Dipole-quadrupole interference spectroscopy: Observation of an autoionizing He 1D Rydberg series

We have used dipole-quadrupole interference spectroscopy to observe an optically forbidden quadrupole Rydberg series of helium autoionizing levels. The technique measures the forward-backward asymmetry of photoelectron angular distributions produced in the vacuum ultraviolet photoionization of helium. The resonant behavior of this asymmetry in the region of a quadrupole autoionizing level enables the determination of the position, width, and Fano line-profile parameter q of the level. We have obtained these quantities for the He ₂(1,0)n+1D Rydberg series for n=2-7. We find that for n≤3 all three quantities have the expected n scaling, with a quantum defect of 0.31. For n≤3 the average q parameter lies close to zero, whereas for n=2 it is negative.