Relative Ionization Cross Sections Research Articles

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).

Single- and double-electron processes in collisions of Xe23+ions with helium

We report the measurements of relative cross sections for single capture (SC), double capture (DC), single ionization (SI), double ionization (DI), and transfer ionization (TI) in collisions of Xe${}^{23+}$ ions with helium atoms in the velocity range of $0.65\ensuremath{-}1.32$ a.u. The relative cross sections show a weak velocity dependence. The cross-section ratio of double- (DE) to single-electron (SE) removal from He, ${\ensuremath{\sigma}}_{\mathrm{DE}}/{\ensuremath{\sigma}}_{\mathrm{SE}}$, is about $0.45$. Single capture is the dominant reaction channel which is followed by transfer ionization, while only very small probabilities are found for pure ionization and double capture. The present experimental data are in satisfactory agreement with the estimations by the extended classical over-barrier (ECB) model.

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.

Absolute cross sections for the ionization-excitation of helium by electron impact

In a recent publication we presented detailed experimental and theoretical results for the electron-impact-induced ionization of ground-state helium atoms. The purpose of that work was to refine theoretical approaches and provide further insight into the Coulomb four-body problem. Cross section ratios were presented for transitions leading to excited states, relative to those leading to the ground state, of the helium ion. We now build on that study by presenting individual relative triple-differential ionization cross sections (TDCSs) for an additional body of experimental data measured at lower values of scattered-electron energies. This has been facilitated through the development of new electron-gun optics which enables us to accurately characterize the spectrometer transmission at low energies. The experimental results are compared to calculations resulting from a number of different approaches. For ionization leading to ${\mathrm{He}}^{+}{(1{s}^{2})}^{1}S$, cross sections are calculated by the highly accurate convergent close-coupling (CCC) method. The CCC data are used to place the relative experimental data on to an absolute scale. TDCSs describing transitions to the excited states are calculated through three different approaches, namely, through a hybrid distorted-$\text{wave}+R$-matrix (close-coupling) model, through the recently developed four-body distorted-wave model, and by a first Born approximation calculation. Comparison of the first- and second-order theories with experiment allows for the accuracy of the different theoretical approaches to be assessed and gives insight into which physical aspects of the problem are most important to accurately model.

Kinetic‐energy‐sensitive mass spectrometry for separation of different ions with the same m/z value

A double-focusing mass spectrometer (MS) equipped with a superconducting-tunnel-junction (STJ) detector has been applied to measure relative ionization cross-sections for the production of ions that are accompanied by different ion species with the same mass-to-charge (m/z) value. The STJ detector fabricated for this study enables kinetic energy (E) measurement of incoming individual ions at a counting rate of up to approximately 100 k ions/s and an energy resolution (DeltaE/E) of 15%. Both high counting rate and high-energy resolution are necessary to independently determine both m and z and not the m/z value only in ion-counting MS experiments. Ions such as (14)N(2) (2+) and (14)N(+) with the same m/z value can be clearly discriminated using a kinetic-energy-sensitive MS. This fine discrimination capability allows direct determination of relative ionization cross-sections of the homonuclear diatomic ions (14)N(2) (2+)/(14)N(2) (+) and (16)O(2) (2+)/(16)O(2) (+), which are difficult to measure due to the strong interference by the signals of their dissociated atomic ions with noticeably large ionization cross-sections. The new instrument requires no low-abundance heteronuclear diatomic molecules of the forms (14)N(15)N or (16)O(17)O to carry out ionization studies and thus, is expected to be useful in fields such as atmospheric science, interstellar science, or plasma physics.

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.

Multiple ionization and fragmentation of fullerene dimers by highly charged ion impact

We have studied the stabilities of multiply charged van der Waals dimers in slow Xe30+ + [C60]2([C60C70], [C70]2) → Xe(30-s)+ + [C60]2r+([C60C70]r+, [C70]2r+) + (r-s)e-electron-transfer collisions (v = 0.4 a.u. and r ≤ 7). The relative ionization cross sections display even-odd variations as functions of r for [C60]2r+, [C60C70]r+, and [C70]2r+. This is in clear contrast to the typical smooth decreasing behavior of the cross sections for multiple ionization of fullerene monomers, which can be explained within the framework of the classical over-the-barrier model. In addition, we report the branching ratios and the kinetic energy releases for the dominant fragmentation processes that yield intact fullerenes. The experimental results are discussed in view of a simple electrostatic model for dimer ionization and recent results from high level density functional theory calculations.

