Figure 3. Vibronic model.

(a) We consider a vibronic model for bands 1 and 3 coupled to a vibrational mode with frequency ν₁≈668 cm⁻¹ (see Supplementary Note 2). The vibronic states |k₀〉 and |k₁〉 denote the vibrational ground and first excited states of an electronic state |k〉, respectively, with the single index states |g〉, |1〉 and |3〉 denoting the electronic ground state and bands 1 and 3, respectively. The exciton energy splitting ΔΩ₃₁=Ω₃−Ω₁ between bands 1 and 3 is quasi-resonant with the vibrational frequency ν₁, where the detuning is denoted by Δν₁=ΔΩ₃₁−ν₁. The exciton–vibrational coupling between uncoupled states |3₀〉 and |1₁〉 leads to vibronic eigenstates and , each of which is a superposition of |3₀〉 and |1₁〉, leading to an energy-level shifting by δω. (b) The time trace of N11 in normalized intensity (Norm. Int.) against waiting time t₂, where the experimental results are shown as light red circles and the theoretical simulation is shown as a full red line. (c) The time trace of R31 where the experimental results are shown as light blue circles and the simulated data are depicted as a full blue line. The root mean squared deviation between the experimental results and theoretical simulation in b and c is 0.92 and 0.59, respectively.