$$\require{cancel}$$
Consider a system whose Hamiltonian can be written $H(t) = H_0 + H_1(t).$ Here, $$H_0$$ is again a simple time-independent Hamiltonian whose eigenvalues and eigenstates are known exactly. However, $$H_1$$ now represents a small time-dependent external perturbation. Let the eigenstates of $$H_0$$ take the form $H_0\,\psi_m = E_m\,\psi_m.$ We know (see Section [sstat]) that if the system is in one of these eigenstates then, in the absence of an external perturbation, it remains in this state for ever. However, the presence of a small time-dependent perturbation can, in principle, give rise to a finite probability that if the system is initially in some eigenstate $$\psi_n$$ of the unperturbed Hamiltonian then it is found in some other eigenstate at a subsequent time (because $$\psi_n$$ is no longer an exact eigenstate of the total Hamiltonian). In other words, a time-dependent perturbation allows the system to make transitions between its unperturbed energy eigenstates. Let us investigate such transitions.
$$\newcommand {\ltapp} {\stackrel {_{\normalsize<}}{_{\normalsize \sim}}}$$ $$\newcommand {\gtapp} {\stackrel {_{\normalsize>}}{_{\normalsize \sim}}}$$ $$\newcommand {\btau}{\mbox{\boldmath\tau}}$$ $$\newcommand {\bmu}{\mbox{\boldmath\mu}}$$ $$\newcommand {\bsigma}{\mbox{\boldmath\sigma}}$$ $$\newcommand {\bOmega}{\mbox{\boldmath\Omega}}$$ $$\newcommand {\bomega}{\mbox{\boldmath\omega}}$$ $$\newcommand {\bepsilon}{\mbox{\boldmath\epsilon}}$$