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Threshold photoelectron-photoelectron coincidence experiments have been used to study the dissociative double ionisation of O2. The results show that the main dissociative process involves the formation of neutral excited oxygen atoms.

Recent experimental investigations of double ionization of rare gases and diatomic molecules induced by photons from a synchrotron radiation source are presented. A novel technique where two electrons with near zero energy are detected in coincidence is described. New experimental developments of the photoelectron-photoelectron coincidence technique, made possible by third generation synchrotron radiation sources, are addressed

We present high-resolution photoelectron spectra (PES) of D2in the photon energy range from 15.648 to 15.677 eV. This range encompasses autoionizing Rydberg states of D2converging on various vibrational levels of the X 2?g+state of the D2+ion. The PES show features that correspond to the transitions, D2X 1?g(v= 0, N= 0) ? D2+X 2?g+(v+= 0, N+= 0, 2, and 4). This is the first time that a change of ?N= N+? N= 4 in rotational angular momentum has been reported in a PES, where the magnetic field was negligible (less than a few milligauss). The autoionizing state giving rise to the ?N= 4 transition is observed at a photon energy of 15.674 eV and is assigned to the R(2) n= 5p? Rydberg state converging to the v+= 4 vibrational level of the D2+ion. It is suggested that the large change in rotational angular momentum results from the relatively long lifetime of this state.

Photoelectron angular distributions in H2and their dependence on photon energy have been measured for the first time with full rotational selection. In particular, the transitions X1?g(? = 0, N= 0) ? X2?+g(?+= 0, N+= 0, 2) have been studied in the energy region from 15.47 to 15.75 eV. This energy region is dominated by autoionizing structure, largely Rydberg states converging on the ?+= 1 threshold of the H2ion. VUV radiation from the GASPHASE beamline at the Elettra Synchrotron Radiation Source has been used, in conjunction with a hemispherical photoelectron spectrometer featuring a magnetic angle-changing device, to measure the photoelectron angular distributions of the rotationally selected states. Variations of the asymmetry parameter, ?, with photon energy have been extracted from the angular distributions. The value of the asymmetry parameter for the 0-2+transition is essentially constant, despite the presence of a large number of resonances, and has a value close to 0.2, consistent with p-wave electron emission during the autoionization process. Although variations in ? are observed for the 0-0+transition, these are attributed to 2-2+contributions and the underlying constant value is also then consistent with p-wave emission.

Photoelectron angular distributions in H₂ and their dependence on photon energy have been measured for the first time with full rotational selection. In particular, the transitions X¹Σ_g(ν = 0, N = 0) → X²Σ⁺_g(ν⁺ = 0, N⁺ = 0, 2) have been studied in the energy region from 15.47 to 15.75 eV. This energy region is dominated by autoionizing structure, largely Rydberg states converging on the ν⁺ = 1 threshold of the H₂ ion. VUV radiation from the GASPHASE beamline at the Elettra Synchrotron Radiation Source has been used, in conjunction with a hemispherical photoelectron spectrometer featuring a magnetic angle-changing device, to measure the photoelectron angular distributions of the rotationally selected states. Variations of the asymmetry parameter, β, with photon energy have been extracted from the angular distributions. The value of the asymmetry parameter for the 0–2⁺ transition is essentially constant, despite the presence of a large number of resonances, and has a value close to 0.2, consistent with p-wave electron emission during the autoionization process. Although variations in β are observed for the 0–0⁺ transition, these are attributed to 2–2⁺ contributions and the underlying constant value is also then consistent with p-wave emission.

Differential cross sections for photoionization into the 5d96s2 2D5/2ionic state of mercury have been obtained over the photon energy range from 15 eV to 17 eV. By measuring the photoelectron emission at the angles of 0oand 90owith respect to the major axis of the polarization ellipse the variation of the asymmetry ?-parameter for the 2D5/2ionic state of mercury has been determined as a continuous function of photon energy over the above range. This energy range contains three series of autoionizing states converging to the 2D3/2ionization limit. The measurements have been performed using the 5 m McPherson normal incidence monochromator at the Daresbury Synchrotron Radiation Source with a photon energy spread of typically 3 meV.

The vibrationally resolved partial ionization cross sections of the X, A and B states of N2+in the photon energy range ~20-35 eV have been characterized by photoelectron spectroscopy. Vibrational intensity ratios of the X, A and B states have been determined. The vibrational intensity ratios of the X and B states show a wealth of both sharp and broad features. In accordance with previous experimental and theoretical studies, these structures have been assigned either to one-electron processes, like the interchannel coupling between the 3?gand 2?uionization channels and the autoionization of excited Rydberg states, or to two-electron processes, like the recently observed non-Rydberg doubly excited resonances (NRDER). The present study provides direct information on the autoionization of the NRDER states and the experimental evidence of their predicted, but not yet verified, selective decay to the X, A and B states of N2+.