Absolute cross sections of ultra-soft x-ray radiation in at electron scattering upon atoms and the soft-photons approximation

The cross sections of ultra-soft x-ray bremsstrahlung in at electron scattering by Ar, Kr and Xe are theoretically calculated. The results are consistent with the absolute values of the differential cross sections measured by Gnatchenko et al [Phys. Rev. A 80, 022707 (2009)] for scattering electrons with an energy of 600 eV on these atoms.