Atomic Lifetimes of Astrophysical Interest in Ions of Fe

Abstract

Multiple charged ions of iron dominate the EUV spectrum of the solar corona. For the interpretation of such spectra, data on both the atomic structure and the transition rate are essential, most of which are provided by theory and computation. The wavelengths of observed spectra are used to test the predicted energy level structure, while the line intensities depend on level lifetimes and branch fractions. A number of electric dipole and higher-order transition rates have been measured over the years in the laboratory, mostly by beam-foil spectroscopy, at heavy-ion storage rings, and at various ion traps. In this paper, the state of the knowledge base on level lifetimes in all ions of Fe is assessed, and the problems of further progress are outlined.

Long-Lived Levels in Multiply and Highly Charged Ions

“…For three of these, lifetime measurements at a heavy-ion storage ring have yielded results with error bars from 5 to 10%. The results agree with theory within 50 to 80%, which is not satisfying, neither for the measurement precision nor for the accuracy need for reliable information (see [44] for references).…”

“…Lifetime measurements of long-lived or metastable levels have variously been summarized in reviews (for example, see [36][37][38][39][40][41]). Key problems in measuring such long lifetimes accurately have been identified in [42][43][44]. Let me use these reviews as proxies in place of citations of the multitude of original publications, with a few topical exceptions.…”

“…The one level of particular interest in the present context of long-lived levels is the 2s2p 3 P o 1 level in low-Z ions, where it is very long-lived, indeed: for B + [45], C 2+ [46], N 3+ , and O 4+ [47] ions the level lifetimes are in the millisecond range in contrast to the nanosecond range for the majority of levels, and they can be measured with an uncommonly high accuracy, with uncertainties of well below 1%. An important point is that there is no particularly accurate clock or fancy complex detection scheme necessary to achieve these accuracies (which are high only in the field of atomic lifetime measurements, see the discussion in [44]); the decisive element is that the lifetime of interest is drastically different from all others in the same atomic system. As soon as there are several lifetimes of the same order of magnitude involved, the statistical ambiguity of the evaluation of superpositions of exponential decay curves severely limits the precision and accuracy of the lifetime result sought.…”