Triple Ionization Research Articles

Dissociative-ionization cross sections for 12-keV-electron impact on CO2

The dissociative ionization of a CO${}_{2}$ molecule is studied at an electron energy of 12 keV using the multiple ion coincidence imaging technique. The absolute partial ionization cross sections and the precursor-specific absolute partial ionization cross sections of resulting fragment ions are obtained and reported. It is found that \ensuremath{\sim}75% of single ionization, 22% of double ionization, and \ensuremath{\sim}2% of triple ionization of the parent molecule contribute to the total fragment ion yield; quadruple ionization of CO${}_{2}$ is found to make a negligibly small contribution. Furthermore, the absolute partial ionization cross sections for ion-pair and ion-triple formation are measured for nine dissociative ionization channels of up to a quadruply ionized CO${}_{2}$ molecule. In addition, the branching ratios for single-ion, ion-pair, and ion-triple formation are also determined.

Production and decay of chlorine ion excited species in an electron cyclotron resonance ion source plasma

The most important processes for the creation of chlorine ion excited states from the ground configurations of Cl10+ to Cl15+ ions in an electron cyclotron resonance ion source, leading to the emission of K x-ray lines, were studied. Theoretical values for inner-shell excitation and ionization cross-sections, including double KL and triple KLL ionization, transition probabilities and energies for the de-excitation processes, were calculated in the framework of the multi-configuration Dirac–Fock method. With reasonable assumptions about the electron energy distribution, a theoretical Kα x-ray spectrum was obtained, which was then compared with recent experimental data.

Angle-resolved photoelectron spectroscopy of sequential three-photon triple ionization of neon at 90.5 eV photon energy

Multiple photoionization of neon atoms by a strong 13.7 nm (90.5 eV) laser pulse has been studied at the FLASH free electron laser in Hamburg. A velocity map imaging spectrometer was used to record angle-resolved photoelectron spectra on a single-shot basis. Analysis of the evolution of the spectra with the FEL pulse energy in combination with extensive theoretical calculations allows the ionization pathways that contribute to be assigned, revealing the occurrence of sequential three-photon triple ionization.

Profiling Cys34 Adducts of Human Serum Albumin by Fixed-Step Selected Reaction Monitoring

A method is described for profiling putative adducts (or other unknown covalent modifications) at the Cys34 locus of human serum albumin (HSA), which represents the preferred reaction site for small electrophilic species in human serum. By comparing profiles of putative HSA-Cys34 adducts across populations of interest it is theoretically possible to explore environmental causes of degenerative diseases and cancer caused by both exogenous and endogenous chemicals. We report a novel application of selected-reaction-monitoring (SRM) mass spectrometry, termed fixed-step SRM (FS-SRM), that allows detection of essentially all HSA-Cys34 modifications over a specified range of mass increases (added masses). After tryptic digestion, HSA-Cys34 adducts are contained in the third largest peptide (T3), which contains 21 amino acids and an average mass of 2433.87 Da. The FS-SRM method does not require that exact masses of T3 adducts be known in advance but rather uses a theoretical list of T3-adduct m/z values separated by a fixed increment of 1.5. In terms of added masses, each triply charged parent ion represents a bin of ±2.3 Da between 9.1 Da and 351.1 Da. Synthetic T3 adducts were used to optimize FS-SRM and to establish screening rules based upon selected b- and y-series fragment ions. An isotopically labeled T3 adduct is added to protein digests to facilitate quantification of putative adducts. We used FS-SRM to generate putative adduct profiles from six archived specimens of HSA that had been pooled by gender, race, and smoking status. An average of 66 putative adduct hits (out of a possible 77) were detected in these samples. Putative adducts covered a wide range of concentrations, were most abundant in the mass range below 100 Da, and were more abundant in smokers than in nonsmokers. With minor modifications, the FS-SRM methodology can be applied to other nucleophilic sites and proteins.

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 hydrogen sulfide

Precursor-specific relative partial ionization cross sections, for all the different fragment ions formed by electron ionization of hydrogen sulfide, 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. These cross sections allow the contribution from single, double and triple ionization to the individual fragment ion yields, following ionization of hydrogen sulfide, to be quantified. To compare our data with the literature we reduce our precursor-specific cross sections, by summing them for each fragment ion, to generate relative partial ionization cross sections. Following this data reduction, good agreement is found between our data and one set of recently published absolute partial ionization cross sections, but discrepancies are observed with another set of recently published data. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% at 100 eV. Given the lack of available information on the fate of the excited electronic states of H 2S 2+, we have extracted kinetic energy releases for the various dicationic fragmentation channels from our coincidence data. From these kinetic energy releases, estimates of the energies of the electronic states of H 2S 2+ which are responsible for the different fragmentation channels can be made. These estimates, in comparison with other data on the electronic states of H 2S 2+, reveal the population of excited states of H 2S 2+ at electron energies above 50 eV. At ionizing electron energies above 50 eV, a significant proportion of the major dissociation channels of H 2S 2+ appear to involve the population of excited electronic states.

