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Controlling paths of high-order harmonic generation from H 2 + is theoretically investigated by numerically solving the time-dependent Schrödinger equation based on the Born–Oppenheimer approximation in orthogonal two-color fields. Our simulations show that the change of harmonic emission paths is dependent on time-dependent distribution of electrons. Compared with one-dimensional linearly polarized long wavelength laser, multiple returns are suppressed and short paths are dominant in the process of harmonic emission by two-dimensional orthogonal ω/2ω laser fields. Furthermore, not only are multiple returns weaken, but also the harmonic emission varies from twice to once in an optical cycle by orthogonal ω/1.5ω laser fields. Combining the time–frequency distributions and the time-dependent electron wave packets probability density, the mechanism of controlling paths is further explained. As a result, a 68-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.

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The high-order harmonic generation from H[Formula: see text] has been theoretically investigated by numerically solving the two-dimensional time-dependent Schrödinger equation. A sharp minimum can be seen in the harmonic spectrum for larger internuclear distance. Moreover, the electron transition process between the ground state and the first excited state competes with the ionization process at larger internuclear distance, which is the main reason of the spectral minimum. By superposing the static electric field and adjusting the orientation angle [Formula: see text], it is further proved that the electron transition frequency between two lowest electronic states can be decoded by the minimum.

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