HIGH-LYING LONG-LIVED DOUBLY-EXCITED STATES NEAR IONIZATION THRESHOLDS OF HELIUM

Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the He+(N = 1,2,3 and 4) ion ization thresholds. The measurements have been made using an unconvent ional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are pr evented from reaching the detector. For N greater than or equal to 2, it appears that the formation, via photoexcitation, of relatively long -lived, doubly excited Rydberg states is a necessary step in the produ ction of the observed signal. Four processes that might account for th e production of the excited neutral particles are considered. The simp lest, that atoms in the initial doubly excited Rydberg states are them selves directly observable, is considered unlikely. However, the lifet imes of the photoexcited double Rydberg states may be increased throug h the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of col lisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpre tation of other photon impact measurements.