Ion Collision Cross Sections Research Articles

Ionization of hydrogen molecular ions by antiproton impacts

Ionization cross sections in collisions of antiprotons $(\overline{p})$ with $\mathrm{H}_{2}{}^{+}$ molecules are calculated for incident energies in the range of $2--500\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ by using a semiclassical impact-parameter method. The electronic motion is solved in a numerically accurate manner by means of a discrete variable representation. A sudden approximation is applied to the description of the molecular rotation and vibration. The present $\overline{p}+\mathrm{H}_{2}{}^{+}$ ionization cross sections are compared with the theoretical results of the ionization in $\overline{p}+{\mathrm{He}}^{+}$ and the experimental results of the production of ${\mathrm{H}}^{+}$ ions in $\overline{p}+{\mathrm{H}}_{2}$.

The hyperthermal ionization and high absolutemeteor velocities observed with HPLA radars

High Power Large Aperture (HPLA) radars generally observe very high meteor velocities averaging over 50 km s−1. There are only a few events recorded around 30 km s−1, while meteors at 20 km s−1 or slower are very rare. This is a clear and debated contradiction to specular meteor radar results. A high plasma density condition contributes, but the dominating phenomenon is the hyperthermal ionization mechanism due to chemical dynamics of the ionization process. The observed high velocities can be explained in terms of high hyperthermal ionization cross-sections for collisions between ablated meteoroid metal atoms such as Na and/or Fe and atmospheric species.

Ion Mobility Spectrometer with Radial Collisional Focusing

An ion mobility spectrometer that has its mobility cell as a 20-segment quadrupole and functionally the q2 of a triple-quadrupole mass spectrometer has been assembled and tested. The combination of high cell pressure (maximum of 4 Torr of helium) and low axial field (20-160 V per 20.2 cm) results in negligible internal excitation of the ions despite applications of rf and axial fields. The presence of collisional focusing ensures efficient ion transmission and good sensitivity. Collision cross sections of atomic, cluster, peptide, and protein ions were measured and found comparable to literature and calculated cross sections.

Ion swarm data for electrical discharge modeling in air and flue gas mixtures

The first step of this work is the determination of the elastic and inelastic ion-molecule collision cross sections for the main ions (N2+, O2+, CO2+, H2O+ and O−) usually present either in the air or flue gas discharges. The obtained cross section sets, given for ion kinetic energies not exceeding 100 eV, correspond to the interactions of each ion with its parent molecule (symmetric case) or nonparent molecule (asymmetric case). Then by using these different cross section sets, it is possible to obtain the ion swarm data for the different gas mixtures involving N2, CO2, H2O and O2 molecules whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in gas mixtures. This also allows us to clearly show that the classical linear approximations usually applied for the ion swarm data in mixtures such as Blanc’s law are far to be valid. Then, the ion swarm data are given in three cases of gas mixtures: a dry air (80% N2, 20% O2), a ternary gas mixture (82% N2, 12% CO2, 6% O2) and a typical flue gas (76% N2, 12% CO2, 6% O2, 6% H2O). From these reliable ion swarm data, electrical discharge modeling for a wire to plane electrode configuration has been carried out in these three mixtures at the atmospheric pressure for different applied voltages. Under the same discharge conditions, large discrepancies in the streamer formation and propagation have been observed in these three mixture cases. They are due to the deviations existing not only between the different effective electron-molecule ionization rates but also between the ion transport properties mainly because of the presence of a highly polar molecule such as H2O. This emphasizes the necessity to properly consider the ion transport in the discharge modeling.

Collision cross sections of gas phase DNA ions

Collision cross sections of negative ions of a 28-, 40- and 55-mer of single stranded DNA have been measured by an energy loss method, and compared to collision cross sections of proteins of nearly the same molecular weight—ubiquitin, cytochrome c and apomyoglobin, respectively. The oligonucleotides produce negative charge states in electrospray ionization (ESI) similar to the positive charge states produced by the proteins denatured in solution. Cross sections for deoxynucleotide ions increase with charge state in a manner similar to those of protein ions. However, for a given molecular weight and charge state, the cross sections of the oligodeoxynucleotide ions are about 22% lower than those of the proteins.

Prediction of peptide ion collision cross sections from topological molecular structure and amino acid parameters.

Quantitative structure-property relationships (QSPRs) have been developed to predict the ion mobility spectrometry (IMS) collision cross sections of singly protonated lysine-terminated peptides using information derived from topological molecular structure and various amino acid parameters. The primary amino acid sequence alone is sufficient to accurately predict the collision cross section. The models were built using multiple linear regression (MLR) and computational neural networks (CNNs). The best MLR model found contains six descriptors and predicts 94 of 113 peptides (83%) to within 2% of their experimentally determined values. The best CNN model using the same six descriptors predicts 105 of the 113 peptides (93%) to within 2% of their experimentally determined values. The best overall CNN model, using a different set of six descriptors, predicts 109 of the 113 peptides (96%) to within 2% of their experimentally determined values. In addition, this model can discriminate among peptides having identical amino acid composition, but differing in primary amino acid sequence. This represents a capability not found in previously described models. The descriptors used in the models presented may provide some insight into the nature of peptide ion folding in the gas phase.

