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

We present numerical results for the photoelectron spectrum in double ionization by keV photons in the quasiequal-energy sharing region. In this region of the spectrum, the relevant ionizing mechanism is due to a mutual sharing of the photon momentum by both electrons, with small momentum transferred to the atomic nucleus. Calculations were performed for photon energies of 25 and 50 keV, where retardation effects are fundamental, while final-state correlations are of minor importance. The spectra present a two-peak structure, with maxima located at the photoelectron energies , with the photon energy in atomic units. We discuss the general features of the spectrum in terms of the picture of the photoionization of two free electrons, and we propose a way of detecting the contribution by experiments. Received 24 January 2000