Bimolecular reactions of CH2CN2+ with Ar, N2 and CO: reactivity and dynamics.

The reactivity, energetics and dynamics of bimolecular reactions between CH2CN2+ and three neutral species (Ar, N2 and CO) have been studied using a position sensitive coincidence methodology at centre-of-mass collision energies of 4.3-5.0 eV. This is the first study of bimolecular reactions involving CH2CN2+, a species relevant to the ionospheres of planets and satellites, including Titan. All of the collision systems investigated display two collision-induced dissociation (CID) channels, resulting in the formation of C+ + CH2N+ and H+ + HC2N+. Evidence for channels involving further dissociation of the CID product HC2N+, forming H + CCN+, were detected in the N2 and CO systems. Proton-transfer from the dication to the neutral species occurs in all three of the systems via a direct mechanism. Additionally, there are product channels resulting from single electron transfer following collisions of CH2CN2+ with both N2 and CO, but interestingly no electron transfer following collisions with Ar. Electronic structure calculations of the lowest energy electronic states of CH2CN2+ reveal six local geometric minima: both doublet and quartet spin states for cyclic, linear (CH2CN), and linear isocyanide (CH2NC) molecular geometries. The lowest energy electronic state was determined to be the doublet state of the cyclic dication. The ready generation of C+ ions by collision-induced dissociation suggests that the cyclic or linear isocyanide dication geometries are present in the [CH2CN]2+ beam.

Bimolecular reactions of S2+ with Ar, H2 and N2: reactivity and dynamics.

The reactivity, energetics and dynamics of bimolecular reactions between S2+ and three neutral species (Ar, H2 and N2) have been studied using a position-sensitive coincidence methodology at centre-of-mass collision energies below 6 eV. This is the first study of bimolecular reactions involving S2+, a species detected in planetary ionospheres, the interstellar medium, and in anthropogenic manufacturing processes. The reactant dication beam employed consists predominantly of S2+ in the ground 3P state, but some excited states are also present. Most of the observed reactions involve the ground state of S2+, but the dissociative electron transfer reactions appear to exclusively involve excited states of this atomic dication. We observe exclusively single electron-transfer between S2+ and Ar, a process which exhibits strong forward scatting typical of the Landau-Zener style dynamics observed for other dicationic electron transfer reactions. Following collisions between S2+ + H2, non-dissociative and dissociative single electron-transfer reactions were detected. The dynamics here show evidence for the formation of a long-lived collision complex, [SH2]2+, in the dissociative single electron-transfer channel. The formation of SH+ was not observed. In contrast, the collisions of S2+ + N2 result in the formation of SN+ + N+ in addition to the products of single electron-transfer reactions.

Bond-forming and electron-transfer reactivity between Ar2+ and N2.

Collisions between Ar2+ and N2 have been studied using a coincidence technique at a centre-of-mass (CM) collision energy of 5.1 eV. Four reaction channels generating pairs of monocations are observed: Ar+ + N2+, Ar+ + N+, ArN+ + N+ and N+ + N+. The formation of Ar+ + N2+ is the most intense channel, displaying forward scattering but with a marked tail to higher scattering angles. This scattering, and other dynamics data, is indicative of direct electron transfer competing with a 'sticky' collision between the Ar2+ and N2 reactants. Here Ar+ is generated in its ground (2P) state and N2+ is primarily in the low vibrational levels of the C2Σu+ state. A minor channel involving the initial population of higher energy N2+ states, lying above the dissociation asymptote to N+ + N, which fluoresce to stable states of N2+ is also identified. The formation of Ar+ + N+ by dissociative single electron transfer again reveals the involvement of two different pathways for the initial electron transfer (direct or complexation). This reaction pathway predominantly involves excited states of Ar2+ (1D and 1S) populating N2+* in its dissociative C2Σu+, 22Πg and D2Πg states. Formation of ArN+ + N+ proceeds via a direct mechanism. The ArN+ is formed, with significant vibrational excitation, in its ground (X3Σ-) state. Formation of N+ + N+ is also observed as a consequence of double electron transfer forming N22+. The exoergicity of the subsequent N22+ dissociation reveals the population of the A1Πu and D3Πg dication states.

