Generation of Isolated Attosecond Pulse from Asymmetric Molecular Ions by Introducing Half-Cycle-Like Laser Fields

We propose an efficient method for the generation of an isolated attosecond pulse from the asymmetric molecular ions HeH²⁺ by adding a half-cycle-like field (HCLF) to the fundamental driving laser field. The high-order harmonic generation (HHG) is investigated by numerically solving the time-dependent Schr-dinger equation. By performing the time-frequency distributions and the electronic wave packet probability densities, we find that the optimizing combined field is not only useful for extending the HHG cutoff, but also for simplifying the recombination channels through controlling the electron localization. In addition, by adjusting the intensity of the HCLF, a dominant short quantum path is selected to contribute the HHG spectrum. As a result, a 75-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.