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 N-2, Cl-2, and I-2 molecules when exposed to inten se, 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 (10(15) and 2 x 10(16) W cm(-2)). W e show that the anisotropy of the emission patterns increases with the numb er of absorbed photons necessary to produce the parent molecular ion. The s et of experimental results suggest that the alignment process occurs in the leading edge of the laser pulse at moderate intensities (<10(14) W cm(-2)) where the multiphoton regime prevails. We explain these observations in te rms of the rotational pumping model, as demonstrated by statistical simulat ions of multiphoton absorption and emission. The present study confirms tha t, in most cases, the confined emission of fragments along the laser electr ic field is due to the alignment of the transient molecular ions. [S1050-29 47(99)02710-9].