The high-order harmonic radiation is generated in atomic gases irradiated by high-power laser pulses. The question of when each harmonic is emitted is investigated and that is related to the dynamics of electrons in atoms under the high-power laser fields. Information on when low-order harmonics are emitted can be obtained by monitoring the temporal variation of low-lying eigenstates of the atom. In order to obtain a comprehensive information on the emission times of both high- and low-order harmonics, however, the time-frequency analysis of the dipole acceleration using the Wigner and Husimi distribution functions is needed. Our numerical computation of the Wigner and Husimi distributions of the dipole acceleration indicates that low-order harmonics are generated mainly via transitions between bound states and are emitted at times when the laser field amplitude takes on its maximum or minimum value. In the midplateau region, the Wigner and Husimi distributions exhibit a characteristic structure, which indicates that photons of a given harmonic frequency are emitted at two different times within one half optical cycle corresponding to the return times of the two electron paths contributing predominantly to the generation of high-order harmonics. The Wigner and Husimi distributions computed quantum mechanically can be explained semiclassically in terms of the three-step model. The phenomenon of high harmonic generation is thus an example that shows a strong degree of quantum-classical correspondence.