Ionisation of PF3: absolute partial electron ionisation cross sections and the formation and reactivity of dication states.

Absolute partial electron ionisation cross sections, and precursor-specific partial electron ionisation cross sections, for the formation of cations from phosphorus trifluoride (PF3) are reported over the electron energy range 50-200 eV. The absolute values are determined by the measurement of cross sections relative to the formation of PF3+ using 2D ion-ion coincidence time-of-flight mass spectrometry and subsequent scaling using binary encounter-Bethe calculations of the total ionisation cross section. This new dataset significantly augments the partial ionisation cross sections for electron ionization of PF3 found in literature, addressing previous discrepancies in the branching ratios of product ions, and provides the first values for the precursor-specific cross sections. Comparisons to calculated cross sections from the literature are encouraging, although there are discrepancies for individual ions. The coincidence experiments indicate that double and triple ionisation generate approximately 20% of the cationic ionisation products at 200 eV electron energy. One dissociative dication state, dissociating to PF2+ + F+, is clearly identified as the lowest triplet state of PF32+ and five different dications (PF32+, PF22+, PF2+, P2+ and F2+) are detected in the mass spectra. The dication energetics revealed by the experiments are supported by a computational investigation of the dication's electronic structure. The cross sections reported will allow more accurate modelling of the role of the ionization of PF3 in energetic environments. A first investigation of the bimolecular reactivity of metastable states of PF32+ is also reported. In collisions with Ar, O2 and CO dissociative single electron transfer dominates the product ion yield, whereas collision-induced dissociation of the dication is important following collisions with Ne. Consideration of the energetics of these processes indicates that the reactant dication beam contains ions in both the ground singlet state and the first excited triplet state. The deduction regarding the longevity of the triplet state is supported by metastable signals in the coincidence spectra. Weak signals corresponding to the formation of ArF+ are detected following PF32+ collisions with Ar, and experimental and computational considerations indicate this new chemical bond is formed via a collision complex.

Bond-forming and electron-transfer reactivity between Ar2+ and O2.

The reactivity, energetics and dynamics of the bimolecular reactions between Ar2+ and O2 have been studied using a position sensitive coincidence methodology at a collision energy of 4.4 eV. Four bimolecular reaction channels generating pairs of product ions are observed, forming: Ar+ + O2+, Ar+ + O+, ArO+ + O+ and O+ + O+. The formation of Ar+ + O2+ is a minor channel, involving forward scattering, and generates O2+ in its ground electronic state. This single electron transfer process is expected to be facile by Landau-Zener arguments, but the intensity of this channel is low because the electron transfer pathways involve multi-electron processes. The formation of Ar+ + O+ + O, is the most intense channel following interactions of Ar2+ with O2, in agreement with previous experiments. Many different combinations of Ar2+ and product electronic states contribute to the product flux in this channel. Major dissociation pathways of the nascent O2+* ion involve the ion's first and second dissociation limits. Unusually, the experimental results clearly show the involvement of a short-lived collision complex [ArO2]2+ in this channel. The formation of O+ and ArO+ involves direct abstraction of O- from O2 by Ar2+. There is scant evidence of the involvement of a collision complex in this bond forming pathway. The ArO+ product appears to be formed in the first excited electronic state (2Π). The formation of O+ + O+ results from dissociative double electron transfer via an O22+ intermediate. The exoergicity of the dissociation of the nascent O22+ intermediate is in good agreement with previous work investigating the unimolecular dissociation of this dication.

Single and double addition of oxygen atoms to propyne on surfaces at low temperatures.

