Molecular reorientation during dissociative multiphoton ionization

The angular distribution of the ion fragments after multiphoton ionization of diatomic molecules shows a pronounced anisotropy. Fragments are detected mainly along the direction of the laser polarization, as is shown for iodine in this paper. We have performed a double-pulse experiment to clarify the underlying mechanism. It shows that the anisotropy cannot be explained in terms of a variation of the ionization rates with the angle between the electric field and the molecular axis. Instead, the anisotropy is caused by the deflection of the ion fragments in the strong field due to the molecular polarizability. Although the molecules are frozen in space on the time scale of the laser pulse, they gain enough angular momentum to affect the trajectories of the fragments. Classical trajectory calculations support this explanation.