Excitation of autoionizing states of helium-like ions by scattering of high-energy particles

Two-electron excitations of atoms and ions induced by impact of charged particles and photons are fundamental processes, which have been studied since decades. Along with double ionization and ionization accompanied by simultaneous excitation, double excitation is caused by the interelectronic interaction. Accordingly, the cross sections of these processes turn out to be extremely sensitive to the quality of which dynamic correlations are taken into account. This allows one to test different theoretical approaches. In experiments, the neutral helium atom is perferably used as a target, since it is the simplest multielectron system. Helium-like ions with moderate values of nuclear charge Z are even more attractive for investigation of correlated processes, since the corresponding cross sections exhibit a universal scaling behavior. However, such processes are rather rare. Their cross sections decrease rapidly with increasing Z values. In a most recent paper [1], we have studied two-electron excitations of helium-like ions into the two lower autoionizing 2s (S) and 2p (S) states (in the following denoted, for simplicity, by numbers 1 and 2, respectively) by collisions with high-energy electrons and photons. The calculations are performed analytically within the framework of non-relativistic perturbation theory with respect to the interelectronic interaction. The atomic targets are assumed to be characterized by the small parameters 1/Z 1 and αZ 1, where α is the fine-structure constant. The Auger widths of the levels are equal to Γ1 = 0.226 eV and Γ2 = 0.011 eV. In high-energy non-relativistic domain characterized by 3/2 ε 2(αZ)−2, the total cross section can be cast into the following scaling form ( = 1, c = 1) [1]: