The population probability of different energy levels (a) |g,0〉, (b) |g,1〉, (c) |e,0〉, (d) |g,2〉, (e) |e,1〉, (f) |g,3〉, (g) |e,2〉, (h) |g,4〉, (i) |e,3〉 against the normalized coupling constant f = λ/ω_c for different α in the resonant case. Red solid lines represent α = 0, blue dotted lines represent |α| = 0.5, and green dot–dashed lines represent |α| = 1. In panels (e), (h), and (i), the differences among the curves are very small.