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...

The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections of high statistical quality are obtained. A subset of the overall data, namely, equal electron-energy sharing, is used to investigate the effects of molecular orientation on the photoelectron angular distribution. Departures from the first-order helium-like model are evident in detection geometries where electron-electron correlation is "frozen."

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The double photoionization of CO(2) molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO(2)(2+) molecular dication, (ii) the production of a metastable (CO(2)(2+))* that dissociates, with an apparent lifetime of 3.1 micros, giving rise to CO(+) and O(+) ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 micros. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO(2), for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO(+) together with a neutral autoionizing oxygen atom.

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

The threshold photoelectron–photoion coincidence method was used to study O+2 dissociation. Previous results were confirmed and a careful analysis of the time of flight peak shapes using a Monte Carlo simulation gave us new results. The lifetime of the B 2Σ−g state was measured to be 70∓25 ns independently of the vibrational quantum number. The III 2∏u state which dissociates towards the O+(2D0)+O(3P) and O+(2P0)+O(3P) limits shows anisotropic distribution of fragment ions. The c 4Σ−u(v=0) state which was previously observed to dissociate into O+(4S0)+O(1D), its adiabatic limit, is seen to predissociate also about 40% towards the ground state limit.

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