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The electron transfer mechanism was investigated for the single-electron-capture process in 3-keV/u ArAr8+ -He collisions with cold target recoil ion momentum spectroscopy. The different state-selective electron capture processes were identified for one 1 s electron of He capture into 4 s, 4 p, 4 d - 4 f, and 5 s - 5 p states of ArAr8+. We observed that 1 s to 4 s capture mainly happens at small transverse recoil ion momentum, while 1 s to 4 p capture mainly happens at large transverse recoil ion momentum. By comparing the measured projectile scattering angle distributions to the theoretical calculations, it is suggested that 1 s to 4 s and 1 s to 4 p state-selective capture processes are mainly caused by radial and rotational coupling effects, respectively, and the 4 p ± 1 quantum states are preferred. [ABSTRACT FROM AUTHOR]

An experimental investigation of the breakup of (CO2)3+ induced by Ne4+ ion impact at incident energies of 1.12 MeV was performed. By analyzing the momentum distributions and the kinetic energies of the three fragment ions, the nonsequential and sequential dissociation mechanisms are verified. In contrast to highly charged ion impact, two different sequential decay pathways were observed in the present experiment. One pathway originates from the primary cation (CO2)3+ populated into 4S+ states by collision charge exchange and its daughter cation (CO)2+ populated into the two excited states (³ and X¹) by the first fragmentation step, resulting in a lower KER peak. The other pathway originates from the primary cation (CO2)3+ locating at 6 state, and its daughter cation (CO)2+ populated into the metastable excited states ³, X¹, and ³S+, leading to the higher KER peak. Our work is a breakup experiment of (CO2)3+ presenting the initial states of the parent cation (CO2)3+ and the metastable states of CO2+ ion simultaneously. [ABSTRACT FROM AUTHOR]

Autoionization decay from doubly excited states of Ne6+[1s²3lnl (n=3,4,5)] (symmetric configurations) as well as the Coster-Kronig transition from doubly excited states of Ne6+[1s²2pnl(n?7)] (asymmetric configurations) are observed in the transfer ionization reaction channel of 80 keV/u Ne8+-He collisions. It has been predicted that the formation of symmetric configurations results from uncorrelated double-electron capture processes [Z. Chen and C. D. Lin, Phys. Rev. A 48, 1298 (1993)], and the formation of asymmetric configurations probably results from correlated double-electron capture caused by the dynamical electron-correlation effects which are attributed to small internuclear distances. However, previous experimental measurements were not able to obtain information about the dependence on internuclear distance which can be reflected in the transversal recoil-ion momentum. In this work, we measure the recoil-ion momentum in coincidence with the ejected electron velocity and find that the observed transverse recoil-ion momentum is smaller for the formation of symmetric Ne6+[1s²3lnl(n=3,4,5)] states than the formation of asymmetric Ne6+[1s²2pnl(n?7)] states. Since large momentum transfer occurs for small internuclear distances (strong electron-electron interactions) and small momentum transfer occurs for large internuclear distances (weak electron-electron interactions), the results indicate that dynamical electron correlation is important for the formation of the asymmetric states. [ABSTRACT FROM AUTHOR]

We experimentally studied the dissociation dynamics of a highly charged Ar3 cluster initiated by a high-energy electron. The dissociation patterns of the correlated ions from a two-body and a three-body Coulombic explosion (CE) of (Ar3)2+ suggest that predissociation alters the evolution of radiative charge transfer. The three-body CE in (Ar3)4+ and (Ar3)5+ is driven, after double ionization of one constituent Ar atom, by single ionization with a simultaneous interatomic Coulombic decay process. [ABSTRACT FROM AUTHOR]

Electron emissions are studied for the double-electron capture with simultaneous single ionization process in 30 keV/u He2+-Ar atom collision using the reaction microscope technique. Double-differential cross sections have been obtained for emission angles of 0°, 20°, 45°, 90°, 128°, and 175° and electron energies ranging from 0 to 80 eV. No cusp-shaped electrons centered at a speed equal to that of the incident projectile in the forward direction are observed, which is contrary to the earlier results [D. Fregenal, J. Fiol, G. Bernardi, S. Suarez, P. Focke, A. D. Gonzalez, A. Muthig, T. Jalowy, K. O. Groeneveld, and H. Luna, Phys. Rev. A 62, 012703 (2000)]. An explanation has been provided to clarify the observed results in the current reaction channel. [ABSTRACT FROM AUTHOR]

