State selection in nonresonantly excited wave packets by tuning from nonadiabatic to adiabatic interaction

We show for rotational alignment of diatomic molecules that the crossover from nonadiabatic to adiabatic limits is well described by a convolution of excitation pulse envelope and sinusoidal molecular response and that it takes place in a uniform way in the region between 0.1 and 1 for the ratio of pulse duration to rotational period. In a nonresonant Raman-type excitation, this crossover is used to manipulate the J composition of a rotational wave packet with respect to the initial 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 nonthermal distributions centered at high J values. With the alignment signal on top of an isotropic background, experimental techniques sensitive to the induced anisotropy are optimally suited for implementation. To demonstrate the efficiency of the method, numerical simulations are carried out for rotational alignment in ¹⁴N₂ at various experimentally relevant laser intensities. The scheme is transferable to quantum systems with a significant variation in transition frequencies between subsequent levels.