Observation of interference between quadrupole and dipole transitions in low-energy (2-eV) photoionization from a sodium Rydberg state

A measurement of the azimuthal dependence of the angular distribution of photoelectrons from ${13}^3$ ${\mathrm{D}}_{3/2}$-state sodium atoms aligned transversely to the direction of propagation of, and parallel to the direction of linear polarization of, 532-nm ionizing laser radiation, is described. The measured distribution is I(φ)=1+0.026(6)cosφ where φ=0 is the direction of the laser beam. This departure from the symmetry I(φ)=const predicted for a pure electric dipole transition is represented theoretically as due to a dipole-quadrupole interference term.