We measured the kinetic energy distributions of the fragment ions of doubly and quadruply ionized argon dimers using 3000 eV electron impact. For the dissociation of (Ar2)2+, the peak that indicates radiative charge transfer is observed, where the outer-shell ionization (dominant in highly charged ion collision) and the inner-shell ionization (preferential in x-ray experiments) have approximately equal contributions. For the dissociation of (Ar2)4+, the interatomic Coulombic decay and electron-transfer-mediated decay are first observed in the electron-impact process. [ABSTRACT FROM AUTHOR]

Electron emission occurring in transfer ionization for He2+ collisions with argon has been investigated using cold target recoil ion momentum spectroscopy. The double differential cross sections for electron capture to the continuum of the projectile (cusp-shaped electrons) are presented for collision energies from 17.5 to 75 keV/u. For an energy of 30 keV/u, we find a maximum in the experimental ratio of the cusp-shaped electron yield to the total electron yield. This result is explained in terms of the velocity matching between the projectile ion and the electron initially bound to the target. One of the important issues for double electron transitions is the role of electron-electron correlation. If this correlation is weak, then the transfer-ionization process can be viewed as two separate sequential processes. If this correlation is strong, then the transfer-ionization process would happen simultaneously and not sequentially. Our experimental and theoretical results indicate that correlation is weak and that the first step is target ionization followed by charge capture. [ABSTRACT FROM AUTHOR]

State-selective electron capture cross sections and projectile scattering-angle differential cross sections for single-electron capture in collisions of protons with He at energies ranging from 50 to 100 keV have been experimentally investigated by means of a reaction microscope. It is shown that for single-electron capture the ground-state transfer is the dominant reaction channel, and excited-state transfer has relative small contributions to the cross sections. The transfer-excitation process has a minor contribution to the total cross sections. As the projectile energy increases, ground-state transfer remains dominant, but the relative contributions of the excited-state transfers become more important. The minor contribution of transfer excitation remains almost unchanged. Furthermore, it was found that for single transfer the present results are in good agreement with independent electron model predictions in the energy regime considered. For transfer excitation, however, there are obvious discrepancies between our measurements and theoretical results. We concluded that the electron-electron-correlation effects may play a role at least in transfer excitation in collisions of protons with He in the energy range considered. [ABSTRACT FROM AUTHOR]

Electron emission from the single-electron capture with simultaneous single ionization in 30 keV/u He2+ on argon was investigated using a reaction microscope, providing the electron energy spectra and momentum distributions. Intensive peaks for electrons with near-zero kinetic energies have been observed. It is demonstrated that mechanisms contributing to the electron emission include direct transfer ionization (DTI), double-electron capture with autoionization (DECA), and single-electron capture with autoionization (SECA) of target. Comparison of resonance energies shows that Ar+ ions in SECA decay mainly through the 3s3p53d states by emitting Auger electrons, and He** in DECA decay through the 2l2l′ states. The dependence of electron emission on the transverse momentum exchange has been studied. In the transfer ionization channel studied here, the DTI process dominates the electron emission, and no saddle point electron mechanism has been found. [ABSTRACT FROM AUTHOR]

We have performed a kinematically complete experiment for transfer ionization, i.e., the capture of one target electron by the projectile accompanied by the ejection of a second electron, for 50-100 keV p-He collisions by means of a reaction microscope. It was found that the electron momentum distribution projected onto the scattering plane is consistent with the prediction of an independent two-step mechanism involving a binary encounter. The deviation of the electron momentum distribution from the ideal binary ridge is attributed to binding energy effects and the interaction between the electron and the residual recoil ion. The good agreement between our measurements with the dynamic-screening classical trajectory Monte-Carlo calculations strongly suggests that electron-electron correlations play an important role in this two-electron process. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study on single capture in 30- and 100-keV He+ on He collisions was performed. By using a reaction microscope, we obtained the state selective cross sections and the angular-differential cross sections. It was found that the experimental state-selective cross sections were in good agreement with the dynamic screening classical trajectory Monte Carlo calculation for 100-keV He+ incident. The comparisons with various versions of such calculations reveal the roles played by different electron-electron correlation effects. Moreover, a prominent oscillatory structure was observed in the angular-differential cross sections for both projectile energies. With the single capture probability distribution obtained from the classical trajectory Monte Carlo calculation, the oscillation structures can be well explained by atomic-size Fraunhofer-type diffraction. [ABSTRACT FROM AUTHOR]