Internuclear-distance dependence of the role of excited states in high-order-harmonic generation of H2+

The interference minimum in the high-order harmonic spectrum of H${}_{2}{}^{+}$ is studied by solving the full three-dimensional time-dependent Schr\"odinger equation for the electronic motion keeping the nuclei fixed. The two-center interference model works well when the internuclear distance is around its equilibrium value where also recombination to the ${}^{2}{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ (1$s$${\ensuremath{\sigma}}_{\mathrm{g}}$) ground state dominates. As the internuclear distance is increased, the minimum first shifts in position compared with the prediction of the two-center interference model and subsequently disappears. These effects are caused by the excited ${}^{2}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ (2$p$${\ensuremath{\sigma}}_{\mathrm{u}}$) state, partly due to the interference between the amplitudes of recombination to the ground and excited states, but also partly due to the signal associated with recombination to the excited state alone. We find that at internuclear distances beyond $R\ensuremath{\simeq}3$ a.u. the signal close to the harmonic cutoff may be completely dominated by recombination into the excited ${}^{2}{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ (2$p$${\ensuremath{\sigma}}_{\mathrm{u}}$) state.