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

We utilize a 3-D momentum imaging technique to study laser-induced dissociation of a metastable NO$^{2+}$ beam into N$^{+}$+ O$^{+}$. Using an estimated initial vibrational population, measured kinetic energy release and angular distribution spectra, and time-dependent Schr\"{o}dinger equation calculations, we identify the most likely dissociation pathways. While lower intensity pulses ($<$10$^{15}$ W/cm$^{2}$) drive perpendicular transitions between the lowest two electronic states, for higher intensity pulses ($\sim $10$^{16}$ W/cm$^{2}$), dissociation parallel to the laser polarization becomes prominent. Contrary to commonly-held intuition that electronic transitions always prevail, we find that the dominant process underlying this highly-aligned feature is a multiphoton permanent dipole transition solely within the electronic ground state, leading to its continuum.