Inner-Shell Photoionization of Argon

Broadband synchrotron radiation was used to photoionize K-shell electrons of Ar^{2+} ions in an ion cloud target. The ion target was produced by electron impact ionization, and contained inside a Penning ion trap. The ionic charge distribution resulting from vacancy cascading and electron shake-off was measured. Following corrections for the effects of electron capture, the experimental results were compared with a simple prediction that the ionic charge distribution would be the same as the distribution resulting from a K-shell vacancy in neutral argon shifted upward by two charge units. The most probable charge state, Ar^{6+}, was correctly predicted, but not the relative abundances of the ionic charge distribution, even when including possible lowered shake-off probabilities. A more detailed analysis of the vacancy cascade and electron shake-off is necessary to explain the experimental results. These are the first measurements of inner-shell photoionization of multi-charged ions using synchrotron radiation. The results are encouraging for future experiments involving inner-shell photoionization of ions using third generation synchrotron radiation facilities, which are presently being constructed world wide. Also measured were the electron capture rate coefficients for Ar^{rm q+} on H_2 (q = 3, 4, 5, and 6), using the same apparatus. These measurements were necessary to properly analyze the charge changing of the photoions before signal detection. For these measurements, the Ar ions were produced by pulsing the synchrotron radiation through the trap ring electrodes, ionizing Ar atoms, creating an ion cloud with a range of charge states 1 <= q <= 8. The results are _{kappa}(3+) = 4.3 times 10^{-9 } cm^3/s, _ {kappa}(4+) = 5.2 times 10^{-9} cm ^3/s, _{kappa} (5+) = 5.9 times 10 ^{-9} cm^3/s and _{kappa}(6+) = 8.5 times 10^{ -9} cm^3/s.