DIQUIS observation of n=2$\to$7 He $2pnp~^1D$ autoionizing resonances.

We have applied the technique of DIpole-QUadrupole Interference Spectroscopy (DIQUIS) to make the first observation of an optically forbidden Rydberg series using photoelectron spectroscopy. The He $2pnp~^1D$ autoionizing levels appear as a series of resonances in the non-dipole $\gamma$ parameter.\footnote{B. Kr\"assig, E.P. Kanter, S.H. Southworth, R. Guillemin, O. Hemmers, D.W. Lindle, R. Wehlitz, and N.L.S. Martin, Phys. Rev. Lett. {\bf 88}, 203002 (2002).} The experiments were carried out at the Synchrotron Radiation Center, University of Wisconsin-Madison using an electron spectrometer system, designed and built at Argonne National Laboratory, to efficiently measure nondipole asymmetries in photoelectron angular distributions. We will present measurements of $\gamma$ for photon energies that cover the $n=2\to7$ resonance region. Fits to the data yield values of level positions, widths, and Fano $q$ parameters.