Relative Partial Ionization Cross Sections Research Articles

Ionization of acetonitrile

The ionization of acetonitrile has been studied using both electron and photon ionization combined with coincidence detection techniques. Relative partial ionization cross sections and relative precursor-specific partial ionization cross-sections are determined for all the cations formed from single, double and triple electron ionization of CH3CN from 30 to 200 eV electron energy. These cross sections characterize, distinguish and quantify the positive ions formed from these levels of ionization over the electron energy range investigated. To complement these electron ionization studies, relative ion yields for the formation of single ions (both monocations and dications) and monocation pairs have been measured following photoionization of CH3CN from 20 to 42 eV using synchrotron radiation. There is a strong similarity between the photoionization data and electron ionization data.

Ionic fragmentation of the CO molecule by impact of 10-keV electrons: Kinetic-energy-release distributions

The ionic fragmentation of a multiply charged CO molecule is studied under impact of 10-keV electrons using recoil-ion momentum spectroscopy. The kinetic-energy-release distributions for the various fragmentation channels arising from the dissociation of CO${}^{q+}$ ($q$ = 2--4) are measured and discussed in light of theoretical calculations available in the literature. It is observed that the present kinetic-energy-release values are much smaller than those predicted by the Coulomb explosion model. The kinetic-energy-release distribution for the C${}^{+}$+O${}^{+}$ channel is suggested to arise from the tunneling process. It is seen that the peak of kinetic-energy-release distribution is larger for that dissociation channel that arises from the same molecular ion which has higher charge on the oxygen atom. Further, the relative ionic fractions for seven ion species originating from ionization and subsequent dissociation of the CO molecule are obtained and compared with the existing data reported at low energy of the electron impact. The precursor-specific relative partial ionization cross sections are also obtained and shown to be about 66.4$%$ from single ionization, 29.9$%$ from double ionization, 3.3$%$ from triple ionization, and about 0.4$%$ from quadruple ionization of the precursor CO molecule contributing to the total fragment ion yield.

Partial ionization cross-sections of H2O molecule by 10–25 keV electron ionization

The relative ionic fractions and the precursor-specific relative partial ionization cross-sections for the fragment ions formed in direct- and dissociative ionization of H2O molecule have been measured for the first time at 10–25keV electron energies using the multiple ion coincidence imaging technique. We have identified six ions (H2O+, OH+, H+, O+, O2+ and H2+) from the ionization of H2O molecule and have found that the relative ionic fractions for these fragments are almost invariant of impact energy. From multiple-ion coincidence spectra, we have identified three distinct fragmentation channels: namely, H++OH+, H++O++H and H++H++O resulting from dissociation of H2O2+ and two channels: H++O2++H and H++H++O+ resulting from the dissociation of H2O3+. From the analysis, we have estimated that about 94.4% of ‘single ionization’, 5.3% of ‘double ionization’ and 0.3% of ‘triple ionization’ of the parent molecule contribute to the total ion yield. It is further noted that the non-dominant fragment ions O2+ and H2+ possess appreciably larger relative ionic fractions than those obtained at lower impact energies.

Momentum spectroscopy of fragment ions of a multiply charged N2O molecule under impact of 10-keV electrons

The dissociative ionization of a N${}_{2}$O molecule is studied at an electron energy of 10 keV using the multiple-ion-coincidence imaging technique. The complete as well as the incomplete Coulomb explosion pathways for N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions are examined and identified. The precursor-specific relative partial ionization cross sections for resulting fragment ions are obtained. It is found that about 81.8$%$ of single ionization, 17.8$%$ of double ionization, and about 0.4$%$ of triple ionization of the parent molecule contribute to the total fragment ion yield. Furthermore, the relative ionic fractions for ion-pair and ion-triple formation are also determined. The kinetic energy release distributions for different coincidence channels are obtained and compared with the available data at lower energies of electron and photon impacts and with that of high-energy ion impact. From the angular correlation studies of fragment ions, it is inferred that states with bent geometries are involved for most of the fragmentation channels of N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions. The concerted and/or sequential nature of the six dissociation pathways is also assigned. No other experimental or theoretical data exist in the literature to compare with the results obtained at the considered impact energy.

