Double Electron Loss Research Articles

Absolute cross sections for electron capture and loss of O+ in water molecule

SynopsisAbsolute cross sections for single and double electron loss and single and double electron capture for O+ projectiles colliding with water vapor have been measured. In addition, we have performed many-body clas-sical trajectory Monte Carlo calculations, where the O+ projectile was either one or more particle, depending on the model used. The water target is modeled as two virtual particles. The calculated cross sections are in agree-ment with the experimental data.

Triple-coincidence experiment to study the correlated electron emission in the collision-induced double electron loss of He−

The double electron loss (DEL) from ${\mathrm{He}}^{\ensuremath{-}}$ has been investigated in collisions with He atoms at 300-keV impact energy and at electron energies that satisfy the cusp condition; i.e., the corresponding electron velocities match the projectile's velocity. In a time-of-flight experiment the energies of both electrons have been determined by detecting triple coincidences between the two electrons and the outgoing ${\mathrm{He}}^{+}$ ion. The coincidence yield shows a peak as a function of the electron energies at the expected cusp position, providing evidence for the existence of the two-electron cusp [L. Sarkadi and A. Orb\'an, Phys. Rev. Lett. 100, 133201 (2008)] in DEL. The observed energy distribution of the electron pairs is compared with the results of four-body classical trajectory Monte Carlo calculations carried out applying the soft-Coulomb potential approximation for the inclusion of the electron-electron interaction.

Electron capture and loss of O+ projectile in collision with water near the Bragg Peak Energies

The combined experimental and theoretical works of the study of the absolute single and double electron loss (stripping) and single and double electron capture cross sections for 0.2–1.2 MeV O+ projectiles colliding with water molecule are presented. In order to mimic the experimental observation we performed three, four and five-body classical trajectory Monte Carlo calculations. In our model the O+ projectile is either one, (two or three) particle(s) depending on the model used (3, 4 or 5 body model, respectively) and the water target is taken into account as two virtual particles. We found that the calculated cross sections are in agreement with the present experimental data. Moreover, the similarity of the present data with an isoelectronic system, the methane molecule allows us to draw a common curve through both data sets.

Scaling law investigation on dissociation ratios of N2O, CF4, SF6 under 15–30 keV H−, D−, C−, O− impact

The relative dissociation ratios of N2O, CF4, and SF6 are measured under 15–30 keV D− impact. The recoil ions emitted from target molecules are detected and recorded by a time of flight mass spectrometer, in coincidence with the detection of scattered projectiles in two channels, single electron loss and double electron loss, respectively. Together with the data of H−, C−, O− incidence, the ratios of atomic ions produced from target molecules can be scaled by nuclear charges of the incident negative ions (Z) and their impact velocities (v). The ratios divided by Z or Z1/3 or Z2/3 all change linearly with v. Moreover, compared with the center atoms in the target molecule, dissociation of the outer atoms show greater dependence on the nuclear charges of incident negative ions.

Relative dissociation fractions ofN2Ounder15–30−keVH−,C−, andO−negative-ion impact

The relative dissociation fractions of ${\mathrm{N}}_{2}\mathrm{O}$ are studied under 15--30-keV negative ions ${\mathrm{H}}^{\ensuremath{-}},{\mathrm{C}}^{\ensuremath{-}}$, and ${\mathrm{O}}^{\ensuremath{-}}$ impact. The recoil ions and ion pairs originating from the target molecule ${\mathrm{N}}_{2}\mathrm{O}$ are detected and identified in coincidence with scattered ions in single electron loss (SL) and double electron loss (DL) channels using a time-of-flight mass spectrometer. The dissociation fractions for the production of the fragment ions are obtained. We find that the relative dissociation fractions in SL are smaller than those in DL, and the degree of fragmentation will become greater with the impact energy increasing. We also analyze the coincident TOF spectra of two fragment ions which are produced from dissociation of ${\mathrm{N}}_{2}{\mathrm{O}}^{2+}$ and give the possible dissociation pathways of ${\mathrm{N}}_{2}{\mathrm{O}}^{2+}$ with $15--30\text{\ensuremath{-}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}^{\ensuremath{-}},{\mathrm{C}}^{\ensuremath{-}}$, and ${\mathrm{O}}^{\ensuremath{-}}$ impact. There are many studies on ${\mathrm{N}}_{2}\mathrm{O}$ with positive-ion, photon, and electron impact, and we compare our results under negative-ion impact with those works.

