We analyze the eigen spectrum of ĤS , i.e., the Tavis-Cummings model, in the ground and single excitation subspace, which is spanned by the zero photon-dressed ground state |G, 0⟩ where all the molecules are in the ground state and no photon in the cavity, one photondressed ground state |G, 1⟩ where all the molecules are in the ground state and one photon is in the cavity, and the single-molecule excited state |E n , 0⟩ where all the molecules are in the ground state except for the n th molecule. These diabatic states are defined as