Spectroscopy of hydrogenlike and heliumlike argon

The x-ray transitions (n=2→n=1) emitted by fast hydrogenlike and heliumlike argon ions have been studied. The absolute energy of the Lyman α lines of hydrogenlike ions has been measured and the value of the ($1s$) Lamb shift of argon evaluated for the first time. The ³P1,2 and ¹P₁ transitions of heliumlike argon have also been studied at very high precision and their energies compared to multiconfiguration Dirac-Fock calculations. The energies of Lyman α lines are of 3323.2±{}0.5 eV (Lyα₁) and 3318.1±{}0.5 eV (Lyα₂), and those of n=2→n=1 transitions for heliumlike argon are of 3123.6±{}0.25 eV (³P₁), 3126.4±{}0.4 eV (³P₂), and 3139.6±{}0.25 eV (¹P₁).