Relative partial ionization cross sections of N2O under 10–25-keV electron impact

The relative partial ionization cross sections for the fragment ions produced in direct and dissociative ionization of a N${}_{2}$O molecule are measured for impact of 10--25-keV electrons by using an electron-ion-coincidence technique with a linear time-of-flight spectrometer. The six ionic fragments of N${}_{2}$O (N${}_{2}$O${}^{+}$, NO${}^{+}$, N${}_{2}$${}^{+}$, O${}^{+}$, N${}^{+}$, and N${}^{2+}$ $+$ O${}^{2+}$) are observed and identified. The impact energy dependence of the partial ionization cross sections for these ions is expressed relative to the cross section of N${}_{2}$O${}^{+}$ and is found to be nearly invariant. The relative ionic fractions for the produced ions of N${}_{2}$O are also obtained and compared with the earlier reported data available at lower energies of electron impact. It is found that the relative ionic fractions for singly charged fragments are almost energy independent. However, for the doubly charged fragment ions (N${}^{2+}$ $+$ O${}^{2+}$), the present data are found to be higher by almost a factor of four compared to the relative ionic fraction reported earlier at a very low impact energy.

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.

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).

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.

Electron ionization of H2O

Relative partial ionization cross-sections and precursor-specific relative partial ionization cross-sections for fragment ions formed by electron ionization of H2O have been measured using time-of-flight mass spectrometry coupled with a 2D ion coincidence technique. We report data for the formation of H+, H2+, O2+, O+ and OH+ relative to the formation of H2O+, as a function of ionizing electron energy from 30 to 200eV. This data includes, for the first time, measurements on the formation all positive ion pairs and ion triples by dissociative multiple electron ionization of H2O. Through determinations of the kinetic energy release involved in ion pair formation we provide further evidence that indirect processes contribute significantly to the yield of H++OH+ ion pairs below the vertical double ionization threshold.

Electron ionization of CO 2

Relative partial ionization cross-sections (PICS) and precursor-specific relative partial ionization cross-sections for fragment ions formed by electron ionization of CO 2 have been measured using time-of-flight mass spectrometry (TOFMS) coupled with a 2-D ion-coincidence technique. We report data for the formation of C +, O +, CO +, C 2+, O 2+ and CO 2 2+ relative to the formation of CO 2 +, as a function of ionizing electron energy from 30 to 200 eV. This data includes, for the first time, measurements on the formation of ion triples by dissociative multiple ionization of CO 2 by electrons. Our analysis of the 2-D coincidence spectra provides new information on the dynamics and energetic of charge separating dissociation of the CO 2 dication and trication. Through determinations of the kinetic energy release (KER) involved in ion pair formation, we also conclude that indirect double ionization processes contribute significantly to the yield of CO + + O + ion pairs at ionizing electron energies below 55 eV.

Electron ionization of acetylene

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.

Electron ionization of N2O

Relative partial ionization cross sections and precursor specific partial ionization cross sections for electron ionization of N2O have been determined using time-of-flight mass spectrometry and ion–ion coincidence techniques. Cross sections are reported for ionizing energies from 30–200 eV. Data are presented for the formation of N2+, O2+, N+, O+, N2O2+, N2+ and NO+ relative to N2O+. Good agreement is found between our data and one set of recently published absolute partial ionization cross sections. However, some discrepancies are observed between the results presented here and two earlier determinations of the absolute partial ionization cross sections. We ascribe these discrepancies to incomplete collection of translationally energetic fragment ions in the earlier studies, providing additional evidence for the incomplete collection of translationally energetic product ions in some determinations of partial ionization cross sections. Our precursor specific cross sections allow the contribution from single, double and triple ionization to the individual fragment ion yields, following ionization of N2O, to be quantified for the first time.

