Corrections to Observed Atomic Resonance

All the atomic standards we shall be dealing with are based, in one form or another, on the resonant excitation of atoms or ions, by which they make transitions from one quantum state to another. From the observed resonance spectrum we must arrive at the intrinsic, or proper, frequency of the atoms' response, as it would be observed if they were at rest and free from any outside perturbation. Such perturbations will alter and broaden the resonance spectrum and put a limit on the degree of precision with which the intrinsic atomic frequency can be deduced.