Relative dissociation fractions of SF6under impact of 15-keV to 30-keV H−and C−negative ions

The relative dissociation fractions for the production of fragment ions and ion pairs of SF${}_{6}$ are studied for H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact in the energy range from 15 to 30 keV. Recoil ions (${\mathrm{SF}}_{4}^{+}$, SF${}_{3}^{+}$, SF${}_{2}^{+}$, SF${}^{+}$, S${}^{+}$, F${}^{+}$, SF${}_{4}^{2+}$, SF${}_{2}^{2+}$) and ion pairs $({\mathrm{SF}}_{3}^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}_{2}^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}^{+}+{\mathrm{F}}^{+},{\mathrm{S}}^{+}+{\mathrm{F}}^{+}$, F${}^{+}+{\mathrm{F}}^{+})$ are detected and identified in coincidence with scattered projectiles in two charge states ($q=0$ and $q=+1$) by using a time-of-flight spectrometer. The relative dissociation fractions are energy dependent for both single-electron-loss (SL) channel and double-electron-loss (DL) channel processes for certain negative ions. It is also found that the relative dissociation fractions for DL are larger than those for SL. In addition, the degree of fragmentation will become greater with a larger mass number of the projectiles at the same impact energy for the same electron-loss channel. A comparison of the time-of-flight spectra is made between that under negative-ion impact and that under electron impact, and it is found that the probability of production of SF${}_{n}^{+}$ ions with $n$ odd is higher than that of similar ions with $n$ even, and the probability of production of SF${}_{n}^{2+}$ ions with $n$ even is higher than that of similar ions with$n$ odd under H${}^{\ensuremath{-}}$, C${}^{\ensuremath{-}}$, positive-ion, and electron impact. We analyze this interesting phenomenon from the bond-dissociation energies of SF${}_{n}^{+}$ and SF${}_{n}^{2+}$. We also analyze the coincident time-of-flight spectra of two fragment ions resulting from double ionization of SF${}_{6}$ by H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact and describe the major dissociation pathways of SF${}_{6}^{2+}$ for H${}^{\ensuremath{-}}$ and C${}^{\ensuremath{-}}$ impact in the energy range from 15 to 30 keV.

Electron loss and capture from low-charge-state oxygen projectiles in methane

Absolute cross sections for single- and double-electron loss and single- and multiple-electron capture of 15–1000 keV oxygen projectiles (q = −1, 0, 1, 2) colliding with the methane molecule are presented. The experimental data are used to examine cross-section scaling characteristics for the electron loss of various projectiles. In addition, a modified version of the free-collision model was employed for the calculation of the single- and total-electron-loss cross sections of oxygen projectiles presented in this work. The comparison of the calculated cross sections with the present experimental data shows very good agreement for projectile velocities above 1.0 au. The comparison of the present single-electron-capture cross sections with other projectiles having the same charge shows good agreement, and a common curve can be drawn through the different data sets.

Ionization of noble gas by impact of hydrogen anions

Relative cross-sections for double ionization and single ionization of targets under the impact of hydrogen anions were measured. Two systems were investigated, H− projectiles with Kr and Xe targets. The energy region was 10–30 keV. Recoil ions originated from target (Kr+, Kr2+, Xe+, Xe2+) were detected in coincidence with projectiles in several final charge states (q = 0 and q = + 1). We compare the present results with the previous results of Ar. The results show that the cross-section ratios, taken between double and single ionization of Ar, Kr and X in the channels of single and double electron loss of H−, become larger and larger with the incident energy increasing, and that the cross-section ratios will cross at some certain energy for the two different electron loss channels. The crossing energy for Xe is about 23 keV.

Experimental study of ionization of Ar by impact of anions

We describe an experimental apparatus used to study the ionization of gas atoms or molecules in collision with anion projectiles. The setup mainly consists of a time of flight measurement system, a micro-channel plate (MCP) position sensitive detector, and an anode plate MCP detector. These different components of the setup are described in this paper. The single and double ionization processes for Ar under the impact of H anions are studied in the 10–30keV energy range. The results show that the cross section ratios, taken between double and single ionization of Ar for single and double electron loss of H−, become larger and larger with increasing incident energy in the 10–30keV energy range. We also compare our results with the existing H−+Ar data, and find a very interesting phenomenon that the cross section ratios will cross at some energy for the two different electron loss channels.

