Transfer ionization and total electron emission for 25 keV amu-1 He2+ colliding on He and H2

Abstract

We have investigated electron emission for transfer ionization (TI) and total electron emission, for He and H2 targets. Double differential cross sections in angle and energy of the ejected electrons have been obtained covering the angular range from 0° to 175° and energies from 2 to 140 eV. Pure ionization differential cross sections, derived by subtraction, show an increasing contribution to the electron emission at low energies. The main observed structure, a cusp in the forward direction for electrons emitted with velocities close to the incident projectile velocity, presents a symmetric shape for TI. This fact, together with the similar shape of the cusp for both He and H2 targets, suggest the presence of a two-electron intermediate state centred at the projectile leading to cusp formation in TI. The cusp for pure ionization, in contrast, shows a strong asymmetry, and a different shape for each target. The differential electron emission measurements allow us to estimate the cross sections for capture to doubly excited states resulting in autoionization of the projectile, which contributes to the TI channel. We obtained a much larger cross section value for H2 than for He targets, in contrast with known cross sections for true double capture having the same order of magnitude for both targets.

Investigation of the transfer ionization process in collisions of partially stripped ions on He

“…In the collisions of 25 keV/u He 2+ on He, the contribution of autoionization after doubly excited capture (ADC) is 5%. [23] For high-charge ions, ADC contributes significantly to TI, even more important than the direct mechanism. [24,25] The cross section ratio R ADC (R ADC = σ ADC /σ sc ) increases with the increase of projectile energy E at low energies.…”

Nonperturbative treatment of multielectron processes in ion-molecule scattering: Application to He2+-H2collisions

“…Total cross sections for double electron capture into doubly excited states of helium are shown in Fig. 7, together with the only three available measurements from [16,42]. The computed cross sections are of the order of 10 −17 cm 2 at 25 keV/u and decrease by a factor of about 10 from 25 keV/u to 100 eV/u impact energy.…”

“…In more recent years double-electron processes, (e.g., double transfer or transfer ionisation) were addressed by several groups [11][12][13][14][15][16][17]. One of the important problems to elucidate was the dominance of single-versus double-transfer processes at low (keV) impact energies (cf.…”

“…Theory: present results (red solid line). Experiments: Frémont et al[42] (squares) and Martinez et al[16] (triangles).…”

Transfer Ionization of Helium in Collisions with C q+ and O q+ (q = 1–3)

“…On the other hand, because the ionization process can not take place for R r < R c according to the Bohr-Lindhard model, the estimated values have reached zero at E(keV/u)/q 1/2 ≈ 35. It may indicate that another mechanism, autoionization (AI) of the projectile after capture of two electrons to a doubly excited state, [17] can contribute to the TI process. In summary, the ratios σ TI /σ SC of TI to SC of He by C q+ and O q+ (q = 1-3) in the energy range of 15-440 keV/u (0.8-4.2 v Bohr ) have been measured.…”