High-order harmonic generation in laser-irradiated homonuclear diatomics: The velocity gauge version of the molecular strong-field approximation

The strong-field process of high-order harmonic generation in laser-irradiated light homonuclear diatomics (H ₂ ⁺ , N₂, and O₂) is studied compared to atomic counterparts (of nearly identical ionization potential) within the velocity gauge version of conventional strong-field approximation. The applied strong-field approach (we alternatively developed earlier to incorporate rescattering effects beyond the quasiclassical saddle-point approximation) is currently extended to molecular case by means of supplementing the standard linear combination of atomic orbitals and molecular orbitals method. The associated model proved to adequately reproduce a general shape and detailed structure of molecular harmonic spectra, which demonstrate a number of remarkable distinctive differences from respective atomic spectra calculated under the same laser pulses. These revealed differences as well as other generic features (such as the extent of the high-frequency plateau and harmonic emission rates) are found to be strongly dependent on internuclear separation and also very sensitive to the orbital and bonding symmetry of the contributing molecular valence shell. In particular, for some group of harmonics, the emission rates were ascertained to dominate by contribution from inner molecular shells of higher binding energy and different orbital symmetry compared to the outermost molecular orbital normally predominantly contributing. Finally, due to a high suppression in ionization of O₂ relative to counterpart atomic Xe, the harmonic spectrum calculated for O₂ proved to demonstrate a noticeably longer plateau extending far beyond the harmonic cutoff in the Xe spectrum, in a fairly good accordance with relevant experiments.