ABSTRACT
When the Hamiltonian of the system depends on time, the solutions of the Schrödinger equation are non-stationary. Thus the concept of bound states with discrete energy levels and stationary eigenstates should be modified. There are three ways to make these modifications corresponding to the particular kind of approximations, depending on the nature of time dependence of the Hamiltonian. They are:
the time dependent perturbation,
the harmonic approximation,
the adiabatic approximation,
the sudden approximation.
