Fig. 2. Measurement and assignment of low-lying electronic states in [Yb(trensal)].

Experimental luminescence (emission) spectrum of [Yb_0.07Lu_0.93(trensal)] (1′) at 5 K¹³. Transitions are from the lowest KD of the excited ²F_5/2 spin–orbit multiplet to the different KDs of the ground ²F_7/2 multiplet (inset left; not to scale) and occur in the near-IR around 980 nm^13,60. The spectrum is plotted as energy differences with respect to the zero-phonon line of the ground KD (peak 1), thus the spectrum is reversed compared to a conventional emission spectrum. The energies of the four KDs of the ²F_7/2 multiplet, as determined from the spectrum relative to the ground KD at 0 cm⁻¹ (1) are: 474 cm⁻¹ (2), 745 cm⁻¹ (3) and 920 cm⁻¹ (4). Additional features are at 169 cm⁻¹ (1a), 207 cm⁻¹ (1b), 247 cm⁻¹ (1c), 302 cm⁻¹ (1d), 403 cm⁻¹ (2a) and 524 cm⁻¹ (2b), 588 cm⁻¹ (2c), 864 cm⁻¹ (4a).