The effect of the method of switching on of a strong electromagnetic laser
field on the shape of dispersion curves in the anomalous dispersion range i
s considered. The smooth function of switching on the wavefront is approxim
ated by a piecewise continuous monotonically increasing function. The form
of the dispersion curves in the resonance region is shown to be determined
by the nonadiabatic mixing of the quasi-energy states, depending on the rat
e of the field switching on.