High-order harmonic generation by one- and two-electron molecular ions with intense laser pulses

Numerical solutions of the time-dependent Schr\"odinger equation for the one- and two-electron linear molecular ions H₃²⁺, H₃⁺, H₄³⁺, H₄²⁺, H₅⁴⁺, and H₅³⁺ at fixed internuclear distance and parallel to the laser polarization have been obtained in intense laser pulses (I>~10¹⁴ W/cm²) in order to examine the effect of electron-electron interactions on high-order harmonic generation in extended (delocalized) systems. It is found that, in general, two-electron effects produce longer plateaus at large internuclear distances, thus enhancing harmonic generation well beyond the Iₚ+3Uₚ atomic maximum order cutoff law, where Iₚ is the ionization potential and Uₚ the ponderomotive energy. A second plateau is usually found at critical distances Rc=(π/2)α₀ with a cutoff at energy Iₚ+6Uₚ in one-electron systems and Iₚ+12Uₚ for two-electron systems, where α₀ is the ponderomotive radius. Initial delocalized states, such as molecular orbitals, are shown to create longer harmonic generation plateaus and one can therefore attribute enhancement of harmonic generation plateaus by electron repulsion due to the increased separation and delocalization of electrons in extended systems by correlation effects.