How Molecular Frisbees Could Encourage Nuclear Proliferation

Anybody who has played with a frisbee will have an intuitive idea of how the angle at which you throw them determines the path they take.

As frisbees get smaller, however, the physics changes. On a tiny scale, the air becomes thicker like syrup and inertia begins to play a much smaller role. So it’s easy to think that there is fundamental limit to how small you can make a frisbee.

Not quite, say Johannes Floss and buddies at the The Weizmann Institute of Science in Israel. It’s actually fairly straightforward to control the trajectory of a spinning molecule just like a frisbee.

In recent years, a number of techniques have emerged to set molecules in a gas spinning with their axes precisely aligned, like a three dimensional array of floating tops. These techniques all zap the molecules with a carefully prepared laser pulse to make them rotate in a certain way.

But how to turn these spinning tops into frisbees? After all, the motion of frisbees is essentially the result of the interaction between the spinning body and air, but aerodynamics cannot play a role at the molecular level.

The answer say Floss and co is to fire the spinning molecules through an electric field produced by another laser. Provided the field has some intensity gradient, it will play a role analogous to air in frisbee flight. When that happens, the inclination of the spinning molecules will determine the trajectory they take.

As Floss and co point out: “A similar technique is used by Frisbee players finessing the tilt of the spinning disc for directing it into a pair of waiting hands.”

This frisbee technique gives remarkable control over the path the molecules take. The trajectory depends on factors such as the strength of the field, the inclination of rotation and the mass of the molecule.

This has important implications for a number of emerging techniques, particularly in areas where ionisation cannot be used. For example, molecular nanofabrication in which tiny structures are built almost brick by brick must use neutral molecules because the build up of charge could distort the shape or even prevent construction entirely.

But perhaps the most important application, at least in the short term, will be isotope separation. Since the trajectory depends on the mass of the molecule, the technique will naturally separate molecules containing different isotopes.

Nuclear scientists will want to investigate this technique’s potential for separating the more fissionable uranium 235 from uranium 238. In recent years, physicists have made great strides in separating these isotopes using lasers to selectively ionise one isotope while leaving the other neutral, which allows them to be separated using an electric field.

The conventional separation techniques rely on giant centrifuges that are difficult and expensive to build and so form an important technology barrier that prevents countries with nuclear aspirations from making their own highly enriched uranium.

But there is a growing fear that laser enrichment will make this much process easier. And now there is a new technique that could make isotope separation even easier.

That makes it easy to predict that molecular frisbees will become the focus of intense interest in the next few years. But how much we’ll hear about these future developments is much harder to say.

Ref: arxiv.org/abs/1010.0887: Molecular Frisbee: Motion of Spinning Molecules in Inhomogeneous Fields