Photoelectron angular distributions for the outer shell of the alkali-metal atoms

Dirac-Fock calculations of the photoionization of the outer ns electrons in Na, K, Rb, and Cs have been performed. The resulting photoelectron angular distribution asymmetry parameters β show strong variations with energy, in contrast to the nonrelativistic prediction of β=2, independent of energy, for s electrons. This deviation is due to the spin-orbit interaction of the continuum electron which gives rise to two final states εp1/2 and εp3/2, in place of the single εp nonrelativistically. The transitions to the εp1/2 and εp3/2 have matrix elements and phase shifts which differ and interfere, thereby resulting in an energy-dependent β. The effects are particularly dramatic between the Cooper minima in the two channels, and they persist even for Z as low as that for Na.