Electron Capture In Collisions Research Articles

Statistical aspects of ion-atom collisions

The maximum-entropy description of ion-atom collisions is reviewed. The method is illustrated by recent analyses of recoil charge distributions is electron-capture collisions and of final-state charge distributions in charge transfer from Rydberg atoms.

Electron capture in collisions of O 8+ with H: Absolute line emission cross sections

We present the first experimental determination of absolute line emission cross sections, resulting from charge changing collisions of bare oxygen ions with atomic hydrogen. The O VIII lines resulting from the dominantly populated n=5 level confirm to a large extent the almost coinciding theoretical data obtained with an MO approach by Shipsey et al. and with an AO+ approach by Fritsch and Lin. Emission from the non-dominantly populated n = 6 level is found to be stronger than predicted by theory and favours the MO data.

Double ionisation energy of methane measured using a double electron capture technique

The vertical double ionisation energy of methane is determined to be 35.6+or-0.25 eV by means of an experimental technique in which the kinetic energy spectrum is measured of product negative ions resulting from double electron capture collisions of OH+, O+ and Cl+ ions with neutral CH4 at 6 keV impact energy.

Near-threshold translational energy spectroscopy (NT TES)

Unknown reaction channels in the translational energy spectrum of Ar2+ ions produced in Ar3 or=+/Ar electron capture collisions are identified by reducing the electron energy in the ion source below the threshold values for the production of different excited Ar3+ projectile states. The advantages of this near-threshold translational energy spectroscopy method, as well as final results for the Ar3+/Ar collision system, are described.

Post-collision line broadening of K-LL Auger transitions induced by electron capture to C5+ions at energies of 25 and 100 keV

K-LL Auger electron emission from C3+ ions induced by electron capture collisions of C4+ and C5+ ions on Xe, H2 and He atoms at energies of 25 and 100 keV was measured at an observation angle of 0 degrees . The peak of promptly decaying (1s2s2) 2S states directly produced in C5+-He collisions exhibited a pronounced low-energy tail, which was attributed to post-collision interactions (PCI) with He2+ ions recoiling at defined relative velocities. From the peak shape a lifetime of T=1.2+or-0.2*10-14s was determined for the (1s2s2) 2S state, which is in agreement with recent calculations which give values for T of 1.337*10-14s.

State-selected electron capture by molecular ions: collisions of CS22+and CS23+with monatomic and diatomic targets

High resolution translation energy change spectra of product ions of electron capture collisions of CS22+ and CS23+ with rare-gas atoms and diatomic molecules, H2, N2 and O2, have been measured using a modified, reversed geometry, double focusing mass spectrometer. Electron capture occurs selectively into the ground electronic state except in the case of CS23+ collisions with Xe and O2. In the former case significant target excitation occurs whereas in the latter case the predominant reaction result is dissociative electron capture.

Subshell-selective electron capture in collisions of C 4+ (0.05 a.u. < V < 0.40 a.u.) with H and H 2

Relative subshell selective single electron capture cross sections for collisions of C 4+ with molecular and atomic hydrogen have been measured by means of VUV spectrometry in the energy range of incident ion beams from 10 q keV down to 0.15 q keV. Despite the very low ion beam currents (down to a few nA), optical measurements were possible with a VUV photon position sensitive detector and results are in good agreement with theoretical calculations in the low energy range.

Traveling-molecular-orbital-expansion studies of electron capture in collisions of fully stripped ions (Z=6-9) with H and H2.

