We have observed photoexcitation of the dipole-forbidden 1s(2) 1S0-->2p(2) 1D2 resonance in helium by measuring the nondipolar forward-backward asymmetry of photoelectron angular distributions in the 2l2l(') autoionizing region. By exploiting the electric dipole-quadrupole interference in the excitation of both the 2s2p 1P1 and 2p(2) 1D2 levels, we have observed the quadrupole resonance in photoabsorption and extracted its Fano line shape parameters and the relative phase of the 1sEp and 1sEd continua. We find the quadrupole line profile index q(2) to be markedly different from theoretical expectations.

We report measurements of the scattering of electrons by helium atoms in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 50 -350 eV and the polarization of the laser was arranged to be parallel to electrons scattered through 135 • . Energyshifted peaks corresponding both to one-and two-photon emission were observed. Calculations using the Kroll-Watson approximation are perfectly consistent with the data.

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We have measured (e, 2e) energy spectra in cadmium for the 4d Ss Sp J =1 autoionizing region.The ejected-electron energy range investigated was from 2.6 to 4.8 eV with an incident electron energy of 150 eV and a scattering angle of 3'. Small difterences were found in spectra taken at ejected-electron directions 180 apart. We ascribe these diA'erences to interference terms arising from coherent excitation of the J =1 and overlapping 5pnp J =0,2 autoionizing levels. A calculation of these eftects is presented that agrees quite well with the experimental data. We obtain experimental excitation amplitude ratios and assign a previously undetected autoionizing level to SPY D2.

Theoretical Fano q-parameters for He autoionizing states excited by electron impact are presented, as extracted from first- and second-order hybrid distorted-wave Born + R-matrix calculations. It is found that the q-parameters for (2s2)1S, (2p2)1D, and (2s2p)1P from the first-order calculations are essentially real quantities, while those from the second-order calculations are complex but with a real part similar to the first-order values. These findings are interpreted in terms of the relative phases of the first- and second-order amplitudes.