  The laser-induced fragmentation dynamics of this most fundamental polar
molecule HeH$^+$ are measured using an ion beam of helium hydride and an
isotopologue at various wavelengths and intensities. 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 nonionizing 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 leading to a more
general picture of strong-field molecular dynamics and facilitating
interpolation to systems between the two extreme cases.