The traveling-molecular-orbital-expansion description has been employed to investigate electron capture in collisions of fully stripped ions (Z = 6--9) with H and H/sub 2/ in the energy regime from 0.14 to 9 keV/amu. The pseudopotential method was used to represent the H/sub 2/ /sup +/ core which enables us to treat the H/sub 2/ molecule as an atom having an ionization potential for H/sub 2/..-->..H/sub 2/ /sup +/. Features of the adiabatic potentials and corresponding radial coupling matrix elements at the ''reaction window'' as well as the quantitative results of relatively small-scale close-coupling calculations provide a rationale for the interpretation of a recent measurement for these systems by Meyer et al. (Phys. Rev. A 32, 3310 (1985)). For collisions of O/sup 8+/ with H, an extended molecular-orbital-expansion approach has been employed (i) to seek an understanding of the discrepancy evident in two recent independent large-scale calculations for n = 6 partial cross sections, and (ii) to obtain a quantitative test for the mechanism suggested in this paper.

Energy-gain spectroscopy of electron-capture collisions between low-energy Ar and Ne projectiles and atomic and molecular deuterium targets

Translational energy-gain spectroscopy is used to determine the final-state (nl) populations following single-electron capture by ${\mathrm{Ar}}^{\mathrm{q}+}$ (q=4--8) and ${\mathrm{Ne}}^{\mathrm{q}+}$ (q=4--7) projectiles in atomic and molecular deuterium targets at an energy of 545q eV. The importance of kinematics in analyzing these systems is discussed. The final-state populations are found to be very sensitive to the energy-level structure of the collision system. The results are interpreted in terms of a ``reaction window'' in the energy gain which is calculated using a multichannel Landau-Zener model.

State-diagnosed electron capture collisions of CS2q+(q=2, 3) with atomic and molecular gases

Relative electron capture cross sections have been measured for CS2q+ (q=2, 3) ions colliding with atomic and molecular targets at 3q keV translational energy. Translational energy gain/loss spectroscopy indicates that capture is predominantly into the ground electronic state. In the case of Cs22+ projectiles, a reaction window occurs for avoided crossings in the range 3-8 AA; for CS23+ projectiles, the reaction window becomes extremely narrow and shifts towards the region of 1.5 AA.

Semiclassical complex energy theory of orbiting resonances in curve crossing systems

The authors discuss orbiting resonances in two-state curve crossing systems. A semiclassical quantisation condition determining complex-energy resonance poles is derived. Applications to the electron capture collision process N3++H to N2++H+ show good agreement with recent exact quantum computations.

Electron capture-induced decomposition (ECID): A new method to study electron-capture collisions

A new method for the study of electron-capture collisions of doubly charged polyatomic ions is discussed. In the collision, not only electron transfer, but also excitation may take place. If so, the singly charged product ion will fragment and this fragmentation will be characteristic of both its structure and its internal energy. The principle of the experiment is to select a doubly charged ion beam, collide it on a neutral gas and look at the fragmentation following electron capture. The process, in analogy with collision-induced decomposition, may be called electron capture-induced decomposition (ECID). The difficulty with the experiment is that there are various interference reactions, which are discussed in detail. From the ECID process, a wide range of information on the collision complex, threshold energetics, the electronic states of the reactants, total and differential cross-sections, the internal energy distribution in the product ions and, above all, on the energy transfer in the collision can be learned. In connection with this, a new method to determine internal energy distribution is also discussed.

Single electron capture by Ar 3+ from rare-gas atoms

Translational energy spectra for single electron capture collisions of 9 keV Ar 3+ ions with He, Ne, Ar, Kr and Xe were measured, using a double-focusing mass spectrometer. The data show a clear trend that as target ionization potential decreases, the degree of excitation of the fast product ions increases. There is clear evidence of the presence of first and second metastable states in the primary ion beam. The cross-sections for single electron capture into particular states of Ar 2+, for the collision of Ar 3+ with He, are reported for the impact energy range 6–18 keV.

Reaction windows for electron capture by highly charged ions

It is pointed out that, when considering electron capture in collisions of highly charged ions with atoms or molecules, a qualitative discussion in terms of a common 'reaction window' is valid only when fully stripped ions are involved. In the case of partly stripped ions with well separated subshell levels it becomes essential to define individual reaction windows for the final states of the captured electron. These reaction windows generally appear at smaller internuclear separations than in the corresponding bare-ion case. The shift is particularly pronounced for final states of high angular momentum. This observation is supported by available experimental data.

