Angular anisotropy parameters for photo-detachment of negative ions

Dipole and nondipole angular anisotropy parameters are investigated in photoionization of negative ions first by deriving general analytical expressions for their near-threshold behavior and then performing numerical calculations for the outer and intermediate subshells of the negative ions $I^-$ and $Si^-$. The results are compared with those of their respective isoelectronic neighbors, Xe and P. The calculations are performed within the one-electron Hartree-Fock approximation and with many-electron correlations considered using the random phase approximation with exchange. The derived analytical expressions for the parameters are used to check the accuracy of our numerical results as well as to demonstrate the range of validity of the Wigner threshold law for negative ions. Differences and similarities in the behavior of the dipole and nondipole parameters for the negative ions and neutral atoms are demonstrated and discussed.