Experiments designed to simulate the low temperature surface chemistry occurring in interstellar clouds provide clear evidence of a reaction between oxygen atoms and propyne ice. The reactants are dosed onto a surface held at a fixed temperature between 14 and 100 K. After the dosing period, temperature programmed desorption (TPD), coupled with time-of-flight mass spectrometry, are used to identify two reaction products with molecular formulae C3H4O and C3H4O2. These products result from the addition of a single oxygen atom, or two oxygen atoms, to a propyne reactant. A simple model has been used to extract kinetic data from the measured yield of the single-addition (C3H4O) product at surface temperatures from 30-100 K. This modelling reveals that the barrier of the solid-state reaction between propyne and a single oxygen atom (160 +/- 10 K) is an order of magnitude less than that reported for the gas-phase reaction. In addition, estimates for the desorption energy of propyne and reaction rate coefficient, as a function of temperature, are determined for the single addition process from the modelling. The yield of the single addition product falls as the surface temperature decreases from 50 K to 30K, but rises again as the surface temperature falls below 30 K. This increase in the rate of reaction at low surface temperatures is indicative of an alternative, perhaps barrierless, pathway to the single addition product which is only important at low surface temperatures. The kinetic model has been further developed to characterize the double addition reaction, which appears to involve the addition of a second oxygen atom to C3H4O. This modelling indicates that this second addition is a barrierless process. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and propyne could occur under on interstellar dust grains on an astrophysical time scale.

Bond-forming reactions of small triply charged cations with neutral molecules.

Time-of-flight mass spectrometry reveals that atomic and small molecular triply charged cations exhibit extensive bond-forming chemistry, following gas-phase collisions with neutral molecules. These experiments show that at collision energies of a few eV, I(3+) reacts with a variety of small molecules to generate molecular monocations and molecular dications containing iodine. Xe(3+) and CS2(3+) react in a similar manner to I(3+), undergoing bond-forming reactions with neutrals. A simple model, involving relative product energetics and electrostatic interaction potentials, is used to account for the observed reactivity.

Electron ionisation of sulfur dioxide.

Relative precursor-specific partial ionisation cross sections for the fragment ions formed following electron ionisation of sulfur dioxide (SO2) have been measured for the first time, from 30 to 200 eV, using time-of-flight mass spectrometry coupled with two-dimensional ion coincidence detection. These data quantify the yields of O(2+), O(+), SO(2+), S(+), O2(+), and SO(+) ions, relative to the formation of SO2(+), via single, double, and triple electron ionisation of SO2. Formation of O(2+), following electron-SO2 collisions, has been quantified for the first time. The data allow a first experimental estimate of the triple ionisation potential of SO2 (69.0 ± 3.6 eV), an energy in good agreement with a value derived in this study via computational chemistry. The triple ion combination S(+) + O(+) + O(+) is clearly detected following electron collisions with SO2 at electron energies markedly below the vertical energy for forming SO2(3 +). This observation is accounted for by the operation of a stepwise pathway to the formation of S(+) + 2O(+) which does not involve the formation of a molecular trication.

Electronic state selectivity in dication-molecule single electron transfer reactions: NO(2+) + NO.

The single-electron transfer reaction between NO(2+) and NO, which initially forms a pair of NO(+) ions, has been studied using a position-sensitive coincidence technique. The reactivity in this class of collision system, which involves the interaction of a dication with its neutral precursor, provides a sensitive test of recent ideas concerning electronic state selectivity in dicationic single-electron transfer reactions. In stark contrast to the recently observed single-electron transfer reactivity in the analogous CO(2)(2+)/CO(2) and O(2)(2+)/O(2) collision systems, electron transfer between NO(2+) and NO generates two product NO(+) ions which behave in an identical manner, whether the ions are formed from NO(2+) or NO. This observed behaviour is in excellent accord with the recently proposed rationalization of the state selectivity in dication-molecule SET reactions using simple propensity rules involving one-electron transitions.

Double ionization of cycloheptatriene and the reactions of the resulting C7H(n)2+ dications (n = 6, 8) with xenon.

