Spatial alignment of diatomic molecules in intense laser fields: I. Experimental results

We present measurements of the laser induced spatial alignment of two diatomic molecules, iodine (I₂) and bromine (Br₂). Dynamic alignment is inferred from the angular distribution of the ionic fragments from multi-electron dissociative ionization (MEDI). The angular distributions were determined for different pulse durations and energies of the short infrared laser pulses that induce the MEDI, and were measured using a velocity map ion imaging detector. The width of the angular distribution of the fragments with respect to the laser polarization axis depends only weakly on the laser pulse energy, and decreases rapidly for longer pulse lengths (a few picoseconds) at constant pulse energy. The interpretation of this result in terms of dynamic alignment is supported by an extended field ionization Coulomb explosion model that includes the rotation of the molecule induced by the laser field.