Rotational excitation and molecular alignment in intense laser fields

Rotational excitation and spatial alignment in moderate intensity radiation fields are studied numerically and analytically, using time-dependent quantum mechanics. Substantial rotational excitation is found under conditions typically used in time-resolved spectroscopy experiments. The broad rotational wave packet excited by the laser pulse is well defined in the conjugate angle space, peaking along the field polarization direction. Both the rotational excitation and the consequent spatial alignment can be controlled by the choice of field parameters. Fragment angular distributions following weak field photodissociation of the rotational wave packet are computed as a probe of the degree of alignment. In the limit of rapid photodissociation the angular distribution is peaked in the forward direction, reflecting the anisotropy of the aligned state. Potential applications of the effect demonstrated range from reaction dynamics of aligned molecules and laser-control to material deposition and laser-assisted isotope separation.