Electron capture collisions of Kr2+(3P) in H2

Single-electron-capture cross sections for Kr2+-H2 collisions in the 1-6 keV energy range are determined using the initial growth curve technique. Energy defect measurements reveal that the incident Kr2+ beam is purely in the ground electronic state and that the product Kr+ ions are predominantly in the 2P and 2S states. Charge transfer occurs via avoided crossings of pseudo-molecular potential energy surfaces as well as by non-crossing, Demkov-type mechanisms (1964).

Recoil ion confinement in a radio frequency quadrupole ion trap

Multicharged Ne q+ recoil ions (3≤q≤8) arising from impact of 1 MeV/u Cl beam ions (mean charge near 11e) on Ne target gas have been produced and observed with good resolution in an axially symmetric radio frequency quadrupole ion trap. Investigations of the stability properties are in agreement with theory, while measurements of ion storage time constants, in comparison with those in other trap types, indicated that ion loss arose from electron capture collisions.

Theory and experiment of electron capture in collisions of multiply charged projectiles with light targets

In this joint theoretical and experimental effort, we examine in detail the capture channel in collisions of multiply charged projectiles with light targets. We report new high-resolution photon-spectroscopic results of electron capture of energetic (2–5 MeV) C 4+ ions with H 2 and He, from which we extract specific ( n, l) capture cross sections. The present study extends our previous work and demonstrates anew the superiority of the multiple scattering theories, here that of the continuum distorted wave (CDW) calculations, to describe the capture process. The multiple scattering contributions (absent in the usual first (single scattering) and second (single and double scattering) Born approximation) are shown to be important not only for the absolute magnitudes but also for the relative populations of the final states.

Double electron capture by Ar3+from rare-gas atoms

The processes of double electron capture in collisions between multiply charged Ar3+ ions and He, Ne, Ar and Kr have been studied using translational energy spectroscopy. The measurements show that the dominant reaction channels are due to capture from the ground state 3p3 4S of Ar3+ ions into high excited states of Ar+; there is clear evidence of the presence of first and second metastable states in the incident ion beam.

Single-electron capture collisions of ground and metastable N2+.ions with atomic and molecular gases

Translational energy spectroscopy, using a double-focusing mass spectrometer is used to study energy-loss/gain spectra for the single electron capture collisions of 6 keV N2+. ions with atomic (He, Ne, Ar, Kr, Xe) and molecular (N2, H2) target gases. The product states are identified by energy defect analysis. With all collision pairs, reaction channels have been observed which indicate the presence of the long-lived metastable state of N2+.(2p2 4P12/) in the incident beam. Comparison with previous data has shown that the fraction of this metastable state is higher in the present data. The measured data show that the probability of electron capture is greatest where there is a crossing of the potential energy curves for the initial and final states which occurs at internuclear distances between 4 and 7 au. New data are presented for Kr, Xe and N2 target gases.

Cross sections σnlfor electron capture collisions between medium velocity, highly charged ions and molecular hydrogen

The population of highly excited n, l states in single-electron capture by Au ions of charge state 12-18 and 2 au velocity from molecular hydrogen has been investigated experimentally for 9<or=n<or=16. Cross sections sigma nl were extracted from measurements of 2000-6000 AA photons emitted after capture. A refocusing technique made possible sufficient resolution for the observation of the different l components of the n to n-1 transitions. A comparison with UDWA and CTMC theoretical calculations for point-like ions colliding with atomic hydrogen shows one of their main conclusions to be true: the preferentially populated states have a high angular momentum. The CTMC l distribution agrees better with the experimental data than does the UDWA result. The validity of the comparison is discussed in detail. As a by-product of the measurement, the dipole polarisabilities of the ions in question were obtained.