FIGURE 2. Low-energy EPR transitions.

These calculated diagrams used parameter values g  1.96, J₀  -2.45 K, D_Z  43 mK and D_XY = 0 (see text for details). a, The magnetic field is parallel to the c axis. Whereas the orbital singlet ⁴A₂ (S = 3/2) gives the superposition of the three transitions 1, 2, and 3, the orbital doublet ²E (S = 1/2) gives two inter-doublet transitions 4 and 6 which are basically allowed, as well as two intra-doublet transitions 5 and 7 (which are respectively allowed by transverse DM interactions and non-symmetrical exchange interactions due to a small deviation from the trigonal symmetry²⁴ (Methods)). Second order zero-field splitting of ⁴A₂ and small splitting of the lines 1, 2, and 3 is not shown. The M_J labels correspond to the quantization axis along the DM anisotropy field. b, Shown are the transitions 4, 5, 6, and 7 for the angle = 45° between the field and cluster C₃-axis. The M_J labels correspond to the quantization axis along the field in the strong field limit. The boundaries of the measured resonance fields of Fig. 4 correspond to the field distribution given by the positions of labels 4, 5, 6 and 7 above the curves. The blue transitions correspond to S = 3/2; the red and the green transitions correspond to S = 1/2.