Alignment of diatomic molecules in a free-jet expansion

Abstract

We describe the results of a polarised fluorescence study of the spatial distribution of alignment of I2 molecules seeded in a free jet of helium gas. We have observed that molecular alignment tends to increase towards the edges of the jet in a manner reminiscent of that observed in fluid flow. This suggests that the alignment of seeded species is induced during the fluid-like flow region that characterises the supersonic jet expansion. The distribution of alignment results from the spatial distribution of shear forces, a consequence of the transverse velocity gradients. This alignment is not lost in the gas-dynamic region since collisions that change the m quantum number are extremely inefficient. This interpretation suggests that the assumption of inviscid flow in the continuum region is inappropriate for discussing molecular alignment though might be valid under high Reynolds number conditions. It may also indicate that careful design of the nozzle or capillary might give internal shear fields that would lead to molecular orientation along with or instead of molecular alignment.