Impulsive longitudinal molecular alignment

We report on measurements of a rotational wavepacket in diatomic nitrogen created impulsively with a circularly polarized 800 nm ultrfast pulse. This wavepacket differs from those typically produced in impulsive alignment experiments with linearly polarized light in that it exhibits transient alignment along the direction of laser propagation. In this experiment, The initial anisotropy in the angular distribution is torroidal within the plane of polarization, wheras the time averaged anisotropy lies perpendicular to this plane. This is in stark contrast to the linearly polarized case where both the initial and time averaged anisotropies lie along the electric field vector. In order to probe the wavepacket evolution in a co-propagating geometry, we use an interferometric measurement of the time varying index of refraction.