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We present experimental evidence for a deceleration of convoy electrons produced by 5 MeV͞u ions (N 71 , Ne 101 , S 131 , Ni 231 , and Ag 371 ) during the interaction with insulator foils at normal incidence. The deceleration first increases with increasing projectile charge, reaches a maximum at a projectile charge of about 16, and seems to approach zero for even higher charges. Different possible mechanisms and quantitative estimates for the slowing down of convoy electrons are presented.[ S0031-9007(97) PACS numbers: 34.50. Fa, 72.20.Jv, 73.61.Ph, 79.20.Rf Electron spectra in ion-solid interactions show, apart from Auger lines and binary-encounter electrons, one prominent structure exactly in the projectile flight direction: the convoy-electron peak. Convoy electrons are fast electrons that leave the solid surface with about the same velocity as the projectile ion. They give rise to a cusp shaped kinematic peak and are related to the attractive Coulomb potential of positive ions. This peak was first measured in ion/atom collisions experiments [1] and shortly after for ion-solid interactions [2]. In both cases convoy electrons are produced by electron capture (ECC) [3] and by electron loss to continuum states of the projectile (ELC) [4,5]. In dense matter, these electrons are subject to a random walk under the influence of the target constituents and the projectile potential [6].Convoy electrons can be accelerated by the imagepotential of the projectile charge, as has been found for ions under glancing-angle scattering conditions at semiconductor and metal targets [7][8][9] and at normal-incidence conditions for highly charged ions at a proton-equivalent energy of 5 MeV͞u [10]. In ion-insulator interactions ionizing collisions result in a positive nuclear-track potential, which can decelerate target Auger-electrons emitted from the insulator surface [11] and accelerate desorbed positive hydrogen ions [12]. In this Letter we present first evidence for a deceleration of convoy electrons induced by 5 MeV͞u highly charged ions traversing insulating polypropylene ͓͑C 3 H 6 ͔ n ͒ foils at normal incidence.A detailed description of the experimental setup has been published previously [10,11]. 5 MeV͞u heavy ions were delivered by the heavy-ion cyclotron of the Ionenstrahl-Labor (ISL) at the Hahn-Meitner Institut. The beam was sent through a post-cyclotron stripper foil and a magnet to select projectiles with a charge-state close to equilibrium. The beam of 0.1 to 10 particle nA was collimated to about 1 mm 2 at the center of the magnetically shielded target chamber with a vacuum of typically 10 26 mbar. During the experiments the targets were wobbled in both directions perpendicular to the beam for an accurate fluence determination and for a reduction of the heat load. For the measurement of convoy-electron spectra an electrostatic zero-degree tandem spectrometer (energy resolution DE͞E 0.6% and solid angle DV 2 3 10 25 sr) was used [13]. The ions pass the first stage of the spectrometer and electrons emitted in t...

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The method of high-resolution Auger spectroscopy was used to study mechanisms for double-electron capture producing the projectile configurations 3lnlЈ and 4lnlЈ (nу4) in Ne 10ϩ ϩHe collisions. Emphasis was given to slow collisions with projectile energies near 1 keV. At low collision energies the production of the configurations 3lnlЈ (nу6) of nonequivalent electrons is found to become dominant. It is shown that dielectronic processes produced by the electron-electron interaction play a major role in the creation of the Ne 8ϩ (3lnlЈ) states.

Different contributions to radiative stabilization in Ne 10ϩ ϩHe collisions at projectile energies of 10 and 150 keV are studied. For both energies, radiative stabilization is found to be equal to Ϸ0.3 when referred to the total double capture. In Ne 10ϩ ϩHe collisions doubly excited states 3lnlЈ and 4lnlЈ (nу4) are either produced by uncorrelated two-electron transitions or by dielectronic mechanisms due to electron-electron interaction. A strong contribution (Ϸ0.15) to the stabilization follows from the decay of near-equivalent electrons 3lnlЈ and 4lnlЈ(nϭ4,5). Another major contribution ͑0.10-0.25͒ to stabilization is due to the decay of configurations 3lnlЈ (nу6) of nonequivalent electrons produced by the dielectronic process of autoexcitation. A small contribution is found to be due to the configurations 3lnlЈ (nϾ9) created by dielectronic phenomena in the postcollisional and asymptotic regions (Ϸ0.04).