Ejection energy of photoelectrons in strong-field ionization

We show that zero ejection energy of the photoelectrons is classically impossible for hydrogenlike ions, even when field ionization occurs adiabatically. To prove this we transform the basic equations to those describing two two-dimensional anharmonic oscillators. The same method yields an alternative way to derive the anomalous critical field of hydrogenlike ions. The analytical results are confirmed and illustrated by numerical simulations.