B 2 1 ( Σ u + 1 ) excited state decay dynamics in CS2

The authors report time resolved photoelectron spectra of the B21(Σu+1) state of CS2 at pump wavelengths in the region of 200nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors’ data reveals a rapid decay pathway (τ<50fs), in addition to a longer lived channel (τ∼350–650fs) that displays a marked change in apparent lifetime when the polarization of the pump laser is rotated with respect to that of the probe. Since the initially populated B21(Σu+1) state may decay to form either S(D1) or S(P3) products (the latter produced via a spin-orbit induced crossing from a singlet to a triplet electronic surface), this lifetime observation may be rationalized in terms of changes in the relative ionization cross section of these singlet and triplet states of CS2 as a function of laser polarization geometry. The experimentally observed lifetime of the longer lived channel is therefore a superposition of these two pathways, both of which decay on very similar time scales.

K-shell and total ionization cross sections following electron-molecule collisions: An empirical scaling law

Collisions between electrons and various molecular targets (H{sub 2}O, CH{sub 4}, C{sub 3}H{sub 4}, N{sub 2}) at projectile energies above the K-shell ionization threshold of the molecule have been investigated experimentally. From electron emission spectra, relative total ionization cross section {sigma}{sub t} and K-shell ionization cross section {sigma}{sub K} are determined. The ratio {sigma}{sub K}/{sigma}{sub t} is then deduced for each target as a function of the projectile energy and compared with those evaluated in the case of atomic targets. Strong differences between atomic and molecular targets are observed in the slope of the ratio at the highest projectile energies. These differences are explained using the well-known Kim-Rudd formula developed for atomic targets. In the projectile energy range we explored, we develop a simple empirical scaling law for the ratio {sigma}{sub K}/{sigma}{sub t} as a function of the projectile energy.

Ion-pair production due to electron-impact dissociative ionization of sulfur hexafluoride molecule

Using the crossed molecular and electron beams technique and the mass-spectrometric analysis of the reaction products, electron-impact dissociative ionization of the sulfur hexafluoride ( SF 6 ) molecule was studied within the incident electron energy range from threshold up to 150 eV. The simultaneous production of the ion fragment pair SF 3 + , F + was observed confirming the mechanism of intermediate doubly charged SF 4 2 + ion formation suggested earlier by several authors. The relative dissociative ionization cross sections were normalized to the absolute scale using known experimental data.

Fast heavy ion collisions with C 60: Collective excitation

Ionization and fragmentation of C 60 in collision with 5 MeV/μm O 6+ ions are studied using recoil ion ToF method. Relative ionization cross sections up to C 60 4+ are determined. The qualitative trend for different C 60 charge states was compared against simple plasmon excitation model.

Absolute electron-impact ionization cross sections of free radicals from threshold to 30eV: CHx=2,3, Si(CH3)x=1,2,3, and H2Si(CH3) radicals

We report absolute electron-impact ionization cross sections from threshold to 30eV for the formation of the parent ions from free radicals. A microwave discharge is used as a source to produce a large amount of these free radicals, and measurements are mainly performed by means of mass spectrometry. We detail the method used to measure electron-impact ionization cross sections of the main radicals produced. First, this method is tested on well known radicals such as CH3 and CH2, produced in a discharge sustained in an Ar–CH4 gas mixture; then it is applied to the free radicals Si(CH3)3,2,1 and H2Si(CH3), which are produced in a discharge sustained in Ar–Si(CH3)4. The experimental results are compared with the calculations performed using the binary-encounter-Bethe model and using a simplified form of this theory, the total ionization cross section model. Absolute electron-impact ionization cross sections have been measured at 30eV for Si(CH3) and CH3 by means of the selective energy transfer reaction process between Ar(P23) metastable species and the tetramethylsilane molecule [Si(CH3)4]. The value measured for Si(CH3) is used as the reference to calculate absolute values from relative ionization cross sections measured for the other radicals, Si(CH3)3,2,1 and H2Si(CH3), from threshold to 30eV. For example, we find at 30eV the absolute cross section values, 13.9×10−20±1.8×10−20, 2.0×10−20±0.3×10−20, 1.5×10−20±0.1×10−20, and 12.8×10−20±1.7×10−20m2 for m∕z=73, 58, 43, and 45, respectively.

