By using Kramers-Henneberger unitary transformation, the rapidly oscillating perturbation is transposed to the argument of a potential function. It is shown that the depth of the potential-energy well in which the electron is localized decreases monotonically with an increase in the strength of the external field. The analytic technique of a shifted 1/N expansion was used to express the position of the ground energy level as a function of the external-source intensity. The critical value of a parameter characterizing the external source is found for which the ground bound quasi-discrete electron level vanishes. The localized-to-delocalized-state transition is identifying as a diffuse nonmetal-metal phase transition.