Some Further Considerations of Correlated Behavior

The direct knockout mechanism had earlier been evoked by Samson [36, 37], who pointed out that the variation of some double photoionization cross sections as a function of energy closely resembles the form of electron impact ionization cross sections of ions. This is, of course, the expected behavior if direct knockout dominates, which is expected in the energy range relatively near threshold. Further consideration of these model mechanisms is left for a later section where the role of initial state electron correlation is discussed. 

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