Differential partial ionization cross sections for 10–24 keV electrons colliding withhelium andneon atoms

The relative differential partial ionization cross sections of helium and neon atomsfor 10–24 keV electron impact have been measured by using an ejectedelectron–ion coincidence technique. Measurements were accomplished byemploying a time-of-flight mass spectrometer for charge analysis of the ionsand a channel electron multiplier working in a pulse-counting mode fordetecting the slow electrons of non-discriminated energies ejected at 90°to the incident electron beam direction. The charge state fractionsFn of ions withcharge states nare found to be nearly invariant with the impact energy. The present resultsshow that the multiply charged ions of the target atoms are produced bydifferent mechanisms, namely by a direct multiple ionization or by inner shellionization followed by non-radiative transitions, for instance, by an Augerprocess combined with shake-off transitions. A good agreement is obtainedbetween our experimental results and the calculations based on inner shellionization cross sections folded with charge state abundances, which resultfrom the decay of the corresponding initial inner shell vacancies.

Partial dissociative ionization ofSF6by electron impact using an ejected electron-ion coincidence technique

The relative differential partial ionization cross sections for the production of ionic fragments of ${\mathrm{SF}}_{6}$ molecule under impact of 10--20 keV electrons have been measured using an ejected electron-produced ion coincidence technique in a crossed beam apparatus with a time-of-flight analysis of the ions. The detection angle of the ejected electrons of nondiscriminated energies was kept at 90\ifmmode^\circ\else\textdegree\fi{} with respect to the incident electron-beam direction. The 14 ionic fragments: ${\mathrm{SF}}_{5}^{+},$ ${\mathrm{SF}}_{4}^{+},$ ${\mathrm{SF}}_{3}^{+},$ ${\mathrm{SF}}_{2}^{+},$ ${\mathrm{SF}}^{+},$ ${\mathrm{S}}^{+},$ ${\mathrm{F}}^{+},$ ${\mathrm{SF}}_{4}^{2+},$ ${\mathrm{SF}}_{3}^{2+},$ ${\mathrm{SF}}_{2}^{2+},$ ${\mathrm{S}}^{2+},$ ${\mathrm{F}}^{2+},$ ${\mathrm{SF}}_{3}^{3+},$ and ${\mathrm{S}}^{3+}$ resulting from the dissociative ionization of the ${\mathrm{SF}}_{6}$ molecule were identified and their relative production cross sections have been measured. The branching ratios of 8 ionic fragments ${\mathrm{SF}}_{m}^{n+}$ $(m=1$ to 5 and $n=1$ to 3) as a function of impact energy have been determined. These ratios are found to have an almost a constant value over the considered impact energy range within the experimental uncertainty of the measurements. For the investigated impact energies, no previous data or theoretical calculations exist for a direct comparison with the present results.

Electron-impact ionization of hydrogen chloride

Relative partial ionization cross sections for the formation of Cl+, Cl2+ and H+ from the hydrogen chloride molecule have been determined, from simultaneously recorded mass and two-dimensional ion–ion coincidence spectra, as a function of the ionizing electron energy from 30 to 200 eV. Precursor specific cross sections have been derived which quantify the contribution to the yield of each fragment ion from each level of ionization and the kinetic energy released in the fragmentation reactions of multiply charged HCl has been used to obtain thermodynamic information on the doubly and triply charged states. Analysis shows that at 100 eV electron energy double ionization contributes 12% of the total ion yield and 3% at 50 eV. At 100 eV the decay of HCl2+ by the loss of an H atom to form Cl2+ is a result of approximately 41% of the double ionization events, comparable to the frequency of charge separating dissociation (52%). The formation of Cl2+ has a threshold of 46.8 ± 1.5 eV and is assigned to population of the c5Σ− dication state. The observed threshold for forming long-lived dications (HCl2+) is consistent with the population of the ground state of the dication, and higher vibrational levels of this state appear to contribute to the yield of H+ + Cl+ ion pairs. The formation of metastable HCl2+ contributes a maximum of 0.6% to the ion yield. Long-lived HCl3+ ions are not detected but dissociative triple ionization makes up approximately 0.7% of the ion yield at 200 eV. We estimate the lowest lying dissociative state of HCl3+ lies at 72 ± 2 eV above the ground state of the neutral molecule.

