Spatial orientation of molecules in strong electric fields and evidence for pendular states

Abstract

IN typical collisional or spectroscopic experiments, molecules rotate freely with random spatial orientations. The resulting isotropic averaging obscures or suppresses much stereodynamical information and has remained a recalcitrant problem. The only practical means for orienting a molecule itself, rather than just its axis of rotation, has been electric field focusing^1,2. But this is applicable only to certain rotational states of symmetric top molecules (or equivalent) that exhibit a first-order Stark effect. Orientation of molecules other than symmetric tops has long been considered to be quite unfeasible³. Recently, however, it has been pointed out^4,5 that by exploiting the extreme rotational cooling that can occur in supersonic molecular beams, substantial orientation of diatomic, linear or asymmetric top molecules should become possible at accessible field strengths. The anisotropy of the Stark effect allows molecules in the lowest few rotational states to be trapped in 'pendular states' and thereby confined to librate (oscillate about the field axis) over a limited angular range. Here we describe an experiment which demonstrates that oriented pendular states can be obtained for a diatomic molecule with modest field strengths. With anticipated improvements, this technique should become widely applicable.