Harmonic generation by theH2+molecular ion in intense laser fields

The harmonic-generation (HG) spectra of the ${\mathrm{H}}_2^{+}$ molecular ion generated by short, intense, linearly polarized laser pulses are calculated nonperturbatively by solving a three-dimensional time-dependent Schrödinger equation. It is found that while ${\mathrm{H}}_2^{+}$ radiates in a similar manner to an atomic system such as H or ${\mathrm{He}}^{+}$ at high frequencies, the molecular ion in high-vibrational states or at large internuclear distances behaves analogously to a two-level system in an intense, relatively-long-wavelength laser field. It will be shown that symmetric molecular ions should in general produce more efficient harmonic generation than atoms, due to the presence of molecular charge-resonance (CR) states, which have no atomic analogues. Two HG plateaus, one molecular due to CR transitions and one atomiclike, appear for a long-wavelength excitation. Each plateau has its own maximum harmonic-number cutoff, which can be rationalized in terms of the maximum energy which can be acquired by an electron in the laser field.