Relativistic, retardation, and multipole effects in photoionization cross sections: Z, n, and l dependence

The limits of validity of the nonrelativistic dipole formulation of the total photoionization cross section are explored. Relativistic and nonrelativistic independent particle model calculations of all subshells of C, Sn, and U are performed from 0 to 200 keV above threshold in each case. The comparison shows a remarkable agreement between nonrelativistic dipole and full relativistic multipole results for s subshells, even at the highest energies investigated where nonrelativistic dipole approximation should show significant breakdown; this phenomenon does not occur for l≠0 subshells or in angular distributions. The phenomenon is explained in terms of an approximate cancellation among relativistic, retardation, and multipole effects which occurs only for s subshells