High-resolution (∼0.05%) red shift of a ∼ 60 keV Kβ line upon ionization

A recent measurement [1] demonstrates that iridium's Kα2-line, centered at ≃63286.96 eV for a cold atom, increases ≃+10 eV in energy when it is emitted by a modestly (∼17×) ionized plasma. This measurement, enabled by a near-coincident lutetium K-edge filter, agrees well with atomic physics computations. Not understood at the time was a similar measurement with a thulium filter at the ≃59370 eV energy of ytterbium's Kβ1 line, which indicated that its photon energy decreases with ionization. The computation reported here shows that the ionization energy shift for Yb's Kβ lines is indeed negative and agrees qualitatively with the measurements. For the K-lines the ionization energy shift may be most interesting in atomic physics, while for the L-lines the ionization energy shift is a promising plasma diagnostic [2].