\begin{equation}
P_{\nu'} - P_{\nu} = \frac{1}{2}\frac{\hbar}{\mu\omega_e}\left[P^{[2]}-\frac{1}{\mu\omega_e^2}P^{[1]}E^{[3]}\right]\Delta\nu;\quad \Delta\nu=\nu' - \nu
\end{equation}