Panning for Gold: New Emission Lines from UV–VIS Spectroscopy of Au i and Au ii

Abstract

The recent detection of a neutron star merger by the LIGO collaboration has renewed interest in laboratory studies of r -process elements. Accurate modeling and interpretation of the electromagnetic transients following the mergers requires computationally expensive calculations of both the structure and opacity of all trans-iron elements. To date, the necessary atomic data to benchmark structure codes are incomplete or, in some cases, absent entirely. Within the available laboratory studies, the literature on Au i and Au ii provides incomplete reports of the emission lines and level structures. We present a new study of Au i and Au ii lines and levels by exposing a solid gold target to plasma in the Compact Toroidal Hybrid (CTH) experiment at Auburn University. A wavelength range from 187 to 800 nm was studied. In Au i , 86 lines are observed, 43 of which are unreported in the literature, and the energies of 18  levels and 16 of the 18 known  levels are corroborated by a least-squares level energy optimization. In Au ii , 76 emission lines are observed, and 51 of the lines are unreported in the literature. For both Au i and Au ii , the new lines predominantly originate from the most energetic of the known levels, and over half of the new Au ii lines have wavelengths longer than 300 nm. For the estimated electron parameters of CTH plasmas at the gold target ( n e  ∼ 10 12 cm −3 , T e  ∼ 10 eV), two-electron transitions are similar in intensity to LS-allowed one-electron transitions.