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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...

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