Ionization of highly excited states of the hydrogen atom by a strong low-frequency field

The probability for ionization of highly excited states of the hydrogen atom by a low-frequency field is estimated by applyjng the quasiclassical wave function for an electron in a Coulomb field or radiation field obtained earlier. The expression predicts an ionization threshold at approximately the same field strengths as those observed experimentally. However the growth of the ionization probability on increase of the field strength in the vicinjty of the threshold is about ten times higher than that observed experimentally. The approximations on which the derivation of the formula for the ionization probability is based are discussed