The Department of Physics, Stockholm University
Friday 13 December
13:00 - 15:00
In this thesis I present measurements using laser photodetachment to investigate negative ions of iridium produced in a cesium sputter ion source and stored in the cryogenic storage ring, DESIREE. The ions are photodetached by either a tunable OPO laser system or a tunable narrow bandwidth Titanium Sapphire laser. The vacuum conditions of DESIREE allowed us to store the ions for several thousand seconds and investigate the lifetimes of longlived excited states. Iridium has an electron affinity of 1.56436(15) eV and the ground state electron configuration is [Xe]4f14 5d8 6s2 3F4 , where the term scheme is determined by the partly filled 5d shell. Predicted bound excited states are metastable as they are of the same configuration and therefore the same parity as the ground state. The 3F3 level can decay to the ground state by a magnetic- dipole transition while the 3P2 level can only decay by an electric-quadrupole transition. The higher lying fine structure components, 3F2 and 3P1 , have been assumed to be unbound. We performed systematic studies of the photodetachment signal as function of photon energy and time after ion injection (milliseconds to 2000 seconds). We locate the threshold for detachment from the 3P2 level and find that this level is bound by 1.00 ± 0.03 eV. The lifetime of this state could only be estimated with a lower bound since it does not decay within the timeframe of the experiment. A lower limit of the lifetime is 1000 s. The previously observed 3F3 state was not observed in this experiment most likely due to a short lifetime of < 100 ms. A third excited state was observed with a lifetime of 146 ± 20 ms. This is most likely the lifetime of one of the two states 3F2 or 3P1 , that was previously predicted to be unbound.