H2andN2ionization and dissociative ionization byC−andO−ions at intermediate velocities: Direct and electron loss channels

Cross sections for single ionization and dissociative ionization of molecular targets under the impact of atomic anions were measured. Three systems were investigated, a ${\mathrm{H}}_{2}$ target with ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{O}}^{\ensuremath{-}}$ projectiles, and a ${\mathrm{N}}_{2}$ target with ${\mathrm{O}}^{\ensuremath{-}}$ projectiles. The velocity range was 1.07--2.14 a.u. Recoil ions originated from the target (${\mathrm{H}}_{2}^{+},{\mathrm{H}}^{+},{\mathrm{N}}_{2}^{+},{\mathrm{N}}^{+}$, and ${\mathrm{N}}^{2+}$) were measured in coincidence with projectiles in several final charge states ($q=\ensuremath{-}1$, 0, $+1$, and $+2$). These states, negative, neutral, and positive, respectively, correspond to direct, single, and multiple electron loss channels. Target ionization is mostly due to the projectile single electron loss and direct processes, while target fragmentation is dominated by the projectile double electron loss. These results point to both target ionization and projectile direct or single electron loss processes being dominated by large impact parameters, while fragmentation and multiple electron loss are associated to small impact parameters.

Electron loss and dissociation in high energy collisions between multiply charged oligonucleotide anions and noble gases

We have measured electron loss cross sections and fragment spectra for collisions between multiply charged oligonucleotide anions and noble gas atoms. The ions were produced in an electrospray source and accelerated through a potential difference of 50 kV before collisions with the target gas. The collision cross sections depend on the charge state of the projectile ions and on the target gas. The fragment spectra provide information about the sequences of oligonucleotides. The major sequence ions are w and a−B ions. Radical ions as a result of electron loss lead to a ions and cleavage of the sugar ring. Double-electron loss is found to be the first step toward formation of b ions.

Electron loss from heavy heliumlike projectiles in ultrarelativistic collisions with many-electron atomic targets

We study single- and double-electron loss from heavy heliumlike projectiles in ultrarelativistic collisions with neutral many-electron target atoms. The simultaneous interaction of the target with two projectile electrons is found to be the dominant process in the double-electron loss provided the atomic number of the projectile, ${Z}_{p},$ that of the target, ${Z}_{t},$ and the collision velocity, $v,$ satisfy the condition ${Z}_{p}{Z}_{t}/vg0.4.$ It is shown that for a wide range of projectile and target atomic numbers the asymptotic double-to-single loss ratio strongly depends on the target atomic number but is nearly independent of the nuclear charge of the projectile. It is also demonstrated that many-photon exchange between the target and each of the projectile electrons considerably influences the double loss in collisions with very heavy targets.

How can a neutral atom capture an electron to continuum states?

We report on a systematic experimental study of the cusp peak associated with the ionization of the target (known as electron capture to the continuum, ECC) by impact of neutral atoms. In a measurement carried out for He 0 on Ar collisions, the dependence of ECC on the impact energy was investigated. The energy was varied between 18.9 and 250 keV/amu. The relative contributions (“fractions”) of the different processes (electron capture and loss to the continuum, double electron loss) to the cusp have been determined. For the ECC channel absolute doubly differential cross sections (DDCS) were also measured and compared to the corresponding DDCS values obtained with He + projectiles at the same experimental conditions. In another experiment an accurate measurement was performed for the shape of the ECC cusp by impact of goound-state and excited (metastable) He projectiles. The aim of this latter experiment was to check the validity of a model based on the formation of a low-lying virtual resonance state of the intermediate e − + He(2 1S) system. The case of the neutral hydrogen as projectile was also considered: the ECC cusp observed from 100 keV H 0 on He collisions is compared with the corresponding ECC cusp obtained with protons.

Single- and double-electron detachment from H- in collisions with He.

The single- and double-electron detachment processes have been studied for 85 keV H{sup {minus}} on He collisions measuring the energy spectra of the electrons emitted in forward direction. In the spectrum belonging to the single-electron loss (SEL) the nonresonant part (cusp) has been resolved from the resonant part [lines from the (2{ital s}2{ital p}){sup 1}{ital P}{sup {ital o}} shape resonance of H{sup {minus}}]. The ratio of the integrated yield of the double-electron loss (DEL) to that of SEL was found to be 0.36{plus_minus}0.02. The yield of the cusp in the SEL spectrum was found to be surprisingly small, only (70{plus_minus}20){percent} of the yield of the cusp in the DEL spectrum. The formation of the cusp in SEL is interpreted as a result of dipolar interaction between the electron and the outgoing H{sup 0} atom. {copyright} {ital 1996 The American Physical Society.}

Multiple-electron ionization, capture, and loss by 19-MeV Fq+ (q=2-9) in collisions with Ne and Ar.

