Innershell Photoionization Studies of Neutral Atomic Nitrogen

Innershell ionization of a $1s$ electron by either photons or electrons is important for X-ray photoionized objects such as active galactic nuclei and electron-ionized sources such as supernova remnants. Modeling and interpreting observations of such objects requires accurate predictions for the charge state distribution (CSD) which results as the $1s$-hole system stabilizes. Due to the complexity of the complete stabilization process, few modern calculations exist and the community currently relies on 40-year-old atomic data. Here, we present a combined experimental and theoretical study for innershell photoionization of neutral atomic nitrogen for photon energies of $403-475$~eV. Results are reported for the total ion yield cross section, for the branching ratios for formation of N$^+$, N$^{2+}$, and N$^{3+}$, and for the average charge state. We find significant differences when comparing to the data currently available to the astrophysics community. For example, while the branching ratio to N$^{2+}$ is somewhat reduced, that for N$^+$ is greatly increased, and that to N$^{3+}$, which was predicted not to be zero, grows to $\approx 10\%$ at the higher photon energies studied. This work demonstrates some of the shortcomings in the theoretical CSD data base for innershell ionization and points the way for the improvements needed to more reliably model the role of innershell ionization of cosmic plasmas.

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