The formation and fragmentation of the molecular dication C(7)H(8)(2+) from cycloheptatriene (CHT) and the bimolecular reactivities of C(7)H(8)(2+) and C(7)H(6)(2+) are studied using multipole-based tandem mass spectrometers with either electron ionization or photoionization using synchrotron radiation. From the photoionization studies, an apparent double-ionization energy of CHT of (22.67 ± 0.05) eV is derived, and the appearance energy of the most abundant fragment ion C(7)H(6)(2+), formed via H(2) elimination, is determined as (23.62 ± 0.07) eV. Analysis of both the experimental data as well as results of theoretical calculations strongly indicate, however, that an adiabatic transition to the dication state is not possible upon photoionization of neutral CHT and the experimental value is just considered as an upper bound. Instead, an analysis via two different Born-Haber cycles suggests (2)IE(CHT) = (21.6 ± 0.2) eV. Further, the bimolecular reactivities of the C(7)H(n)(2+) dications (n = 6, 8), generated via double ionization of CHT as a precursor, with xenon as well as nitrogen lead, inter alia, to the formation of the organo-xenon dication C(7)H(6)Xe(2+) and the corresponding nitrogen adduct C(7)H(6)N(2)(2+).

Formation of organoxenon dications in the reactions of xenon with dications derived from toluene.

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Electron ionization of SiCl4.

Relative partial ionization cross sections (PICS) for the formation of fragment ions following electron ionization of SiCl(4), in the electron energy range 30-200 eV, have been determined using time-of-flight mass spectrometry coupled with an ion coincidence technique. By this method, the contributions to the yield of each fragment ion from dissociative single, double, and triple ionization, are distinguished. These yields are quantified in the form of relative precursor-specific PICS, which are reported here for the first time for SiCl(4). For the formation of singly charged ionic fragments, the low-energy maxima appearing in the PICS curves are due to contributions from single ionization involving predominantly indirect ionization processes, while contributions to the yields of these ions at higher electron energies are often dominated by dissociative double ionization. Our data, in the reduced form of relative PICS, are shown to be in good agreement with a previous determination of the PICS of SiCl(4). Only for the formation of doubly charged fragment ions are the current relative PICS values lower than those measured in a previous study, although both datasets agree within combined error limits. The relative PICS data presented here include the first quantitative measurements of the formation of Cl(2) (+) fragment ions and of the formation of ion pairs via dissociative double ionization. The peaks appearing in the 2D ion coincidence data are analyzed to provide further information concerning the mechanism and energetics of the charge-separating dissociations of SiCl(4) (2+). The lowest energy dicationic precursor state, leading to SiCl(3) (+) + Cl(+) formation, lies 27.4 ± 0.3 eV above the ground state of SiCl(4) and is in close agreement with a calculated value of the adiabatic double ionization energy (27.3 eV).

Electron ionization of methane: the dissociation of the methane monocation and dication.

Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2).

Bonding analysis of the [C(2)O(4)](2+) intermediate formed in the reaction of CO(2)(2+) with neutral CO(2).

The bonding patterns of the [C(2)O(4)](2+) dication formed upon interaction of CO(2)(2+) with neutral CO(2) are investigated using the analysis of domain-averaged Fermi holes (DAFHs). The DAFH approach provides an explanation for the previously observed "asymmetry" of the energy deposition in the pair of CO(2)(+) monocations formed in the thermal reaction CO(2)(2+) + CO(2) --> [C(2)O(4)](2+) --> 2 CO(2)(+), specifically that the CO(2)(+) monocation formed from the dication dissociates far more readily than the CO(2)(+) monocation formed from the neutral molecule. The bonding pattern is consistent with a description of intermediate [C(2)O(4)](2+) as a complex between the triplet ground state of CO(2)(2+) with the singlet ground state of neutral CO(2), which can, among other pathways, smoothly proceed to a nondegenerate pair of (4)CO(2)(+) + (2)CO(2)(+) where the former stems from the dication and the latter stems from the neutral reactant. Hence the "electronic history" of the components is retained in the [C(2)O(4)](2+) intermediate. In addition, dissociation of (4)CO(2)(+) is discussed based on CCSD and CASSCF calculations. Equilibrium geometries for the ground electronic states of CO(2)(0/+/2+) and some other relevant structures of CO(2)(+) are determined using the MRCI method.

Crossed-beam scattering studies of electron-transfer processes between the dication CO2(2+) and neutral CO2: electronic states of reactants and products involved.

