Strong-field dissociation dynamics of NO$^{2+}$: A multiphoton electronic or vibrational excitation?*

A 3-D momentum imaging technique is employed to study intense ultrafast laser-induced dissociation of a metastable NO$^{2+}$ beam. We focus on N$^{+ }$+ O$^{+}$ coincidences and explore possible dissociation pathways using estimates of the initial vibrational population and transition rates between the X $^{2}\Sigma ^{+ }$and A $^{2}\Pi $ states together with kinetic energy release and angular distribution spectra. Our analysis suggests that lower intensity pulses ($<$10$^{15}$ W/cm$^{2})$ drive perpendicular transitions between these states. Higher intensity pulses ($\sim $10$^{16}$ W/cm$^{2})$, on the other hand, yield a prominent contribution from molecules breaking parallel to the polarization. While the results are preliminary, an intriguing possibility is that this feature is due to a direct 2-photon transition to the vibrational continuum of the X $^{2}\Sigma ^{+}$ state, $i.e.$, a multiphoton vibrational excitation on the electronic ground state.