Term Analysis of Fe IV

The continued analysis of the spectrum of triply ionized iron, Fe IV, has led to a practically complete description of the three lowest electron configurations. Of 3 d 5 all 37 levels are established, of 3 d 4 4 s only one of the 63 levels is missing and of 3 d 4 4 p 177 of the 180 possible levels are found. The structure of the three configurations is analysed by least-squares fits to the observed levels and by Hartree-Fock calculations. The level values have been determined with an uncertainty of less than 0.4 cm -1 from 706 lines of the transition group 3 d 5 -3 d 4 4 p , extending over the region 446-789 Å, and 560 lines of the group 3 d 4 4 s -3 d 4 4 p in the region 1247-2028 Å. The results are based on spectrograms obtained by means of a low-voltage sliding-spark discharge and the two 10.7 m vacuum spectrographs at the National Bureau of Standards in Washington, DC Predicted wavelengths and intensities of the forbidden transitions within 3 d 5 are collected in a list containing 95 entries from 1370 to 9393 Å. A comparison with Thackeray's line list for RR Telescopii shows an excellent agreement between prediction and observation as regards both wavelengths and intensities.