Single and multiple photoionisation of H2S by 40–250 eV photons

Multi-electron coincidence measurements on photoionisation of H(2)S have been carried out at photon energies from 40 to 250 eV. They quantify molecular field effects on the Auger process in detail and are in good agreement with the existing theory. Spectra of core-valence double ionisation of H(2)S are presented and partially analysed. Auger decays from the core-valence states produce triply charged product spectra with unexplained and surprising intensity distributions. Triple ionisation by the double Auger process from 2p hole states shows little effect of the molecular field splitting, but includes a substantial contribution from cascade processes, some involving dissociation in intermediate states. The onset of triple ionisation at the molecular geometry is determined as 61 ± 0.5 eV.

Metastable ClO2+ and ClO3+ ions in the gas phase: a combined theoretical and mass spectrometric investigation

We have performed a detailed theoretical study applying large ab initio computations of the low lying electronic states of ClO(2+) and ClO(3+) and an experimental search for these ClO(2+) and ClO(3+) gas-phase species. For both species, we have found electronic states with potential barriers with respect to dissociation, where these multiply positively charged diatomic ions can exist in the gas phase as long-lived metastable molecules. Our potential energy curves are used to predict the double-ionization spectrum of ClO and to derive a set of spectroscopic parameters for the metastable bound states of these species. At the MRCI + Q/aug-cc-pV5Z level, the adiabatic double and triple ionization energies of ClO are computed to be 32.4 eV and 65.0 eV, respectively. Experimentally, we confirm the existence of ClO(2+) using mass spectrometry. ClO(2+) could be produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of PdCl(2) and NH(4)Cl powders and survived a flight time of ~9 μs. We report also the experimental observation of the ClN(2+), PdCl(2+) and InO(2+) diatomic doubly charged ions.

Triple ionization of lithium-like ions by photon impact

Within the framework of sudden approximation, the non-relativistic cross sections for the triple ionization of Li-like ions caused by absorption and Compton scattering of high-energy photons are calculated. The ratios of the triple-to-single and triple-to-double ionization cross sections are obtained. For neutral lithium atom, our predictions for σ3+/σ+ and σ3+/σ2+ are compared both with available experimental and theoretical results.

Single and multiple ionization ofC60fullerenes and collective effects in collisions with highly charged C, F, and Si ions with energy 3 MeV/u

We have measured absolute cross sections for single, double, triple, and quadruple ionization of ${\mathrm{C}}_{60}$ in collisions with 3 MeV/u C, F, and Si projectile ions at various projectile charge states. The experiment was performed using the recoil-ion time-of-flight technique. Projectile charge state dependence of the ionization yields was compared mainly with a model based on the giant dipole plasmon resonance (GDPR). In some cases, the continuum-distorted-wave--eikonal-initial-state (CDW-EIS) model which is normally applied for ion-atom collisions was also used as a reference. An excellent qualitative agreement between the experimental data for single and double ionization and the GDPR model predictions was found for all projectile charge states.

Multielectron coincidence study of the double Auger decay of3d-ionized krypton

Multielectron coincidence data for triple ionization of krypton have been recorded above the 3d ionization threshold at two photon energies (140 and 150 eV). Three principal transition pathways hav ...

CDW-EIS calculations for multiple ionization of Ne, Ar, Kr and Xe by the impact of H+ and He+, including post-collisional electron emission

We present theoretical single to quintuple ionization cross sections for Ne, Ar, Kr and Xe bombarded by H+ and He+. Post-collisional contributions due to Auger-like processes are taken into account using recent photoionization data. The present continuum distorted wave-eikonal initial state (CDW-EIS) and first Born approximation results are compared with the experimental data available in the energy range of 50–10 000 keV amu−1 for H+ on Ne and Ar, and 50–1000 keV amu−1 for the other cases. In general, the combination of the CDW-EIS with the post-collisional branching ratios describes well the multiple ionization data above 300 keV amu−1, showing a clear tendency to coalesce with the first Born approximation at high energies. The surprising result of this work is the good performance of the first Born approximation which describes rather well the experimental data of double and triple ionization, even in the intermediate energy range (50–300 keV amu−1), where direct ionization is the dominant contribution.

