Highly excited states: New experimental windows in photoexcitation

In a recent series of experiments, it has been demonstrated that highly excited states, formed through photoexcitation using vacuum-ultra-violet (VUV) synchrotron radiation, have a significant probability of decaying via the fluorescence decay route. This decay route has products of VUV fluorescence photons and, in some cases, long lived metastable atoms. In helium, studies using high resolving powers at third generation synchrotron light sources utilising this decay route to the metastable atoms have enabled the identification of three series of triplet doubly-excited states below the N = 2 ionisation threshold which are excited by single photon impact from the singlet ground state. The pulsed nature of synchrotron light and the detection of fluorescence photons has been utilised to enable fluorescence lifetime measurements and the determination of the (2p3d)1P lifetime. Fluorescence timing measurements have also allowed the nearly-energy-degenerate ion states 32S, 32P and 32D to be separated through their different lifetimes to allow ion state specific cross sections as a function of energy to be extracted. These groups of measurements have stimulated new theoretical work which explicitly includes both the fluorescence and autoionisation decay routes to reveal for some states a branching ratio in favour of fluorescence.