Differential cross sections for electron impact excitation of the first four excited states of helium have been measured for electron impact energies of 30, 40 and 50 eV. A novel `magnetic angle-changing' technique has been used which allows the full angular range from to to be covered for the first time. Serious discrepancies between theory and experiment have been found at angles that were previously inaccessible experimentally.

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The angular distribution parameter for the Xe ion state has been studied for the Xe autoionization resonances in the energy range between 20 and 24 eV using tunable synchrotron radiation from the Daresbury Laboratory SRS. A new magnetic angle-changing technique has been used and is reported for the first time in photoionization measurements. This technique allows the angular measurements to be made without any movement of the electron spectrometer. The present measurements have been made as continuous functions of both photon energy and photoelectron ejection angle which highlights the dramatic effect of those autoionizing resonances on the angular distribution parameter.

Autoionization processes in Oz have been studied by measuring photoelectrons in the photon energy region between the 0; X VIs and a ionic states (12.4-15 ev). The present experimental results provide comprehensive information about autoionization dynamics since the decay routes from the spin-orbit components ofthe neutral autoionizing states to the two spin-orbit components of the ground ionic state are resolved. The intensity ratio of the 0 : X'n,,, and ' I I , , > components has been determined and strong Sa -+ Sl' selectivity has been observed. The observed selectivity is explained in terms of a model in which the orbital angular momentum along the intemuclear axis of the positive ion core remains unchanged during the autoionization process. Using this model, Rydberg orbitals of the I, I' and I" autoionizing states are reassigned as 4sc,, 3d6, and 3doE,, respectively, and the spin-orbit constant of the J state is determined.

Rotationally selective vibrational autoionization in molecular hydrogen has been investigated using two-dimensional photoelectron spectroscopy. Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron VUV radiation between the upsilon =0 and upsilon =1 vibrational thresholds of H2+X2 Sigma g+, 15.45-15.70 eV. Constant rotational transition energy spectra have been extracted revealing the decay routes of autoionizing states. No large changes in the rotational quantum number of the ion core during autoionization were observed.