Observation of the spin–orbit activated interchannel coupling in the 3d photoionization of caesium atoms

The ionization cross-section of the 3d spin–orbit components of the Cs atom has been measured from about 12 to 70 eV above their respective thresholds. The measured relative ionization cross-section of the 3d5/2 channel exhibits a pronounced minimum above threshold followed by a second maximum near the 3d3/2 ionization onset and thus qualitatively confirms the theoretical predictions of a spin–orbit activated interchannel coupling (Amusia et al 2002 Phys. Rev. Lett 88 093002).

Multidetection (e,2e) electron spectrometer

An electron-pair coincidence spectrometer, using only a single electrostatic toroidal analyzer, has been developed for the study of final-state electron pairs following ionization by electron impact. The system allows the angular and energy distribution of electron pairs emitted in a selected plane to be detected by a single planar microchannel plate stack followed by two position-sensitive semicircular wedge-and-strip anodes. The output signals are decoded into energy, angle and nanosecond coincidence timing information from which relative energy and angular triple differential ionization cross sections are determined. The performance of the instrument is indicated by He I photoionization of krypton and the first measurements of differential cross sections for electron-impact ionization of the P3∕22 and P1∕22 states of krypton. For perpendicular-plane scattering and an excess energy of 10 eV for the P3∕22 state and 0.67 eV less for the P1∕22 state, these cross sections are significantly different for the two states and indicate the importance of the final state of the ion in the scattering process.

Photoelectron spectroscopy of irontricarbonyl complexes of vinylnaphthalenes: Quantum-chemical calculations by the density functional method

The HeI photoelectron spectra of the complexes η4-1-vinylnaphthalene-Fe(CO)3 and η 4-2-vinylnaphthalene-Fe(CO)3 and the free organic ligands 1-vinylnaphthalene and 2-vinylnaphthalene are measured. The spectra are interpreted on the basis of quantum-chemical calculations by the density functional method in the SVWN/6-31G(d) local density approximation and with the B3LYP/6-31G(d) hybrid exchange-correlation functional. The Kohn-Sham orbital energies with calibration corrections are shown to reproduce well the vertical ionization potentials of both the irontricarbonyl π complexes under study and their ligands. The relative ionization cross sections σi of the valence orbitals of the complexes are calculated in terms of the Gelius model. These quantities prove to agree perfectly with the relative ionization cross sections σ i exp determined experimentally from direct estimation of the band areas of the photoelectron spectra.

Core level ionization dynamics in small molecules studied by x-ray-emission threshold-electron coincidence spectroscopy

X-ray-emission threshold-electron coincidence spectroscopy has been applied to ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}\mathrm{O}$. The main features of the spectra can be interpreted as conventional core level threshold electron spectra, free from postcollision interaction effects. The relative cross sections for adiabatic ionization of close-lying core hole states and vibrational substates are presented. The results are compared to theoretical predictions and state-of-the-art photoelectron spectra and are discussed in terms of direct threshold ionization and core vacancy rearrangement processes.

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.

Photoelectron spectroscopy of molecular systems and quantum-chemical calculations in terms of the density functional theory: Iron π complexes L-Fe(CO)3

The HeI and NeI photoelectron spectra of irontricarboniyl complexes with cyclic diene ligands—αmethylstyrene, orthoquinodimethane, and cyclooctatetraene—were obtained. The results of quantum-chemical calculations of the molecules of these compounds in the approximation of the density functional theory (B3LYP/6-31G*) are presented. It is shown that this approximation describes well the excited ionic states of the π complexes under study. The relative ionization cross sections σπ and σ3d , which characterize the probability of removal of electrons from the molecular π-ligand and 3d-metal orbitals, are estimated. The mechanism of the selective coordination of the Fe(CO)3 groups to corresponding organic ligands is discussed. The energies of the L-Fe(CO)3 chemical bond are calculated.

Single ionization of calcium by electron impact

Relative cross sections for the single ionization of calcium were measured inthe energy range from threshold to 100.7 eV using a time-of-flight iondetection method. Simultaneous detection of the ions from ionization of watermolecules was used to monitor and eliminate energy-dependent effects. Presentcross sections are normalized at 40 eV using previous experimental data.A comparison is made with existing partial cross sections for Ca+as well as total ionization cross sections to highlight the significant differencewhich exists in the energy shape and magnitude of the cross section maximum.