Distinguishing two mechanisms for enhanced ionization of H2+ using orthogonal two-color laser fields

We theoretically study the ionization enhancement of the diatomic molecular ion ${\mathrm{H}}_{2}{}^{+}$ at two critical internuclear distances $R$, using orthogonal two-color laser fields. The polarization of the fundamental infrared laser field and a weak second-harmonic field is parallel and perpendicular to the molecular axis, respectively. It is observed that adding the second-harmonic field raises slightly the first ionization peak at the smaller critical $R$, whereas it enhances the second one at the larger critical $R$ significantly. We further analyze the observable evidence which distinguishes two underlying mechanisms responsible for the enhanced ionization of ${\mathrm{H}}_{2}{}^{+}$: (i) the resonant excitation along with the coherent interference of the ionizing wave packets from the $1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u}$ states and (ii) the easier ionization from the up-field site of the molecule.