Mutual Ionization in 200 keV H - He Collisions

By now many experimental data on fully differential cross sections (FDCS) exist for single target ionization covering a broad range of different collision systems. At this stage it therefore seems reasonable to turn to somewhat more complex ionization processes and to fully exploit the opportunities provided by heavy projectiles, in order to advance our understanding of collision-induced few-particle quantum-dynamics. One fundamental step forward is to study double ionization of the target atom, where electron-electron correlations have to be taken into account [1]. Another important step is to investigate mutual ionization of both collision partners, whereby twocenter effects can be studied. Although this process is more complicated than double ionization in that it requires a structured projectile, one important advantage is that the mutual ionization can proceed through a single interaction (between the two electrons to be ionized), while double ionization requires at least two interactions. In that sense mutual ionization is the more simple process. Experimentally, following numerous total cross section studies, multiple differential data became only available quite recently [2, 3]. In fact, the very first measurement of FDCS for MeV C + He collisions was performed at GSI just a few years ago [3]. There the question was addressed to what extend mutual ionization proceeds through the interaction between the two active electrons and how this particular reaction channel can be separated from other ionization mechanisms. However, because of the comparable binding energies of projectile and target (48 and 24 eV), a considerable contribution of nucleus-electron interaction was found. Only under certain kinematical conditions features typical for electron impact ionization (e,2e) were observed. In the present work we chose 200 keV H as projectiles and investigated the simultaneous ionization of the projectile and the He target by means of a kinematically complete experiment using a reaction microscope [4]. Here, compared to all previous studies, the binding energy of the projectile electron is much smaller (0.7 eV). Thus, one might expect that during the collision the weakly bound quasi free projectile electron interacts with the target giving rise to ionization of both, projectile and target. Since the projectile nucleus is just a spectator in this scenario we expect to approach a situation very similar to free electron impact ionization. Theoretically, if the above assumptions hold, a first order treatment in the projectile-target interaction should be adequate to describe the H He collisions. We use a first Born approximation (FBA) where the projectile and the target are assumed to have just one active electron and mutual ionization can only proceed through an interaction between these active electrons. The initial and final electronic states in H were found by solving the Schrödinger equation for a Yukawa-like potential used to model the short-range force experienced by the active electron in H. In Fig. 1 the experimental longitudinal momentum spectra of electrons ionized form the target and the projectile are shown in comparison with our FBA calculation (thick lines). The projectile electron distribution exhibits a main peak at about 2.3 a.u. as well as a “bump” approximately at the projectile velocity (indicated by the arrow in Fig.1). Both features are very well reproduced by our first order theory. The projectile electron has to deliver the energy to ionize the He (24.7 eV) which leads to a change in the projectile electron momentum from initially vp to at least 2.48 a.u. in agreement with experiment. Because of very similar arguments a forward shifted momentum distribution for the target electrons is predicted (thick line in Fig.1). However, the experimental data show a pronounced cusp shape with a maximum at pz = 0. This discrepancy can only be removed if a more advanced theory is used for the target ionization. Taking a CDW-EIS calculation for pure target ionization, where the H projectile is treated as an anti-proton (thin curve in Fig. 1) the data are very well described. It appears that the post-collisioninteraction of the target with the projectile electron, which is not included in the FBA, is essential.