Formation of doubly excited two-electron ions during ${\mathrm{F}}^{8+}$+He, ${\mathrm{F}}^{8+}$+Ne, and ${\mathrm{F}}^{8+}$+Ar collisions

High-resolution x-ray spectroscopy has been used to measure the cross sections for forming doubly excited ${\mathrm{F}}^{7+}$ ions in the 2s2p and 2$p^2$ configurations following ${\mathrm{F}}^{8+}$+He, ${\mathrm{F}}^{8+}$+Ne, and ${\mathrm{F}}^{8+}$+Ar collisions over a projectile-energy range of 13–31 MeV. The intermediate states are found to be formed predominantly by projectile-electron excitation coupled with target-to-projectile electron transfer to an excited state during a single ion-atom collision. This process is termed nonresonant transfer and excitation (NTE) [P. L. Pepmiller et al., IEEE Trans. Nucl. Sci. NS-30, 1002 (1983)]. A competing process for formation of the doubly excited states is resonant transfer and excitation (RTE) [D. Brandt, Phys. Rev. A 27, 1314 (1983)], a process analogous to dielectronic recombination. Although the projectile energies spanned the region where resonant transfer and excitation are expected to maximize, no resonant contribution to the measured cross sections could be positively identified for the collision systems studied here. This result is consistent with the relative magnitudes of the theoretical cross sections for resonant and nonresonant transfer and excitation presented in the present work.