Double-ionization mechanisms of the argon dimer in intense laser fields

We have measured the two-site double ionization of argon dimers by ultrashort laser pulses leading to fragmentation into two singly charged argon ions. Contrary to the expectations from a pure Coulomb explosion following rapid removal of one electron from each of the atoms, we find three distinct peaks in the kinetic energy release (KER) distribution. By measuring the angular distribution of the fragment ions and the vector momentum of one of the emitted electrons for circular and linear laser polarization, we are able to unravel the ionization mechanisms leading to the three features in the KER. The most abundant one results from tunnel ionization at one site followed by charge-enhanced tunnel ionization of the second atom. The second mechanism, which leads to a higher KER we identify as sequential tunnel ionization of both atoms accompanied by excitation. The third mechanism is present with linearly polarized light only. It is most likely a frustrated triple ionization, where the third electron does not escape but is trapped in a Rydberg state.

Mechanisms underlying strong-field double ionization of argon dimers

We investigate double ionization of argon dimers in high-intensity ultrashort Ti:sapphire laser pulses. We are able to identify several strong-field excitation pathways of the dimer that terminate in atomic ion pairs from a Coulomb explosion. The explosion starts from two-site double-ionized dimers and from one-site double-ionized ones after radiative charge transfer at small internuclear separation. One-site double ionization is accomplished by laser-induced charge transfer in the high-intensity laser pulse following two-site double ionization. The highest energy ion pairs we observed can be attributed to ``frustrated triple ionization'' of the argon dimer.

Complex sub-laser-cycle electron dynamics in strong-field nonsequential triple ionizaion

Using the full three-dimensional classical ensemble model, we have investigated nonsequential triple ionization (NSTI) of Ne by intense linearly polarized laser fields systematically. Trajectory back analysis enables us to identify the various NSTI channels at different intensities in an intuitive way. The momentum distributions of the triply ionized ions calculated by this model agree well with the experimental results over a wide range of laser intensities [J. Phys. B 41, 081006 (2008)]. With this classical model we achieve insight into the complex sub-laser-cycle dynamics of the correlated three electrons in NSTI.

Multiple ionization of oxygen studied by coincident measurement

We experimentally study double and triple ionization of oxygen using a reaction microscope. The kinetic energy releases (KERs) and angular distributions are obtained through coincidentally measuring the ionic fragments of doubly or triply charged parent ions. The pathway O(2+)2 ? O (+) O(+)?proceeds through some excited electronic states. The KERs exhibit definite structures independent of the laser intensity and the pulse duration. However, the angular distribution of coincident O(+) reflects the symmetry of the highest occupied molecular orbital (HOMO) only for few-cycle laser pulses at low intensity. The pathways O(2+) 2 --> O(2+) +O and O(3+) 2 --> O(2+) + O(+) occur through some repulsive states. The KERs show a single broad peak and decrease with increasing the pulse duration. The decrease of KER comes from the stretch of the internuclear distance in intense laser fields.

Spectra of the triply charged ion CS23+ and selectivity in molecular Auger effects

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

Triple ionization spectra by coincidence measurements of double Auger decay: The case of OCS

By combining multiple electron coincidence detection with ionization by synchrotron radiation, we have obtained resolved spectra of the OCS(3+) ion created through the double Auger effect. The form of the spectra depends critically on the identity of the atom bearing the initial hole. High and intermediate level electron structure calculations lead to an assignment of the resolved spectrum from ionization via the S 2p hole. From the analysis it appears that the double Auger effect from closed shell molecules favors formation of doublet states over quartet states. Molecular field effects in the double Auger effect are similar to those in the single Auger effect in linear molecules.

Triple ionisation of methane by double Auger and related pathways

Triple ionisation of methane by decay of the singly charged ion with a 1s vacancy produces a trication spectrum starting near 70 eV binding energy. Vibrational excitation in the initial hole state broadens and shifts the triple ionisation bands. Ionisation through core-valence doubly ionised states gives lower triple ionisation onsets and changes the spectral intensity pattern in accordance with retention of the initial valence holes in course of the double Auger effect. The double Auger effects occur both directly and as cascades, the different pathways producing different electron distributions and final state spectra.

Polarization dependence in non-resonant photo-triple-ionization of CO2

We investigate the angular dependence of the triple fragmentation of CO3+2, formed by the absorption of linearly polarized 200 eV photons. Complete momentum mapping of all fragment ions (C+, O+, O+) enables us to determine the plane of fragmentation and its orientation w.r.t. the polarization axis. While the C+ ions are found to be preferentially ejected perpendicular to the polarization axis, the plane spanned by the velocity vectors of the three fragments is found to contain the polarization axis. The distribution of the angles between the velocity vectors is mapped to the O = C = O bond angle of the precursor under the Coulomb explosion model. The mapping shows that the triple ionization at this energy proceeds from the normal state of CO2. In the light of this we are led to the surprising conclusion that non-resonant photo-triple-ionization preferentially occurs in those CO2 molecules whose C2v symmetry axis is nearly perpendicular to the polarization axis.