Gauge invariant interpretation of multiphoton transition probabilities

It is shown by an elementary discussion that the proper discrimination between the energy operator of the system in a specified gauge and the so-called unperturbed Hamiltonian, as recently emphasised by Yang (1976), resolves apparent difficulties with the gauge invariant interpretation of multiphoton transition probabilities in some of the current literature. In particular, conditions more general than those usually quoted are established, under which either of the interactions -qx.E and -qA.p/m gives the same transition probabilities with the same unperturbed wavefunctions.