Double multiphoton ionization, shake-off and the time-dependent Hartree-Fock mechanisms

There are a variety of mechanisms leading to double multiphoton ionization. We show that two of them, the shake-off mechanism and the time-dependent Hartree-Fock process are, in fact, identical. In the limit in which electron-electron correlations are negligible they are the only surviving mechanism. For a helium target in a low-frequency limit, they are the dominant double-ionization process but in this domain the process is much too small to be observable.