Angular distributions of ejected electrons from autoionizing 3pndstates of magnesium

We have measured the angular distributions of electrons ejected from doubly excited autoionizing Rydberg 3pnd J=1 and 3 states of magnesium, where n=10–18. To excite the aligned 3snd ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ states of Mg, we used two dye lasers crossed with an atomic beam. A third laser, linearly polarized in the same direction as the other two, excited the atoms in the 3snd states to the 3pnd states. The angular distributions of ejected electrons are given by dσ/dΩ=${\mathit{tsum}}_{\mathit{k}}$ ${\mathit{a}}_{\mathit{k}}$ ${\mathit{P}}_{\mathit{k}}$(cosθ), where k=0,2,4,6. We measured the angular distribution parameters ${\mathit{a}}_{\mathit{k}}$ as a function of the energy of the third laser photon using an electron detector and a ${\mathrm{Mg}}^{+}$ ion detector. We compare our results with the predictions of an R-matrix multichannel quantum-defect theory. We find good agreement between experiment and theory.