Excitation-ionization processes in K-shell vacancy production in Li by fast bare oxygen ions: doubly-differential cross sections

Recent theoretical work has demonstrated that $K$-shell vacancy production in Li by 1.5 MeV/amu O$^{8+}$ impact cannot be understood as a simple one-electron process. Rather, a certain two-electron excitation-ionization process, in which the valence electron is removed, while one of the $K$-shell electrons makes a transition to an excited state, was found to give the dominant contribution to the singly-differential cross section at low to intermediate energies of the outgoing electron [1]. In this work, we extend the calculations to the doubly-differential level and present cross sections which are differential in the electron energy and the transverse momentum transfer [2]. The calculation involves the combination of impact-parameter-dependent single-electron amplitudes and a two-dimensional Fourier transformation of the resulting multielectron amplitudes to obtain momentum-transfer-dependent transition matrix elements. Results are found to be in good agreement with recent measurements, especially at low outgoing electron energy, and underline the importance of two-electron excitation ionization in this collision system. [1] T. Kirchner et al., Phys. Rev. A 89, 062702 (2014); [2] M. D. \'Spiewanowski et al., Phys. Rev. A 93, 012707 (2016)