The harmonic-generation (HG) spectra of the H-2+ molecular ion generat
ed by short, intense, linearly polarized laser pulses are calculated n
onperturbatively by solving a three-dimensional time-dependent Schrodi
nger equation. It is found that while H-2+ radiates in a similar manne
r to an atomic system such as H or He+ at high frequencies, the molecu
lar ion in high-vibrational states or at large internuclear distances
behaves analogously to a two-level system in an intense, relatively-lo
ng-wavelength laser field. It will be shown that symmetric molecular i
ons 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 excita
tion. Each plateau has its own maximum harmonic-number cutoff, which c
an be rationalized in terms of the maximum energy which can be acquire
d by an electron in the laser field.