Wavelengths of the 3d6(5D)4s a6D-3d5(6S)4s4p y6P multiplet of Fe II (UV 8)

Weinvestigate the wavenumber scale of Fe I and Fe II lines using new spectra recorded with Fourier transform-spectroscopy and a reanalysis of archival spectra. We find that standards in Ar II, Mg I, Mg II, and Ge I give a consistent wavenumber calibration. We use the recalibrated spectra to derive accurate wavelengths for the a(6)D - y(6)P multiplet of Fe II (UV 8) using both directly measured lines and Ritz wavelengths. Lines from this multiplet are important for astronomical tests of the invariance of the fine-structure constant on a cosmological time scale. We recommend a wavelength of 1608.45081 angstrom with one standard deviation uncertainty of 0.00007 angstrom for the a(6)D(9/2) - y(6)P(7/2) transition.