Charge‐Exchange Spectra of Hydrogenic and He‐like Iron

We present H-like Fe XXVI and He-like Fe XXV charge-exchange spectraresulting from collisions of highly charged iron with N2 gas at an energy of 10eV/amu in an electron beam ion trap. Although individual high-n emission linesare not resolved in our measurements, we observe that the most likely level forFe25+ --> Fe24+ electron capture is n~9, in line with expectations, while themost likely value for Fe26+ --> Fe25+ charge exchange is significantly higher.In the Fe XXV spectrum, the K-alpha emission feature dominates, whetherproduced via charge exchange or collisional excitation. The K-alpha centroid islower in energy for the former case than the latter (6666 versus 6685 eV,respectively), as expected because of the strong enhancement of emission fromthe forbidden and intercombination lines, relative to the resonance line, incharge-exchange spectra. In contrast, the Fe XXVI high-n Lyman lines have asummed intensity greater than that of Ly-alpha, and are substantially strongerthan predicted from theoretical calculations of charge exchange with atomic H.We conclude that the angular momentum distribution resulting from electroncapture using a multi-electron target gas is significantly different from thatobtained with H, resulting in the observed high-n enhancement. A discussion ispresented of the relevance of our results to studies of diffuse Fe emission inthe Galactic Center and Galactic Ridge, particularly with ASTRO-E2/Suzaku.Comment: 16 pages, 4 figures (3 color), accepted by Ap