Direct evidence of the dominant role of multiphoton permanent-dipole transitions in strong-field dissociation of NO2+

We study laser-induced dissociation of a metastable NO2+ ion-beam target into N+ + O+, focusing on the prominent contribution by molecules breaking parallel to the polarization at high peak laser intensity (similar to 10(15) W/cm(2)). Our experimental results and time-dependent Schrodinger equation calculations show that, contrary to commonly held intuition that electronic transitions always prevail, the dominant process underlying this highly aligned dissociation is a multiphoton permanent-dipole transition involving only the electronic ground state and leading to its vibrational continuum. Strong-field permanent-dipole transitions should thus be considered generally, as they may play a significant role in other heteronuclear molecules. Moreover, their role should only grow in importance for longer wavelengths, a trending direction in ultrafast laser studies.