Indications of Nuclear-Track-Guided Electrons Induced by Fast Heavy Ions in Insulators

Abstract

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...

Search for short-time phase effects in the electronic damage evolution – A case study with silicon

“…The electron temperature leads also to an enhanced absorption of Auger electrons during their transport to the surface and modifies the corresponding Auger angular distribution [17]. The nucleartrack potential resulting from ionization and charge separation in the ion track, leads to decelerated convoy electrons [11,12] and Auger electrons [9,10,12]. The corresponding positive potential attracts electrons and repels positively charged target ions [18], especially the light ones.…”

“…The corresponding positive potential attracts electrons and repels positively charged target ions [18], especially the light ones. One may estimate that a strong ion-track potential [9][10][11][12] existing for more than 10 fs will lead to significant atomic motion of light atoms, thereby suppressing any influence of hot electrons that return to the track at a too late stage of the evolution.…”

“…In previous investigations, it was possible to distinguish between the appearance of a nuclear-track potential in some polymers [9][10][11][12], related to Coulomb explosion, and also high electron temperatures in semi-metals (graphite and graphite-like amorphous carbon) [12][13][14], metals (Al, Be) and some metallic glasses [15]. These quantities are related to the energetic positions and widths of Auger lines that yield different snapshots of the short-time evolution, some 10 −15 s after the passage of heavy ions.…”

Grazing scattering of 1–2MeV HeH+ ions from KCl(001): Effect of surface track potential

“…As a result, a strong surface track potential is created behind the projectile ion on insulator surfaces. Similar track potentials induced behind the projectile ions upon transmitting through foils were already observed through measurements of energy shifts of Auger [4] and convoy electrons [5]. A recent Monte Carlo simulation can calculate temporal evolution of the track potential [6].…”

Ultrafast electronic processes in an insulator: The Be and O sites in BeO

“…Such a potential may finally lead to materials modifications via the so-called Coulomb explosion 24 of target atoms, but typically the mechanism cannot be traced back from the resulting atomic rearrangements. For shorter times, however, the potential itself shows up in the surface-scattering of molecules 25 , or in the penetration of convoy electrons through the surface (polypropylene, PP) 26 , or in a strong energy reduction of target Auger electrons (in PP and mylar) 27 in the sub-fs-and fs-time domains. This effect seems to be found mainly for large-band-gap insulators, where electrons may be trapped and holes have low mobility.…”