Ion-molecule Charge Research Articles

Molecular photodissociation dynamics revealed by Coulomb explosion imaging.

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.

Ion-molecule charge exchange in Io's extended atmosphere: Velocity dependence

The Io plasma torus is mostly composed of electrons and ions of O and S, singly and multiply-charged. These ions interact with the neutral species of Io's atmosphere (S, O, SO2 and SO) through symmetrical (e.g., O+ + O ⇒ O + O+) and asymmetrical (e.g., S+ + O ⇒ S + O+) charge exchange reactions. Moreover, molecular ions (SO2+, SO+) produced in Io's atmosphere by ionization or charge exchange are accelerated as they are picked up by the local plasma flow and eventually also charge exchange with the atmospheric neutrals (e.g., SO2+ + SO2 ⇒ SO2 + SO2+).Cross sections for many charge exchange reactions were estimated in Johnson and Strobel (1982) and McGrath and Johnson (1989) for ion speeds of 60 km/s appropriate for torus/neutral cloud interactions where the plasma is approximately at corotation in Io's reference frame. But close to Io, the flow velocity and ion temperature are drastically reduced (Vflow < 10 km/s and Tion < 50 eV) and greatly increased on its flanks (∼80 km/s and 400 eV). Thus, velocity-dependent charge exchange cross sections are critical for numerical simulations of the Io plasma/atmosphere interaction.We provide estimates of charge exchange cross sections of ions on molecular neutrals and of molecular ions on neutrals for the range of velocity [5–120] km/s representative of ion velocities at Io. We combine several methods valid at the low and higher velocities in this range: Two-state impact parameter approximation, quasi-thermal rate constant extrapolation, Langevin orbiting cross sections and scaling from experimental data. We describe these methods in details.Our estimates at 60 km/s are 2–3 times smaller or larger than previously published values, depending on the reaction. Most are generally large (∼60–90 Å2) at small velocities. We discuss the most important reactions relevant to Io's interaction and where improvements and experimental values are needed. We anticipate the effect of these velocity-dependent cross sections in numerical simulations of Io's interaction and show that some of these reactions are of critical importance regarding the plasma composition close to Io, the plasma supply to the torus and the neutral supply to Io's corona and to the giant neutral clouds that extend along Io's orbit.

Ion Momentum Imaging Study of the Ion-Molecule Charge Exchange Reaction of Ar+ + CO2.

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and it is applied for the ion-molecule charge exchange reaction between Ar+ and CO2. In a center-of-mass collision energy range of 7.23-15.96 eV, CO2+ products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu, B2Σu+, and C2Σg+; the multielectron excited states of CO2+ are also found at the higher collision energies. The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2, implying that the charge transfer from Ar+ would be not fast as expected. The strong electron correlations in the short-lived intermediate (Ar-CO2)+ should be responsible for the CO2+ yields at the multielectron excited states.

Electron transfer with no dissociation ion mobility–mass spectrometry (ETnoD IM-MS). The effect of charge reduction on protein conformation

A novel mass spectrometry approach is reported which investigate how ion-molecule charge reduction reactions between radical anions and protein cations modulate protein conformation. An electron transfer reagent (1,3-dicyanobenzene) transfers electrons to positively charged proteins and there are no observable products of dissociation. ETnoD product ions are detected as charged-reduced species with the same molecular weight as the precursor ion, and no significant evidence for proton transfer. We present collision cross section distributions of precursor and product ions before and after exposure to radical ions. Cytochrome c and myoglobin are examined as exemplar systems under both aqueous salt and denaturing conditions before and after exposure to radical anions. We consistently observe depletion of the more compact precursor ion conformers on exposure to the ETD reagent. Remarkably, by examining the collision cross section distributions of the product ions it can be seen that the addition of a single electron can cause a dramatic rearrangement in protein conformation for charge states that are highly populated when sprayed from salty aqueous conditions. Furthermore, a given net charge on an exposed precursor and product ion favours a preferred collision cross section distribution, indicating that the distribution of charge on proteins in the gas phase dictate their conformation. An exception is reported for the low charge state of cytochrome c where compaction was seen in the radical formed post reduction compared to the electrospray generated ion under ETnoD optimised conditions. We propose a model that postulates how electron transfer to conformation stabilising salt bridges may explain our observations.

