Photoionization of argon at high resolution: Collisional processes leading to formation of Ar+2 and Ar+

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Abstract
The photoionization mass spectrum of argon is presented at a photon resolution width of 0.02 Å from threshold to beyond the 2P1/2 continuum. A natural linewidth of ∼0.4 meV is estimated for the first sharp, s-like autoionization peak. Collisional processes leading to formation of Ar+ below threshold are attributed to a kind of Penning ionization of very high Rydberg states. The associative ionization process Ar*+Ar→Ar2+ +e− is also observed below threshold. In the competition between the two collisional processes, the formation of Ar+ is much more probable at high principle quantum number, but this cross section declines exponentially with energy deficit. The cross section for formation of Ar2+ surpasses that for Ar+ below n=13, but it declines at very high n values, asymptotically vanishing at the ionization threshold.