A two-level atom driven by a near-resonant field undergoes Rabi oscillations. Population |c₂|² oscillates at the generalized Rabi frequency Ω_R = √(Ω₀² + Δ²) where Δ is the detuning.
On resonance (Δ=0): complete population inversion every T=π/Ω₀. Off resonance: incomplete inversion, faster oscillation.