Resonant Transfer and Excitation of First-Row Ions Using Zero-Degree Auger Projectile Spectroscopy

Resonant transfer and excitation (RTE) is a correlated two-electron process mediated by the two-center electron-electron interaction: A projectile electron is excited while a target electron is captured, forming doubly excited states. These decay via X-ray (RTEX) or Auger (RTEA) emission. For fast enough collisions with light targets, RTE becomes analogous to dielectronic capture (DC)—a key plasma process—and is described by the impulse approximation (IA). Early (1983–1992) RTEX and the more stringent, state-selective RTEA measurements at accelerator facilities provided indirectly, essential DC cross section information before direct electron-ion DC measurements became available. The 1992 review [1], focusing on zero-degree Auger projectile spectroscopy (ZAPS) of state-selective KLL D states, validated the IA for low-Z_p ions (Z_p ≤ 9). However, a puzzling systematic discrepancy was revealed: IA cross sections were consistently larger than experiment, with the disagreement increasing as projectile atomic number Z_p decreased. This review updates RTEA progress since 1992: Refinements to IA calculations include the use of more accurate Auger rates, considerations of Auger anisotropic emission, novel target binding corrections and even an exact IA formulation. Experimental ZAPS improvements feature a hemispherical spectrograph and a proven in situ more accurate standardized absolute cross section calibration using binary encounter electrons. A methodical analysis demonstrates impressive agreement across all measurements spanning both pre- and post-1992 eras including measurements presented here for the first time, eliminating systematic discrepancies. IA validity is confirmed down to boron ions, with He⁺ ions as the sole clear exception together with some borderline Li-like ion cases. Recently, a rigorous ion-atom collision treatment has also emerged: Nonperturbative close-coupling calculations of transfer excitation of He-like ions in collisions with He confirms RTE dominance via two-center electron-electron interactions at large impact parameters, while providing unexpected insights into many-body collision dynamics at the lowest collision energies.