Lifetime Measurements of Highly Charged Ions

Abstract

How long does an atom “live” in an excited state, when due to conservation rules and symmetry principles the “normal” electric dipole decay to the ground state is forbidden? Then spin-changing “intercombination” transitions and electric- dipole “forbidden” decays occur that are important for the diagnostics of low-density solar coronal and terrestrial plasmas. Regular “allowed” and intercombination transitions in highly charged ions (picosecond and nanosecond lifetime range) have previously been measured with straight, foil-excited, fast ion beams. For ions with only one or two electrons in the valence shell, very long sections of the isoelectronic sequence have been covered this way, yielding experimental tests of theoretical predictions of transition rates that are good to a few percent. Recently the situation has become much better than this for lifetimes in the millisecond range. Here the technique of optical observation at a heavy-ion storage ring permits atomic lifetime measurements on intercombination and forbidden transitions with a precision that often is better than 1%, which is clearly surpassing the precision range of typical conventional ion trap data. The electron beam ion trap (EBIT) exhibits a similar precision in the “X-ray” range, and it has reached uncertainties of a few percent in some measurements of optical transitions.