J. Phys. IV France 9 (1999) Pr6-101 Correlation effects in double ionization of magnesium atom S. El Ghazouani, P.A. Hervieux, C. Dal Cappello and J. Langlois* LPMC, Institut de Physique, Technopôle 2000, 57078 Metz, France * Laboratoire des Collisions Électroniques et Atomiques, Faculté des Sciences et Techniques, 6 avenue Le Gorgeu, 29285 Brest cedex, France Abstract. Electron-impact double ionization of magnesium atom (3s 2 ) is investigated theoretically for the case of high incident energies (745 eV). An ab-initio calculation is carried out including electronic correlations in the initial-state wave function. The predictions are compared with the recent experimental data on magnesium atom by El Marji et al. PACS. 34.80.Dp Atomic excitation and ionization by electron impact 1. INTRODUCTION Double ionization of atoms is an attractive study because the electron-electron correlation plays an important role. For instance the double ionization of an atom is forbidden within the independent-particle model [1]. Double ioniza- tion is much less probable than single ionization. Until the last eight years only total cross sections or partial differential cross sections had been measured [2,3]. Re- cently Lahmam-Bennani et al. have performed measure- ments of the fully (five-fold) differential cross section on argon and on krypton [4,5]. These difficult experiments are very sensitive to the finer details in the double ion- ization dynamics because the three outgoing electrons are detected in coincidence. In these so-called (e,3e) experi- ments the directions and the energies of the three final electrons are measured. More recently El Marji et al. [6] have performed four-fold differential cross sections mea- surements on helium where only the two ejected electrons are detected in coincidence. Ford et al. [7,8] have also per- formed the same kind of experiments on magnesium. In these so called (e,3-le) experiments the energy of the scat- tered electron (which is not detected here) is fixed by the energy conservation law and the directions and energies of the two ejected electrons are also measured. Three mecha- nisms are considered to be responsible for the double ion- ization. The first, called the shake-off (SO) mechanism [1], is a single interaction between the incident electron and one target electron [9,10]. In this process, the ejection of two electrons is due to their mutual correlations. The sec- ond mechanism, called two-step 1 (TS1) mechanism [11], consists of a first interaction between the incoming elec- tron and one target electron. It leads to a first ejected electron that interacts with another target electron. Fi- nally, the third mechanism, called two-step 2 (TS2) mech- anism [11], takes into account two interactions between the scattered electron and two electrons of the target. These two processes TS1 and TS2 can be described by the second Born approximation because two interactions are involved. Popov et al. [12] have shown that TS1 is not negligible, even at high incident energies, when the experiments are made with small momentum transfer. El Mkhanter and Dal Cappello [13] have recently shown that TS2 is not negligible neither and that the recent (e,3e) experiments are not the most informative about electron- electron correlations in the initial state because the TS1 and TS2 mechanisms partially destroy this information. In this paper we discuss our results obtained for the double ionization of magnesium atom by using either a correlated initial-state wave function computed within the framework of a configuration-interaction (CI) model, or a Hartree-Fock (3s) wave function. These calculations are presented in the frame of the new experimental situation proposed by El Marji et al. [14]. In this scattering geom- etry the scattered angle is fixed at 18° and the azimuthal direction of the scattered electron is integrated over. Only the two ejected electrons are normally detected along the surface of a 45° cone (see Fig. 1). The ejected electrons energies are fixed at 55 eV. 2. THEORY We restrict our study to the case of high incident energy, which allows us to describe the incoming and scattered electrons by plane waves. The first Born approximation is used because the SO is supposed to be the dominant process [9]. Within this theoretical framework, the 8DCS may be written as Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1999623