Wavelengths of the 3d6(5D)4s a6D - 3d6(5D)4p y6P Multiplet of Fe II (UV 8)

TL;DR

This paper refines the wavelength measurements of the Fe II UV 8 multiplet using Fourier transform spectroscopy, providing highly accurate data crucial for testing fundamental physics in cosmology.

Contribution

It presents a recalibration of Fe II spectral lines with improved accuracy, aiding astronomical measurements of the fine structure constant over cosmic time.

Findings

Recalibrated wavelengths for Fe II UV 8 multiplet.

Recommended wavelength for a6D9/2-y6P7/2 transition: 1608.45081 Å.

Uncertainty of 0.00007 Å in the wavelength measurement.

Abstract

We investigate the wavenumber scale of Fe I and Fe II lines using new spectra recorded with Fourier transform spectroscopy and using a re-analysis 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 a6D-y6P 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 {\AA} with a one standard deviation uncertainty of 0.00007 {\AA} for the a6D9/2-y6P7/2 transition.