Experimental evidence of the asymmetry of the soft electron peak in ion-atom ionization

Abstract

We have measured the doubly differential energy and angular distribution of low-energy electrons emitted in collisions of H + and 3 He 2+ on Ne at 106 keV/u. In this way, we are able to obtain information about the shape of the soft electron ionization peak. Against current belief, but in accordance with a two-Coulomb-center interaction of the emitted electron, we find it to be strongly asymmetric in the forward-backward direction. We discuss the shape of the cross section and introduce a parametric expression for fitting the data.PACS numbers: 34.50.FaThere is no doubt about the existence of a strong soft electron (SE) peak in the spectra of electrons emitted in ion-atom collisions. The corresponding cross section do/dv, doubly differential in the direction and modulus of the electron velocity (v), is expected to have a l/v divergence in the limit v-• 0. Consequently, the energy distribution dcr/dEdfi should be a smooth function for small electron energies (E). Until now, no measurements have been presented that permit a discussion of the shape of the SE peak, mainly because of the well-known experimental difficulties for the accurate analysis of electrons propagating with small velocities.The SE peak is one of the three important features of the spectra which give direct evidence of the electron-ion Coulomb interaction, in ion-atom and atom-atom ionization. Meanwhile much attention has been paid to the two remaining reactions, i.e., electron capture to the continuum (ECC) and electron loss to the continuum (ELC). This paper is the first attempt to study the shape of the SE peak.Fast collisions accompanied with small momentum transfer are usually treated with the first Born approximation (FBA), in which the triply differential cross section (TDCS), differential in the velocity (v) of the emitted electron and momentum transfer (K) of the scattered bare ionic projectile, is given (in atomic units) by [1] where/ V ,/(K) is the generalized oscillator strength. Here Z and V are the charge and initial velocity of the projectile, K the momentum transfer during the collision, and Ei the energy of the initial bound target state, with quantum numbers i =« ,/,ra.The oscillator strength can be expanded in spherical harmonics, giving [2] /*,/ -X^/G>,*)/V(cos0 K ) ,J where #K is the angle between v and K. The doubly differential cross section (DDCS) in the velocity v is obtained by integration on K and gives [2]L where 0 is the angle between V and v. We write explicitly the \/v divergence associated with the asymptotic Coulomb potential of the residual target ion. For a \s initial state and a Coulomb final wave for the electron, Eq. (3) can be written as a double series expansion [3,4],which, within the considered FBA, has the constraint B[ k) =0 for k+L odd and for L > k + 2. For a generic initial state Burgdorfer et al. [5] studied the limit v-• 0 of Eq. (3) and obtained a finite expansion In da/dv = (\/v) S C L (i 9 V,v-0)P L (cosO) 9 (5) L(cven)where CLU,V,V =0) =2?i 0) . For a \s initial state only two multipoles contrib...

Young Type Interference Effect on the Forward-Backward Asymmetry Parameter in Electron Emission from H2Under Fast Ion Impact

“…the soft collision, two center effect, binary encounter and electron capture in continuum [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Since the two indistinguishable H-atoms may be termed as a coherent source of electrons in a large impact parameter collision with a fast projectile, their contributions to the ionization add coherently and an interference effect may be expected and has been observed [1,2].…”

“…We have now demonstrated [41] an independent way of obtaining the interference structure by using the forward backward asymmetry parameter of electron DDCS which is independent of the theoretical model calculations and does not require a complementary experiment with atomic H. The experiments with atomic H as a target become difficult owing to the involved procedure of production of atomic H from the dissociation of H 2 . It is known that a large forward backward angular asymmetry is mainly caused by the two-center mechanism in heavy ion collisions [5][6][7][8][9][10][11][12][13][14]. The enhancement in the forward directions and depletion in the backward angles is caused by the long range post collisional interaction among the three particles in the continuum (electron-projectile ion-recoil ion) of the final states and qualitatively explained by the continuum distorted wave models such as CDW-EIS (eikonal initial state), at least for an atomic target like He [5][6][7] or H [8][9][10].…”

The importance of continuum eigenstates in electron spectroscopy

“…(1) persists. Measurements of the SE peak shape are much more difficult than those of the ELC cusp shape because the energy of the electrons is in the eV region only (as compared to hundreds of eV for the ELC cusp), which requires extremely thin targets [3,30]. Fig.…”

“…These electrons are caused by direct interaction with the projectile and populate the low continuum states of the target. Conventionally, the first-order Born approximation for direct ionisation [2] has been used for their description, but the new experimental discovery of the peak asymmetry [3] caused by a strong interaction with the projectile field, calls for an improved theoretical model.…”

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

“…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.…”