Fragment-emission patterns from the Coulomb explosion of diatomic molecules in intense laser fields

We have systematically studied emission patterns of fragments arising from the Coulomb explosion of N₂, Cl₂, and I₂ molecules when exposed to intense, linearly polarized laser fields. The experiments are performed at 395, 610, and 790 nm, using two different pulse durations (130 fs and 2 ps) and choosing two different peak intensities (10¹⁵ and 2×10¹⁶Wcm^-2). We show that the anisotropy of the emission patterns increases with the number of absorbed photons necessary to produce the parent molecular ion. The set of experimental results suggest that the alignment process occurs in the leading edge of the laser pulse at moderate intensities (<10¹⁴Wcm^-2) where the multiphoton regime prevails. We explain these observations in terms of the rotational pumping model, as demonstrated by statistical simulations of multiphoton absorption and emission. The present study confirms that, in most cases, the confined emission of fragments along the laser electric field is due to the alignment of the transient molecular ions.