PHOTO-IONIZATION OF ARGON AT HIGH-RESOLUTION - COLLISIONAL PROCESSES LEADING TO FORMATION OF AR+2 AND AR+

The photoionization mass spectrum of argon is presented at a photon resolution width of 0.02 Å from threshold to beyond the 2P 1/2 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. © 1979 American Institute of Physics.