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

Numerical solutions of the time-dependent Schrodinger equation for the one- and two-electron linear molecular ions H-3(2+), H-3(+), H-4(3+), H-4(2+), H-5(4+), and H-5(3+) at fixed internuclear distance and parallel to the laser polarization have been obtained in intense laser pulses (I greater than or equal to 10(14) W/cm(2)) 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 I-p + 3Up atomic maximum order cutoff law, where Ip is the ionization potential and U-p the ponderomotive energy. A second plateau is usually found at critical distances R-c = (pi/2)alpha(0) with a cutoff at energy I-p + 6U(p) in one-electron systems and I-p + 12U(p) for two-electron systems, where alpha(0) 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. [S1050-2947 (99)02501-9].