E. Y. Kamber

Single-electron capture by He2+ ions from triatomic molecules

Abstract State-selective single-electron capture (SEC) processes in low-energy collisions of He 2+ ions with H 2 O and CO 2 have been studied experimentally at impact energies between 0.1 and 1 keV by means of translational energy-gain spectroscopy. At the lowest collision energy, 100 eV, the energy-gain spectra indicate that the dominant reaction channels are dissociative transfer ionization (DTI), due to SEC into the n =1 states of He + product ions with simultaneous ionization of the target products. As the projectile impact energy is increased, DTI channel remains dominant for the He 2+ –CO 2 collision system. However, for the He 2+ –H 2 O collision system, non-dissociative SEC into n =2 states of He + with production of H 2 O + in the ground state is also observed and found to become the dominant process at collision energies in excess of 800 eV. The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the Landau–Zener model and the extended version of the classical over-the-barrier model.

Contribution of transfer ionization to cusp electron production in proton on argon collisions

Cusp-electron emission in proton on argon collisions has been investigated at impact energies 33, 50 and 75 keV. By detecting the electrons in coincidence with the charge-state analysed outgoing H+,0 projectiles, the process of electron capture to the continuum (ECC), as well as the process of simultaneous ECC and bound-state capture (transfer ionization, TI) were identified. The ratios of the yields for cusp-electron production by TI and ECC are found to decrease with increasing projectile energies. At the lowest proton energy (33 keV) the measured TI/ECC ratio is about four times smaller than the corresponding ratio for incident He+ measured in earlier work. Significant is the finding that there exists a cusp associated with TI events, and, moreover, that this cusp is present for neutral outgoing projectiles.

Experimental study of single-electron capture by low-energy Neq+ recoil ions (q=3-6) from He, Ne and Ar using translational energy spectroscopy

The translational energy spectroscopy technique has been used to study single-electron capture processes for collisions of slow Neq+ ions (q=3-6, where q is the projectile charge state), produced in a recoil ion source, with He, Ne and Ar at laboratory impact energies between 100 and 600 eV and scattering angles between 0 degrees and 6 degrees . The measurements show that the dominant reaction channels are due to capture into excited states of the projectile products and show a dependence on scattering angle. There is clear evidence of the presence of the first and second metastable states in the incident Ne3+ ion beam. A reasonable description of the dominant final states is obtained in terms of the reaction windows, which are calculated using the single-crossing Landau-Zener model. The measured differential cross sections show that the projectile products are distributed with maximum intensity near a scattering angle theta c which corresponds to capture at an impact parameter equal to the crossing radius.

Capture and ionization processes studied with COLTRIMS

Abstract A review of some basic features of the COLTRIMS technique is presented. Examples of recent work on low-energy electron capture and ionization collisions and for high-velocity ionization from Kansas State University are given.

Dissociative and non-dissociative charge-changing processes in 1.0-2.0 MeV/u O5+ + O2 collisions

In the present work molecular ionization and dissociation correlated with projectile single electron capture and loss is investigated as a function of energy for O5+ + O2 collisions. Coincidence recoil-ion spectra as well as total projectile charge-changing yields were measured for projectile energies of 1.0, 1.5, and 2.0 MeV/u. Charge states for dissociative and non-dissociative recoiling products of the target molecular oxygen were identified. Ratios of charged molecular fragment yields to single non-dissociative ionization yields as a function of projectile energy were determined as well as total cross sections for projectile single electron capture and loss.

State-selective single-electron capture in Ne4+-He collisions

Abstract A cold-target recoil-ion momentum spectroscopy technique (COLTRIMS), capable of measuring simultaneously the energy-gain and the scattering angle of the projectile products, has been used for the study of state-selective single-electron capture from He by 8 keV Ne4+ ions. The dominant reaction channel is due to single-electron capture into the 3s 4P state of the Ne3+ from the ground state incident Ne4+ ions. Also, there is clear evidence of capture into the 3s 2P state. State-selective differential cross-sections for capture into the 3s 4P and 2P states have also been measured and compared with the semi-classical multichannel Landau–Zener calculations.

Single-electron capture collisions of ground and metastable Ne2+ ions with molecular gases

Abstract Using the translational energy-gain spectroscopy technique, we have measured theenergy-gain spectra and absolute total cross sections for single-electron capture in collisions ofNe 2+ with N 2 , CO 2 and H 2 O at laboratory impact energies between 50 and 400eV and 0 ◦ scattering angles. In all the collision systems studied here, reaction channels have beenobserved which indicate the presence of the long-lived metastable states of (2s 2 2p 4 1 D and 1 S)in the Ne 2+ incident beam. These measurements also indicate that capture from the metastablestates into excited states of the projectile product ions is the most important inelastic process.Contributions from capture accompanied by the excitation and ionization of the target productare also detected. In addition, the energy dependence of the total single-electron capture crosssections is studied and found to slowly increase with increasing impact energy. The presentdata are compared with the theoretical calculations of the classical over the barrier, extendedclassical over the barrier and Landau–Zener models.PACS numbers: 34.70.+e, 82.30.Fi(Some figures may appear in colour only in the online journal)

Electron capture processes in slow collisions of Ne6+ ions with CO2 and H2O

Energy-gain spectra and absolute total cross-sections for single-, double-, and triple-electron capture processes in collisions of Ne6+ ions with CO2 and H2O at laboratory impact energies between 450 and 2400 eV, have been studied experimentally by means of a translational energy-gain spectroscopy technique. The energy-gain spectra for single-electron capture show that the dominant reaction channels are due to capture into the n=4 state of Ne5+, in agreement with classical over-the-barrier model calculations. In both cases, contributions due to transfer excitation into the 2s2p (1,3P) 3 l states are also detected. The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the single-crossing Landau-Zener (LZ) model and the extended version of the classical over-the-barrier (ECOB) model. The energy dependence of cross-sections for electron capture are also measured and found to be slowly increased with increasing collision energy. The data for single-electron capture are also compared with theoretical results based on the multi-channel Landau-Zener (MCLZ) model.

Experimental evidence of transfer excitation in Ar6+-He collisions

High-resolution cold-target recoil-ion momentum spectroscopy (COLTRIMS) has been used to study single-electron capture processes in collisions of Ar 6+ ions with He at an impact energy of 9 keV and scattering angles between 0 and 3 mrad. The energy-gain spectra show that the capture occurs mainly into the 4s state with a significant contribution involving capture into the 4p state. Reaction channels associated with transfer excitation into the 3s3p3d states of Ar 5+ are also observed for the first time, accounting for 6 .0:4/% of the total cross sections for single- electron capture. Experimental results are found to be in good agreement with the two-electron atomic orbital close-coupling calculations.

State-selective single-electron stripping processes of Ar2+ ions in collisions with He and Ar

State-selective single-electron stripping processes of Ar2+ ions in collisions with He and Ar atoms have been studied experimentally at laboratory impact energies of 6 and 8 keV by means of translational energy spectroscopy. These measurements show that the dominant reaction channels are due to ionisation of long-lived highly excited states with excitation energies very close to the ionisation potential of Ar2+. Weaker processes are also observed due to ionisation of metastable and ground state Ar2+ ions.