Secondary ion-molecule reactions in matrix-assisted laser desorption/ionization

Ion-molecule charge- and proton-transfer reactions in the desorption plume are considered for the case of matrix-assisted laser desorption/ionization (MALDI) with ultraviolet laser excitation, and it is proposed that they are major determinants of the observed mass spectrum. Specific MALDI phenomena which are discussed include the dominance of singly charged ions and analyte-matrix or analyte-analyte signal suppression. Should any be formed, highly charged products can be reduced by reaction with neutral matrix, yet singly charged ions cannot generally be neutralized in the same manner. Ion suppression effects can also be explained by similar reactions, which in some cases involve interconversion of dissimilar ion types. The plume is proposed often to be more under thermodynamic rather than kinetic control owing to these secondary reactions. UV/MALDI mass spectra should therefore be largely predictable, given sufficient thermodynamic information, and appropriate experimental conditions of sufficient analyte and plume density. Copyright 2000 John Wiley & Sons, Ltd.

Supression of electron-induced positive ion emission by a molecular overlayer: Ion-molecule charge exchange at a surface.

We report the suppression of electron-stimulated desorption of positive ions (${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$) from a ${\mathrm{TiO}}_{2}$ (110) surface caused by adsorption of a fractional monolayer of molecular ${\mathrm{NH}}_{3}$. A linear decrease of ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ emission with ${\mathrm{NH}}_{3}$ coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or with adsorbed molecules. We propose a novel charge exchange mechanism where electron transfer from the occupied orbitals of the adsorbed molecules to the desorbing ions causes the decrease in detected ion yield. Charge transfer cross sections of 2.8(\ifmmode\pm\else\textpm\fi{}0.5) and 2.7(\ifmmode\pm\else\textpm\fi{}1) (\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$) ${\mathrm{cm}}^{2}$ have been determined for ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ ions, respectively.

Coupled wavepackets study of the dynamics of a model ion-molecule charge exchange

The dynamics of the ion-molecule charge exchange process A + BC + → A + + BC is investigated in a semiclassical framework. The treatment makes use of a semiclassical approximation whereby the collisional motion evolves along a classical trajectory, whereas the vibrational wavefunctions associated with each electronic state are treated quantally. The wavefunctions are defined over a grid and their time evolution is handled numerically. Besides its ability to solve the collision equations, the wavepacket method is shown to provide a useful physical picture of the collision. The goal of this paper is to put forward the basic characteristics of charge exchange processes in term of coupled wave-packets dynamics. In order to do that, a very simple model case has been investigated in the frame of the impact parameter approximation. This model provides deep physical insight into the dynamics of the process, especially in the intermediate energy domain where the non-adiabatic transitions are neither vertical, nor horizontal.

A comparison of approximate techniques for the determination of potential energy surfaces of ion–molecule charge transfer systems

The (HO2)+ molecular ion is used to experiment two approximate procedures which aim at reducing the computation effort that is needed for the determination of potential energy surfaces of ion–molecule charge transfer systems. The two procedures involve configuration interaction (CI) calculations of moderate sizes and are based on diagonal corrections of the electronic Hamiltonian matrix in a basis of projected-valence bond (PVB) configuration-state functions (CSF). The PVB-CSF’s used in practice correspond to a full valence CI for each ionic or neutral partner as well as single excitations accounting for polarization and electron transfer. The diagonal corrections are of two sorts: (i) if insufficiently large orbital expansion bases are used they remove the relative ion–molecule basis set superposition error; (ii) if asymptotic energy levels of the involved neutrals or ions in their ground or valence excited states are misplaced they properly adjust these levels. When applied to (HO2)+ using a minimal or an extended orbital basis set expansion the proposed approaches yield concording results. The results also agree with the effective model potential (EMP) data of Grimbert et al. [Chem. Phys. 124, 187 (1988)] which have proved successful in the description of the H++O2 charge transfer dynamics. Comparison with fragmentary results from MRD-CI calculations by Vazquez et al. [Mol. Phys. 59, 291 (1986)] and Schneider et al. [Chem. Phys. 128, 311 (1988)] is somewhat mitigated. The method should be particularly useful for bulky ion–molecule systems.

Coupled wave packets study of the dynamics of dissociative ion–molecule charge exchange

A coupled wave packet method is presented which allows us to treat exactly the vibrational and dissociative motions in nonadiabatic atom–molecule collisions, whose relative motion is described by a classical trajectory. It consists of a time propagation of the coupled vibrational wave functions defined over a grid. It is applied to the dissociative charge exchange (DCE) process in atom–molecule collisions. Model cases are investigated that put forward the basic characteristics of the DCE process. Analysis of the time evolution of the wave packets leads to a direct view of the collision dynamics and to an interpretation of the dissociation mechanisms. Striking phenomena are revealed, that were not predictable by the previously existing approximate methods.

Effect of quantization of rotational energy on the ion-molecule capture

Failure to account for the quantized character of the energy of the molecule is one of the main reasons for the differences between measured and classically calculated rate constants for the ion-molecule charge transfer reactions. The quantization is especially important at low temperatures even for molecules that feature small dipole momenta.

