where k runs over the two quartet states. D 0 is the residual splitting of the quartets due to secondorder interaction with higher-lying states, ∆ is the initial splitting between the spatial quartet states, and γ is the (imaginary part of the) first-order spin-orbit coupling between the quartets. By using Eq. 3, the energy spectrum of 1 can be fit very accurately, see Fig. ); the corresponding values are listed in Table .