Inner-shell ionization of potassium atoms ionized by a femtosecond laser

With a femtosecond laser pulse we rapidly ionize potassium atoms (K-0) in the gas phase, generating potassium ions (K+), and monitor the altered energy-level scheme with a subsequent hard x-ray pulse. Removal of the potassium 4s valence electron increases the binding energies of both the valence and the 1s core levels, and induces an ultrafast change of the 1s-4p x-ray transition energy by about 2.8 eV. We simultaneously observe a 50% increase in oscillator strength of K+ over K-0 for that transition.