Using Bohmian trajectory(BT) method, we investigate the dynamic interference in high-order harmonic generation from diatomic molecular ions. It is demonstrated that the main characteristics of the molecular harmonic spectrum can be well reproduced by only two BTs which are located at the two ions. This happens because these two localized trajectories can receive and store the whole collision information coming from all of the other re-collision trajectories. Therefore,the amplitudes and frequencies of these two trajectories represent the intensity and frequency distribution of the harmonic generation. Moreover, the interference between these two trajectories shows a dip in the harmonic spectrum, which reveals the molecular structure information.
Using the frequency-domain theory, we investigate the above-threshold ionization(ATI) process of an atom in twocolor laser fields. When both photon energies of the two-color laser fields are much smaller than the atomic ionization threshold, the ATI spectrum depends on the angle between the two lasers' polarization directions. While when the photon energy of one laser is comparable with or larger than the atomic ionization threshold, the ATI spectrum is independent of the angle, and only several dips appear at certain angles. By analyzing the contributions of different quantum channels, we find that, for the case that both frequencies of the two color lasers are low, the quantum interferences between the channels are strong, and hence the spectrum changes with the angle between the two lasers' polarization directions. While for the case that the frequency of one of the two color lasers is high, the contributions of the channels to the ATI spectrum decrease dramatically with increasing channel order, hence the interferences between the channels disappear, and the ATI spectrum has a step-like structure, which is independent of the angle between the two lasers' polarizations. These results can shed light on the study of the corresponding relation between classical and quantum mechanisms of the matter–laser interaction in high-frequency laser fields.
