VELOCITY DEPENDENCE OF COLLISIONAL ALIGNMENT OF OXYGEN MOLECULES IN GASEOUS EXPANSIONS

THE orientational dependence of molecular interactions has long been r ecognized as central to an understanding of reaction mechanisms and of collisions in the gas phase and at surfaces. Studies of orientation e ffects have recently become possible owing to the development of techn iques for aligning molecules. 'Brute-force' methods using electric or magnetic fields can induce alignment of molecules with dipole moments( 1,2), and polarized-absorption approaches(3) can be used in cases wher e there are suitable molecular transitions; but one of the simplest an d most general methods involves the supersonic expansion of molecular beams seeded with molecules that induce rotational alignment-selection of specific rotational states-by collisions(4-12). Here we use such a n approach to induce strong rotational alignment of oxygen molecules i n a beam seeded with various other gases at close to atmospheric press ure. Most significantly, we find that the degree of alignment depends on the velocity of the molecules in the supersonic expansion-fast mole cules are much more highly aligned than slower ones, and the velocity of maximum alignment can be altered by changing the gas mixture. In th is way, we can prepare rotationally aligned molecules with well define d velocities, opening up new possibilities for experiments in molecula r dynamics.