Grotrian diagram showing the lowest energy levels in Y⁺. The transitions between energy levels are indicated, with the line transparency set by the transition strength, log gf. The 4d5p–4d² (28 000 cm⁻¹–14 000 cm⁻¹) transitions are shown with thick lines in blue. Radiative transfer modelling analysis in Vieira et al. (2023) tentatively links the 4d5p–4d² (28 000 cm⁻¹–8000 cm⁻¹) transitions to absorption at λ < 450 nm in the epoch 1 spectrum; other notable transitions are around 300 nm, too short to be observed by X-shooter (4d5p–4d5s, 28 000 cm⁻¹–1000 cm⁻¹), and signatures of the 5s5p–4d² transitions (24 000 cm⁻¹–14 000 cm⁻¹) at 1 μm, which are much weaker than the Sr⁺ line transitions at similar wavelengths. The 4d5p–4d² transitions have lower oscillator strength than several of the Y⁺ lines at shorter wavelengths, but due to the rapid decline of opacity with wavelength for typical r-process elements, these transitions have higher opacity compared to the continuum at 760 nm. When no lines are drawn between levels, the transition is negligible with log gf < −2.