Note the similarity to Doppler cooling in the unbalanced radiation pressures due to the atomic motion. The unbalanced pressure in PG cooling is not due to a Doppler shift but an induced population imbalance. Doppler cooling depends on the parameter k v Γ Γ where Γ Γ is the scattering rate, whereas PG cooling depends on k v Δ Δ 0 ′ . At low intensity Δ Δ 0 ′ ≪ ≪ Γ Γ and thus PG cooling works at lower atomic velocities (temperatures) than Doppler Cooling.