Enhanced harmonic generation in extended molecular systems by two-color excitation

Numerical simulations of high-order harmonic generation in the molecular ions ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{2+}$ show that two-color intense $\left(I\simeq {10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2\right)$ short $(<\mathrm{}1-\mathrm{psec})$ laser pulse excitation produces harmonics at large internuclear distances with larger efficiencies (intensities) at ${6U}_p,$ ${8U}_p,$ and ${10U}_p$ energies than in single-atom single-frequency excitation. The two-color excitation scheme creates efficient high harmonics by first ionizing electrons with a high-frequency laser pulse and accelerating them with lower frequency towards neighboring atoms. Such two-color schemes should be useful for enhancing high-order harmonic generation in clusters.