Laboratory measurements of Fe XXIV line emission:: 3 → 2 transitions near excitation threshold

Using the Electron Beam Ion Trap facility at Lawrence Livermore National Laboratory, we have measured relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. The measurements include line formation by direct electron impact excitation (DE), radiative cascades, resonant excitation (RE), and dielectronic recombination (DR) satellites with captured electrons in n greater than or equal to 5 levels. Good agreement with R-matrix and distorted wave calculations is found. In collisionally ionized plasmas, at temperatures near where the ion abundance peaks (kT(e) similar to 1.7 keV), the RE contributions are found to be less than or similar to 5% of the line emission, while the DR satellites contribute less than or similar to 10%. While good agreement with state-of-the-art atomic physics calculations is found, there is less good agreement with existing spectral synthesis codes in common astrophysical use. For the Fe XXIV 3p(3/2) --> 2s(1/2), 3p(1/2) --> 2s(1/2), and 3d(5/2) --> 2p(3/2) transitions, the synthesis code MEKAL underestimates the emissivity in coronal equilibrium by similar to 20% at temperatures near where the ion abundance peaks. In situations where the ionization balance is not solely determined by the electron temperature, RE and DR satellites may contribute a considerable fraction of the line emission.