Single-electron transfer between Ar2+(3P, 1D) and He at low collision energies

The single-electron transfer reactions between helium atoms and the ground (P-3) and first (D-1) excited states of Ar2+(3p(-2)) have been investigated at a state-resolved level employing a position-sensitive coincidence mass spectrometer. Using this apparatus, the complete angular distributions of the Ar+(P-2) and He+(S-2) product ions arising from this electron transfer reaction have been determined at centre-of-mass collision energies ranging from 0.4 to 1.2 eV. The Ar+ product ions formed by the reaction of the P-3 state of Ar2+ are predominantly forward scattered at collision energies between 1.2 and 0.6 eV, the distribution broadening with decreasing collision energy, and are scattered isotropically at the lowest collision energy investigated (0.4 eV). Product Ar+ ions arising from the reaction of He with the D-1 state of Ar2+ have angular distributions which vary strongly with the collision energy over the range studied. No reactivity of the second (S-1, 3p(-2)) excited state of Ar2+ is observed, allowing an upper limit of 0.02 to be placed on the relative reaction cross-section for this state with respect to that of the D-1 state of Ar2+.