Two-photon absorption in atomic or molecular systems

In this paper the possibility of observing two-photon absorption, due to the term (e2/2mc2)A2 in the interaction Hamiltonian of an atomic or molecular system with an electromagnetic field, is considered. The dependence of these processes on the coherence and polarization of radiation is studied. It is found that two-photon absorption between states with different parity should be readily observable by using a laser.