Harmonic generation in free molecules

A molecular theory of harmonic generation in free molecules is developed using the principles of quantum electrodynamics. A Cartesian tensor formalism is employed and emphasis is placed on the rotational average which is required to account for the random molecular orientations in the pump beam. It is demonstrated that even harmonics are forbidden to all orders of the multipolar interaction Hamiltonian and in the appendix a new relation involving the rotational average of an arbitrary rank Cartesian tensor is presented.