Ne ionization induced by the impact of 106-keV/u H+ and 3He2+: Two-center effects on the soft-electron emission.

We present an experimental study of the electron emission induced by the collisional ionization of Ne gas by 106-keV/u H[sup +] and [sup 3]He[sup 2+] projectiles. Cross sections, doubly differential in electron energy [ital E][sub [ital e]] and angle [theta], were measured throughout the ranges 0.5[le][ital E][sub [ital e]][le]440 eV and 0[degree][le][theta][le]180[degree]. We show that measurements at electron energies down to 0.5 eV were possible after a reduction of the physical extension of the gas target to a minimum. We find a remarkable dependence of the measured distribution of soft electrons with the extension of the target. We try a model according to which this effect is attributed to the scattering of electrons into the acceptance cone defined by the detector. Our measurements enable us to perform a quantitative analysis of the shape of the soft-electron peak. Contrary to current belief, this peak is found to be strongly asymmetric. The magnitude of this asymmetry is comparable to that of the cusp resulting from electron capture into the continuum. This puts into evidence that, also for these low-energy electrons, the interaction with the two Coulomb centers, projectile and residual target ion, is essential. By analyzing the ratio of the emission obtainedmore » with [sup 3]He[sup 2+] and H[sup +], we also study the dependence of the doubly differential cross section on the projectile charge.« less