Choice of gauge and multiphoton transitions

In a recent paper, Kobe (see Phys. Rev. Lett., vol.40, no.9, p.538 (1978)) has stated that the result obtained for non-resonant two-photon absorption depends on the choice of the coupling between the atom and the electromagnetic field (choice of gauge). The authors believe that his demonstration, even if mathematically exact, has no physical implications. They demonstrate that there are small terms due to relaxation processes omitted by Kobe which exactly compensate the difference between the values obtained, with the different couplings.