Electron-impact ionization of the chlorine molecule

Relative partial ionization cross sections for the formation of Cl2+, Cl+ and Cl2+ from molecular chlorine have been recorded as a function of the ionizing electron energy. In these measurements particular attention has been paid to the efficient collection of fragment ions with high translational energies and the minimization of any mass-dependent discrimination effects. The cross sections show that at electron energies above the double ionization threshold the yield of fragment ions can be comparable with the ion yield of nondissociative ionization. Further analysis shows that at electron energies above 50 eV the yield of fragment ions from multiple ionization is comparable with the yield of fragment ions from single ionization: dissociative multiple ionization contributes 14% of the ion yield at 50 eV electron energy and 26% at 100 eV. The decay of Cl22+ by heterolytic cleavage to form Cl2+ is a result of approximately 5% of the dissociative double ionization events. This heterolytic process has a threshold of 41.8±1.5 eV. Electron-impact induced triple ionization to form long-lived Cl23+ ions has been detected for the first time. This nondissociative triple ionization process makes up approximately 2% of the triple ionization events and triple ionization is responsible for approximately 2% of the ion yield above 100 eV. The threshold for dissociative triple ionization is determined to be 65.3±1.5 eV, a value in good agreement with a trication precursor state energy derived from the kinetic energy release for the fragmentation of Cl23+ to Cl2+ and Cl+, which provides the first experimental estimate of the triple ionization energy of molecular chlorine.

Single and double electron-impact ionization of dichlorine monoxide (Cl 2O)

Electron-impact single and double ionization of dichlorine monoxide (Cl 2O) have been investigated using time-of-flight mass spectrometry and ion–ion coincidence techniques. Relative partial single ionization cross sections of Cl 2O have been determined for incident electron energies from 30 to 450 eV using time-of-flight mass spectrometry. The experiments detect stable parent ions, Cl 2O +, and the fragmentation products Cl +, ClO +, and Cl 2 +. The formation and fragmentation of the Cl 2O dication has been investigated using ion–ion coincidence techniques coupled with time-of-flight mass spectrometry. The coincidence spectra show that Cl 2O 2+ dissociates to form the following pairs of ions: Cl + + Cl +, O + + Cl + and Cl + + ClO +. Interpretation of the coincidence spectra suggests that there is a multiple kinetic energy release upon dicationic dissociation to form O + + Cl +, indicating that two distinct electronic states of Cl 2O 2+ dissociate to form this ion pair. The experiments also yield a determination of the excitation energy required to form the lowest energy dication electronic state (30.6 ± 1 eV) which dissociates to form Cl + + ClO +. Comparison of this excitation energy with estimated values of the double ionization energy of Cl 2O indicates that this state is probably the ground electronic state of Cl 2O 2+ and, hence, these investigations provide a first estimate for the double ionization energy of Cl 2O.

Single and double electron-impact ionization of chlorine dioxide

Electron-impact single and double ionization of chlorine dioxide, OClO, have been investigated using time-of-flight mass spectrometry and ion–ion coincidence techniques. Relative partial single ionization cross sections of OClO have been determined for incident electron energies from 30 to 450 eV using time-of-flight mass spectrometry. The experiments detect stable parent ions, OClO+, and the fragmentation products O+, O2+, Cl+ and ClO+. The formation and fragmentation of the OClO dication has been investigated using ion–ion coincidence techniques coupled with time-of-flight mass spectrometry. The coincidence spectra show that OClO2+ dissociates to form the following pairs of ions: O++ClO+ and O++Cl+. Interpretation of the coincidence spectra suggests that there is a multiple kinetic energy release upon dicationic dissociation to form the O++Cl+ ion pair, and also yields a determination of the excitation energy required to form the lowest energy dication electronic states which dissociate to form O++ClO+ and O++Cl+. The similarity of these excitation energies indicate that these dissociation reactions might occur from the same electronic state of OClO2+. Comparison of these excitation energies with estimated values of the double ionization energy of OClO indicate that this state is probably the ground electronic state of OClO2+ and, hence, these investigations provide a first estimate for the double ionization energy of OClO.

Electron-impact ionization of nitric acid

Relative partial ionization cross sections of nitric acid have been determined for incident electron energies from 40 to 450 eV using time-of-flight mass spectrometry. The experiments detect stable parent ions, HNO2, and the fragmentation products H +; N +; O +; OH +; NO + and NO 2- +. The ionic decay channel involving the formation of H + has been observed for the first time and the appearance energy of this fragment has been determined to lie between 55 and 60 eV.