Measurement and calculation of absolute cross sections for excitation of the 3 s 2 3 p P 1 ∕ 2 o 2 − 3 s 2 3 p P 3 ∕ 2 o 2 fine-structure transition in Fe 13 +

Experimental cross sections are reported for the ${\mathrm{Fe}}^{13+}$ coronal green line transition $3{s}^{2}3p\phantom{\rule{0.3em}{0ex}}^{2}P_{1∕2}^{o}\ensuremath{-}3{s}^{2}3p\phantom{\rule{0.3em}{0ex}}^{2}P_{3∕2}^{o}$ at $\ensuremath{\lambda}5303\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}\phantom{\rule{0.3em}{0ex}}(2.338\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The center-of-mass interaction energies are in the range $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (below threshold) through threshold, to $6.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ($2.9\ifmmode\times\else\texttimes\fi{}$threshold). Data are compared with results of a 135-level Breit-Pauli $R$-matrix theory. Present experiment detects a strong maximum in the excitation cross section of magnitude $15\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ at $2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Smaller structures are observed between $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $6.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with a maximum cross section never exceeding about $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$. All features are due to enhancement of the direct excitation via a multitude of narrow, closely-spaced resonances calculated by the theory, the effects of which are convoluted by the $125\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ energy resolution of the present experiment. Iron is present in practically every astrophysical object, as well as being an impurity in fusion plasmas. Present results are the highest charge state in any ion for which an absolute excitation cross section has been measured.