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Angular correlation in the two-electron continuum of the He double photoionization has been studied, both experimentally and theoretically, for equal and unequal energy sharing conditions at the photon energy of 40 eV above the threshold. The triple differential cross sections have been measured in the plane perpendicular to the photon direction using the multi-coincidence detection technique of Reddish et al at the Daresbury Synchrotron Radiation Source. Recent modifications to the bending-magnet beam line allowed an effective cancellation of the circular polarization at the target, leaving a relatively high degree of linear polarization (Stokes parameters: S 3 = S 2 ∼ = 0, S 1 = 0.8). The measured cross sections are compared with the calculations using the 3C method of Maulbetsch and Briggs and the convergent close coupling method of Kheifets and Bray. Good agreement between theory and experiment has been found in most cases, except for the unequal energy sharing when one of the escaping electrons is detected in a direction close to the polarization axis.

A versatile electron-ion coincidence spectrometer for photoionization studies is described. The spectrometer incorporates a 127 degrees cylindrical electrostatic energy analyser and a time-of-flight energy analyser. The two analysers are mounted on a turntable that can be rotated with respect to the polarization axis of the photon beam. Both analysers have a penetrating field stage which provides very high detection efficiency over a large solid angle and excellent threshold energy resolution ( approximately 3 meV) for the detection of charged particles. The analysers can also detect energetic particles. The time-of-flight analyser is used specifically for the separation and detection of different charge states of molecular photodissociation products. The use of the spectrometer to observe photoelectron-photo-ion coincidences and photoelectron-photoelectron coincidences in a variety of experiments (TPEPICO, PEPICO, TPEPECO) is illustrated by a study of the photo-double ionization of argon and of the dissociation products from the v=0 level of the c state of oxygen at 24.56 eV. Additionally, a spectrum of coincidences between threshold photoelectrons (TPESCO) is presented in the region of the 3P double ionization potential of argon.

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Double ionization of O2 induced by photons from a synchrotron radiation source has been studied in an electron-electron coincidence experiment. A new technique has been developed wherein only electrons with near zero energy are detected by use of the penetrating field method. In this way high sensitivity and energy resolution have been attained allowing nineteen vibrational levels of the (>2 2+ ground state to be observed and molecular parameters of this state to be obtained.