Inelastic processes in the interactions of partially stripped ions ofcarbon, nitrogen and oxygen with atomic hydrogen at intermediate and highenergies

The classical trajectory Monte Carlo (CTMC) simulation method has beenemployed to study the sub-shell distributions of total charge transfercross sections and total ionization cross sections in collisions ofpartially stripped ions of carbon, nitrogen and oxygen in different chargestates (q ⩽ 5) with ground state atomic hydrogen in the energy range10-200 keV amu-1. The total cross section for chargetransfer and ionization has been found to be in good agreement with theexperimental observations as well as with other theoretical findings. Dueto the non-availability of any other experimental results for sub-shelldistributions of charge transfer cross sections, comparisons have beenmade with other existing theoretical results. The observations have beenfound to be satisfactory. However, an oscillatory structureof the charge-state dependence of the total cross sectionshas not been found in our calculation.

Eikonal scattering cross section for elastic electron–ion collisions in nonideal plasmas

Modified eikonal approximation is applied to investigate the elastic electron–ion collisions in nonideal plasmas. An effective pseudopotential model, taking into account the plasma screening and collective effects, is applied to describe the electron–ion interaction potential in a classical nonideal plasma. The semiclassical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the total elastic scattering cross section as a function of the projectile energy, nonideal plasma parameter, and Debye length. The results show that the scaled total cross section in ideal plasmas described by the Debye–Hückel potential model is found to be greater than that in nonideal plasmas described by the pseudopotential model. It is also found that the scaled cross section is decreased with increasing the nonideal plasma parameter (γ), i.e., the nonideality of plasmas reduces the electron–ion collision cross section.

Classical treatment of collisions

Using a classical treatment in the frame of an independent particle model with an effective Hamiltonian, we calculate capture and ionization cross sections for collisions in the impact energy range , and compare our data with those of three sets of experiments and with those for collisions.

Applications of Collision Dynamics in Quadrupole Mass Spectrometry

Some applications of collision dynamics in the field of quadrupole mass spectrometry are presented. Previous data on the collision induced dissociation of ions in triple quadrupole mass spectrometers is reviewed. A new method to calculate the internal energy distribution of activated ions directly from the increase in the cross section for dissociation with center of mass energy is presented. This method, although approximate, demonstrates explicitly the high efficiency of transfer of translational to internal energy of organic ions. It is argued that at eV center of mass energies, collisions between protein ions and neutrals such as Ar are expected to be highly inelastic. The discovery and application of collisional cooling in radio frequency quadrupoles is reviewed. Some previously unpresented data on fragment ion energies in triple quadrupole tandem mass spectrometry are shown that demonstrate directly the loss of kinetic energy of fragment ions in the cooling process. The development of the energy loss method to measure collision cross sections of protein ions in triple quadrupole instruments is reviewed along with a new discussion of the effects of inelastic collisions in these experiments and related ion mobility experiments.

Ionization mechanisms in CRRES chemical releases: 1. In situ measurements and model results

The quadrupole ion mass spectrometer (QIMS) measured fluxes of barium and strontium ions during the CRRES G‐1 chemical release in full Sun and the G‐11b release in darkness. The barium ion fluxes detected in darkness were only a factor of 10 lower than the photoionization fluxes, indicating that the rate of nonsolar‐UV ionization was at least a factor of 8 higher than literature values of charge transfer and collisional ionization cross sections would suggest. Model calculations of the QIMS ion fluxes using new, state‐specific, calculations of the cross sections for ion‐pair formation and charge transfer [Wolf and Hunton, this issue] demonstrate that the large nonphotoionization rates are consistent with significant metastable excitation in the neutral cloud but not with a ground state population. Upper limits to the collisional ionization rates for the conditions of the releases are 4.3 × 10−3 s−1 for ion‐pair production and 2.2 × 10−3 S−1 for charge transfer. Collisions of high‐energy electrons with the neutrals early in the cloud expansion is suggested as a mechanism for producing the metastable metal atoms. A similar analysis of the strontium data indicates that photoionization, charge transfer, and collisional ion‐pair production alone cannot explain the large fluxes of strontium ions detected in the sunlit release. Associative ionization may therefore be an important ionization mechanism for that metal in sunlight. The implications of these results for analysis of space critical ionization velocity experiments are discussed.

A segmented radiofrequency-only quadrupole collision cell for measurements of ion collision cross section on a triple quadrupole mass spectrometer

The gas collision cell of a triple quadrupole mass spectrometer has been modified to consist of ten short quadrupole rod segments that allow an axial field to be applied to the cell in order to make measurements of ion mobility. The radiofrequency (rf)-quadrupole field provides effective radial confinement that greatly reduces diffusional losses at low pressure. The mobilities of mass-selected ions from an ionspray source have been measured at a pressure of 8 × 10−3 torr at electric fields of 0. 1 to 3 V/cm, and used to calculate the collision cross sections of the ions. The measured cross sections compare well with those measured by other techniques.

