We study the perturbation caused by the variation of parameters defining the quantum system itself, as would be the case of a temporal change in the nuclear charge of a hydrogenic atom. It is shown that the system remains practically in the original state whenever the variation of the parameter in relation with the period of the quantum oscilations is small. This is not the case for a sudden variation - as in the case of a nuclear b desintegration -, when finite excitations do occur. In this case is easier to match by invoking the continuity of the wave function during the sudden parameter change in order to obtain the excited amplitudes.
parametric perturbation; time-dependent quantum perturbation
