At the same time, there will be a process of atomic absorption which removes energy from the field while raising electrons from the lower state to the upper state. Its rate is given by an essentially identical equation, ∂ ∂ N 2 ∂ ∂ t = − − ∂ ∂ N 1 ∂ ∂ t = B 12 ρ ρ ( ν ν ) N 1 .