Spectra of lutetium obtained with a sliding spark were photographed in the wavelength range 400–3200 Å. Wavelengths of 246 lines, comprising all lines identified as Lu iv appearing in the spark at 50-A peak current, are given. Fifty-seven energy levels were deduced from these lines, including nearly all levels of the 4f13(5d, 6s, 6p, 6d, and 7s) configurations and their connection to the 4f14 1S0 ground state. An ionization energy of 364 500±200 cm−1 was derived. The energy levels are interpreted by means of a theoretical analysis utilizing fitted radial parameters. With these results, trends of Slater and spin–orbit parameters and configuration energy differences for fourth spectra of the rare earths are found. The fundamental energy differences between lowest levels of the 4fN and 4fN−15d configurations for the fourth and fifth spectra are deduced.
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