Charge-Exchange Spectra of Hydrogenic and He-like Iron

We present H-like Fe XXVI and He-like Fe XXV charge-exchange spectra resulting from collisions of highly charged iron with N2 gas at an energy of ~10 eV amu-1 in an electron beam ion trap. Although individual high-n emission lines are not resolved in our measurements, we observe that the most likely level for Fe+25 → Fe+24 electron capture is nmax ~ 9, in line with expectations, while the most likely value for Fe+26 → Fe+25 charge exchange is significantly higher. In the Fe XXV spectrum, the Kα emission feature dominates, whether produced via charge exchange or collisional excitation. The Kα centroid is lower in energy for the former case than for the latter (6666 vs. 6685 eV, respectively), as expected because of the strong enhancement of emission from the forbidden and intercombination lines, relative to the resonance line, in charge-exchange spectra. In contrast, the Fe XXVI high-n Lyman lines have a summed intensity greater than that of Lyα and are substantially stronger than predicted from theoretical calculations of charge exchange with atomic H. We conclude that the angular momentum distribution resulting from electron capture using a multielectron target gas is significantly different from that obtained with H, resulting in the observed high-n enhancement. A discussion is presented of the relevance of our results to studies of diffuse Fe emission in the Galactic center and Galactic ridge, particularly with Astro-E2/Suzaku.