  New approach to study the spontaneous emission of the atomic system in the
presence of the high-intensity laser field is used to study the process of
harmonic generation. The analysis is based on the consideration of quantum
system interaction with quantized field modes beeing in vacuum state, while the
intense laser field is considered classically beyond the perturbation theory.
The numerical analysis of the emission from the single one-electron one
dimensional atom irradiated by the femtosecond laser pulse of Ti-Sa laser is
discussed. It is demonstrated that not only odd but also even harmonics can be
emitted if the laser field is strong enough. The origin of appearance of even
harmonics is studied. The obtained results are compared with that found in the
frames of semiclassical approach widely used to study the harmonic generation.
It is found that semiclassical approach is inapplicable in the strong-field
limit.
