Threshold krypton charge-state distributions coincident with K-shell fluorescence

Measurements of Kr^q+ yields in coincidence with K-shell fluorescence, as incident x-ray energy is varied across the K-shell threshold, are reported. Near threshold, we observe slight variations in the branching ratios as a function of energy, which are connected with the different behaviors of the flux-normalized partial yields for each q. The lower-q yields show a resonance peak near threshold superimposed on a smoothly rising edge, whereas the higher-q yields show only a smooth rise. A simple model is developed which accounts for these features, incorporating both the threshold photoexcitation and the cascade behavior of the spectator electrons.