Contribution of the quasifree mechanism to the ratio of double-to-single ionization

Abstract

We evaluate the contribution of the quasifree (QF) mechanism to the ratio of double-to-single ionization by one photon. This mechanism is expected to take place in the interaction of a high-energy photon with an atom, and results in the mutual sharing of the photon momentum by two electrons, with small momentum transferred to the nucleus. In this case the electrons should be ejected in nearly opposite directions, sharing the energy nearly symmetrically. By using retardation, we show that the QF contribution displays a ${\ensuremath{\omega}}^{\ensuremath{-}5/2}$ energy dependence, in contrast to the well-known ${\ensuremath{\omega}}^{\ensuremath{-}7/2}$ energy dependence of the shake-off term. Our calculation shows that the QF mechanism does not give a leading contribution to the ratio of double-to-single ionization for keV photon energies, and is smaller than previously predicted.