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Ionic fragmentation of SiH, has been studied in the vicinity of the Si : K-edge in the photon energy range 1800-1900eV. Ionic fragments observed at photon energies above the Si: Is * U* excitation are mostly atomic ions Siq+ (q = 1-3) and H + . Ratios of the abundances for Si+, and Si2+ and Si3+ are roughly 4: 4: I, and the abundance of H+ is much higher than the sum of the Si+, Si2+ and Si3+ abundances. The averaged kinetic energy release given to H+ is estimated to be about 12eV. This complete decomposition of SiH, with the ejection of energetic H + is interpreted as Coulomb explosion of the multiply-charged (mainly + 4 or + 5) parent ion produced via the vacancy cascade (successive Auger and/or Coster-Kronig transitions).

We have measured yield curves of the multiply-charged ions produced by the photoercitation in the vicinity of the Ar K-shell ionization threshold using monochromatized undulator radiation and a time-of-flight mass spectrometer. The charge distribution of the multiply-charged Ar ions produced by the K-shell excitation suggests that both the e l e ~t m n in the outer shell and the exei!ed Rydberg electron are shaken off with high probability during S U C C ~S E ~V ~ electronic relaxation processes. Post-collision interaclion efFeca are also seen at the photon energies just above the K threshold. The fluorescence yield for the K hole is estimated to be 0.135+0.03 from the charge distribution above the K threshold.

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