Mechanisms for Rapid Electron Transfer in Ionic Solids Following Multiple Ionization by Heavy-Ion Impact

Systematic measurements of the spectra of fluorine K¿ x-rays produced by the bombardment of alkali and alkaline earth fluorides with 2-MeV/amu ions ranging from He to Ar have given new information on the mechanisms for rapid electron transfer within the K-hole lifetime (~10-13sec.). Spectra obtained with He ions provide clear evidence of the occurrence of resonant electron transfer (RET) following 1s2p ionization of F- in the compounds KF and SrF2. Calculations employing a point-charge model support the conclusion that the anomalously low KL1 x-ray satellite intensities observed for these two compounds are the result of a resonant exchange of an electron between the outermost p level of the metal ion and the 2p level of the fluorine ion. Such a transfer mechanism requires a close match between the energies of the two levels involved. Recent measurements using 22-MeV carbon ions have revealed that the KL2 satellite intensity in CaF2 is much lower than that displayed by any of the other group II fluorides, thereby suggesting RET also occurs in association with this double L-hole state. When high states of ionization are produced, rapid electron transfer to the fluorine L-shell becomes highly probable even in the absence of level matching. The mechanism for this probable involves collision induced transitions from the fluorine 2p band. At the same time, population of the M-shell levels of the ionized fluorine occurs, as is evidenced by the observation of large x-ray energy shifts and intense Kß hypersatellite structure.