Gauge invariance and photon emission from ground state atoms in the presence of external electromagnetic field

The relevance of gauge invariance in considering the question of emission of photons in the presence of external electromagnetic field from the ground state of quantum mechanical systems is studied. With an exact numerical calculation for atomic hydrogen, it is shown that the photons cannot be unequivocally assigned to have been emitted from the ground state due to the invariance of gauge of the probability for the process