Alignment enhancement by the combination of a short and a long laser pulse

The alignment dynamics of polyatomic molecules subject to two temporally overlapping nonresonant laser pulses is studied theoretically and experimentally. We examine the potential advantage of combining a long (compared to the molecular rotational periods) with a short pulse, where both laser fields are linearly polarized with common polarization direction. Experimentally, iodobenzene molecules are irradiated by a 2 ps pulse at 800 nm synchronized to the peak of a 9 ns pulse at 1064 nm. The alignment dynamics following the short laser pulse is measured using time-resolved Coulomb explosion with a delayed 25 fs -long intense laser pulse. The alignment attained with the two-pulse combination is significantly enhanced compared to that attainable with either a short or a long pulse alone under nonionizing conditions. Numerically, we solve the time dependent Schrödinger equation nonperturbatively for asymmetric top, symmetric top, and linear molecules subject to similar combinations of long and short excitation pulses. The alignment dynamics, and in particular the alignment enhancement, are explained and their generality is tested.