Threshold Double Photoexcitation of Argon with Synchrotron Radiation

Auger satellites have been measured to determine the probability of $M$-shell excitation accompanying $K$-shell photoionization of Ar, as a function of photon energy. The theoretically predicted difference between the dependence of shakeup and shakeoff probabilities on the photon energy near threshold is demonstrated for the first time. Results are critically compared with calculations.