Dipole-quadrupole interference spectroscopy: Observation of an autoionizing He1DRydberg series

Abstract

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 ${}_{2}{(1,0)}_{n}^{+}{\phantom{\rule{0.28em}{0ex}}}^{1}\phantom{\rule{-0.16em}{0ex}}D$ Rydberg series for $n=2$--7. We find that for $n\ensuremath{\ge}3$ all three quantities have the expected $n$ scaling, with a quantum defect of 0.31. For $n\ensuremath{\ge}3$ the average $q$ parameter lies close to zero, whereas for $n=2$ it is negative.