[en] We describe within a unified nonrelativistic approach ionization (single and multiple) by photoabsorption at high incident photon energies ω (but still ω << m), and related processes like radiative single-electron or multi-electron capture. These processes, involving a high energy photon, can be understood in terms of singularities of the many-body Coulombic potential. These singularities, at points where any two particles (e-e and e-N) coalesce, are reflected in the singularities of the initial and final state wave functions (at these points wave functions are non-differentiable), and the singularities of the electron-radiation interaction. The matrix element for these radiation processes, involving singular functions, can be understood as a Fourier transform (in large electron momenta) of a function with singularities. Since photoabsorption and (through time reversal related) radiative capture at high photon energies require at least one large electron momentum, the analysis is equivalent to the analysis of the asymptotics of Fourier transforms. The asymptotic behavior of Fourier transforms of singular functions is determined, according to Fourier transform theory, by the singularities of these functions. Our discussion is general and does not depend on the choice of the form (length, velocity, acceleration, etc.) of the photoionization (and radiative capture) matrix element. We use this approach here to study the high energy total cross sections for single ionization and the spectrum and total cross section for double ionization of a two-electron atom and total cross section for radiative capture of one and two electrons with emission of one photon. We are able to extract the slowly converging Stobbe factor from all of the cross sections thereby achieving fast convergence of the results at high energies. We explain the general advantage of using acceleration form for high energy calculations. Within our unified approach we explain both (for single ionization) the persistent high energy deviations from independent particle approximation predictions and (for double ionization) the shake-off (SO) and quasi-free (QF) contributions. We also discuss the relative importance of the related contributions for the radiative two electron capture by a bare nucleus as a function of the nuclear charge. (author)