Velocity dependence of collisional alignment of oxygen molecules in gaseous expansions

Abstract

THE orientational dependence of molecular interactions has long been recognized as central to an understanding of reaction mechanisms and of collisions in the gas phase and at surfaces. Studies of orientation effects have recently become possible owing to the development of techniques for aligning molecules. 'Brute-force' methods using electric or magnetic fields can induce alignment of molecules with dipole moments1,2, and polarized-absorption approaches3 can be used in cases where there are suitable molecular transitions; but one of the simplest and most general methods involves the supersonic expansion of molecular beams seeded with molecules that induce rotational alignment—selection of specific rotational states—by collisions4–12. Here we use such an approach to induce strong rotational alignment of oxygen molecules in a beam seeded with various other gases at close to atmospheric pressure. Most significantly, we find that the degree of alignment depends on the velocity of the molecules in the supersonic expansion—fast molecules are much more highly aligned than slower ones, and the velocity of maximum alignment can be altered by changing the gas mixture. In this way, we can prepare rotationally aligned molecules with well defined velocities, opening up new possibilities for experiments in molecular dynamics.