Charge-exchange processes in fast collisions of F{sup {ital q}+} ({ital q}=2--9) on Ne and Ar gas targets have been studied using the projectile--recoil-ion coincidence method. The target ionization without a projectile charge change increases with increasing projectile charge approximately as {similar_to}{ital q}{sup 1.4}. This dependence is weaker than the {ital q}{sup 2} dependence predicted by the first Born approximation because of the large ionization probabilities involved. The single-electron-capture cross sections follow the known {ital q}{sup 3} scaling law, while the double-electron-capture cross sections display a {ital q}{sup 6} dependence, as predicted by the independent-electron approximation for small capture probabilities. Single- and double-electron loss from the {ital L} shell decreases with increasing {ital q} mainly because of the decrease in the number of {ital L}-shell electrons, while binding-energy effects play only a minor role. In contrast, the large sudden decrease in electron-loss cross sections once {ital K}-shell electrons must be removed is due to the large increase in binding energy.

Observation of collisionally induced (1s2p2p')4Pe shape resonance of He-

Autodetachment via the collisionally populated (1s2p2p'${)}^{4}$${\mathit{P}}^{\mathit{e}}$ shape resonance of ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ has been observed by means of 0\ifmmode^\circ\else\textdegree\fi{} electron spectroscopy in a measurement of the double-differential cross section for single and double electron loss to the continuum in ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ on Ar collisions at 75-keV/u projectile energy. An alternative parametrization has been developed for the autodetachment lines emitted with low energy in the projectile reference frame. The determined position and width of the shape resonance are in agreement with the results of theoretical calculations and photodetachment measurements.

Measurements of electron capture and loss cross sections in collisions of Ar + and Ar 2+ with He in the energy range of 50–300 keV

Cross sections for the single electron capture (σ 10), single electron loss (σ 12) and double electron loss (σ 13) in Ar + +He collisions in the energy range 50–250 keV are presented. Also cross sections for the single electron capture (σ 21), double electron capture (σ 20) and single electron loss (σ 23) in the Ar 2+ + He system in the collision energy range of 100–300 keV are reported. The growth curve technique has been used in the data analysis for obtaining these cross sections. The present measurements are compared with some available data.

Cross Sections of Charge Exchange for Fast He Ions Passing through Zn Vapor

Single and double electron loss and capture cross sections have been measured for He 0 , He 1+ and He 2+ ions passing through a Zn vapor target at energies ranging from 0.8 to 2.0 MeV. Uncertainties of the cross sections were improved by applying the Rutherford scattering method for the determination of the target thickness. When combined with other previous experimental data, obtained cross sections support the oscillatory dependence on target atomic number. In addition, the equilibrium mean charges of He ions were compared for solid and gas phases of the Zn element, but no density effect has been observed.

Measurement of charge-changing cross sections in collisions of He and He+ with H2, O2, CH4, CO and CO2

Single- and double-electron loss cross sections of He, and single-electron capture and single-electron loss cross sections of He+ in collisions with H2, O2, CH4, CO and CO2 have been measured in the energy range 0.3-1.8 MeV. From the measured cross sections, charge-changing cross sections in collisions of He and He+ with C have been determined using the Bragg rule. Scaling is examined for electron capture cross sections.

Collisional formation and destruction of fast negative hydrogen ions in He, Ne, and Ar targets.

Measurements are reported of the cross sections for collisional destruction (single- and double-electron loss) of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ and for its formation through single- and double-electron capture by H and ${\mathrm{H}}^{+}$, respectively. Three different targets were employed (He, Ne, and Ar) and the velocity (in a.u.) of the projectiles was 2.4\ensuremath{\le}v\ensuremath{\le}12.6 for ${\mathrm{H}}^{+}$ and 4<v<9 for H and ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$. The charge states emerging from the target were simultaneously detected and the data were analyzed by the growth rate and attenuation methods from which individual cross sections could be deduced. Special attention was paid to the double-capture process in Ar where a very pronounced shell effect was firmly established on experimental grounds and was successfully described by theoretical estimates based on independent-electron probabilities of capture and simple scaling rules. Difficulties for obtaining an overall description of the cross sections for either destruction or formation of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ based on simple models are discussed.