Spectrum of two quasiequal-energy electrons ejected by a high-energy photon

We present numerical results for the photoelectron spectrum in double ioniz ation by keV photons in the quasiequal-energy sharing region. In this regio n of the spectrum, the relevant ionizing mechanism is due to a mutual shari ng of the photon momentum by both electrons, with small momentum transferre d to the atomic nucleus. Calculations were performed for photon energies of 25 and 50 keV, where retardation effects are fundamental, while final-stat e correlations are of minor importance. The spectra present a two-peak stru cture, with maxima located at the photoelectron energies epsilon(1) = omega /2 +/- root omega(3)/8c(2) with omega the photon energy in atomic units. We discuss the general features of the spectrum in terms of the picture of th e photoionization of two free electrons, and we propose a way of detecting the contribution by experiments.