The non-adiabatic alignment of diatomic molecules is examined in experiment and numerical simulations using as an example the nitrogen molecule. Numerical simulations are carried out to elucidate alignment of rotationally excited molecules, in particular for laser pulse duration and in the transition region between the non-adiabatic and adiabatic regimes. Depending on the M quantum number, the interaction with the laser field can involve emission of energy as well as absorption, and these two

more »

... ocesses lead to opposite motion of the molecular axis with respect to the laser polarization direction. For a moderately strong laser pulse with a duration comparable to the molecular rotational period, we show that the crossover from non-adiabatic to adiabatic limits is well described by a convolution of excitation pulse envelope and sinusoidal molecular response and it takes place in a uniform way in the region between 0.1 and 1 for the ratio of pulse duration to rotational period. For higher intensities, the rotational wave packet contains a greater manifold of coupled rotational levels, for which the degree of adiabaticity varies. Differences in the rotational period in the excited transitions for appropriate laser pulse parameters lead to the formation of a region in which anti-alignment, or rotational cooling, dominates over alignment, or rotational heating. Based on the dependence of post-pulse alignment on the ratio of rotational time and pulse duration, a method for controlling the J composition of a rotational wave packet is proposed. The laser pulse duration is used as parameter to achieve selective non-adiabatic interaction in the rotationally cold part of the thermal distribution. By optimizing the duration of a single pulse, arbitrarily narrow distributions at low J levels can be formed. A double pulse excitation, where a longer second pulse acts as a selective dump pulse, allows to prepare non-thermal distributions centered at high J values. Experimentally, the decay of alignment in nitrogen and mixtures of [...]