ass spectra, the values of a / ' for OCIO for incident electron energies o f 70, 100, 250 and 320 eV, where the contribution of any fragm ent ions from dissociative double ionization is assessed and subtracted, have been determined.
2D coincidence experim ents were perform ed to investigate the form ation and fragm entation of OCIO^^. Peaks due to the form ation of + CIO^, 4- CC and the previously unobserved identical ion pair were detected in the 2D coincidence spectra. Contrary to the one dim ensional investigation o f OCIO^^, the three-body dication dissociation reaction form ing + CC was the m ost frequently occurring reaction channel. The increased detection efficiency of the energetic fragm ents not form ed along the axis of the TO FM S such as those from sequential dication dissociation reactions experienced using the 2D apparatus set-up accounts for the increased abundance o f the + CC ion pair.
From the slopes of the peaks corresponding to 4- CC, it is not possible to discern whether the dication fragm entation pathw ay form ing this ion pair is a direct m echanism or a sequential m echanism involving an initial charge separation and subsequent m onocation dissociation. H owever, it is apparent that the dissociation is either a true C oulom b explosion or a fast sequential mechanism. There is some evidence in the 2D coincidence spectrum o f a slow er sequential dissociation of OCIO^^ to form + CC proceeding via an initial tw o-body charge-separating reaction, form ing and a longer-lived CIO^ interm ediate w hich then dissociates to form Cl"^. Therefore, the dissociation of OCIO^^ to form O'*" + CC probably occurs via a fast sequential m echanism involving an initial tw o-body charge separation w ith a fraction of the ion pairs being form ed by a slow er sequential dication dissociation.
The first estim ate of the total double-to-single ionization cross section ratio shows that m ultiple ionization is a significant proportion (15%) of the total ionization o f OCIO. Therefore, w hen extracting any data from mass spectra, it is essential to consider the possible contribution of dissociative m ultiple ionization to fragm ent ion signals in the spectra.
6.5 Overall Conclusions
This chapter describes the investigation of the single and double ionization o f chlorine dioxide, OCIO. The single ionization of OCIO has been investigated using tim e-of-flight mass
spectrom etry in order to determ ine values of the relative partial single ionization cross section for the fragm ent ions of OCIO. The double ionization o f OCIO has been investigated using ion-ion coincidence techniques to derive inform ation concerning the m echanism s and energetics of the dication dissociation reactions.
From the single ionization o f OCIO, the parent ion OCIO^ and fragm ent ions O^, 0%^, CC and CIO^ and their isotopes were observed in the tim e-of-flight m ass spectrum . F rom the spectra, values of a / ‘, relative to the parent ion, w ere derived. The tw o different approaches used in the one and tw o-dim ensional investigations yielded values of o / ' for O^, 0 %^ and CIO^ in good agreem ent with one another. H ow ever, the value derived for CC from the one-dim ensional m ass spectrum does in fact contain a contribution from dissociative double ionization to the ion signal in the mass spectrum. Therefore, the low er value of a / ' derived from the 2D study is a m ore reliable indication o f the value of o / ' for the CC fragm ent of OCIO. Thus, although the assum ption that energetic ions from m ultiple ionization are discrim inated against in the one-dim ensional study does generally hold true, any dicationic dissociation occurring along the axis of the TO FM S will result in a contribution of fragm ents from m ultiple ionization to the one-dim ensional tim e-of-flight m ass spectra.
C oincidence techniques were em ployed to investigate the form ation and fragm entation of OCIO^^. In the one and tw o-dim ensional coincidence spectra, the dissociation o f OCIO^^ leads to the form ation o f + CIO^, 4- CC and ion pairs. The three-body dissociation reaction form ing 4- CC is the m ost frequently occurring dication decay channel.
Investigations of the energetics of the dissociation reactions of OCIO^^ indicate that the form ation o f the tw o-body dissociation reaction 4- CIO^ occurs from the ground electronic state of the dication. Thus a first estim ate o f the double ionization energy o f OCIO has been determ ined as 35 ± 2 eV. F or the three-body dissociation reaction form ing O^ 4- CC, the fragm entation pathw ay follow ed is a sequential m echanism involving an initial tw o-body charge separation follow ed by secondary dissociation of the m olecular m onocation to form the detected ion pair. In addition, there are two distinct dication states responsible for the form ation of the 4- Cl"^ ion pair and the low est o f these dication states may be the ground state of the dication. N o dication dissociation m echanism could be assigned for the form ation o f the 4- ion pair, although the m ost likely fragm entation pathw ay is, as for the 4- CC ion pair, a sequential m echanism involving secondary decay.
The first estim ate o f the ratio o f the total double-to-single ionization cross sections indicates that double ionization contributes around 15% to the total ionization cross section of OCIO. Therefore, m ultiple ionization m ust be considered w hen investigating the single ionization of molecules.
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