A new system for photon induced fluorescence spectroscopy applied to the study of doubly excited states of helium

A new system for photon induced fluorescence spectroscopy (PIFS) at the gas phase photoemission beamline (Elettra, Trieste) is described. It consists of a novel non-dispersive detector for VUV photons and a spectrometer for UV/visible light with cooled CCD detector. To illustrate the performance of the apparatus, the fluorescence decay of the doubly excited states of helium below the N= 2 ionization threshold has been studied. Fluorescence emission from both singlet and triplet states has been observed, and in particular weak visible emission resulting from decay of 73D and 83D triplet states confirmed the recently published assignment of these states. This illustrates how PIFS can be used to provide a direct and unambiguous verification of the assignments of states observed in photoabsorption.