Ch 21.3 The transition rate from state $|i\rangle$ to a continuum of final states $|f\rangle$ under a perturbation $\hat{V}$: $\Gamma_{i\to f} = \frac{2\pi}{\hbar}|\langle f|\hat{V}|i\rangle|^2\rho(E_f)$, where $\rho(E_f)$ is the density of final states. *See also:* Time-dependent perturbation theor