Direct models at high energies

Direct Double Photoionization. This is the sole mechanism for He, and it is present (though sometimes only weakly) for other species. At high energies it occurs mainly by shake-off, which gives deeply concave electron distributions. Nearer threshold, direct knockout is the main mechanism and gives flatter distributions. The very low intensity of direct ionization to molecular dication states with separated charges on distant atoms accords with the knockout model. 

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