Electron capture inH++H2

Electron capture in ${\mathrm{H}}^{+}$+${\mathrm{H}}_{2}$ collisions is studied at energies of 1.0, 2.0, and 3.0 keV in the reduced angular range $\ensuremath{\tau}\ensuremath{\le}3.0$ keV deg. Time-of-flight techniques are used for state identification, and the probability of electron capture to $\mathrm{H}(1s)+{\mathrm{H}}_{2}^{ +}(^{2}\ensuremath{\Sigma}_{g}^{+})$ in an exchange collision is determined. This is shown to be the dominant exchange channel at small scattering angles. The "summed" exchange channels have a reduced cross section that is found to peak in the forward direction and shows no structure, while the cross section for capture to final ground-state channels shows some small structure at 3.0 keV for $\ensuremath{\tau}\ensuremath{\ge}1.5$ keV deg. The reduced cross section, at 1.0 keV, for capture to excited states of H (with the ${\mathrm{H}}_{2}^{+}$ in its ground state) is found to have a maximum at $\ensuremath{\tau}=1$ keV deg. The results are in agreement with a previously proposed model for ion-molecule charge exchange.

Applications of Atom-Photon Coincidence Measurements to Charge Transfer

Measurements have been made of the polarization of characteristic radiation of excited states, formed in charge exchange collisions, in delayed time coincidence with the neutral particles formed and scattered during the collision. Such information directly provides ratios of populations of orbital magnetic sublevels and phase differences between scattering amplitudes. This relative phase data, in conjunction with an appropriate model of the collision process, has provided insights into the evolution of the electronic charge distribution during the collision process. These techniques are also being applied to ion-molecule charge transfer collisions.

Ion-Molecular Reactions in Plasma Containment Devices

We investigate ion-molecule charge exchange followed by dissociative electron recombination with molecular ions due to the presence of common impurities in a plasma-containment device. For a ${\mathrm{He}}^{+}$ plasma with ${\mathrm{N}}_{2}$ impurity, the ionic constituency of plasmas is changed from ${\mathrm{He}}^{+}$ to nitrogen ions with a time constant ${\ensuremath{\tau}}^{*}$ given by ${\ensuremath{\tau}}^{*}P({\mathrm{N}}_{2})\ensuremath{\simeq}24$ msec $\ensuremath{\mu}$ Torr. However, those ion-molecular recombination processes are not responsible for the enhanced plasma loss in the Princeton spherator at the best vacuum condition.

Ion production and recombination in flames

Experimental results for hydrocarbon-air or oxygen flames are presented at pressures down to 3 mm Hg. These are interpreted to yield ion recombination coefficients which are independent of pressure and equal to 2×10−7 cc/sec, i.e., the rate of ion recombination is second order with respect to the ion concentration. The rate of ion formation is proportional to the pressure squared so the formation process is second order. The importance of ambipolar diffusion at pressures below 66 mm Hg is deduced, so it is necessary to consider diffusion in interpreting ion decay processes behind the flame front. These results with other available information are discussed in terms of ion production by chemi-ionization, e.g. by CH+O→CHO++e−, At 66 mm Hg, if the reactants are present in equal concentration, the reactant concentration would have to be 1011 molecules/cc. The initially produced ion rapidly reacts with H2O by an ionmolecule charge exchange reaction to produce H2O+. The half-life of the initially produced ion is very short, approximately 10−7 sec, so it is present in only small concentrations. In summary, a picture has been formulated of the mechanism of ion formation and removal in flames which is consistent with the known facts. Some details are still fuzzy, and the specific elementary steps to be chosen are still open to question. In such a complex system as a flame, with many reactions occurring, it is surpring that such an apparently simple picture seems to fit the experimental observations. With more accurate and detailed data, additional facets of the problem can be expected to appear.

Ion-Molecule Charge Transfer Reactions in the Alpha Radiolysis of Various Hydrocarbons in a Mass Spectrometer

The new alpha-particle mass spectrometer proves to be a powerful tool for the study of charge transfer between gaseous ions and molecules. Energetically possible charge-transfer reactions have been differentiated into probable and improbable reactions by employing binary mixtures of hydrocarbon gases in this instrument. The alpha-particle induced charge-transfer reaction, C2H2++C6H6→C2H2+C6H6+, is shown to be very efficient. In contrast, the charge-transfer reaction, C2H2++C2H4→C2H2+C2H4+, is shown to be inefficient. This inefficiency is discussed in terms of the competing chemical reaction, C2H2++C2H4→C3H3+ +CH3.