The effects of vibrational mode, spin–orbit state, and collision energy on collision‐induced dissociation and predissociation of OCS+

The mechanisms of collisional activation of ions in mass spectrometry

This article is a review of the mechanisms responsible for collisional activation of ions in mass spectrometers. Part I gives a general introduction to the processes occurring when a projectile ion and neutral target collide. The theoretical background to the physical phenomena of curve-crossing excitation (for electronic and vibrational excitation), impulsive collisions (for direct translational to vibrational energy transfer), and the formation of long-lived collision intermediates is presented. Part II highlights the experimental and computational investigations that have been made into collisional activation for four experimental conditions: high (>100 eV) and intermediate (1-100 eV) center-of-mass collision energies, slow heating collisions (multiple low-energy collisions) and collisions with surfaces. The emphasis in this section is on the derived post-collision internal energy distributions that have been found to be typical for projectile ions undergoing collisions in these regimes.

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+–rare gas collisions: An experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.

Reaction of formaldehyde cation with molecular hydrogen: effects of collision energy and H2CO+ vibrations

The effects on the title reaction of collision energy (E(col)) and five H(2)CO(+) vibrational modes have been studied over a center-of-mass E(col) range from 0.1 to 2.3 eV. Electronic structure and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important. Only the hydrogen abstraction (HA) product channel is observed, and despite being exoergic, HA has an appearance energy of approximately 0.4 eV, consistent with a transition state found in the electronic structure calculations. A precursor complex-mediated mechanism might possibly be involved at very low E(col), but the dominant mechanism is direct over the entire E(col) range. The magnitude of the HA cross section is strongly, and mode specifically affected by H(2)CO(+) vibrational excitation, however, vibrational energy has no effect on the appearance energy.

Vibrational mode and collision energy effects on reaction of H2CO+ with CO2

The effects of collision energy (Ecol) and five different modes of H2CO+ vibration on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 3.2 eV, including measurements of product ion recoil velocity distributions. Electronic structure and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important along the reaction coordinate. Two product channels are observed, corresponding to Hydrogen Transfer (HT) and Proton Transfer (PT). Both channels are endothermic with similar onset energies of approximately 0.9 eV; however, HT dominates over the entire Ecol range and accounts for 70-85% of the total reaction cross section. Both HT and PT occur by direct mechanisms over the entire Ecol range, and have similar dependence on reactant vibrational and collision energy. Despite these similarities, and the fact that the two channels are nearly isoenergetic and differ only in which product moiety carries the charge, their dynamics appear quite different. PT occurs primarily in large impact parameter stripping collisions, where most of the available energy is partitioned to product recoil. HT, in contrast, results in internally hot products with little recoil energy and a more forward-backward symmetric product velocity distribution. Vibration is found to affect the reaction differently in different collision energy regimes. The appearance thresholds are found to depend only on total energy, i.e., all modes of vibration are equivalent to Ecol. With increasing Ecol, vibrational energy becomes increasingly effective, relative to Ecol, at driving reaction. For HT, this transition occurs just above threshold, while for PT it begins at roughly twice the threshold energy.

Imaging the Mode-Selected Predissociation of OCS+[(v1v2v3)B̃2Σ+]

A new experimental approach to explore the mode-selected chemistry is proposed and demonstrated here. In this approach a double-resonance excitation scheme is exploited to prepare a well-defined mode or state of a parent ion. A time-sliced velocity imaging technique interrogates the fragment ion from the subsequent predissociation. Application to the title process reveals remarkable mode-specific behaviors despite the long dissociation time associated with the indirect bond-breaking process. A qualitative interpretation of the major findings is surmised.

Is collision-induced dissociation of low-energy carbonyl sulfide cations adversely affected by asymmetry?

We have measured relative abundances of fragment ions resulting from collision-induced dissociation of OCS(+) ions in collision with xenon neutrals as a function of ion kinetic energy and scattering angle. The lowest energy dissociation product, S(+), dominates at all energies up to 53 eV kinetic energy studied here. Surprisingly, the second most abundant dissociation channel is CS(+) and not CO(+) even though the thermochemical threshold for CO(+) is lower than that for CS(+) and CO(+) is more abundant than CS(+) in the normal mass spectrum of OCS. We do not observe any significant abundance of CO(+) in this energy range, suggesting that collision-induced excitation and dissociation of OCS(+) is significantly different to that of symmetric triatomic ions. A possible role of asymmetry in the molecular ion's collisional activation via neutral collision is suggested for the different behavior.