Crossed-beam scattering experiments were carried out at collision energies of 4.51 and 2.71 eV to elucidate the electronic states involved in the nondissociative and dissociative electron-transfer reactions observed following CO(2)(2+)/CO(2) collisions. Specifically, we focus on the observation that, in the dissociative electron-transfer reaction, forming CO(+), the majority of the CO(+) product ions are formed via electron capture by the CO(2)(2+) rather than via ejection of an electron from the neutral CO(2) reaction partner. The main channels resulting in nondissociative electron transfer are reactions of the ground (X(3)Sigma(g)(-)) and excited states of CO(2)(2+) to give different combinations of the ground and excited states of the product pair of CO(2)(+) ions in which the combination AA appears to be significant. The CO(+) ions appear mainly to arise from slow dissociation of CO(2)(+)(b(4)Pi(u)) formed following electron capture by the ground state of the dication reactant (X(3)Sigma(g)(-)), with possible contributions from electron capture by higher triplet excited states of the dication.

Selective dissociation in dication-molecule reactions.

The single electron transfer reactions between (13)CO(2)(2+) and (12)CO(2) and between (18)O(2)(2+) and (16)O(2) have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the (13)CO(2)(+) ion (the capture monocation) formed following electron transfer to (13)CO(2)(2+). In our studies of the carbon dioxide collision system, in agreement with previous work, the capture monocation shows a greater propensity to dissociate than the monocation formed from the neutral, (12)CO(2)(+) (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the (13)CO(2)(+) and (12)CO(2)(+) ions are different and are consistent with the ejection monocation dissociating via population of the C(2)Sigma state, whilst the capture ion is predominantly directly formed in dissociative quartet states. This state assignment is in accord with an expected preference for one-electron transitions in the electron transfer process. A propensity for one-electron transitions also rationalizes our observation that following dissociative single electron transfer between (18)O(2)(2+) and (16)O(2) the ejection monocation ((16)O(2)(+)) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the (16)O(2)(+) dissociation results from population of the B((2)Sigma) state. The less favoured dissociation of the capture monocation clearly involves population of a different electronic state(s) to those populated in the ejection ion. Indeed, the experimental data are consistent with the dissociation of the capture monocation via predissociated levels of the b((4)Sigma) state. Since the population of the B((2)Sigma) state from the neutral O(2) molecule involves a one-electron transition, and the population of the valence dissociative states of O(2)(+) from the dication are multi-electron processes, the preferential dissociation of the ejection monocation in this collision system can be rationalized by the same principles used to explain the electron transfer reactivity of CO(2)(2+) with CO(2).

Studies of the fragmentation of the monocation and dication of methanol.

Relative partial ionization cross sections and precursor-specific relative partial ionization cross sections for fragment ions formed by electron ionization of methanol have been measured using time-of-flight mass spectrometry coupled with a two-dimensional ion coincidence technique. Relative cross sections are reported for ionizing energies from 30 to 200 eV. Good agreement is found between our data and one set of recently published absolute partial ionization cross sections. Conversely, discrepancies are observed with another set of recently published data; we attribute these discrepancies to the loss of translationally energetic fragment ions. Our precursor-specific cross sections allow the contribution from single and double ionization to the individual fragment ion yields, following ionization of methanol, to be quantified for the first time. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% between 150 and 200 eV.

The diatomic dication PO2+.

The diatomic dication PO(2+) has been generated by the sputtering of surface-oxidized InP wafers and by electron ionization of gaseous trimethyl phosphate. According to ab initio calculations, the dication PO(2+) is metastable with respect to dissociation into P(+) + O(+), and the calculated ionization energy of the PO(+) monocation to form the dicationic species is ca. 22.6 eV. The potential energy functions for the ground state and twelve low-lying excited electronic states of the PO(2+) dication have been calculated using high-level ab initio methods with adiabatic excitation energies of PO(2+), spectroscopic constants and the ionization energies of PO and PO(+) being determined. Upon collision with xenon, electron transfer to PO(2+) takes place to form the PO(+) monocation together with a small amount of dissociative electron transfer to form P(+). Upon collisions of PO(2+) with deuterium, charge separation is accompanied by a bond-forming reaction to yield monocationic POD(+).