Vertical triple ionization of ethyne molecules in triple-electron-transfer collisions with O2+ beam ions

Abstract Triple-electron-transfer (TET) collisions of 6 keV O 2+ beam ions with gas-phase ethyne molecules were studied with mass spectrometric techniques. Measurement of the kinetic energies of the O − ions produced yielded triple-ionization energies (TIEs) of ethyne to states of its triply charged positive ion, once those transitions corresponding to the transfer of three electrons in a single collision were identified: eight such peaks were found between 65.1 and 80.1 eV. Possible O 2+ states in the beam include 3 P, 1 D, 1 S; according to spin conservation the singlet ions should populate only doublet states of C 2 H 2 3+ , while projectiles in triplet states would populate both its doublet and quadruplet states. Using ab initio propagator calculations for the triple-ionization transitions, the TIEs and quantum numbers for the states of C 2 H 2 3+ associated with each peak observed in the TET spectrum could be identified. The overall match between the theoretical and experimental energies was good, so that it could be established that the population of quadruplet states was comparable in intensity with that for doublets. This is consistent with previous evidence for O 2+ ( 3 P) being the dominant state in the projectile beam used.