The heteronuclear limit of strong-field ionization: HeH$^+$

A sophisticated setup for measuring laser-induced fragmentation of an ion beam of helium hydride and an isotopologue at various wavelengths and intensities enables us to study the dynamics of this most fundamental polar molecule. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for non-ionizing dissociation, single ionization and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems yielding a much clearer picture of strong-field molecular dynamics in general and facilitating interpolation to other systems.