Photorecombination of Metastable Beryllium-like Xenon Ions

Dielectronic recombination (DR) of beryllium-like Xe was investigated by using the ESR electron cooler as a free electron target. Three series of resonances associated with excitations from the 2s ground state could be measured (Fig. 1), namely 2s → 2s2p P1, 2s → 2s2p P1 and 2s → 2s2p P2. The resonances of the 2s2p P2 nj manifolds are much weaker than the ones of the other series. No signatures of DR resonances with excitation from 2s to the P0 state were observed. In addition to the three ground state DR sequences (Fig. 1) we found resonances associated with initially metastable Xe(2s2p P0) ions (Fig. 2). The metastable state is populated during the stripping process and survived the beam preparation, cooling and storage time in the ESR. In the present case, the Xe isotope A = 136 with nuclear spin I = 0 was used. In absence of nuclear spin, the P0 level in Be-like ions is known to be extremely long-lived because a single photon transition to the 2s S0 ground state is forbidden. For I 6= 0 the hyperfine interaction quenches the P0-state which leads to a substantially accelerated decay [1]. For I = 0, as a consequence of strong scaling with the nuclear charge Z , the E1M1 double photon transition becomes a relevant but still very slow decay channel for heavy ions [1, 2, 3]. In contrast to He-like ions (see e.g. [2, 4]), in Be-like ions the