Double-excitation and double-escape processes studied by photoelectron spectroscopy near threshold

Recent experimental work using photons to study double excitation, double escape and related processes near their respective thresholds is reviewed. These processes are of significant current interest because they are completely determined by electron-electron correlations. In particular, experimental studies based on the techniques of low-energy electron spectroscopy are described. By using selected examples from coincidence and non-coincidence experiments the various kinds of dynamic and spectroscopic information that can be obtained are illustrated. Recent results, mainly in the rare gases, are described and their interpretation is discussed. Future perspectives are considered.