Isolation of s-states in He-like uranium: Two-photon decay
 
In this study projectile x-ray emission from relativistic collisions of initially Lithium-like uranium (U89+) with gas target atoms is measured in coincidence with up-charged projectiles (ions which lost one electron; U90+). The K-shell ionizarion occuring in such collisions leads to formation of excited states He-like ions. Surprisingly, when observing the x-rays from subsequent decays of these states, only two out of all possibe transitions can be indetified; namely the M1 decay of the 3S1 and the two-photon (2E1) decay of the 1S0 state. As a consequence, K-shell ionization of high-Z Li-like ions in fast atomic collisions turned out to be a highly selective mechanism for the population of the 2S-states in the He-like species, namely the 1s2s[1S0] and the 1s2s[3S1] state. Besides providing a direct access to two-photon decay investigations, this mechanism is of importance for selective production and precision spectroscopy of excited states in high-Z He-like systems where up to now very few experimental data exist.