Towards a measurement of the 2s2p ³P₀ → 2s^{2 1}S₀ E1M1 two photon transition rate in Be-like xenon ions

Dielectronic recombination (DR) of Be-like ¹³⁶Xe⁵⁰⁺ was measured by employing the electron-ion merged-beam technique at the storage ring ESR of GSI in Darmstadt, Germany. We present results covering the center-of-mass energy range 0 − 4 eV. In addition to Rydberg series of resonances associated with ground-state intra L-shell excitations, resonances were found which are formed during DR of initially metastable ¹³⁶Xe⁵⁰⁺(2s2p³P₀) parent ions. This state is the first excited state and is long lived because the J=0 → J=0 transition and the absence of nuclear spin make the rare E1M1 two-photon transition the lowest-order decay channel. Future measurements which will monitor the strength of the resonances as function of storage time may be exploited for a measurement of the E1M1 two photon transition rate.