Projectile scattering angle dependence of multiple ionization cross sections in collisions of C3+ with Ar

We have measured capture- and loss-ionization cross sections differential in the projectile scattering angle for 2-MeV C3+ −Ar collisions. The scattering angles ranged from 0.1-1.0 mrad. The obtained differential cross sections were reproduced fairly well by a classical-trajectory Monte Carlo (nCTMC) method.

Absolute cross-section measurements for ionization of He Rydberg atoms in collisions with K.

Absolute cross sections for ionization of 1.5--10.0 keV/amu Rydberg helium atoms in principal quantum states 12\ensuremath{\le}n\ensuremath{\le}15 due to collisions with potassium have been measured. The data are compared with the free-electron cross section at equal velocity. Our results for the collisional ionization cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$) agree both in shape and absolute magnitude with the data available for the total electron-scattering cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{e}}$) and support recent theoretical models for ionization of Rydberg atoms with neutral perturbers.

Relative collision cross sections of organic ions

The relative collision cross sections σ c of selected precursor ions that collide with helium at high ion kinetic energies (kiloelectronvolts) have been obtained by monitoring the ion beam attenuation at different collision pressures. A value for the total collision cross section is obtained, which includes contributions from different processes that lead to fragmentation, scattering, charge exchange, charge inversion, and stripping. For n-alkanes and n-alkylbenzenes the σ c value increases linearly by 0.1 per methylene group. Linear relationships exist between relative collision cross section and ionization cross section σ i, and polarizability α of the corresponding neutral molecule, but different equations are required for the description of σ c in each chemical series. A simple relationship, which requires only the chemical formula of the precursor ion, has been determined that can predict all relative collision cross sections with an average error of 3.5%. The total relative collision cross section increases with the size of the ion. The experimental results are in agreement with the predictions of the hard-sphere collision theory. The approach can be applied to determine the relative collision cross sections at different ion kinetic energy regimes and collisions with different target gases.

Collision cross sections for protein ions

A method for the determination of cross sections for gas-phase protein ions, based on the energy loss of ions as they pass through a collision gas, is described. A simple model relates the energy loss to the number of collisions and hence the cross section. Results from a Monte Carlo model that support the validity of this approach are described. Experimental cross sections are reported for motilin, ubiquitin, cytochrome c, myoglobin, and bovine serum albumin. Cross sections range from approximately 800 Å 2 for motilin to approximately 14,000 Å 2 for bovine serum albumin and generally increase with the number of charges on the ion. Cytochrome c ions from aqueous solution show somewhat smaller cross sections than ions formed from solutions of higher organic content, suggesting that the gas-phase ions may retain some memory of their solution conformation.

The determination of collision cross-sections

Collision cross-sections of ions, σ0t, can be accurately obtained by extrapolating σt to zero target gas pressure, thus eliminating the error in the collision path length, l(n), caused by target gas leaking out of the collision cell. σ0t values for various ions have been determined and contain structural information. For normal alkanes (n = 1–10) σOt steadily increases from 3.4 to 14.2Å2 by 1.2Å2 per methylene unit. The observation that ions with cyclic structures have smaller total collision cross-sections than corresponding non-cyclic species could be general, as observed for cyclohexane, cyclohexanone and the C2H3O+ ions from oxirane with respect to acetyl cations. The benzene and pyridine dications cannot be considered as pure six-membered ring species since their σ0t values are significantly larger than those of the singly charged ions.

Coupled subshell description of the L-shell ionization in asymmetric ion-atom collisions

L-subshell ionization cross sections for collisions of Be, C, F, Mg, Si and S ions with Au and Th m the energy range of 0.5–2.5 MeV/amu are calculated within the coupled Subshell approximation and compared to the experimental data. Fair agreement is obtained once the matrix elements of the projectile perturbing potential are renormalized to account for the screening due to the target spectator electrons. The present calculations provide the first consistent description of the direct Coulomb ionization and vacancy sharing among the L-subshells during an asymmetric ion-atom collision. The remaining discrepancies suggest further complexity inherent to such a process.

Electron ion interactions in crystal channels: collisions in ultra-dense electron media

Dielectronic excitation of H-like S, Ca and Ti is shown to occur in the dense electron gas of a crystal channel. Cross sections for collisional ionization of the short lived excited states can then be determined. Ionic excitation can also be achieved by resonant coherent excitation in which case specific m states can be excited for further study.

Charge Transfer and Ionization Cross Sections for Collisions of Tiq, Crq+, Feq+ and Niq+ Ions with Atomic Hydrogen

Using the classical Monte Carlo method and a Thomas-Fermi type pseudo-potential for the incompletely stripped ion, total electron capture and ionization cross sections have been calculated for the collisions of Tiq+, Crq+, Feq+ and Niq+ ions with hydrogen atoms for a large number of selected charge states in the energy range from 10 to 103keV/amu. The charge transfer cross sections for q > 6 confirm the scaling σcx/q vs. E/q0.5 in the entire energy range.