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The three body fragmentation of methane dication has been studied using the technique of cold target recoil ion momentum spectroscopy. The process is initiated by impact of energetic Ar9+ ions on neutral methane and the data is subsequently collected in coincidence with Ar8+ projectile. By analysing the dissociation channels leading to (H + H+ + CH 2 + ) and (H + H 2 + + CH+) fragments, it is concluded that these fragments are formed in a sequential manner via formation of molecular intermediates CH 3 + and CH 2 + respectively. It is shown that these molecular intermediates carry a few eVs as their internal energies, part of which is released when they emit an H-atom with the open possibility that the final detected fragments may still be internally excited. This was accomplished by analysing the two-steps of the sequential process in their own native frames. For a molecular system having three-dimensional structure, our results prove to be an ideal example to highlight the importance of using native frames for correct interpretation of the obtained results. Our results indicate that the dissociation of methane dication can be a major source of production of H-atoms in addition to H+ fragments with the probability of the two being of similar order. [ABSTRACT FROM AUTHOR]

The three-body breakup of [ C 2 H 2 ] 3 + formed in collision with Xe9+ moving at 0.5 atomic units of velocity is studied by using recoil ion momentum spectroscopy. Three-body breakup channels leading to (H+, C+, CH+) and (H+, H+, C 2 + ) fragments are observed in the experiment and their kinetic energy release is measured. The breakup into (H+, C+, CH+) occurs via concerted and sequential modes, whereas the breakup into (H+, H+, C 2 + ) shows only the concerted mode. By collecting events coming exclusively from the sequential breakup leading to (H+, C+, CH+), we have determined the kinetic energy release for the unimolecular fragmentation of the molecular intermediate, [ C 2 H ] 2 + . By using ab initio calculations, the potential energy surface for the lowest electronic state of [ C 2 H ] 2 + is generated, which shows the existence of a metastable state with two possible dissociation pathways. A discussion on the agreement between our experimental results and these ab initio calculations is presented. [ABSTRACT FROM AUTHOR]

We report on the direct experimental evidence of hydrogen migration in triply charged acetylene. The roaming hydrogen atom in a triply charged molecular ion is counter intuitive. The three body breakup channel C 2 H 2 3 + → H + + C + + C H + is studied using the technique of recoil ion momentum spectroscopy. The triply charged ion was generated in collisions of the neutral parent with a slow highly charged Xe9+ ion. Three different dissociation pathways have been identified and separated, namely, concerted breakup in an acetylene configuration, concerted breakup in a vinylidene configuration, and sequential breakup via a [ C 2 H ] 2 + intermediate, and the branching ratio for all three pathways are determined. [ABSTRACT FROM AUTHOR]

The ion-induced fragmentation of C H 4 2 + into H+ and C H 3 + is studied using a cold target recoil ion momentum spectroscopy in coincidence with the charge state of the post-collision projectile. Using constant velocity Ar9+ and N3+, results from four different datasets are presented, with a selection on the final charge state of the projectile (Ar8+ or Ar7+ and N2+ or N+). Three distinct dissociation pathways (I, II, and III) are observed for each dataset, with the mean kinetic energy release values of around 4.7, 5.8, and 7.9 eV, respectively. The electronic states that are populated correspond to electronic configurations (1 t 2 ) − 2 and (2 a 1 ) − 1 (1 t 2 ) − 1 of the methane dication, C H 4 2 + . The relative branching ratios between the three pathways are discussed as a function of the charge state of the post-collision projectile, and a strong correlation with the specific nature of the ion–molecule interaction is found. The existing ab initio calculations have provided an explanation only for pathway II. In this article, we propose an explanation for pathway III, but pathway I still remains unexplained and requires further theoretical efforts. A discussion of the dependence of dissociation on the mode of excitation is presented. [ABSTRACT FROM AUTHOR]

The technique of recoil ion momentum spectroscopy is employed to determine the complete momentum vectors for three fragment dissociation channels, [D2O](q+2) → (D+ + D+ + Oq+) with q = 1, 2, or 3 formed in collisions of isolated water molecules with 450 keV Xe9+ ions. The kinetic energy released in each of these dissociation channels is measured and angular correlations between the fragment momenta are determined. From the angular correlations of the three fragment ions with the direction of the incoming beam, a strong anisotropy in the emission of recoil fragments is reported. It is inferred that the molecular plane prefers to lie orthogonal to the incoming beam direction with certain orientations being more preferred than others and a clear signature of non-coplanar dissociation is also observed. [ABSTRACT FROM AUTHOR]

We report experimental evidence for molecular deformation due to a vibrationally active transition state of multiply charged acetylene molecules under the impact of low energy Ar8+ projectiles. “Butterflylike” structures are observed in the experimental coincidence spectra between hydrogen and carbon ionic fragments. Such structures can be generated by numerical simulations and are found to originate from the bending motion of the dissociating molecule. Angular distributions for dissociation products from triply charged C2H2 ion are reported. [ABSTRACT FROM AUTHOR]

Events from the sequential and concerted modes of the fragmentation of OCS3+ that result in coincident detection of fragments C+, O+, and S+ have been separated using a newly proposed representation. An ion beam of 1.8 MeV Xe9+ is used to make the triply charged molecular ion, with the fragments being detected by a recoil ion momentum spectrometer. By separating events belonging exclusively to the sequential mode of breakup, the electronic states of the intermediate molecular ion (CO2+ or CS2+) involved are determined, and from the kinetic energy release spectra, it is shown that the low lying excited states of the parent OCS3+ are responsible for this mechanism. An estimate of branching ratios of events coming from sequential versus concerted mode is presented. [ABSTRACT FROM AUTHOR]

We report the formation of energetic neutral Rydberg hydrogen atoms and transient Rydberg molecular ions, [(H2O)q+][open_star] in ion-impact dissociation of isolated water molecules. The kinetic energy spectra of the neutral Rydberg H atoms are determined from the complete study of (H*, H+, O+) dissociation channel. This channel of water dissociation is suggested as a possible additional source of the energetic neutrals detected in upper atmospheres of extra solar planets, and of slow electrons which are known to play a major role in radiation induced damage to living cells. [ABSTRACT FROM AUTHOR]

Events from the sequential and concerted modes of the fragmentation of OCS 3+ that result in coincident detection of fragments C + , O + , and S + have been separated using a newly proposed representation. An ion beam of 1.8 MeV Xe 9+ is used to make the triply charged molecular ion, with the fragments being detected by a recoil ion momentum spectrometer. By separating events belonging exclusively to the sequential mode of breakup, the electronic states of the intermediate molecular ion (CO 2+ or CS 2+ ) involved are determined, and from the kinetic energy release spectra, it is shown that the low lying excited states of the parent OCS 3+ are responsible for this mechanism. An estimate of branching ratios of events coming from sequential versus concerted mode is presented.