Multicoincidence mass spectrometry applied to hexamethyldisilane excited around the silicon 2p edge

Abstract

Photoelectron-photoion-photoion coincidence (PEPIPICO) mass spectrometry is applied to Si 2p core ionization.The ion yield spectrum is compared to the spectrum of the tetramethylsilane molecule in order to point out resonances due to the Si-Si chemical bond. Simple coincidence mass spectra are dominated by the SiC3HgC fragment ion and do not show a strong dependence on photon wavelength. PEPIPICO spectra demonstrate that dissociation dynamics is dominated by stepwise fragmentation of SiC3H9+ and that double ionization always involves S i S i chemical bond rupture, shown to be faster than the Si-C rupture. We discuss the results in term of a fast decay of Si-Si into singly and doubly charged molecules followed by a cascade of slow fragmentation and isomerization of SiC3Hg+. IntroductionIn the past few years, considerable interest has been developed in the study of the dissociation processes of core excited moleculesId because of possible site-selective fragmentation pathways. Our recent work on tetrahedral silicon compound molecules such as SiH4,' Si(CH3)4,8 and SiF49 photoexcited near the Si 2p ionization edge shows that the nature of and the intensity ratio between single-and double-ionization decay channels vary strongly with the photon energy in the region of resonances, especially when the comparison is made below and above the core ionization limit, leading thus to different dissociation channels. For a discrete core-excited state, the excited electron in the valence electron cloud, which acts as a spectator or a nonspectator during the electronic decay channels, controls the nature of the final electronic states of the ion mostly with a single positive charge ion and its subsequent fragmentation. In contrast, in the core ionization continuum, normal Auger (including cascade Auger) processes explain the enhancement of double-(or triple-) ionization channels at the expense of single-ionization ones, giving rise to the observation of lighter fragments.Mass spectrometry of polymethylsilanes and siloxanes has been the subject of many studies because of the very high stability of the trimethylsilyl, Si(CH3)3+,In the present work, we report new mass spectrometry measurements with the multicoincidence technique known as PEPIPICO mass spectrometry or charge separation mass spectrometry)I3 (CSMS) applied to hexamethyldisilane, Si2(CH3)6 (HMDS), photoexcited near the Si 2p edge (i.e., from 100-to 130-eV photon energy). The interest of this molecule compared to the previously studied monosilane molecules is the presence of S i S i and Si-C bonds with different strengths. The different bonding pattern of the silicon atoms in HMDS is shown to affect the resonance pattern near the Si 2p edge compared to those of tetramethylsilane8 (Me&), for which the silicon atom is bound only to carbon atoms. The main purpose of the present work is to study the dissociation dynamics of such a core-excited molecule after single, double, and triple ionization, though this technique also allows analysis of metastable states. The problem of...

Nuclear motion in carbonyl sulfide induced by resonant core electron excitation

“…The time-of-flight correlation plots from the triple coincidences follow the same pattern since the slope ⌬T S + / ⌬T O + Ϸ −1.2 because the time deviation is proportional to the momentum deviation. 27 We also studied Dalitz plots for the dicationic three-body breakups and found that both C / O + / S + and C + / O + / S display uncorrelated patterns similar to those in Figs. 3͑c͒ and 3͑d͒, thus confirming the concerted dissociation.…”

Employing Soft X-Rays in Experimental Astrochemistry

“…The Coulomb explosion associated with the Auger process should explain the increase in kinetic energy of the ionic fragments, reflected in the increasing broadening of several fragments. This broadening observed in simple coincidence spectra (PEPICO) and its consequence on the shape of peaks in mass spectra was discussed by Simon et al (1993).…”

“…Following Eland (1989) and Simon et al (1993) the dissociation dynamics of double charge species can be studied by analyzing the coincidence figure (see inset Fig. 14a) of the ions in double coincidence mode in PE2PICO spectra.…”

“…Following Simon et al (1993), the slope of coincidence figure in PE2PICO spectra in this case is given by: for or for…”

Coincidence spectroscopy: Past, present and perspectives

“…The major difference between the two methods is that, in PEPICO the information is extracted from the correlation of the cation with the electron. By contrast, in a PEPIPICO experiment, the information on the various fragmentation pathways is inferred from the correlation maps of the fragment ions, defined by plotting the arrival time of the second fragment at the detector versus the flight time of the first ion [45] [53]. Depending on the progress of the fission, the pattern of the coincidence map will be different, as described in a very comprehensive manner in ref.…”