Double Scattering Effects in the Ionization Spectrum Produced by Single Energetic Atomic Collisions

Abstract

We present experimental evidence of double scattering processes in the ionization of hydrogenic projectiles by single collisions with He targets at intermediate energies. We observe a distinctive shoulder in the electron velocity distribution in the forward direction at a velocity approximately 3 times greater than the velocity of the projectile. We interpret this structure as due to the emission of projectile electrons which have undergone two consecutive binary scattering processes: one with the target in first place, followed by a second one with the projectile nucleus. [S0031-9007(96)00587-X] PACS numbers: 34.50.FaThe velocity distribution of electrons emitted in energetic atomic collisions has been actively investigated during the past three decades (see, for instance, the review paper by Rudd et al. [1]). This continuing interest has been prompted by information on the interactions and mechanisms involved in the ionization process which underlie these structures. The traditional picture contains three main features of the ionization cross section ds͞d 3 v e . They are the binary encounter (BE) sphere [2] which can be produced by target or projectile ionization, and two sharp peaks which may appear, one centered in velocity space at the origin [3] and the other at the velocity v p of the incident projectile [4][5][6][7]. These structures are closely related to two-body interactions of the ejected electron with the target and the projectile. In any case, the cusp-shaped peaks have been traditionally attributed to a mechanism where the ejected electron ends up in a low lying continuum state of the "charged" residual target or projectile. On the other hand, the binary encounter process occurs as the result of an impulsive two-body interaction when an electron initially bound to one atom (target or projectile) is expelled by an elastic binary encounter with the other atom (projectile or target). This process gives rise to a peak in the velocity distribution located on a "sphere" of radius approximately equal to y p and centered either about v e v p or v e 0, depending on whether the electron is originally attached to the target or the projectile, respectively. The velocity distribution of this initial bound state determines the shape of the shoulder in the ionization cross section ds͞d 3 v e .In due time, it became clear that the cusp-shaped peaks could not be properly described in terms of a simple twobody interaction mechanism of the electron with either the target or the projectile, respectively. Thus the prominent asymmetry of the peak at v e v p in isolated ion-atom collisions was interpreted as being due to the interaction of the electron with the residual target ion [8]. This effect showed the importance of thinking of the final continuum state as a three-body, not a two-body, system and led to an active search for other three-body effects. Experimental evidence of such an effect was given by the observation of an enhanced emission at speeds in between the target and projectile velocities [9]. Even though...

Evidence for Fermi-Shuttle Ionization in Intermediate VelocityC++XeCollisions

“…1 transformed to electron velocity components parallel ͑y z ͒ and perpendicular ͑y x ͒ to the beam direction (similar to Refs. [1,2,21]), and both y components have been divided by the projectile velocity V. In this normalized velocity space, an elastic scattering by the target and the projectile center is represented by a circle centered at the t͑0, 0͒ and the p͑1, 0͒ points, respectively. The T process in Fig.…”

“…In recent years, several laboratories have started to study the emission of fast electrons in ion-atom [1][2][3] and ionsolid [4][5][6][7][8] collisions. In some cases, distinct signatures of Fermi-shuttle processes have been found in the electron spectra [1,2,8].…”

“…In some cases, distinct signatures of Fermi-shuttle processes have been found in the electron spectra [1,2,8]. Fermi [9] had proposed the mechanism as a possible origin of cosmic rays; specifically, giant magnetic fields, moving against each other in space, can accelerate charged particles to very high energies in long sequences of reflections.…”

“…Fermi-shuttle acceleration was first identified by Suárez et al [1], as the T -P process in H 1 He collisions. The first observation of the P-T process in gaseous targets was reported by Bechthold et al [2] in collisions with 5.9 MeV͞u U 271 ions.…”

“…To investigate the mechanism in more detail, it looks most promising to proceed with thin gaseous targets. In such experiments, only double scattering processes have been observed earlier [1,2].…”